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Atmospheric gas-to-particle conversion is a crucial or even dominant contributor to haze formation in Chinese megacities in terms of aerosol number, surface area and mass. Based on our comprehensive observations in Beijing during 15 January 2018-31 March 2019, we are able to show that 80-90% of the aerosol mass (PM2.5) was formed via atmospheric reactions during the haze days and over 65% of the number concentration of haze particles resulted from new particle formation (NPF). Furthermore, the haze formation was faster when the subsequent growth of newly formed particles was enhanced. Our findings suggest that in practice almost all present-day haze episodes originate from NPF, mainly since the direct emission of primary particles in Beijing has considerably decreased during recent years. We also show that reducing the subsequent growth rate of freshly formed particles by a factor of 3-5 would delay the buildup of haze episodes by 1-3 days. Actually, this delay would decrease the length of each haze episode, so that the number of annual haze days could be approximately halved. Such improvement in air quality can be achieved with targeted reduction of gas-phase precursors for NPF, mainly dimethyl amine and ammonia, and further reductions of SO2 emissions. Furthermore, reduction of anthropogenic organic and inorganic precursor emissions would slow down the growth rate of newly-formed particles and consequently reduce the haze formation.

Secondary aerosol formation in the aging process of primary emission is the main reason for haze pollution in eastern China. Pollution evolution with photochemical age was studied for the first time at a comprehensive field observation station during winter in Beijing. The photochemical age was used as an estimate of the time scale attributed to the aging process and was estimated from the ratio of toluene to benzene in this study. A low photochemical age indicates a fresh emission. The photochemical age of air masses during new particle formation (NPF) days was lower than that on haze days. In general, the strongest NPF events, along with a peak of the formation rate of 1.5 nm(J(1.5)) and 3 nmparticles (J(3)), were observed when the photochemical age was between 12 and 24 h while rarely took place with photochemical ages less than 12 h. When photochemical age was larger than 48 h, haze occurred and NPF was suppressed. The sources and sinks of nanoparticles had distinct relation with the photochemical age. Our results show that the condensation sink (CS) showed a valley with photochemical ages ranging from 12 to 24 h, while H2SO4 concentration showed no obvious trend with the photochemical age. The high concentrations of precursor vapours within an air mass lead to persistent nucleation with photochemical age ranging from 12 to 48 h in winter. Coincidently, the fast increase of PM2.5 mass was also observed during this range of photochemical age. Noteworthy, CS increased with the photochemical age on NPF days only, which is the likely reason for the observation that the PM2.5 mass increased faster with photochemical age on NPF days compared with other days. The evolution of particles with the photochemical age provides new insights into understanding how particles originating from NPF transform to haze pollution. (C) 2020 Elsevier B.V. All rights reserved.

Intense and frequent new particle formation (NPF) events have been observed in polluted urban environments, yet the dominant mechanisms are still under debate. To understand the key species and governing processes of NPF in polluted urban environments, we conducted comprehensive measurements in downtown Beijing during January–March, 2018. We performed detailed analyses on sulfuric acid cluster composition and budget, as well as the chemical and physical properties of oxidized organic molecules (OOMs). Our results demonstrate that the fast clustering of sulfuric acid (H2SO4) and base molecules triggered the NPF events, and OOMs further helped grow the newly formed particles toward climate- and health-relevant sizes. This synergistic role of H2SO4, base species, and OOMs in NPF is likely representative of polluted urban environments where abundant H2SO4 and base species usually co-exist, and OOMs are with moderately low volatility when produced under high NOx concentrations. publishedVersion

The climate and air quality effects of aerosol particles depend on the number and size of the particles. In urban environments, a large fraction of aerosol particles originates from anthropogenic emissions. To evaluate the effects of different pollution sources on air quality, knowledge of size distributions of particle number emissions is needed. Here we introduce a novel method for determining size-resolved particle number emissions, based on measured particle size distributions. We apply our method to data measured in Beijing, China, to determine the number size distribution of emitted particles in a diameter range from 2 to 1000 nm. The observed particle number emissions are dominated by emissions of particles smaller than 30 nm. Our results suggest that traffic is the major source of particle number emissions with the highest emissions observed for particles around 10 nm during rush hours. At sizes below 6 nm, clustering of atmospheric vapors contributes to calculated emissions. The comparison between our calculated emissions and those estimated with an integrated assessment model GAINS (Greenhouse Gas and Air Pollution Interactions and Synergies) shows that our method yields clearly higher particle emissions at sizes below 60 nm, but at sizes above that the two methods agree well. Overall, our method is proven to be a useful tool for gaining new knowledge of the size distributions of particle number emissions in urban environments and for validating emission inventories and models. In the future, the method will be developed by modeling the transport of particles from different sources to obtain more accurate estimates of particle number emissions.

