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A dilute plasma is continuously maintained in the troposphere by ionising particle radiation from galactic cosmic rays and radon decay. Small ions in the 1–2 nm size range play an important role in atmospheric processes such as ion-induced nucleation of aerosol particles. Consequently there is a need for precise and robust instruments to measure small ions both for atmospheric observations and for laboratory experiments that simulate the atmosphere. Here we describe the design and performance of the Cluster Ion Counter (CIC, Airel OÜ), which simultaneously measures the number concentrations of positively- and negatively-charged ions and particles below 5 nm mobility diameter, with low noise and fast time response. The detection efficiency is above 80 % for ions and charged particles between 1.2 and 2.0 nm, and above 90 % between 2.0 and 3.0 nm. The ion concentrations measured by the CIC agree well with reference instruments. The noise level (1 σ of background measurements) is typically between 20 and 30 ions cm-3 at 1 Hz sampling rate and an air flow rate of 7 l min-1 per analyzer. The noise level improves when higher flow rates and longer sampling periods are used. The CIC responds rapidly with 1 s time resolution to pulses of ionisation produced in the CLOUD chamber by a CERN particle beam. [ABSTRACT FROM AUTHOR]

Chemical ionisation inlets are fundamental instrument components in chemical ionisation mass spectrometry (CIMS). However, the sample gas and reagent ion trajectories are often understood only in a general and qualitative manner. Here, we evaluate two atmospheric pressure interface chemical ionisation inlets (MION2 and Eisele type inlet) with 3D computational fluid dynamics physicochemical models regarding the reagent ion and sample gas trajectories and estimate their efficiencies of reagent ion production, reagent ion delivery from the ion source volume into the ion–molecule mixing region, and the interaction between reagent ions and target molecules. The models are validated by laboratory measurements and quantitatively reproduce observed sensitivities to tuning parameters, including ion currents and changes in mass spectra. The study elucidates how the different transport and chemical reactions proceed within the studied inlets, where space charge can already be relevant at ion concentrations as low as 10 7 cm-3 , and compares the two investigated inlet models. The models provide insights into how to operate the inlets and will help in the development of future inlets that further enhance the capability of CIMS. [ABSTRACT FROM AUTHOR]

Nitrous acid (HONO) is an important precursor of OH radicals which affects not only the sinks of primary air pollutants but also the formation of secondary air pollutants, but its source closure in the atmosphere is still controversial due to a lack of experiment validation. In this study, the HONO budget in Beijing has been analyzed and validated through the coronavirus disease (COVID-19) lockdown event, which resulted in a significant reduction in air pollutant emissions, providing a rare opportunity to understand the HONO budget in the atmosphere. We measured HONO and related pollutants from 1 January to 6 March 2020, which covered the Chinese New Year (CNY) and the COVID-19 lockdown. The average concentration of HONO decreased from 0.97 ± 0.74 ppb before CNY to 0.53 ± 0.44 ppb during the COVID-19 lockdown, accompanied by a sharp drop in NO x and the greatest drop in NO (around 87 %). HONO budget analysis suggests that vehicle emissions were the most important source of HONO during the nighttime (53 ± 17 %) before CNY, well supported by the decline in their contribution to HONO during the COVID-19 lockdown. We found that the heterogeneous conversion of NO 2 on ground surfaces was an important nighttime source of HONO (31 ± 5 %), while that on aerosol surfaces was a minor source (2 ± 1 %). Nitrate photolysis became the most important daytime source during the COVID-19 lockdown compared with that before CNY, resulting from the combined effect of the increase in nitrate and the decrease in NO. Our results indicate that reducing vehicle emissions should be an effective measure for alleviating HONO in Beijing. [ABSTRACT FROM AUTHOR]

