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Aerosol particles play critical roles in climate and human health. However, aerosol composition and evolution, particularly secondary organic aerosol (SOA), and aerosol interactions with clouds in high-altitude background areas in China remain less understood. Here, we conducted real-time measurements of submicron aerosols (PM 1) using aerosol mass spectrometers at a forested mountain site (1128 m a.s.l.) in southeastern China in November 2022. The average (±1σ) PM 1 mass concentration was 4.3±4.8 µ g m -3 , which was ubiquitously lower than that at other mountain sites in China. Organic aerosol (OA) constituted the largest fraction of PM 1 (42.9 %) and was predominantly secondary, as indicated by the high oxygen-to-carbon (O / C) ratio (0.85–0.96) and carbon oxidation state (0.21–0.49). Notably, the remarkably enhanced PM 1 concentrations observed during the daytime on cloudless days were identified to be likely to be produced from cloud evaporation. While more oxidized oxygenated OA was scavenged efficiently during cloud events, cloud evaporation was found to release a significant amount of less oxidized oxygenated OA from air masses transported from polluted regions. The distinct decrease in OA / Δ CO with the increase in O / C during the cloud evaporative period further demonstrates that OA remaining in cloud droplets is generally in a moderate oxidation state. Moreover, organic nitrates were also estimated and showed a higher contribution to the total nitrate during the cloudy period (27 %) than during the evaporative period (3 %). Overall, our results demonstrate the importance of SOA and the influences of cloud processes in regional mountain areas in southeastern China. [ABSTRACT FROM AUTHOR]

Secondary organic aerosol (SOA) constitutes an important component of atmospheric particulate matter, with a substantial influence on air quality, human health and the global climate. The volatility basis set (VBS) framework has provided a valuable tool for better simulating the formation and evolution of SOA where SOA precursors are grouped by their volatility. This is done in order to avoid the computational cost of simulating possibly hundreds of atmospheric organic species involved in SOA formation. The accuracy of this framework relies upon the accuracy of the volatility distribution of the oxidation products of volatile organic compounds (VOCs) used to represent SOA formation. However, the volatility distribution of SOA-forming vapours remains inadequately constrained within global climate models, leading to uncertainties in the predicted aerosol mass loads and climate impacts. This study presents the results from simulations using a process-scale particle growth model and a global climate model, illustrating how uncertainties in the volatility distribution of biogenic SOA precursor gases affect the simulated cloud condensation nuclei (CCN). We primarily focused on the volatility of oxidation products derived from monoterpenes as they represent the dominant class of VOCs emitted by boreal trees. Our findings reveal that the particle growth rate and their survival to CCN sizes, as simulated by the process-scale model, are highly sensitive to uncertainties in the volatilities of condensing organic vapours. Interestingly, we note that this high sensitivity is less pronounce in global-scale model simulations as the CCN concentration and cloud droplet number concentration (CDNC) simulated in the global model remain insensitive to a 1-order-of-magnitude shift in the volatility distribution of organics. However, a notable difference arises in the SOA mass concentration as a result of volatility shifts in the global model. Specifically, a 1-order-of-magnitude decrease in volatility corresponds to an approximate 13 % increase in SOA mass concentration, while a 1-order-of-magnitude increase results in a 9 % decrease in SOA mass concentration over the boreal region. SOA mass and CCN concentrations are found to be more sensitive to the uncertainties associated with the volatility of semi-volatile compounds, with saturation concentrations of 10 -1 µgm-3 or higher, than the low-volatility compounds. This finding underscores the importance of having a higher resolution in the semi-volatile bins, especially in global models, to accurately capture SOA formation. Furthermore, the study highlights the importance of a better representation of saturation concentration values for volatility bins when employing a reduced number of bins in a global-scale model. A comparative analysis between a finely resolved nine-bin VBS setup and a simpler three-bin VBS setup highlights the significance of these choices. The study also indicates that radiative forcing attributed to changes in SOA over the boreal forest region is notably more sensitive to the volatility distribution of semi-volatile compounds than low-volatility compounds. In the three-bin VBS setup, a 10-fold decrease in the volatility of the highest-volatility bin results in a shortwave instantaneous radiative forcing (IRFari) of - 0.2 ± 0.10 Wm-2 and an effective radiative forcing (ERF) of + 0.8 ± 2.24 Wm-2 , while a 10-fold increase in volatility leads to an IRFari of + 0.05 ± 0.04 Wm-2 and an ERF of + 0.45 ± 2.3 Wm-2 over the boreal forest region. These findings underscore the critical need for a more accurate representation of semi-volatile compounds within global-scale models to effectively capture the aerosol loads and the subsequent climate effects. [ABSTRACT FROM AUTHOR]

Understanding the compositions and evolution of atmospheric organic vapors is crucial for exploring their impact on air quality. However, the molecular and seasonal characteristics of organic vapors in urban areas, with complex anthropogenic emissions and high variability, remain inadequately understood. In this study, we conducted measurements in urban Beijing during 2021-2022 covering four seasons using a Vocus-PTR, an improved Proton Transfer Reaction-Mass Spectrometry (PTR-MS). During the measurement period, a total of 895 peaks are observed, and 543 of them can be assigned to formulas. The contribution of CxHyOz species is most significant, which compose up to 53.7% of the number and 76.0% of the mass of total organics. With enhanced sensitivity and mass resolution, various sub-ppt level species and organics with multiple oxygens (≥3) were discovered. When counting the species number, 42.2% of the organics measured are at sub-ppt level and 37.8% of the species contain 3-8 oxygens. Organic vapors with multiple oxygens mainly consist of intermediate volatility and semi-volatile compounds, and many of them are found to be the multi-generational oxidation products of various volatile organic precursors. In summer, the fast photooxidation process generates organic vapors with multiple oxygens, and leads to an increase in both their concentration and proportion. While in other seasons, the variations of organic vapors with multiple oxygens are closely correlated with those of organic vapors with 1-2 oxygens, which could be heavily influenced by primary emissions. Organic vapors with low oxygen content (≤ 2 oxygens) are comparable to the results obtained by traditional PTR-MS measurements in both urban Beijing and neighboring regions. [ABSTRACT FROM AUTHOR]

Atmospheric new particle formation (NPF) and associated production of secondary particulate matter dominate aerosol particle number concentrations and submicron particle mass loadings in many environments globally. Our recent investigations show that atmospheric NPF produces a significant amount of particles on days when no clear NPF event has been observed/identified. Furthermore, it has been observed in different environments all around the world that growth rates of nucleation mode particles vary little, usually much less than the measured concentrations of condensable vapors. It has also been observed that the local clustering, which in many cases acts as a starting point of regional new particle formation (NPF), can be described with the formation of intermediate ions at the smallest sizes. These observations, together with a recently developed ranking method, lead us to propose a paradigm shift in atmospheric NPF investigations. In this opinion paper, we will summarize the traditional approach of describing atmospheric NPF and describe an alternative method, covering both particle formation and initial growth. The opportunities and remaining challenges offered by the new approach are discussed. [ABSTRACT FROM AUTHOR]

Aerosol particles play crucial roles in both climate dynamics and human health. However, there remains a significant gap in our understanding of aerosol composition and evolution, particularly regarding secondary organic aerosols (SOA), and their interaction with clouds in high-altitude background areas in China. Here we conducted real-time measurements of submicron aerosols (PM1) using aerosol mass spectrometers at a forested mountain site (1128 m a.s.l.) in southeastern China in November 2022. Our results revealed that organic aerosol (OA) constituted a substantial portion of PM1 (41.1 %), with the OA being primarily of secondary origin, as evidenced by a high oxygen-to-carbon (O/C) ratio (0.85–0.96). Positive matrix factorization resolved two distinct SOA factors: less oxidized oxygenated OA (LO-OOA) and more oxidized OOA (MO-OOA). Interestingly, MO-OOA was scavenged efficiently during cloud events, while cloud evaporation contributed significantly to LO-OOA. The ratio of OA/ΔCO increased with a decrease in the O/C ratio, suggesting that OA remaining in cloud droplets generally maintained a moderate oxidation state. Furthermore, our results indicated a higher contribution of organic nitrates to total nitrate during cloudy periods (27 %) compared to evaporative periods (3 %). Notably, a substantial contribution of nitrate in PM1 (20.9 %) was observed, particularly during high PM periods, implying that nitrate formed in polluted areas interacted with clouds and significantly impacted the regional background site. Overall, our study underscores the importance of understanding the dynamics of secondary organic aerosols and the impacts of cloud processing in regional mountainous areas in southeastern China. [ABSTRACT FROM AUTHOR]

Aerosol particles play crucial roles in both climate dynamics and human health. However, there remains a significant gap in our understanding of aerosol composition and evolution, particularly regarding secondary organic aerosols (SOA), and their interaction with clouds in high-altitude background areas in China. Here we conducted real-time measurements of submicron aerosols (PM1) using aerosol mass spectrometers at a forested mountain site (1128 m a.s.l.) in southeastern China in November 2022. Our results revealed that organic aerosol (OA) constituted a substantial portion of PM1 (41.1 %), with the OA being primarily of secondary origin, as evidenced by a high oxygen-to-carbon (O/C) ratio (0.85-0.96). Positive matrix factorization resolved two distinct SOA factors: less oxidized oxygenated OA (LO-OOA) and more oxidized OOA (MO-OOA). Interestingly, MO-OOA was scavenged efficiently during cloud events, while cloud evaporation contributed significantly to LO-OOA. The ratio of OA/ACO increased with a decrease in the O/C ratio, suggesting that OA remaining in cloud droplets generally maintained a moderate oxidation state. Furthermore, our results indicated a higher contribution of organic nitrates to total nitrate during cloudy periods (27 %) compared to evaporative periods (3 %). Notably, a substantial contribution of nitrate in PM1 (20.9 %) was observed, particularly during high PM periods, implying that nitrate formed in polluted areas interacted with clouds and significantly impacted the regional background site. Overall, our study underscores the importance of understanding the dynamics of secondary organic aerosols and the impacts of cloud processing in regional mountainous areas in southeastern China. [ABSTRACT FROM AUTHOR]

Secondary organic aerosol (SOA) constitutes an important component of atmospheric particulate matter, with substantial influence on air quality, human health and the global climate. Volatility basis set (VBS) framework has provided a valuable tool for better simulating the formation and evolution of SOA where SOA precursors are grouped by their volatility. This is done in order to avoid the computational cost of simulating possibly hundreds of atmospheric organic species involved in SOA formation. The accuracy of this framework relies upon the accuracy of the volatility distribution of the oxidation products of volatile organic compounds (VOCs) used to represent SOA formation. However, the volatility distribution of SOA forming vapours remains inadequately constrained within global climate models, leading to uncertainties in the predicted aerosol mass loads and climate impacts. This study presents the results from simulations using a process-scale particle growth model and a global climate model, illustrating how uncertainties in the volatility distribution of biogenic SOA precursor gases affects the simulated cloud condensation nuclei (CCN). We primarily focused on the volatility of oxidation products derived from monoterpenes as they represent the dominant class of VOCs emitted by boreal trees. Our findings reveal that the particle growth rate and their survival to CCN sizes, as simulated by the process scale model, are highly sensitive to uncertainties in the volatilities of condensing organic vapours. Interestingly, we note that this high sensitivity is less pronounce in global scale model simulations, as the CCN concentration and cloud droplet number concentration (CDNC) simulated in the global model remain insensitive to a one order of magnitude shift in the volatility distribution of organics. However, a notable difference arises in the SOA mass concentration as a result of volatility shifts in the global model. Specifically, a one order of magnitude decrease in volatility corresponds to an approximate 13 % increase in SOA mass concentration, while, a one order of magnitude increase results in a 9 % decrease in SOA mass concentration over the boreal region. SOA mass and CCN concentrations are found to be more sensitive to the uncertainties associated with the volatility of semi-volatile compounds than the low-volatile compounds. Furthermore, the study highlights the importance of a better representation of saturation concentration values for volatility bins when employing a reduced number of bins in a global scale model. A comparative analysis between a finely resolved 9-bin VBS setup and a simpler 3-bin VBS setup highlights the significance of these choices. The study also indicates that radiative forcing attributed to changes in SOA is notably more sensitive to the volatility distribution of semi-volatile compounds than low-volatile compounds. In the 3-bin VBS setup, a ten-fold decrease in volatility of the highest volatility bin results in a shortwave instantaneous radiative forcing (IRF ari) of -0.2 ± 0.10 Wm−2 and an effective radiative forcing (ERF) of +0.8 ± 2.24 Wm−2, while a ten-fold increase in volatility leads to an IRF ari of +0.05 ± 0.04 Wm−2 and ERF of +0.45 ±2.3 Wm−2. These findings underscore the critical need for a more accurate representation of semi-volatile compounds within global scale models to effectively capture the aerosol loads and the subsequent climate effects. [ABSTRACT FROM AUTHOR]

Secondary organic aerosol (SOA) constitutes an important component of atmospheric particulate matter, with substantial influence on air quality, human health and the global climate. Volatility basis set (VBS) framework has provided a valuable tool for better simulating the formation and evolution of SOA where SOA precursors are grouped by their volatility. This is done in order to avoid the computational cost of simulating possibly hundreds of atmospheric organic species involved in SOA formation. The accuracy of this framework relies upon the accuracy of the volatility distribution of the oxidation products of volatile organic compounds (VOCs) used to represent SOA formation. However, the volatility distribution of SOA forming vapours remains inadequately constrained within global climate models, leading to uncertainties in the predicted aerosol mass loads and climate impacts. This study presents the results from simulations using a process-scale particle growth model and global climate model, illustrating how uncertainties in the volatility distribution of biogenic SOA precursor gases affects the simulated cloud condensation nuclei (CCN). We primarily focused on the volatility of oxidation products derived from monoterpenes as they represent the dominant class of VOCs emitted by boreal trees. Our findings reveal that the particle growth rate and their survival to CCN sizes, as simulated by the process scale model, are highly sensitive to uncertainties in the volatilities of condensing organic vapours. Interestingly, we note that this high sensitivity is less pronounce in global scale model simulations, as the CCN concentration and cloud droplet number concentration (CDNC) simulated in the global model remain insensitive to a one order of magnitude shift in the volatility distribution of organics. However, a notable difference arises in the SOA mass concentration as a result of volatility shifts in the global model. Specifically, a one order of magnitude decrease in volatility corresponds to an approximate 13% increase in SOA mass concentration, while, a one order of magnitude increase results in a 9% decrease in SOA mass concentration over the boreal region. SOA mass and CCN concentrations are found to be more sensitive to the uncertainties associated with the volatility of semi-volatile compounds than the low-volatile compounds. Furthermore, the study highlights the importance of a better representation of saturation concentration values for volatility bins when employing a reduced number of bins in a global scale model. A comparative analysis between a finely resolved 9-bin VBS setup and a simpler 3-bin VBS setup highlights the significance of these choices. The study also indicates that radiative forcing attributed to changes in SOA is notably more sensitive to the volatility distribution of semi-volatile compounds than low-v olatile compounds. In the 3-bin VBS setup, a ten-fold decrease in volatility of the highest volatility bin results in a shortwave instantaneous radiative forcing (IRFari) of -0.2 ± 0.10 Wm-2 and an effective radiative forcing (ERF) of +0.8 ± 2.24 Wm-2, while a ten-fold increase in volatility leads to an IRFari of +0.05 ± 0.04 Wm-2 and ERF of +0.45 ± 2.3 Wm-2. These findings underscore the critical need for a more accurate representation of semi-volatile compounds within global scale models to effectively capture the aerosol loads and the subsequent climate effects. [ABSTRACT FROM AUTHOR]

We present a novel photolytic source of gas-phase NO3 suitable for use in atmospheric chemistry studies that has several advantages over traditional sources that utilize NO2 + O3 reactions and/or thermal dissociation of dinitrogen pentoxide (N2O5). The method generates NO3 via irradiation of aerated aqueous solutions of ceric ammonium nitrate (CAN, (NH4)2Ce(NO3)6) and nitric acid (HNO3) or sodium nitrate (NaNO3). We present experimental and model characterization of the NO3 formation potential of irradiated CAN / HNO3 and CAN / NaNO3 mixtures containing [CAN] = 10 −3 to 1.0 M, [ HNO3 ] = 1.0 to 6.0 M, [ NaNO3 ] = 1.0 to 4.8 M, photon fluxes (I) ranging from 6.9 × 10 14 to 1.0 × 10 16 photons cm -2 s -1 , and irradiation wavelengths ranging from 254 to 421 nm. NO3 mixing ratios ranging from parts per billion to parts per million by volume were achieved using this method. At the CAN solubility limit, maximum [ NO3 ] was achieved using [ HNO3 ] ≈ 3.0 to 6.0 M and UVA radiation (λmax⁡ = 369 nm) in CAN / HNO3 mixtures or [ NaNO3 ] ≥ 1.0 M and UVC radiation (λmax⁡ = 254 nm) in CAN / NaNO3 mixtures. Other reactive nitrogen (NO2 , N2O4 , N2O5 , N2O6 , HNO2 , HNO3 , HNO4) and reactive oxygen (HO2 , H2O2) species obtained from the irradiation of ceric nitrate mixtures were measured using a NOx analyzer and an iodide-adduct high-resolution time-of-flight chemical ionization mass spectrometer (HR-ToF-CIMS). To assess the applicability of the method for studies of NO3 -initiated oxidative aging processes, we generated and measured the chemical composition of oxygenated volatile organic compounds (OVOCs) and secondary organic aerosol (SOA) from the β -pinene + NO3 reaction using a Filter Inlet for Gases and AEROsols (FIGAERO) coupled to the HR-ToF-CIMS. [ABSTRACT FROM AUTHOR]

The multi-scheme chemical ionisation inlet 1 (MION1) enables rapid switching between the measurement of atmospheric ions without chemical ionisation and neutral molecules using various atmospheric pressure chemical ionisation methods. In this study, we introduce the upgraded version, the multi-scheme chemical ionisation inlet 2 (MION2). The new design incorporates enhanced ion optics, resulting in increased reagent ion concentration, ensuring a robust operation, and enabling the use of multiple chemical ionisation methods with the same ionisation time. In order to simplify the regular calibration of MION2, we developed an open-source flow reactor chemistry model called MARFORCE. This model enables quantification of the chemical production of sulfuric acid (H2SO4), hypoiodous acid (HOI), and hydroperoxyl radical (HO2). MARFORCE simulates the convection–diffusion–reaction processes occurring within typical cylindrical flow reactors with uniform inner diameters. The model also includes options to simulate chemical processes in the following two scenarios: (1) when two flow reactors with different inner diameters are connected and (2) when two flows are merged into one using a Y-shaped tee, although with reduced accuracy. Furthermore, the chemical mechanism files in the model are compatible with the widely used Master Chemical Mechanism (MCM), allowing for future adaptation to simulate other chemical processes in flow reactors. Furthermore, we conducted a comprehensive characterisation of the bromide (Br-) and nitrate (NO3-) chemical ionisation methods with different ionisation times. We performed calibration experiments for H2SO4 , HOI , and HO2 by combining gas kinetic experiments with the MARFORCE model. The evaluation of sulfur dioxide (SO2), water (H2O), and molecular iodine (I2) involved dilution experiments from a gas cylinder (SO2), dew point mirror measurements (H2O), and a derivatisation approach combined with a high-performance liquid chromatography quantification (I2), respectively. Our findings indicate that the detection limit is inversely correlated with the fragmentation enthalpy of the analyte–reagent ion (Br-) cluster. In other words, stronger binding (resulting in a larger fragmentation enthalpy) leads to a lower detection limit. Additionally, a moderately longer ionisation time enhances the detection sensitivity, thereby reducing the detection limit. For instance, when using the Br- chemical ionisation method with a 300 ms ionisation time, the estimated detection limit for H2SO4 is 2.9×104 molec. cm -3. Notably, this detection limit is even superior to that achieved by the widely used Eisele-type chemical ionisation inlet (7.6×104 molec. cm -3), as revealed by direct comparisons. While the NO3- chemical ionisation method remains stable in the presence of high humidity, we have observed that the Br- chemical ionisation method (Br- –MION2) is significantly affected by the air water content. Higher levels of air water lead to reduced sensitivity for HO2 and SO2 under the examined conditions. However, we have found that a sharp decline in sensitivity for H2SO4 , HOI , and I2 occurs only when the dew point exceeds 0.5–10.5 ∘ C (equivalent to 20 %–40 % RH; calculated at 25 ∘ C throughout this paper). For future studies utilising the atmospheric pressure Br- chemical ionisation method, including Br- –MION2, it is crucial to carefully consider the molecular-level effects of humidity. By combining approaches such as the water-insensitive NO3- –MION2 with Br- –MION2, MION2 can offer more comprehensive insights into atmospheric composition than what can be achieved by either method alone. By employing instrument voltage scanning, chemical kinetic experiments, and quantum chemical calculations, we have conclusively established that the presence of iodine oxides does not interfere with the detection of HIO3. Our comprehensive analysis reveals that the ions IO3- , HIO3⚫NO3- , and HIO3⚫Br- , which are detected using the Br- and NO3- chemical ionisation methods, are primarily, if not exclusively, generated from gaseous HIO3 molecules within atmospherically relevant conditions. [ABSTRACT FROM AUTHOR]