Sub-10 nm metal-based nanoparticles have garnered immense interest due to their unique properties and versatile applications. In this study, we created sub-10 nm Ag-based particles with a spark discharge generator and explored the parameters impacting their size distribution and charging properties, including carrier gas flow rates, spark discharge voltage, electrode gap distances, and capacitance. Our findings illuminate that there is a comparable influence of different factors on both self-charged and neutral particles. Among the different factors, carrier gas flow rates emerging as a paramount determinant in particle size. While increasing spark discharge voltage and capacitance within the spark circuit increases particle concentrations, the associated changes in particle size prove to be less straightforward. Significant differences between the concentration of positive and negative self-charged particles manifest when the carrier gas flow rate surpasses 5.0 L min−1, with positive particles ranging from 0.8 to 1.2 nm and negative particles spanning 0.8 to 3.0 nm. Self-charged particles close to 1 nm tend to exhibit positive charges, whereas those larger than 2 nm tend to acquire negative charges, which suggests the growth of negative particles is faster than positive ones in the spark chamber. Nevertheless, these disparities between bipolar particles diminish with the increase of residence time, leading to the observation of similar particle size distributions. Positive particles consistently bear a single charge, while some negative particles exceeding 3 nm exhibit multiple charges, primarily under carrier gas flow rates exceeding 7.5 L min−1. This study provides insights into the control of properties of nano-sized metal particles, which are crucial for their practical utilization. Copyright © 2024 American Association for Aerosol Research [ABSTRACT FROM AUTHOR]

Aerosol particles undergo continuous changes in their chemical composition and physical properties throughout their lifecycles, leading to diverse climate and health impacts. In particular, organic nanoparticle's surface equilibrium vapor pressure stands as a critical factor for gas–particle partitioning and is pivotal for understanding the evolution of aerosol properties. Herein, we present measurements of evaporation kinetics and surface equilibrium vapor pressures of a wide array of laboratory-generated organic nanoparticles, employing the dynamic-aerosol-size electrical mobility spectrometer (DEMS) methodology, a recent advancement in aerosol process characterization. The DEMS methodology is founded on the principle that the local velocity of a size-changing nanoparticle within a flow field has a one-to-one correspondence with its local size. Consequently, this approach can facilitate the in situ probing of rapid aerosol size-changing processes by analyzing the trajectories of size-changing nanoparticles within the classification region of a differential mobility analyzer (DMA). We employ the DEMS with a tandem DMA setup, where a heated sheath flow in the second DMA initiates particle evaporation in its classification region. Through analysis of the DEMS response and the underlying mechanism governing the evaporation process, we reconstruct temporal radius profiles of evaporating nanoparticles and derive their surface equilibrium vapor pressures across various temperatures. Our results demonstrate a good agreement between the vapor pressures deduced from DEMS measurements and those documented in literature. We discuss the measurable vapor pressure range achievable with DEMS and elucidate associated uncertainties. Furthermore, we outline prospective directions for refining this methodology and anticipate its potential to contribute to the characterization of aerosol-related kinetic processes with currently unknown mechanisms. [ABSTRACT FROM AUTHOR]

Isolation of extracellular vesicles (EV) has been developing rapidly in parallel with the interest in EVs. However, commonly utilized protocols may not suit more challenging samplematrixes and could potentially yield suboptimal results. Knowing and assessing the pitfalls of isolation procedure to be used, should be involved to some extent for EV analytics. EVs in cow milk are of great interest due to their abundancy and large-scale availability as well as their cross-species bioavailability and possible use as drug carriers. However, the characteristics of milk EVs overlap with those of other milk components. This makes it difficult to isolate and study EVs individually. There exists also a lack of consensus for isolation methods. In this study, we demonstrated the differences between various differential centrifugation-based approaches for isolation of large quantities of EVs from cow milk. Samples were further purified with gradient centrifugation and size exclusion chromatography (SEC) and differenceswere analyzed. Quality measurements were conducted onmultiple independent platforms. Particle analysis, electron microscopy and RNA analysis were used, to comprehensively characterize the isolated samples and to identify the limitations and possible sources of contamination in the EV isolation protocols. Vesicle concentration to protein ratio and RNA to protein ratios were observed to increase as samples were purified, suggesting co-isolation with major milk proteins in direct differential centrifugation protocols. We demonstrated a novel size assessment of vesicles using a particle mobility analyzer that matched the sizing using electron microscopy in contrast to commonly utilized nanoparticle tracking analysis. Based on the standards of the International Society for Extracellular Vesicles and the quick checklist of EVTrack. org for EV isolation, we emphasize the need for complete characterization and validation of the isolation protocol with all EV-related work to ensure the accuracy of results and allow further analytics and experiments. [ABSTRACT FROM AUTHOR]

Particulate matter has major climate and health impacts, and it is therefore of utmost importance to be able to measure the composition of these particles to gain insights into their sources and characteristics. Many methods, both offline and online, have been employed over the years to achieve this goal. One of the most recent developments is the Vaporization Inlet for Aerosols (VIA) coupled to a nitrate Chemical Ionization Mass Spectrometer (NO 3 -CIMS), but a thorough understanding of the VIA–NO 3 -CIMS system remains incomplete. In this work, we ran a series of tests to assess the impacts from different systems and sampling parameters on the detection efficiency of highly oxygenated organic molecules (HOMs) in the VIA–NO 3 -CIMS system. Firstly, we found that the current VIA system (which includes an activated carbon denuder and a vaporization tube) efficiently transmits particles (> 90 % for particles larger than 50 nm) while also removing gaseous compounds (> 97% for tested volatile organic compounds – VOCs). One of the main differences between the VIA and traditional thermal desorption (TD) techniques is the very short residence time in the heating region, on the order of 0.1 s. We found that this short residence time, and the corresponding short contact with heated surfaces, is likely one of the main reasons why relatively reactive or weakly bound peroxides, for example, were observable using the VIA. However, the VIA also requires much higher temperatures in order to fully evaporate the aerosol components. For example, the evaporation temperature of ammonium sulfate particles using the VIA was found to be about 100–150 °C higher than in typical TD systems. We also observed that the evaporation of particles with larger sizes occurred at slightly higher temperatures compared to smaller particles. Another major aspect that we investigated was the gas-phase wall losses of evaporated molecules. With a more optimized interface between the VIA and the NO 3 -CIMS, we were able to greatly decrease wall losses and thus improve the sensitivity compared to our earlier VIA work. This interface included a dedicated sheath flow unit to cool the heated sample and provide the NO 3 -CIMS with the needed high flow (10 L min -1). Our results indicate that most organic molecules observable by the NO 3 -CIMS can evaporate and be transported efficiently in the VIA system, but upon contact with the hot walls of the VIA, the molecules are instantaneously lost. This loss potentially leads to fragmentation products that are not observable by the NO 3 -CIMS. Thermograms, obtained by scanning the VIA temperature, were found to be very valuable for both quantification purposes and for estimating the volatility of the evaporating compounds. We developed a simple one-dimensional model to account for the evaporation of particles and the temperature-dependent wall losses of the evaporated molecules, and we thereby estimate the concentration of HOMs in secondary organic aerosol (SOA) particles. Overall, our results provide much-needed insights into the key processes underlying the VIA–NO 3 -CIMS method. Although there are still some limitations that could be addressed through hardware improvements, the VIA–NO 3 -CIMS system is a very promising and useful system for fast online measurements of HOMs in the particle phase. [ABSTRACT FROM AUTHOR]