We present a novel photolytic source of gas-phase NO3 suitable for use in atmospheric chemistry studies that has several advantages over traditional sources that utilize NO2 + O3 reactions and/or thermal dissociation of dinitrogen pentoxide (N2O5). The method generates NO3 via irradiation of aerated aqueous solutions of ceric ammonium nitrate ((NH4)2Ce(NO3)6, "CAN") and nitric acid (HNO3) or sodium nitrate (NaNO3). We present experimental and model characterization of the NO3 formation potential of irradiated CAN/HNO3 and CAN/NaNO3 mixtures containing [CAN] = 10−3 to 1.0 M, [HNO3] = 1.0 to 6.0 M, [NaNO3] = 1.0 to 4.8 M, photon fluxes (I) ranging from 6.9×1014 to 1.0×1016 photons cm−2 s−1, and irradiation wavelengths ranging from 254 to 421 nm. NO3 mixing ratios ranging from parts per billion to parts per million by volume were achieved using this method. At the CAN solubility limit, maximum [NO3] was achieved using [HNO3] ≈ 3.0 to 6.0 M and UVA radiation (λmax = 369 nm) in CAN/HNO3 mixtures or [NaNO3] ≥ 1.0 M and UVC radiation (λmax = 254 nm) in CAN/NaNO3 mixtures. Other reactive nitrogen (NO2, N2O4, N2O5, N2O6, HNO2, HNO3, HNO4) and reactive oxygen (HO2, H2O2) species obtained from the irradiation of ceric nitrate mixtures were measured using a NOx analyzer and an iodide adduct high-resolution time-of-flight chemical ionization mass spectrometer (HR-ToF-CIMS). To assess the applicability of the method for studies of NO3-initiated oxidative aging processes, we generated and measured the chemical composition of oxygenated volatile organic compounds and secondary organic aerosols from the β -pinene + NO3 reaction using a Filter Inlet for Gases and Aerosols (FIGAERO) coupled to the HR-ToF-CIMS. [ABSTRACT FROM AUTHOR]

In the lower troposphere, rapid collisions between ions and trace gases result in the transfer of positive charge to the highest proton affinity species and negative charge to the lowest proton affinity species. Measurements of the chemical composition of ambient ions thus provide direct insight into the most acidic and basic trace gases and their ion–molecule clusters – compounds thought to be important for new particle formation and growth. We deployed an atmospheric pressure interface time-of-flight mass spectrometer (APi-ToF) to measure ambient ion chemical composition during the 2016 Holistic Interactions of Shallow Clouds, Aerosols, and Land Ecosystems (HI-SCALE) campaign at the United States Department of Energy Atmospheric Radiation Measurement facility in the Southern Great Plains (SGP), an agricultural region. Cations and anions were measured for alternating periods of ∼ 24 h over 1 month. We use binned positive matrix factorization (binPMF) and generalized Kendrick analysis (GKA) to obtain information about the chemical formulas and temporal variation in ionic composition without the need for averaging over a long timescale or a priori high-resolution peak fitting. Negative ions consist of strong acids including sulfuric and nitric acid, organosulfates, and clusters of NO 3- with highly oxygenated organic molecules (HOMs) derived from monoterpene (MT) and sesquiterpene (SQT) oxidation. Organonitrates derived from SQTs account for most of the HOM signal. Combined with the diel profiles and back trajectory analysis, these results suggest that NO 3 radical chemistry is active at this site. SQT oxidation products likely contribute to particle growth at the SGP site. The positive ions consist of bases including alkyl pyridines and amines and a series of high-mass species. Nearly all the positive ions contained only one nitrogen atom and in general support ammonia and amines as being the dominant bases that could participate in new particle formation. Overall, this work demonstrates how APi-ToF measurements combined with binPMF analysis can provide insight into the temporal evolution of compounds important for new particle formation and growth. [ABSTRACT FROM AUTHOR]

Highly oxygenated organic molecules (HOMs) are important for the formation of secondary organic aerosol (SOA), which poses serious health risks and exerts great influence on Earth's climate. However, the speciation of particle-phase HOMs and its relationship with gas-phase HOM formation has been limited by the lack of suitable analytical techniques. Here, combining a novel particle evaporation inlet, the VIA (Vaporization Inlet for Aerosols), with a nitrate chemical ionization mass spectrometer (NO3 -CIMS), gas- and particle-phase HOM products of α -pinene ozonolysis were studied under different conditions. Within the 50 min residence time of our Teflon chamber, we observed enhancement of C16 – C19 HOM dimers in particles compared to the HOMs that were condensing. In particular, gas-phase dimer formation was considerably suppressed in experiments with the addition of CO or NO, but dimers still made up a considerable fraction of the observed SOA. In addition to the generally shorter carbon skeletons of the particle-phase dimers (i.e., C16 – C19) compared to the gas phase (C19 – C20), average O/C ratios of the HOMs (especially in the dimer range) also decreased slightly in the particle phase. C17H26Oz compounds, which have often been reported by previous offline measurements, dominate the particle-phase HOM mass spectra in α -pinene ozonolysis experiments. Our results indicate that these C17 compounds might be related to particle-phase processes within 1 h after HOM condensation. However, the new VIA– NO3 -CIMS system used in this work will require more detailed characterization to better understand how the thermal desorption and wall effects may modify the measured particle-phase HOM distributions. Nevertheless, organic nitrate, for example, measured by this novel VIA– NO3 -CIMS system was consistent with the measurements of an Aerodyne aerosol mass spectrometer (AMS), showing the capability of this system as a promising technique for particle-phase HOM measurements. Taken together, we believe that this system is a promising technique for combined online gas- and particle-phase HOM measurements. [ABSTRACT FROM AUTHOR]

The multi-scheme chemical ionisation inlet 1 (MION1) allows fast switching between measuring atmospheric ions without chemical ionisation and neutral molecules by multiple chemical ionisation methods. In this study, the upgraded multischeme chemical ionisation inlet 2 (MION2) is presented. The new design features improved ion optics that increase the reagent ion concentration, a generally more robust operation and the possibility to run multiple chemical ionisation methods with the same ionisation time. To simplify the regular calibration of MION2, we developed an open-source flow reactor chemistry model (MARFORCE) to quantify the chemical production of sulfuric acid (H2SO4), hypoiodous acid (HOI) and hydroperoxyl radical (HO2). MARFORCE simulates convection-diffusion-reaction processes inside typical cylindrical flow reactors with uniform inner diameters. The model also provides options to simulate the chemical processes 1) when two flow reactors with different inner diameters are connected together and 2) when two flows are merged into one (connected by a Y-shape tee), but with reduced accuracy. Additionally, the chemical mechanism files in the model are compatible with the widely-used Master Chemical Mechanism, thus allowing future adaptation to simulate other chemical processes in flow reactors. We further carried out detailed characterisation of the bromide (Br-) and nitrate (NO3-) chemical ionisation methods with different ionisation times. We calibrated H2SO4, HOI and HO2 by combining gas kinetic experiments with the MARFORCE model. Sulfur dioxide (SO2), water (H2O) and molecular iodine (I2) were evaluated using dilution experiments from a gas cylinder (SO2), dew point mirror measurements (H2O), and a derivatization approach in combination with high-performance liquid chromatography quantification (I2), respectively. We find that the detection limit is negatively correlated with the fragmentation enthalpy of the analyte-reagent ion (Br-) cluster, i.e., a stronger binding (larger fragmentation enthalpy) leads to a lower detection limit. Additionally, a moderately longer reaction time enhances the detection sensitivity thus decreasing the detection limit. For example, the detection limit for H2SO4 is estimated to be 2.9x104 molec. cm-3 with a 300 ms ionisation time. A direct comparison suggests that this is even better than the widely-used Eisele-type chemical ionisation inlet. While the NO3- chemical ionisation method is generally more robust, we find that the Br- chemical ionisation method (Br--MION2) is significantly affected by air water content. Higher air water content results in lower sensitivity for HO2 and SO2 within the examined conditions. On the other hand, a steep sensitivity drop of H2SO4, HOI and I2 is only observed when the dew point is greater than 0.5-10.5 °C (equivalent to 20-40 % RH; calculated at 25 °C hereafter). Future studies utilising atmospheric pressure Br- chemical ionisation method, including Br--MION2, should carefully address the humidity effect on a molecular basis. By combining methods such as water-insensitive NO3--MION2 with Br--MION2, MION2 should be able to provide greater details of air composition than either of these methods alone. Combining instrument voltage-scanning, chemical kinetic experiments and quantum chemical calculations, we find that the HIO3 detection is not interfered with by iodine oxides under atmospherically relevant conditions. The IO3-, HIO3NO3- and HIO3Br- ions measured using the Br- and NO3- chemical ionisation methods are primarily, if not exclusively, produced from gaseous HIO3 molecules. [ABSTRACT FROM AUTHOR]

In the lower troposphere, rapid collisions between ions and trace gases result in the transfer of positive charge to the highest proton affinity species and negative charge to the lowest proton affinity species. Measurements of the chemical composition of ambient ions thus provide direct insight into the most acidic and basic trace gases and their ion-molecule clusters — compounds thought to be important for new particle formation and growth. We deployed an atmospheric pressure interface time-of-flight mass spectrometer (APi-ToF) to measure ambient ion chemical composition during the 2016 Holistic Interaction of Shallow Clouds, Aerosols, and Land Ecosystems (HI-SCALE) campaign at the United States Department of Energy Atmospheric Radiation Measurement facility in the Southern Great Plains, an agricultural region. Cations and anions were measured for alternating periods of ~24 hours over one month. We use binned positive matrix factorization (binPMF) and scaled Kendrick mass defect (SKMD) analysis to obtain information about the chemical formulas and temporal variation in ionic composition without the need for averaging over a long timescale or a priori high-resolution peak fitting. Negative ions consist of strong acids including sulfuric and nitric acid, organosulfates, and clusters of NO3- with highly oxidized molecules (HOMs) derived from monoterpene and sesquiterpene oxidation. Organic nitrates derived from sesquiterpenes account for most of the HOM signal. Combined with the diel profiles and back trajectory analysis, these results suggest that nitrate radical chemistry is active at this site. Sesquiterpene oxidation products likely contribute to particle growth at the SGP site. The positive ions consist of bases including alkylpyridiniums and amines and a series of high mass species. Nearly all the positive ions contained only one nitrogen atom and in general support ammonia and amines as being the dominant bases that could participate in new particle formation. Overall, this work demonstrates how APi-ToF measurements combined with binPMF analysis can provide insight into the temporal evolution of compounds important for new particle formation and growth. [ABSTRACT FROM AUTHOR]