Nitrous acid (HONO) is an important precursor of OH radicals which affects not only the sinks of primary air pollutants but also the formation of secondary air pollutants, whereas its source closure in the atmosphere is still controversial due to a lack of experiment validation. In this study, the HONO budget in Beijing has been analyzed and validated through the coronavirus disease (COVID-19) lockdown event, which resulted in the largest changes in air pollutant emissions in the history of modern atmospheric chemistry. A home-made Water-based Long-Path Absorption Photometer (LOPAP) along with other instruments were used to measure the HONO and related pollutants from January 1, 2020 to March 6, 2020, which covered the Chinese New Year (CNY) and the COVID-19 lockdown. The average concentration of HONO decreased from 0.97 ± 0.74 ppb before CNY to 0.53 ± 0.44 ppb during the COVID-19 lockdown, accompanied by a sharp drop of NOx and the greatest drop of NO (around 87%). HONO budget analysis suggests that vehicle emissions were the most important source of HONO during the nighttime (53%) before CNY, well supported by the decline of their contribution to HONO during the COVID-19 lockdown. We found that the heterogeneous conversion of NO2 on ground surfaces was an important nighttime source of HONO (31%), while that on aerosol surfaces was a minor source (2%). Nitrate photolysis became the most important daytime source during the COVID-19 lockdown compared with that before CNY, resulting from the combined effect of the increase in nitrate and the decrease in NO. Our results indicate that reducing vehicle emissions should be an effective measure for alleviating HONO in Beijing. [ABSTRACT FROM AUTHOR]

Nitrous acid (HONO) is an important precursor of OH radicals which affects not only the sinks of primary air pollutants but also the formation of secondary air pollutants, whereas its source closure in the atmosphere is still controversial due to a lack of experiment validation. In this study, the HONO budget in Beijing has been analyzed and validated through the coronavirus disease (COVID-19) lockdown event, which resulted in the largest changes in air pollutant emissions in the history of modern atmospheric chemistry. A home-made Water-based Long-Path Absorption Photometer (LOPAP) along with other instruments were used to measure the HONO and related pollutants from January 1, 2020 to March 6, 2020, which covered the Chinese New Year (CNY) and the COVID-19 lockdown. The average concentration of HONO decreased from 0.97 ± 0.74 ppb before CNY to 0.53 ± 0.44 ppb during the COVID-19 lockdown, accompanied by a sharp drop of NOx and the greatest drop of NO (around 87 %). HONO budget analysis suggests that vehicle emissions were the most important source of HONO during the nighttime (53 %) before CNY, well supported by the decline of their contribution to HONO during the COVID-19 lockdown. We found that the heterogeneous conversion of NO2 on ground surfaces was an important nighttime source of HONO (31 %), while that on aerosol surfaces was a minor source (2 %). Nitrate photolysis became the most important daytime source during the COVID-19 lockdown compared with that before CNY, resulting from the combined effect of the increase in nitrate and the decrease in NO. Our results indicate that reducing vehicle emissions should be an effective measure for alleviating HONO in Beijing. [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]

Differential mobility particle size spectrometers (DMPSs) are widely used to measure the aerosol number size distribution. Especially during new particle formation (NPF), the dynamics of the ultrafine size distribution determine the significance of the newly formed particles within the atmospheric system. A precision quantification of the size distribution and derived quantities such as new particle formation and growth rates is therefore essential. However, size-distribution measurements in the sub-10 nm range suffer from high particle losses and are often derived from only a few counts in the DMPS system, making them subject to very high counting uncertainties. Here we show that a CPC (modified Airmodus A20) with a significantly higher aerosol optics flow rate compared to conventional ultrafine CPCs can greatly enhance the counting statistics in that size range. Using Monte Carlo uncertainty estimates, we show that the uncertainties of the derived formation and growth rates can be reduced from 10 %–20 % down to 1 % by deployment of the high statistics CPC on a strong NPF event day. For weaker events and hence lower number concentrations, the counting statistics can result in a complete breakdown of the growth rate estimate with relative uncertainties as high as 40 %, while the improved DMPS still provides reasonable results at 10 % relative accuracy. In addition, we show that other sources of uncertainty are present in CPC measurements, which might become more important when the uncertainty from the counting statistics is less dominant. Altogether, our study shows that the analysis of NPF events could be greatly improved by the availability of higher counting statistics in the used aerosol detector of DMPS systems. [ABSTRACT FROM AUTHOR]