Highly oxygenated organic molecules (HOM) are important for the formation of secondary organic aerosol (SOA), which poses serious health risks and exerts great influence on Earth's climate. However, the speciation of particle-phase HOM and its relationship with gas-phase HOM formation has been limited by the lack of suitable analytical techniques. Here, combining a novel particle evaporation inlet VIA (Volatilization Inlet for Aerosols) with a nitrate chemical ionization mass spectrometer (NO3-CIMS), gas- and particle-phase HOM products of α-pinene ozonolysis were studied under different conditions. Within the 50-min residence time of our Teflon chamber, we observed enhancement of C16-C19 HOM dimers in particles compared to the HOM that were condensing. In particular, gas-phase dimer formation was considerably suppressed in experiments with the addition of CO or NO, but dimers still made up a considerable fraction of the observed SOA. In addition to the generally shorter carbon skeletons of the particle phase dimers (i.e. C16-C19) compared to the gas phase (C19-C20), average O / C ratios of the HOM (especially in the dimer range) also decreased slightly in the particle phase. C17H26Oz compounds, which have often been reported by previous offline measurements, dominate the particle-phase HOM mass spectra in α-pinene ozonolysis experiments. Our results indicate that these C17 compounds might be related to particle-phase processes within one hour after HOM condensation. However, the new VIA-NO3-CIMS system used in this work will require more detailed characterization to better understand how the thermal desorption and wall effects may modify the measured particle-phase HOM distributions. Nevertheless, for example organic nitrate measured by this novel VIA-NO3-CIMS system was consistent with the measurements of an Aerodyne Aerosol Mass Spectrometer (AMS), showing the capability of this system as a promising technique for particle-phase HOM measurements. Taken together, we believe that this system is a promising technique for combined online gas- and particle-phase HOM measurements. [ABSTRACT FROM AUTHOR]

At SMEAR II research station in Hyytiälä, located in the Finnish boreal forest, the process of new particle formation and the role of ions has been investigated for almost 20 years near the ground and at canopy level. However, above SMEAR II, the vertical distribution and diurnal variation of these different atmospheric ions are poorly characterized. In this study, we assess the atmospheric ion composition in the stable boundary layer, residual layer, mixing layer, and free troposphere, and the evolution of these atmospheric ions due to photochemistry and turbulent mixing through the day. To measure the vertical profile of atmospheric ions, we developed a tailored set-up for online mass spectrometric measurements, capable of being deployed in a Cessna 172 with minimal modifications. Simultaneously, instruments dedicated to aerosol properties made measurements in a second Cessna. We conducted a total of 16 measurement flights in May 2017, during the spring, which is the most active new particle formation season. A flight day typically consisted of three distinct flights through the day (dawn, morning, and afternoon) to observe the diurnal variation and at different altitudes (from 100 to 3200 m above ground), to capture the boundary layer development from the stable boundary layer, residual layer to mixing layer, and the free troposphere. Our observations showed that the ion composition is distinctly different in each layer and depends on the air mass origin and time of the day. Before sunrise, the layers are separated from each other and have their own ion chemistry. We observed that the ions present within the stable layer are of the same composition as the ions measured at the canopy level. During daytime when the mixing layer evolved and the compounds are vertically mixed, we observed that highly oxidized organic molecules are distributed to the top of the boundary layer. The ion composition in the residual layer varies with each day, showing similarities with either the stable boundary layer or the free troposphere. Finally, within the free troposphere, we detected a variety of carboxylic acids and ions that are likely containing halogens, originating from the Arctic Sea. [ABSTRACT FROM AUTHOR]

Understanding chemical processes leading to the formation of atmospheric aerosol particles is crucial to improve our capabilities in predicting the future climate. However, those mechanisms are still inadequately characterized, especially in polar regions. In this study, we report observations of neutral and charged aerosol precursor molecules and chemical cluster composition (qualitatively and quantitatively), as well as air ions and aerosol particle number concentrations and size distributions from the Marambio research station (64 ∘ 15 ′ S, 56 ∘ 38 ′ W), located north of the Antarctic Peninsula. We conducted measurements during the austral summer, between 15 January and 25 February 2018. The scope of this study is to characterize new particle formation (NPF) event parameters and connect our observations of gas-phase compounds with the formation of secondary aerosols to resolve the nucleation mechanisms at the molecular scale. NPF occurred on 40 % of measurement days. All NPF events were observed during days with high solar radiation, mostly with above-freezing temperatures and with low relative humidity. The averaged formation rate for 3 nm particles (J3) was 0.686 cm -3 s -1 , and the average particle growth rate (GR 3.8–12 nm) was 4.2 nm h -1. Analysis of neutral aerosol precursor molecules showed measurable concentrations of iodic acid (IA), sulfuric acid (SA), and methane sulfonic acid (MSA) throughout the entire measurement period with significant increase in MSA and SA concentrations during NPF events. We highlight SA as a key contributor to NPF processes, while IA and MSA likely only contribute to particle growth. Mechanistically, anion clusters containing ammonia and/or dimethylamine (DMA) and SA were identified, suggesting significant concentration of ammonia and DMA as well. Those species are likely contributing to NPF events since SA alone is not sufficient to explain observed nucleation rates. Here, we provide evidence of the marine origin of the measured chemical precursors and discuss their potential contribution to the aerosol phase. [ABSTRACT FROM AUTHOR]

At SMEAR II research station in Hyytiälä, located in the Finnish boreal forest, the process of new particle formation and the role of ions has been investigated for almost 20 years near the ground and at canopy level. However, above SMEAR II, the vertical distribution and diurnal variation of these different atmospheric ions are poorly characterized. In this study, we assess the atmospheric ion composition in the stable boundary layer, residual layer, mixing layer and free troposphere, and the evolution of these atmospheric ions due to photochemistry and turbulent mixing through the day. To measure the vertical profile of atmospheric ions, we developed a tailored setup for online mass spectrometric measurements, capable of being deployed in a Cessna 172 with minimal modifications. Simultaneously, instruments dedicated to aerosol properties measured in a second Cessna. We conducted a total of 16 measurement flights in May 2017, during the spring, which is the most active new particle formation season. A flight day typically consisted of three distinct flights through the day (dawn, morning and afternoon) to observe the diurnal variation and at different altitudes (from 100 m to 3200 m above ground), and to capture the boundary layer development from stable boundary layer, residual layer to mixing layer, and the free troposphere. Our observations showed that the ion composition is distinctly different in each layer and depends on the air mass origin and time of the day. Before sunrise, the layers are separated from each other and have their own ion chemistry. We observed that the ions present within the stable layer are of the same composition as the ions measured at the canopy level. During daytime when the mixing layer evolved and the compounds are vertically mixed, we observed that highly oxidised organic molecules are distributed to the top of the boundary layer. The ion composition in the residual layer varies with each day, showing similarities with either the stable boundary layer or the free troposphere. Finally, within the free troposphere, we detected a variety of carboxylic acids and ions that are likely containing halogens, originating from the Arctic Sea. [ABSTRACT FROM AUTHOR]

The metallurgical industry in the Kola Peninsula, north-west Russia, form, after Norilsk, Siberia, the second largest source of air pollution in the Arctic and subarctic domain. Sulfur dioxide (SO 2) emissions from the ore smelters are transported to wide areas, including Finnish Lapland. We performed investigations on concentrations of SO 2 , aerosol precursor vapours, aerosol and ion cluster size distributions together with chemical composition measurements of freshly formed clusters at the SMEAR I station in Finnish Lapland relatively close (∼ 300 km) to the Kola Peninsula industrial sites during the winter 2019–2020. We show that highly concentrated SO 2 from smelter emissions is converted to sulfuric acid (H 2 SO 4) in sufficient concentrations to drive new particle formation hundreds of kilometres downwind from the emission sources, even at very low solar radiation intensities. Observed new particle formation is primarily initiated by H 2 SO 4 –ammonia (negative-)ion-induced nucleation. Particle growth to cloud condensation nuclei (CCN) sizes was concluded to result from sulfuric acid condensation. However, air mass advection had a large role in modifying aerosol size distributions, and other growth mechanisms and condensation of other compounds cannot be fully excluded. Our results demonstrate the dominance of SO 2 emissions in controlling wintertime aerosol and CCN concentrations in the subarctic region with a heavily polluting industry. [ABSTRACT FROM AUTHOR]

Chemical ionization mass spectrometry (CIMS) using iodide as a reagent ion has been widely used to classify organic compounds in the atmosphere by their elemental formula. Unfortunately, calibration of these instruments is challenging due to a lack of commercially available standards for many compounds, which has led to the development of methods for estimating CIMS sensitivity. By coupling a thermal desorption aerosol gas chromatograph (TAG) simultaneously to a flame ionization detector (FID) and an iodide CIMS, we use the individual particle-phase analytes, quantified by the FID, to examine the sensitivity of the CIMS and its variability between isomers of the same elemental formula. Iodide CIMS sensitivities of isomers within a formula are found to generally vary by 1 order of magnitude with a maximum deviation of 2 orders of magnitude. Furthermore, we compare directly measured sensitivity to a method of estimating sensitivity based on declustering voltage (i.e., "voltage scanning"). This approach is found to carry high uncertainties for individual analytes (0.5 to 1 order of magnitude) but represents a central tendency that can be used to estimate the sum of analytes with reasonable error (∼30 % differences between predicted and measured moles). Finally, gas chromatography (GC) retention time, which is associated with vapor pressure and chemical functionality of an analyte, is found to qualitatively correlate with iodide CIMS sensitivity, but the relationship is not close enough to be quantitatively useful and could be explored further in the future as a potential calibration approach. [ABSTRACT FROM AUTHOR]

New particle formation (NPF) occurs frequently in various atmospheric environments and it is a major source of ultrafine particles. This study proposes an indicator, I, for the occurrence of NPF in the atmosphere based on the mechanism of H2SO4–amine nucleation. It characterizes the synergistic effects of the governing factors for H2SO4–amine nucleation, including H2SO4 concentration, amine concentrations, the stability of H2SO4–amine clusters, and aerosol surface area concentration. Long-term measurements in urban Beijing were used to validate this indicator. Good consistency was found between this indicator and the occurrence of NPF. NPF was usually observed with I > 1 for typical conditions in urban Beijing. The derivation and expressions of I also indicate a good positive association between the H2SO4 dimer concentration and NPF, as also verified by measurements. I was shown to be also applicable in urban Shanghai. Copyright © 2021 American Association for Aerosol Research [ABSTRACT FROM AUTHOR]

We compared observations of aerosol particle formation and growth in different parts of the planetary boundary layer at two different environments that have frequent new particle formation (NPF) events. In summer 2012 we had a campaign in Po Valley, Italy (urban background), and in spring 2013 a similar campaign took place in Hyytiälä, Finland (rural background). Our study consists of three case studies of airborne and ground-based measurements of ion and particle size distribution from ∼1 nm. The airborne measurements were performed using a Zeppelin inside the boundary layer up to 1000 m altitude. Our observations show the onset of regional NPF and the subsequent growth of the aerosol particles happening almost uniformly inside the mixed layer (ML) in both locations. However, in Hyytiälä we noticed local enhancement in the intensity of NPF caused by mesoscale boundary layer (BL) dynamics. Additionally, our observations indicate that in Hyytiälä NPF was probably also taking place above the ML. In Po Valley we observed NPF that was limited to a specific air mass. [ABSTRACT FROM AUTHOR]

Gaseous hydrochloric (HCl) and hydrobromic acid (HBr) are vital halogen species that play essential roles in tropospheric physicochemical processes. Yet, the majority of the current studies on these halogen species were conducted in marine or coastal areas. Detection and source identification of HCl and HBr in inland urban areas remain scarce, thus limiting the full understanding of halogen chemistry and potential atmospheric impacts in the environments with limited influence from the marine sources. Here, both gaseous HCl and HBr were concurrently measured in urban Beijing, China, during winter and early spring of 2019. We observed significant HCl and HBr concentrations ranging from a minimum value at 1 × 10 8 molecules cm -3 (4 ppt) and 4 × 10 7 molecules cm -3 (1 ppt) up to 6 × 10 9 molecules cm -3 (222 ppt) and 1 × 10 9 molecules cm -3 (37 ppt), respectively. The HCl and HBr concentrations are enhanced along with the increase of atmospheric temperature, UVB and levels of gaseous HNO 3. Based on the air mass analysis and high correlations of HCl and HBr with the burning indicators (HCN and HCNO), gaseous HCl and HBr are found to be related to anthropogenic burning aerosols. The gas–particle partitioning may also play a dominant role in the elevated daytime HCl and HBr. During the daytime, the reactions of HCl and HBr with OH radicals lead to significant production of atomic Cl and Br, up to 2 × 10 4 molecules cm -3 s -1 and 8 × 10 4 molecules cm -3 s -1 , respectively. The production rate of atomic Br (via HBr + OH) is 2–3 times higher than that of atomic Cl (via HCl + OH), highlighting the potential importance of bromine chemistry in the urban area. On polluted days, the production rates of atomic Cl and Br are faster than those on clean days. Furthermore, our observations of elevated HCl and HBr may suggest an important recycling pathway of halogen species in inland megacities and may provide a plausible explanation for the widespread halogen chemistry, which could affect the atmospheric oxidation in China. [ABSTRACT FROM AUTHOR]

Chemical ionization mass spectrometry (CIMS) using iodide as a reagent ion has been widely used to classify organic compounds in the atmosphere by their elemental formula. Unfortunately, calibration of these instruments is challenging due to a lack of commercially available standards for many compounds, which has led to the development of methods for estimating CIMS sensitivity. By coupling a Thermal desorption Aerosol Gas chromatograph (TAG) simultaneously to a flame ionization detector (FID) and an iodide CIMS, we use the individual particle-phase analytes, quantified by the FID, to examine the sensitivity of the CIMS and its variability between isomers of the same elemental formula. Iodide CIMS sensitivities of isomers within a formula are found to generally vary by one order of magnitude with a maximum deviation of two orders of magnitude. Furthermore, we compare directly measured sensitivity to a method of estimating sensitivity based on declustering voltage (i.e., "voltage scanning"). This approach is found to carry high uncertainties for individual analytes (half to one order of magnitude), but represents a central tendency that can be used to estimate the sum of analytes with reasonable error (~30% differences between predicted and measured moles). Finally, GC retention time, which is associated with vapor pressure and chemical functionality of an analyte, is found to qualitatively correlate with iodide CIMS sensitivity, but the relationship is not close enough to be quantitatively useful and could be explored further in the future as a potential calibration approach. [ABSTRACT FROM AUTHOR]

Iodine species are important in the marine atmosphere for oxidation and new-particle formation. Understanding iodine chemistry and iodine new-particle formation requires high time resolution, high sensitivity, and simultaneous measurements of many iodine species. Here, we describe the application of a bromide chemical ionization mass spectrometer (Br-CIMS) to this task. During the iodine oxidation experiments in the Cosmics Leaving OUtdoor Droplets (CLOUD) chamber, we have measured gas-phase iodine species and sulfuric acid using two Br-CIMS, one coupled to a Multi-scheme chemical IONization inlet (Br-MION-CIMS) and the other to a Filter Inlet for Gasses and AEROsols inlet (Br-FIGAERO-CIMS). From offline calibrations and intercomparisons with other instruments, we have quantified the sensitivities of the Br-MION-CIMS to HOI, I 2 , and H 2 SO 4 and obtained detection limits of 5.8 × 10 6 , 3.8 × 10 5 , and 2.0 × 10 5 molec. cm -3 , respectively, for a 2 min integration time. From binding energy calculations, we estimate the detection limit for HIO 3 to be 1.2 × 10 5 molec. cm -3 , based on an assumption of maximum sensitivity. Detection limits in the Br-FIGAERO-CIMS are around 1 order of magnitude higher than those in the Br-MION-CIMS; for example, the detection limits for HOI and HIO 3 are 3.3 × 10 7 and 5.1 × 10 6 molec. cm -3 , respectively. Our comparisons of the performance of the MION inlet and the FIGAERO inlet show that bromide chemical ionization mass spectrometers using either atmospheric pressure or reduced pressure interfaces are well-matched to measuring iodine species and sulfuric acid in marine environments. [ABSTRACT FROM AUTHOR]

Particulate organic nitrates (pON) are an important component of secondary organic aerosol in biogenic-emission-dominant environments and play a critical role in NO x cycles. However, estimation of pON has been a challenge in polluted environments, e.g., North China Plain, with high concentrations of inorganic nitrate and NO x. Here we developed a method for estimation of pON from the measurements of high-resolution aerosol mass spectrometer coupled with a thermodenuder based on the volatility differences between inorganic nitrate and pON. The results generally correlated well with those estimated from positive matrix factorization of combined organic and inorganic mass spectra and from the ratio of NO + to NO 2+ (NO x+ ratio), yet they had improvements in reducing negative values due to the influences of high concentration of inorganic nitrate and constant NO x+ ratio of organic nitrates (RON). By applying this approach to the measurements at an urban (Beijing) and a rural site (Gucheng) in summer and winter in the North China Plain, we estimated that the average mass concentrations of NO 3,org (1.8 µ g m -3 vs. 1.0 µ g m -3) and pON to OA (27.5 % vs. 14.8 %) were higher in summer than in winter in Beijing, indicating more pON formation in biogenically and anthropogenically mixed environments. In addition, the average NO 3,org loading in Gucheng was 1.9 µ g m -3 , and the pON at the rural site also showed higher contribution to OA than that in Beijing during wintertime due to higher primary emissions and gaseous precursors in Gucheng. In addition, RON was determined and showed considerable differences between day–night and clean–polluted periods, highlighting the complexity of pON compounds from different chemical pathways (e.g., OH and NO 3 oxidation) and sources. [ABSTRACT FROM AUTHOR]

Atmospheric oxidation products of volatile organic compounds consist of thousands of unique chemicals that have distinctly different physical and chemical properties depending on their detailed structures and functional groups. Measurement techniques that can achieve molecular characterizations with details down to functional groups (i.e., isomer-resolved resolution) are consequently necessary to provide understandings of differences of fate and transport within isomers produced in the oxidation process. We demonstrate a new instrument coupling the thermal desorption aerosol gas chromatograph (TAG), which enables the separation of isomers, with the high-resolution time-of-flight chemical ionization mass spectrometer (HR-ToF-CIMS), which has the capability of classifying unknown compounds by their molecular formulas, and the flame ionization detector (FID), which provides a near-universal response to organic compounds. The TAG-CIMS/FID is used to provide isomer-resolved measurements of samples from liquid standard injections and particle-phase organics generated in oxidation flow reactors. By coupling a TAG to a CIMS, the CIMS is enhanced with an additional dimension of information (resolution of individual molecules) at the cost of time resolution (i.e., one sample per hour instead of per minute). We found that isomers are prevalent in sample matrix with an average number of three to five isomers per formula depending on the precursors in the oxidation experiments. Additionally, a multi-reagent ionization mode is investigated in which both zero air and iodide are introduced as reagent ions, to examine the feasibility of extending the use of an individual CIMS to a broader range of analytes with still selective reagent ions. While this approach reduces iodide-adduct ions by a factor of 2, [M - H] - and [M + O 2 ] - ions produced from lower-polarity compounds increase by a factor of 5 to 10, improving their detection by CIMS. The method expands the range of detected chemical species by using two chemical ionization reagents simultaneously, which is enabled by the pre-separation of analyte molecules before ionization. [ABSTRACT FROM AUTHOR]

We compared observations of aerosol particle formation and growth in different parts of the planetary boundary layer at two different environments that have frequent new particle formation (NPF) events. In summer 2012 we had a campaign in Po Valley, Italy (urban background) and in spring 2013 a similar campaign took place in Hyytiälä, Finland (rural background). Our study consists of airborne and ground-based measurements of ion and particle size distribution from ~1 nm. The airborne measurements were performed using a Zeppelin inside the boundary layer up to 1000 m altitude. Our observations show the onset of regional NPF and the subsequent growth of the aerosol particles happening uniformly inside the mixed layer (ML) in both locations. However, in Hyytiälä we noticed local enhancement in the intensity of NPF caused by mesoscale BL dynamics. Additionally, our observations indicate that in Hyytiälä NPF was probably also taking place above the ML. In Po Valley we observed NPF that was limited to a specific air mass. [ABSTRACT FROM AUTHOR]

Volatility and viscosity have substantial impacts on gas–particle partitioning, formation and evolution of aerosol and hence the predictions of aerosol-related air quality and climate effects. Here aerosol volatility and viscosity at a rural site (Gucheng) and an urban site (Beijing) in the North China Plain (NCP) in summer and winter were investigated by using a thermodenuder coupled with a high-resolution aerosol mass spectrometer. The effective saturation concentration (C*) of organic aerosol (OA) in summer was smaller than that in winter (0.55 µ g m -3 vs. 0.71–0.75 µ g m -3), indicating that OA in winter in the NCP is more volatile due to enhanced primary emissions from coal combustion and biomass burning. The volatility distributions varied and were largely different among different OA factors. In particular, we found that hydrocarbon-like OA (HOA) contained more nonvolatile compounds compared to coal-combustion-related OA. The more oxidized oxygenated OA (MO-OOA) showed overall lower volatility than less oxidized OOA (LO-OOA) in both summer and winter, yet the volatility of MO-OOA was found to be relative humidity (RH) dependent showing more volatile properties at higher RH. Our results demonstrated the different composition and chemical formation pathways of MO-OOA under different RH levels. The glass transition temperature (Tg) and viscosity of OA in summer and winter are estimated using the recently developed parameterization formula. Our results showed that the Tg of OA in summer in Beijing (291.5 K) was higher than that in winter (289.7–290.0 K), while it varied greatly among different OA factors. The viscosity suggested that OA existed mainly as solid in winter in Beijing (RH = 29 ± 17 %), but as semisolids in Beijing in summer (RH = 48 ± 25 %) and Gucheng in winter (RH = 68 ± 24 %). These results have the important implication that kinetically limited gas–particle partitioning may need to be considered when simulating secondary OA formation in the NCP. [ABSTRACT FROM AUTHOR]