Atmospheric cluster ions are important constituents in the atmosphere, and their concentrations and compositions govern their role in atmospheric chemistry. However, there is currently limited quantitative research on atmospheric ion compositions, sources, and sinks, especially in the urban atmosphere where pollution levels and human populations are intense. In this study, we measured the compositions of negative cluster ions and neutral molecules using an atmospheric pressure interface high-resolution time-of-flight mass spectrometer (APi-TOF) and a chemical ionization mass spectrometer in urban Beijing. Quantitative analysis of cluster ions was performed by their comparison with condensation sink (CS), reagent ions, and neutral molecules. We demonstrate the feasibility of quantifying cluster ions with different compositions using in situ-measured ion mobility distributions from a neutral cluster and air ion spectrometer (NAIS). The median concentration of negative cluster ions was 85 (61–112 for 25 %–75 %) cm -3 during the measurement period, which was negatively correlated with CS. The negative cluster ions mainly consisted of inorganic nitrogen-containing ions, inorganic sulfur-containing ions, and organic ions in the form of adducts with NO 3- or HSO 4-. The CHON-related organic ions accounted for over 70 % of the total organic ions. Although the molecules clustered with NO 3- and HSO 4- had similar compositions, we found that HSO 4- clustered more efficiently with CHO and CHON nonNPs species (CHON excluding nitrated phenols), while NO 3- clustered more efficiently with nitrated phenols (CHON NPs). Additionally, most organic ions were positively correlated with neutral molecules, resulting in similar diurnal cycles of organic ions and neutral molecules. However, an exception was found for CHON NPs , the concentration of which is also significantly influenced by the reagent ions NO 3-. The charge fractions are generally higher for molecules with higher molecular weight and a higher oxidation state, and the opposite diurnal variations in charging fractions between H 2 SO 4 and organic species indicate a charging competition between them. Finally, we choose HSO 4- and C 3 H 3 O 4- as representatives to calculate the contribution of different formation and loss pathways. We found their losses are condensational loss onto aerosol particles (73 %–75 %), ion–molecule reaction losses (19 %), and ion–ion recombination losses (6 %–8 %). [ABSTRACT FROM AUTHOR]

New particle formation (NPF) is a global phenomenon that significantly influences climate. NPF also contributes to haze, with pronounced negative impacts on human health. Theory and observations both show that nucleation is favored during clean days and inhibited during haze episodes due to a high pre‐existing condensation sink (CS). Here we show that the surprising occurrence of NPF during haze days in Beijing is associated with a high concentration of sulfuric acid dimers. With both field observations and model simulations, we demonstrate that downward mixing of sulfur dioxide (SO2) from the residual layer aloft enhances ground level SO2, which in turn elevates sulfuric acid dimer after rapid SO2 oxidation in the polluted air. Our results address a key gap between the source of SO2 and its atmospheric oxidation products during haze conditions in a megacity, Beijing, providing a missing link in a complete chain describing NPF in the polluted atmosphere. Plain Language Summary: New particle formation is a global phenomenon with notable effects on climate and health. Recent studies have argued that atmospheric nucleation during haze episodes with high aerosol loading should not occur. In spite of this, we have observed intensive nucleation events during haze episodes in Beijing and found an unexpected high concentration of sulfur dioxide (SO2), along with a decreased condensation sink (CS) owing to the development of boundary layer. The SO2 originates from emissions by tall stacks outside of Beijing and is subsequently transported downward from the morning residual layer to the surface, this contributes to peaks of SO2 during the day. A decreased loss of sulfuric acid dimer owing to a decreased CS drives nucleation during haze episodes. As the phenomenon of a later morning SO2 peak exactly coincide with nucleation has been widely reported in eastern China, we highlight the importance of emissions of SO2 from tall smokestacks in atmospheric nucleation events. Key Points: Sulfuric acid dimer is involved in new particle formation (NPF) events during haze episodes in BeijingField observation and model simulation confirm downward mixing of sulfur dioxide from residual layerHigh formation rate of sulfuric acid dimer owing to a decreased condensation sink promotes NPF during haze [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]