New particle formation (NPF) is one of the major sources of atmospheric ultrafine particles. Due to the high aerosol and trace gas concentrations, the mechanism and governing factors for NPF in the polluted atmospheric boundary layer may be quite different from those in clean environments, which is however less understood. Herein, based on long-term atmospheric measurements from January 2018 to March 2019 in Beijing, the nucleation mechanism and the influences of H2SO4 concentration, amine concentrations, and aerosol concentration on NPF are quantified. The collision of H2SO4 –amine clusters is found to be the dominating mechanism to initialize NPF in urban Beijing. The coagulation scavenging due to the high aerosol concentration is a governing factor as it limits the concentration of H2SO4 –amine clusters and new particle formation rates. The formation of H2SO4 –amine clusters in Beijing is sometimes limited by low amine concentrations. Summarizing the synergistic effects of H2SO4 concentration, amine concentrations, and aerosol concentration, we elucidate the governing factors for H2SO4 –amine nucleation for various conditions. [ABSTRACT FROM AUTHOR]

Metallurgical industry in Kola peninsula, North-West Russia, form a second largest source of air pollution in the Arctic and sub-Arctic domain. Sulphur dioxide emissions from the ore smelters are transported to wide areas including Finnish Lapland. We performed investigations on concentrations of SO2 and aerosol precursor vapours, aerosol and ion cluster size distributions together with chemical composition measurements of freshly formed clusters at SMEAR I station in Finnish Lapland relatively close (~300 km) to Kola peninsula industrial sites during winter 2019-2020. We show that highly concentrated SO2 from smelter emissions is converted to sulphuric acid (H2SO4) with sufficient concentrations to drive new particle formation hundreds of kilometres downwind of the emission sources even with very low solar radiation intensities. Observed new particle formation is primarily initiated by H2SO4 - ammonia (negative-) ion induced nucleation. Particle growth to cloud condensation nuclei (CCN) sizes was concluded to result from sulphuric acid condensation. However, airmass advection had a large role in modifying aerosol size distributions and other growth mechanisms cannot be fully excluded. Our results demonstrate the dominance of SO2 emissions in controlling winter-time aerosol and CCN concentrations in the subarctic region with heavily polluting industry. [ABSTRACT FROM AUTHOR]

The Filter Inlet for Gases and AEROsols (FIGAERO) coupled with a time-of-flight chemical ionization mass spectrometer (ToF-CIMS) enables online measurements of both gas-phase and particle-phase chemical constituents of ambient aerosols. When properly calibrated, the incorporated particle filter collection and subsequent thermal desorption enable the direct measurement of volatility of said constituents. Previously published volatility calibration results however differ from each other significantly. In this study we investigate the reason for this discrepancy. We found a major source of error in the widely used syringe deposition calibration method that can lead to an overestimation of saturation vapour pressures by several orders of magnitude. We propose a new method for volatility calibration by using atomized calibration compounds that more accurately captures the evaporation of chemical constituents from ambient aerosol particles. For example, we found a difference of ∼ 15 ∘C in observed Tmax⁡ values between the atomizer method and syringe method using the lowest solution concentration (0.003 gL-1). This difference translates into a difference of up to 3 orders of magnitude in saturation concentration C∗ space. We justify our claim with evaporation modelling and direct scanning electron microscopy imaging, while also presenting possible error sources of the atomizer method. We finally present how typical calibration parameters derived with both methods impact the volatility basis set (VBS) derived from measurements of secondary organic aerosols (SOAs). [ABSTRACT FROM AUTHOR]

We have developed a field-deployable gas chromatograph (GC) with thermal desorption preconcentration (TDPC), which is demonstrated here with automatic detector switching between two high-resolution time-of-flight mass spectrometers (TOF-MSs) for in situ measurements of volatile organic compounds (VOCs). This system provides many analytical advances, including acquisition of fast time–response data in tandem with molecular speciation and two types of mass spectral information for each resolved GC peak: molecular ion identification from Vocus proton transfer reaction (PTR) TOF-MS and fragmentation pattern from electron ionization (EI) TOF-MS detection. This system was deployed during the 2018 ATHLETIC campaign at the University of Colorado Dal Ward Athletic Center in Boulder, Colorado, where it was used to characterize VOC emissions in the indoor environment. The addition of the TDPC-GC increased the Vocus sensitivity by a factor of 50 due to preconcentration over a 6 min GC sample time versus direct air sampling with the Vocus, which was operated with a time resolution of 1 Hz. The GC-TOF methods demonstrated average limits of detection of 1.6 ppt across a range of monoterpenes and aromatics. Here, we describe the method to use the two-detector system to conclusively identify a range of VOCs including hydrocarbons, oxygenates, and halocarbons, along with detailed results including the quantification of anthropogenic monoterpenes, where limonene accounted for 47 %–80 % of the indoor monoterpene composition. We also report the detection of dimethylsilanediol (DMSD), an organosiloxane degradation product, which was observed with dynamic temporal behavior distinct from volatile organosiloxanes (e.g., decamethylcyclopentasiloxane, D5 siloxane). Our results suggest DMSD is produced from humidity-dependent heterogeneous reactions occurring on surfaces in the indoor environment, rather than formed through gas-phase oxidation of volatile siloxanes. [ABSTRACT FROM AUTHOR]

Particulate organic nitrates (pON) are an important component of secondary organic aerosol in biogenic emission dominant environments, and play a critical role in NOx cycles. However, estimation of pON has been a challenge in polluted environments, e.g., North China Plain with high concentrations of inorganic nitrate and NOx. Here we developed a method for estimation of pON from the measurements of high-resolution aerosol mass spectrometer coupled with a thermodenuder based on the volatility differences between inorganic nitrate and pON. The results generally correlated well with those estimated from positive matrix factorization of combined organic and inorganic mass spectra and from the ratio of NO+ to NO2+ (NOx+ ratio), yet had improvements in reducing negative values due to the influences of high concentration of inorganic nitrate and constant NOx+ ratio of organic nitrates (RON). By applying this approach to the measurements at an urban (Beijing) and a rural site (Gucheng) in summer and winter in North China Plain, we estimated that the average mass concentrations of NO3,org (1.8 µg m-3 vs.1.0 µg m-3) and pON to OA (27.5% vs. 14.8%) were higher in summer than in winter in Beijing, indicating more pON formation in biogenically and anthropogenically mixed environments. In addition, the average NO3,org loading in Gucheng was 1.9 µg m-3, and the pON at the rural site also showed higher contribution to OA than that in Beijing during wintertime due to higher primary emissions and gaseous precursors in Gucheng. In addition, RON was determined and showed considerable differences between day-night and clean-polluted periods, highlighting the complexity of pON compounds from different chemical pathways (e.g., OH and NO3 oxidation) and sources. [ABSTRACT FROM AUTHOR]

Iodine species are important in the marine atmosphere for oxidation and new-particle formation. Understanding iodine chemistry and iodine new-particle formation requires high time resolution, high sensitivity, and simultaneous measurements of many iodine species. Here, we describe the application of bromide chemical ionization mass spectrometers (Br-CIMS) to this task. During iodine new-particle formation experiments in the Cosmics Leaving OUtdoor Droplets (CLOUD) chamber, we have measured gas-phase iodine species and sulfuric acid using two Br-CIMS, one coupled to a Multi-scheme chemical IONization inlet (Br-MION-CIMS) and the other to a Filter Inlet for Gasses and AEROsols inlet (Br-FIGAERO-CIMS). From offline calibrations and inter-comparisons with other instruments attached to the CLOUD chamber, we have quantified the sensitivities of the Br-MION-CIMS to HOI, I2, and H2SO4 and obtain detection limits of 5.8 x 106, 6.3 x 105, and 2.0 x 105 molec cm-3, respectively, for a 2-min integration time. From binding energy calculations, we estimate the detection limit for HIO3 to be 1.2 x 105 molec cm-3, based on an assumption of maximum sensitivity. Detection limits in the Br-FIGAERO-CIMS are around one order of magnitude higher than those in the Br-MION-CIMS; for example, the detection limits for HOI and HIO3 are 3.3 x 107 and 5.1 x 106 molec cm-3, respectively. Our comparisons of the performance of the MION inlet and the FIGAERO inlet show that bromide chemical ionization mass spectrometers using either atmospheric pressure or reduced pressure interfaces are well-matched to measuring iodine species and sulfuric acid in marine environments. [ABSTRACT FROM AUTHOR]

Gaseous hydrochloric (HCl) and hydrobromic acid (HBr) are vital halogen species that play essential roles in tropospheric physicochemical processes. Yet, majority of the current studies on these halogen species were conducted in marine or coastal areas. Detection and source identification of HCl and HBr in inland urban areas remain scarce, thus, limiting the full understanding of halogen chemistry and potential atmospheric impacts in the environments with limited influence from the marine sources. Here, both gaseous HCl and HBr were concurrently measured in urban Beijing, China during winter and early spring of 2019. We observed significant HCl and HBr concentrations ranged from a minimum value at 1.3 x 108 cm-3 and 4.3 x 107 cm-3 up to 5.9 x 109 cm-3 and 1.2 x 109 cm-3, respectively. The HCl and HBr concentrations are enhanced along with the increase of atmospheric temperature, UVB, and levels of gaseous HNO3. Based on the air mass analysis and high correlations of HCl and HBr with the burning indicators (HCN and HCNO), the gaseous HCl and HBr are found to be related to anthropogenic burning aerosols. The gas-aerosol partitioning may also play a dominant role in the elevated daytime HCl and HBr. During the daytime, the reaction of HCl and HBr with OH radicals lead to significant production of atomic Cl and Br, up to 1.7 x 104 cm-3 s-1 and 7.9 x 104 cm-3 s-1, respectively. The production rate of atomic Br (via HBr + OH) are 2-3 times higher than that of atomic Cl (via HCl + OH), highlighting the potential importance of bromine chemistry in the urban area. Furthermore, our observations of elevated HCl and HBr may suggest an important recycling pathway of halogen species in inland megacities, and may provide a plausible explanation for the widespread of halogen chemistry, which could affect the atmospheric oxidation in China. [ABSTRACT FROM AUTHOR]