Highly oxygenated compounds are important contributors to the formation and growth of atmospheric organic aerosol and thus have an impact on Earth's radiation balance and global climate. However, knowledge of the contribution of highly oxygenated compounds to organic aerosol and their fate after condensing into the particle phase has been limited by the lack of suitable detection techniques. Here, we present a new online method for measuring highly oxygenated compounds from organic aerosol. The method includes thermal evaporation of particles in a new inlet, the vaporization inlet for aerosols (VIA), followed by identification of the evaporated highly oxygenated compounds by a nitrate chemical ionization mass spectrometer (NO 3 -CIMS). The method does not require sample collection, enabling highly time-resolved measurements of particulate compounds. We evaluate the performance of the method by measuring the detection limit and performing background measurements. We estimate a detection limit of below 1 ng m -3 for a single compound and below 1 µ g m -3 for SOA with the sampling setup used here. These detection limits can be improved upon by optimizing the flow setup. Furthermore, we detect hundreds of particulate highly oxygenated compounds from organic aerosol generated from different precursors. Our results are consistent with previous studies showing that the volatility of organic compounds decreases with increasing m/z ratio and level of oxygenation and that organic aerosol consists of monomers and oligomeric compounds. By comparing the gas- and particle-phase compounds, we found indications of potential particle-phase reactions occurring in organic aerosol. Future work will focus both on further improving the sampling design and on better understanding the evaporation dynamics of the system, but already these initial tests show that the VIA coupled to the NO 3 -CIMS is a promising method for investigating the transformations and fate of the compounds after condensing into the particle phase. [ABSTRACT FROM AUTHOR]

Forests emit large quantities of volatile organic compounds (VOCs) to the atmosphere. Their condensable oxidation products can form secondary organic aerosol, a significant and ubiquitous component of atmospheric aerosol (1,2), which is known to affect the Earth's radiation balance by scattering solar radiation and by acting as cloud condensation nuclei (3). The quantitative assessment of such climate effects remains hampered by a number of factors, including an incomplete understanding of how biogenic VOCs contribute to the formation of atmospheric secondary organic aerosol. The growth of newly formed particles from sizes of less than three nanometres up to the sizes of cloud condensation nuclei (about one hundred nanometres) in many continental ecosystems requires abundant, essentially nonvolatile organic vapours (4-6), but the sources and compositions of such vapours remain unknown. Here we investigate the oxidation of VOCs, in particular the terpene α-pinene, under atmospherically relevant conditions in chamber experiments. We find that a direct pathway leads from several biogenic VOCs, such as monoterpenes, to the formation of large amounts of extremely low-volatility vapours. These vapours form at significant mass yield in the gas phase and condense irreversibly onto aerosol surfaces to produce secondary organic aerosol, helping to explain the discrepancy between the observed atmospheric burden of secondary organic aerosol and that reported by many model studies (2). We further demonstrate how these low-volatility vapours can enhance, or even dominate, the formation and growth of aerosol particles over forested regions, providing a missing link between biogenic VOCs and their conversion to aerosol particles. Our findings could help to improve assessments of biosphere-aerosol-climate feedback mechanisms (6-8), and the air quality and climate effects of biogenic emissions generally., We studied the oxidation of several monoterpenes and other VOCs under atmospherically relevant conditions in the highly instrumented Julich Plant Atmosphere Chamber, JPAC (9), a continuously stirred tank reactor (see [...]

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]

The study of atmospheric processes related to climate requires a multidisciplinary approach, encompassing physics, chemistry, meteorology, forest science, and environmental science. The Academy of Finland Centre of Excellence in atmospheric sciences (CoE ATM) responded to that need for 18 years and produced extensive research and eloquent results, which are summarized in this review. The work in the CoE ATM enhanced our understanding in biogeochemical cycles, ecosystem processes, dynamics of aerosols, ions and neutral clusters in the lower atmosphere, and cloud formation and their interactions and feedbacks. The CoE ATM combined continuous and comprehensive long-term in-situ observations in various environments, ecosystems and platforms, ground- and satellitebased remote sensing, targeted laboratory and field experiments, and advanced multi-scale modeling. This has enabled improved conceptual understanding and quantifications across relevant spatial and temporal scales. Overall, the CoE ATM served as a platform for the multidisciplinary research community to explore the interactions between the biosphere and atmosphere under a common and adaptive framework. [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]

The survival probability of freshly nucleated particles governs the influences of new particle formation (NPF) on atmospheric environments and the climate. It characterizes the probability of a particle avoiding being scavenged by the coagulation with pre-existing particles and other scavenging processes before the particle successfully grows up to a certain diameter. Despite its importance, measuring the survival probability has been challenging, which limits the knowledge of particle survival in the atmosphere and results in large uncertainties in predicting the influences of NPF. Here we report the proper methods to retrieve particle survival probability using the measured aerosol size distributions. Using diverse aerosol size distributions from urban Beijing, the Finnish boreal forest, a chamber experiment, and aerosol kinetic simulations, we demonstrate that each method is valid for a different type of aerosol size distribution, whereas misapplying the conventional methods to banana-type NPF events may underestimate the survival probability. Using these methods, we investigate the consistency between the measured survival probability of new particles and the theoretical survival probability against coagulation scavenging predicted using the measured growth rate and coagulation sink. With case-by-case and time- and size-resolved analysis of long-term measurement data from urban Beijing, we find that although both the measured and theoretical survival probabilities are sensitive to uncertainties and variations, they are, on average, consistent with each other for new particles growing from 1.4 (the cluster size) to 100 nm. [ABSTRACT FROM AUTHOR]

Atmospheric cluster ions are important constituents in the atmosphere. Concentrations and compositions of cluster ions govern their effects on atmospheric chemistry, air quality, and human health. However, quantitative research on ion composition is rare, especially in an urban atmosphere where pollution levels and human populations are intense. In this study, we measure negative cluster ion compositions using an atmospheric pressure interface high-resolution time-of-flight mass spectrometer in urban Beijing. We demonstrate the feasibility of quantifying cluster ion compositions with simultaneous in-situ measurements by a neutral cluster and air ion spectrometer. The median concentrations of negative cluster ions smaller than 1.6 nm were 85 (61–112 for 25–75 %) cm-3, decreasing significantly with an increasing condensation sink (CS). These concentrations are far lower than those observed at comparatively clean sites due to the higher CS in polluted environments. The ions NO3- and HSO4-, together with organic ions with the adducts of NO3- and HSO4-, were the most abundant in urban Beijing, and the organic ions in the atmosphere were similar in composition to those oxygenated organic molecules charged in a chemical ionization mass spectrometer with NO3- as the reagent ions. It was shown that the ambient atmosphere is a natural ion-molecular reaction chamber with NO3- and HSO4- as the main reagent ions. Compared to the clean sites, negative cluster ions in Beijing are composed of more NO3- and CHON organic ions due to higher NOx concentrations and higher fractions of CHON molecules in overall oxygenated organic species. Using dynamic equilibrium equations to examine the fate of HSO4- and C3H3O4- in the atmosphere, we found that their main sources to be the ionization of H2SO4 and C3H4O4 by NO3- and their main loss being the condensational loss onto aerosol particles (73–75 %), followed by ion-molecule reaction losses (19 %), and ion-ion recombination losses (6–8 %). [ABSTRACT FROM AUTHOR]