Volatility and viscosity have substantial impacts on gas-particle partitioning, formation and evolution of aerosol, and hence the predictions of aerosol related air quality and climate effects. Here aerosol volatility and viscosity at a rural site (Gucheng) and an urban site (Beijing) in North China Plain (NCP) in summer and winter were investigated by using a thermodenuder coupled with high resolution aerosol mass spectrometer. The effective saturation concentration (C*) of organic aerosol (OA) in summer was smaller than that in winter (0.55 µg m-3 vs. 0.71-0.75 µg m-3), indicating that OA in winter in NCP is more volatile due to enhanced primary emissions from coal combustion and biomass burning. The volatility distributions varied largely different among different OA factors. In particular, we found that hydrocarbon-like OA (HOA) contained more non-volatile compounds compared to coal combustion related OA. The more oxidized oxygenated OA (MO-OOA) showed overall lower volatility than less oxidized OOA (LO-OOA) in both summer and winter, yet the volatility of MO-OOA was found to be relative humidity (RH) dependent showing more volatile properties at higher RH. Our results demonstrated the different composition and chemical formation pathways of MO-OOA under different RH levels. The glass transition temperature (Tg) and viscosity of OA in summer and winter are estimated using the recently developed parameterization formula. Our results showed that the Tg of OA in summer in Beijing (291.5 K) was higher than that in winter (289.7-290.0 K), while it varied greatly among different OA factors. The viscosity suggested that OA existed mainly as solid in winter in Beijing, but as semi-solids in Beijing in summer and Gucheng in winter. These results have important implications that kinetically limited gas-particle partitioning may need to be considered when simulating secondary OA formation in NCP. [ABSTRACT FROM AUTHOR]

Ammonia (NH 3), a gaseous compound ubiquitously present in the atmosphere, is involved in the formation of secondary organic aerosol (SOA), but the exact mechanism is still not well known. This study presents the results of SOA experiments from the photooxidation of α -pinene in the presence of NH 3 in the reaction chamber. SOA was formed in in nucleation experiments and in seeded experiments with ammonium sulfate particles as seeds. The chemical composition and time series of compounds in the gas and particle phase were characterized by an online high-resolution time-of-flight proton-transfer-reaction mass spectrometer (HR-ToF-PTRMS) and a high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS), respectively. Our results show that the mass concentration of ammonium (NH 4+) was still rising even after the mass concentration of the organic component started to decrease due to aerosol wall deposition and evaporation, implying the continuous new formation of particle-phase ammonium in the process. Stoichiometric neutralization analysis of aerosol indicates that organic acids have a central role in the formation of particle-phase ammonium. Our measurements show a good correlation between the gas-phase organic mono- and dicarboxylic acids formed in the photooxidation of α -pinene and the ammonium in the particle phase, thus highlighting the contribution of gas-phase organic acids to the ammonium formation. The work shows that the gas-phase organic acids contribute to the SOA formation by forming organic ammonium salts through acid–base reaction. The changes in aerosol mass, particle size and chemical composition resulting from the NH 3 –SOA interaction can potentially alter the aerosol direct and indirect forcing and therefore alter its impact on climate change. [ABSTRACT FROM AUTHOR]

New particle formation (NPF) is one of the major sources of atmospheric ultrafine particles. Due to the high aerosol and trace gas concentrations, the mechanism and governing factors for NPF in the polluted atmospheric boundary layer may be quite different from those in clean environments, which is however less understood. Herein, based on long-term atmospheric measurements from January 2018 to March 2019 in Beijing, the nucleation mechanism and the influences of H2SO4 concentration, amine concentrations, and aerosol concentration on NPF are quantified. The collision of H2SO4-amine clusters is found to be the dominating mechanism to initialize NPF in urban Beijing. The coagulation scavenging due to the high aerosol concentration is a governing factor as it limits the concentration of H2SO4 amine clusters and new particle formation rates. Besides, the effective amine concentration is another limiting factor in Beijing because amine is sometimes insufficient for nucleation at the kinetic limit. Based on the synergistic effects of these factors on H2SO4-amine nucleation, governing factors for H2SO4-amine nucleation for different conditions are summarized. [ABSTRACT FROM AUTHOR]

Aromatic volatile organic compounds (VOCs) are key anthropogenic pollutants emitted to the atmosphere and are important for both ozone and secondary organic aerosol (SOA) formation in urban areas. Recent studies have indicated that aromatic hydrocarbons may follow previously unknown oxidation chemistry pathways, including autoxidation that can lead to the formation of highly oxidised products. In this study we evaluate the gas- and particle-phase ions measured by online mass spectrometry during the hydroxyl radical oxidation of substituted C9 -aromatic isomers (1,3,5-trimethylbenzene, 1,2,4-trimethylbenzene, propylbenzene and isopropylbenzene) and a substituted polyaromatic hydrocarbon (1-methylnaphthalene) under low- and medium- NOx conditions. A time-of-flight chemical ionisation mass spectrometer (ToF-CIMS) with iodide–anion ionisation was used with a filter inlet for gases and aerosols (FIGAERO) for the detection of products in the particle phase, while a Vocus proton-transfer-reaction mass spectrometer (Vocus-PTR-MS) was used for the detection of products in the gas phase. The signal of product ions observed in the mass spectra were compared for the different precursors and experimental conditions. The majority of mass spectral product signal in both the gas and particle phases comes from ions which are common to all precursors, though signal distributions are distinct for different VOCs. Gas- and particle-phase composition are distinct from one another. Ions corresponding to products contained in the near-explicit gas phase Master Chemical Mechanism (MCM version 3.3.1) are utilised as a benchmark of current scientific understanding, and a comparison of these with observations shows that the MCM is missing a range of highly oxidised products from its mechanism. In the particle phase, the bulk of the product signal from all precursors comes from ring scission ions, a large proportion of which are more oxidised than previously reported and have undergone further oxidation to form highly oxygenated organic molecules (HOMs). Under the perturbation of OH oxidation with increased NOx , the contribution of HOM-ion signals to the particle-phase signal remains elevated for more substituted aromatic precursors. Up to 43 % of product signal comes from ring-retaining ions including HOMs; this is most important for the more substituted aromatics. Unique products are a minor component in these systems, and many of the dominant ions have ion formulae concurrent with other systems, highlighting the challenges in utilising marker ions for SOA. [ABSTRACT FROM AUTHOR]

Atmospheric oxidation products of volatile organic compounds consist of thousands of unique chemicals that have distinctly different physical and chemical properties depending on their detailed structures and functional groups. Measurement techniques that can achieve molecular characterizations with details down to functional groups (i.e., isomer-resolved resolution) are consequently necessary to provide understandings of differences of fate and transport within isomers produced in the oxidation process. We demonstrate a new instrument coupling the thermal desorption aerosol gas chromatograph (TAG), which enables the separation of isomers, with the high-resolution time-of-flight chemical ionization mass spectrometer (HR-ToF-CIMS), which has the capability of classifying unknown compounds by their molecular formulas, and the flame ion detector (FID), which provide a near-universal response to organic compounds. The TAG-CIMS/FID is used to provide isomer-resolved measurements of samples from liquid standard injections and particle-phase organics generated in oxidation flow reactors. By coupling a TAG to a CIMS, the CIMS is enhanced with an additional dimension of information (resolution of individual molecules) at the cost of time resolution (i.e., one sample per hour instead of per minute). We found that isomers are prevalent in sample matrix with an average number of three to five isomers per formula depending on the precursors in the oxidation experiments. Additionally, a multi-reagent ionization mode is investigated in which both zero air and iodide are introduced as reagent ions, to examine the feasibility of extending the use of an individual CIMS to a broader range of analytes with still selective reagent ions. While this approach reduces iodide-adduct ions by a factor of two, [M-H]- and [M+O2]- ions produced from lower-polarity compounds increase by a factor of five to ten, improving their detection by CIMS. The method expands the range of detected chemical species by using two chemical ionization reagents simultaneously, enabled by the pre-separation of analyte molecules before ionization. [ABSTRACT FROM AUTHOR]

Anthropogenic emissions in Asia have significantly increased during the last two decades; as a result, the induced air pollution and its influences on radiative forcing and public health are becoming increasingly prominent. The Aerodyne Aerosol Mass Spectrometer (AMS) has been widely deployed in Asia for real-time characterization of aerosol chemistry. In this paper, we review the AMS measurements in Asia, mainly in China, Korea, Japan, and India since 2001 and summarize the key results and findings. The mass concentrations of non-refractory submicron aerosol species (NR-PM1) showed large spatial distributions with high mass loadings occurring in India and north and northwest China (60.2–81.3 μg m−3), whereas much lower values were observed in Korea, Japan, Singapore and regional background sites (7.5–15.1 μg m−3). Aerosol composition varied largely in different regions, but was overall dominated by organic aerosols (OA, 32–75%), especially in south and southeast Asia due to the impact of biomass burning. While sulfate and nitrate showed comparable contributions in urban and suburban regions in north China, sulfate dominated inorganic aerosols in south China, Japan and regional background sites. Positive matrix factorization analysis identified multiple OA factors from different sources and processes in different atmospheric environments, e.g., biomass burning OA in south and southeast Asia and agricultural seasons in China, cooking OA in urban areas, and coal combustion in north China. However, secondary OA (SOA) was a ubiquitous and dominant aerosol component in all regions, accounting for 43–78% of OA. The formation of different SOA subtypes associated with photochemical production or aqueous-phase/fog processing was widely investigated. The roles of primary emissions, secondary production, regional transport, and meteorology on severe haze episodes, and different chemical responses of primary and secondary aerosol species to source emission changes and meteorology were also demonstrated. Finally, future prospects of AMS studies on long-term and aircraft measurements, water-soluble OA, the link of OA volatility, oxidation levels, and phase state were discussed. [ABSTRACT FROM AUTHOR]

The Filter Inlet for Gases and AEROsols (FIGAERO) coupled with a Time-of-Flight Chemical Ionization Mass Spectrometer (ToF- CIMS) enables online measurements of both gas-phase and particle phase chemical constituents of ambient aerosols. When properly calibrated, the incorporated particle filter collection and subsequent thermal desorption enable the direct measurement of volatility of said constituents. Previously published volatility calibration results however differ from each other significantly. In this study we investigate the reason for this discrepancy. We found a major source of error in the widely used syringe deposition calibration method that can lead to an overestimation of saturation vapour pressures by several orders of magnitude. We propose a new method for volatility calibration by using atomized calibration compounds that more accurately captures the evaporation of chemical constituents from ambient aerosol particles. We justify our claim with evaporation modelling and direct Scanning Electron Microscopy imaging, while also presenting possible error sources of the atomizer method. We finally present how typical calibration parameters derived with both methods impact the Volatility Basis Set (VBS) derived from measurements of Secondary Organic Aerosols (SOA). [ABSTRACT FROM AUTHOR]

Single particle mass spectrometer (SPMS) instruments have been used for in-situ chemical characterization of atmospheric aerosols, both in the field and laboratory, for over two decades. SPMSs typically combine precise optical particle sizing with laser desorption and ionization followed by time of flight mass spectrometry. Among the advantages of SPMSs over other aerosol chemistry measurement techniques are their single particle resolution and high sensitivity to trace chemical species. The AeroMegt Laser Ablation Aerosol Particle Time of Flight Mass Spectrometer (LAAPToF) is a commercially available member of this instrument class, aiming for a compact size and simplicity for the end user. This article quantifies the performance of LAAPToF with an emphasis on optical counting efficiency. Recommendations for improving detection compared to the base LAAPToF hardware are described. Our results show that changes to the optical detection scheme can lead to over two orders of magnitude improvement in optical counting efficiency in the size range 500–2000 nm vacuum aerodynamic diameter. We also present mass spectral performance for characterizing atmospherically relevant particles in a comparison to a current SPMS design, the Particle Analysis by Laser Mass Spectrometry. Copyright © 2020 American Association for Aerosol Research [ABSTRACT FROM AUTHOR]

The measurements of aerosol particles with a filter inlet for gases and aerosols (FIGAERO) together with a chemical ionisation mass spectrometer (CIMS) yield the overall chemical composition of the particle phase. In addition, the thermal desorption profiles obtained for each detected ion composition contain information about the volatility of the detected compounds, which is an important property for understanding many physical properties like gas–particle partitioning. We coupled this thermal desorption method with isothermal evaporation prior to the sample collection to investigate the chemical composition changes during isothermal particle evaporation and particulate-water-driven chemical reactions in α -pinene secondary organic aerosol (SOA) of three different oxidative states. The thermal desorption profiles of all detected elemental compositions were then analysed with positive matrix factorisation (PMF) to identify the drivers of the chemical composition changes observed during isothermal evaporation. The keys to this analysis were to use the error matrix as a tool to weight the parts of the data carrying most information (i.e. the peak area of each thermogram) and to run PMF on a combined data set of multiple thermograms from different experiments to enable a direct comparison of the individual factors between separate measurements. The PMF was able to identify instrument background factors and separate them from the part of the data containing particle desorption information. Additionally, PMF allowed us to separate the direct desorption of compounds detected at a specific elemental composition from other signals with the same composition that stem from the thermal decomposition of thermally instable compounds with lower volatility. For each SOA type, 7–9 factors were needed to explain the observed thermogram behaviour. The contribution of the factors depended on the prior isothermal evaporation. Decreased contributions from the factors with the lowest desorption temperatures were observed with increasing isothermal evaporation time. Thus, the factors identified by PMF could be interpreted as volatility classes. The composition changes in the particles due to isothermal evaporation could be attributed to the removal of volatile factors with very little change in the desorption profiles of the individual factors (i.e. in the respective temperatures of peak desorption, Tmax). When aqueous-phase reactions took place, PMF was able to identify a new factor that directly identified the ions affected by the chemical processes. We conducted a PMF analysis of the FIGAERO–CIMS thermal desorption data for the first time using laboratory-generated SOA particles. But this method can be applied to, for example, ambient FIGAERO–CIMS measurements as well. There, the PMF analysis of the thermal desorption data identifies organic aerosol (OA) sources (such as biomass burning or oxidation of different precursors) and types, e.g. hydrocarbon-like (HOA) or oxygenated organic aerosol (OOA). This information could also be obtained with the traditional approach, namely the PMF analysis of the mass spectra data integrated for each thermogram. But only our method can also obtain the volatility information for each OA source and type. Additionally, we can identify the contribution of thermal decomposition to the overall signal. [ABSTRACT FROM AUTHOR]

Source apportionment of organic aerosol (OA) from aerosol mass spectrometer (AMS) or aerosol chemical speciation monitor (ACSM) measurements relies largely upon mass spectral profiles from different source emissions. However, the changes in mass spectra of primary emissions from AMS–ACSM with the newly developed capture vaporizer (CV) are poorly understood. Here we conducted 21 cooking, crop straw, wood, and coal burning experiments to characterize the mass spectral features of OA and water-soluble OA (WSOA) using SV-AMS and CV-ACSM. Our results show overall similar spectral characteristics between SV-AMS and CV-ACSM for different primary emissions despite additional thermal decomposition in CV, and the previous spectral features for diagnostics of primary OA factors are generally well retained. However, the mass spectral differences between OA and WSOA can be substantial for both SV-AMS and CV-ACSM. The changes in f55 (fraction of m/z 55 in OA) vs. f57 , f44 vs. f60 , and f44 vs. f43 in CV-ACSM are also observed, yet the evolving trends are similar to those of SV-AMS. By applying the source spectral profiles to a winter CV-ACSM study at a highly polluted rural site in the North China Plain, the source apportionment of primary OA was much improved, highlighting the two most important primary sources of biomass burning and coal combustion (32 % and 21 %). Considering the rapidly increasing deployments of CV-ACSM and WSOA studies worldwide, the mass spectral characterization has significant implications by providing essential constraints for more accurate source apportionment and making better strategies for air pollution control in regions with diverse primary emissions. [ABSTRACT FROM AUTHOR]

Ammonia (NH3), a gasous compound ubiquitiously present in the atmosphere, is involved in the formation of secondary organic aerosol (SOA), but the exact mechanisum is still not well known. This study presents the results of SOA experiments from the photooxidation of a-pinene in the presence of NH3 in the reaction chamber. SOA was formed in nucleation experiment and in seeded experiment with ammonium sulfate particles as seeds. The chemical composition and time-series of compounds in the gas- and particle- phase were characterized by an on-line highresolution time-of-flight proton transfer reaction mass spectrometer (HR-ToF-PTRMS) and a high-resolution time of-flight aerosol mass spectrometer (HR-ToF-AMS), respectively. Our results show that for the aerosol particles in cloud condensation nuclei (CCN) size, the mass concentration of ammonium (NH4+) was still rising even after the mass concentration of organic component started to decrease due to aerosol wall deposition and evaporation, implying the continuous new formation of particle phase ammonium in the process. Stoichiometric neutralization analysis of aerosol indicates that organic acids have a central role in the formation of particle phase ammonium. Our measurements show a good correlation between the gas phase organic mono- and di-carboxylic acids formed in the photooxidation of a-pinene and the ammonium in the particle phase, thus highlighting the contribution of gas-phase organic acids to the ammonium formation in the CCN-size SOA particles. The work shows that the gas-phase organic acids contribute to the SOA formation by forming ammonium salts through acid-base reaction. The changes in aerosol mass, particle size and chemical composition resulting from the NH3-SOA interaction can potentially alter the aerosol direct and indirect forcing and therefore alter its impact on climate change. [ABSTRACT FROM AUTHOR]

A new capture vaporizer (CV) has been developed and used recently in the Aerodyne aerosol mass spectrometer (AMS) and aerosol chemical speciation monitor (ACSM) instead of the standard vaporizer (SV) to reduce the particle bounce. It is important to characterize the CV performance in different environments. In this study, we characterized specific organic aerosols (OAs) from vehicle, cooking, biomass burning, and coal burning emissions by a time-of-flight ACSM (TOF-ACSM) with the CV. Their corresponding marker ions that have been defined in the previous SV-based analysis are still valid in the CV mass spectra. Spectra of OAs from cooking and vehicle exhaust show similarities in distinct alkyl fragments but different ratios of m/z 55 and 57. Ions related to polycyclic aromatic hydrocarbons are present in the OA spectra obtained from burning lignite and bituminous coal but not in the spectra obtained from burning anthracite. Although the relative intensities of m/z 60 and 73 are much lower in the CV spectra than in the SV spectra for biomass burning OA, they are still relatively greater compared with the spectra for other sources. Our data suggest an atmospheric background of f60 of below 0.03 % for CV. Moreover, we deployed the CV TOF-ACSM along with a SV AMS in urban Beijing during the winter of 2017 to characterize ambient OA with strong anthropogenic influences. The CV TOF-ACSM shows a collection efficiency (CE) of about unity. The CV and SV data show consistent mass concentrations of sulfate, nitrate, ammonium, and OA. Six OA factors are identified by the positive matrix factorization (PMF) analysis for both the CV and the SV data. The SV and CV PMF factors show good correlations in mass concentrations. The SV and CV factors related to coal combustion and cooking differ significantly in loadings, explained by the PMF uncertainty and the lack of understanding of the relative ionization efficiency and CE for primary OA. The CV factors related to secondary sources show greater loadings than the SV factors, which may be associated with the changes in signal-to-noise ratios of various ions in the PMF analysis. Our results support improved mass quantification and useful source identification by the CV for ambient particles in the polluted urban environment. The difference in factor loadings between SV and CV should be considered when interpreting or comparing the PMF results among studies. [ABSTRACT FROM AUTHOR]

Secondary organic aerosol (SOA) is an important constituent of the atmosphere where SOA particles are formed chiefly by the condensation or reactive uptake of oxidation products of volatile organic compounds (VOCs). The mass yield in SOA particle formation, as well as the chemical composition and volatility of the particles, is determined by the identity of the VOC precursor(s) and the oxidation conditions they experience. In this study, we used an oxidation flow reactor to generate biogenic SOA from the oxidation of Scots pine emissions. Mass yields, chemical composition and volatility of the SOA particles were characterized and compared with SOA particles formed from oxidation of α -pinene and from a mixture of acyclic–monocyclic sesquiterpenes (farnesenes and bisabolenes), which are significant components of the Scots pine emissions. SOA mass yields for Scots pine emissions dominated by farnesenes were lower than for α -pinene but higher than for the artificial mixture of farnesenes and bisabolenes. The reduction in the SOA yield in the farnesene- and bisabolene-dominated mixtures is due to exocyclic C = C bond scission in these acyclic–monocyclic sesquiterpenes during ozonolysis leading to smaller and generally more volatile products. SOA particles from the oxidation of Scots pine emissions had similar or lower volatility than SOA particles formed from either a single precursor or a simple mixture of VOCs. Applying physical stress to the Scots pine plants increased their monoterpene, especially monocyclic β -phellandrene, emissions, which further decreased SOA particle volatility and increased SOA mass yield. Our results highlight the need to account for the chemical complexity and structure of real-world biogenic VOC emissions and stress-induced changes to plant emissions when modelling SOA production and properties in the atmosphere. These results emphasize that a simple increase or decrease in relative monoterpene and sesquiterpene emissions should not be used as an indicator of SOA particle volatility. [ABSTRACT FROM AUTHOR]