Understanding the atmospheric new particle formation (NPF) process within the global range is important for revealing the budget of atmospheric aerosols and their impacts. We investigated the seasonal characteristics of NPF in the urban environment of Beijing. Aerosol size distributions down to similar to 1 nm and H2SO4 concentration were measured during 2018-2019. The observed formation rate of 1.5 nm particles (J(1.5)) is significantly higher than those in the clean environment, e.g., Hyytiala, whereas the growth rate is not significantly different. Both J(1.5) and NPF frequency in urban Beijing show a clear seasonal variation with maxima in winter and minima in summer, while the observed growth rates are generally within the same range around the year. We show that ambient temperature is a governing factor driving the seasonal variation of J(1.5). In contrast, the condensation sink and the daily maximum H2SO4 concentration show no significant seasonal variation during the NPF periods. In all four seasons, condensation of H2SO4 and (H2SO4)(n)(amine)(n) clusters contributes significantly to the growth rates in the sub-3 nm size range, whereas it is less important for the observed growth rates of particles above 3 nm. Therefore, other species are always needed for the growth of larger particles.

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The spatial and temporal variability of the number size distribution of aerosol particles is an indicator of the dynamic behavior of Beijing's atmospheric pollution cocktail. This variation reflects the strength of different primary and secondary sources, such as traffic and new particle formation, as well as the main processes affecting the particle population. In this paper, we report size-segregated particle number concentrations observed at a newly developed Beijing station during the winter of 2018. Our measurements covered particle number size distributions over the diameter range of 1.5 nm–1 µm (cluster mode, nucleation mode, Aitken mode and accumulation mode), thus being descriptive of a major fraction of the processes taking place in the atmosphere of Beijing. Here we focus on explaining the concentration variations in the observed particle modes, by relating them to the potential aerosol sources and sinks, and on understanding the connections between these modes. We considered haze days and new particle formation event days separately. Our results show that during the new particle formation (NPF) event days increases in cluster mode particle number concentration were observed, whereas during the haze days high concentrations of accumulation mode particles were present. There was a tight connection between the cluster mode and nucleation mode on both NPF event and haze days. In addition, we correlated the particle number concentrations in different modes with concentrations of trace gases and other parameters measured at our station. Our results show that the particle number concentration in all the modes correlated with NOx, which reflects the contribution of traffic to the whole submicron size range. We also estimated the contribution of ion-induced nucleation in Beijing, and we found this contribution to be negligible.

Aerosol number concentration varying spatially and temporally is a good indicator of the dynamic behavior of Beijing's atmospheric cocktail. This variation represents the strength of different contributing primary and secondary sources such as traffic and new particle formation, respectively. In this paper we report size-segregated particle number concentrations observed at newly developed Beijing station during winter 2018. Our measurements cover number size distributions of particles in a diameter range between 1.5 nm and 1 μm (cluster mode, nucleation mode, Aitken mode and accumulation mode), thus being descriptive of a major fraction of the processes happening in the atmosphere of Beijing. Here we aim to explain the concentration variation in the observed modes by relating them to potential aerosol sources as well as to understand the connection between the modes. We focused on two types of days (haze and new particle formation) and divided the data accordingly. Our results show that during new particle formation (NPF) days, an increase in the cluster mode particles was observed. In contrast, during haze days we observed a high concentration of accumulation mode particles. There was a clear correlation between the cluster and nucleation modes during NPF days, while it was absent during haze days. In addition, we correlated the different modes with concentrations of trace gases and other parameters measured at our station. Our results show that all modes in the sub-micron size range correlated with NOx which clearly reflects the contribution of traffic to all particle sizes.

Interest in understanding gas-to-particle phase transformation in several disciplines such as atmospheric sciences, material synthesis, and combustion has led to the development of several distinct instruments that can measure the particle size distributions down to the sizes of large molecules and molecular clusters, at which the initial particle formation and growth takes place. These instruments, which include the condensation particle counter battery, a variety of electrical mobility spectrometers and the particle size magnifier, have been usually characterized in laboratory experiments using carefully prepared calibration aerosols. They are then applied, alone or in combination, to study the gas-to-particle transition in experiments that produce particles with a wide range of compositions and other properties. Only a few instrument intercomparisons in either laboratory or field conditions have been reported, raising the question: how accurately can the sub-10 nm particle number size distributions be measured with the currently available instrumentation? Here, we review previous studies in which sub-10 nm particle size distributions have been measured with at least two independent instruments. We present recent data from three sites that deploy the current state-of-the-art instrumentation: Hyytiala, Beijing, and the CLOUD chamber. After discussing the status of the sub-10 nm size distribution measurements, we present a comprehensive uncertainty analysis for these methods that suggests that our present understanding on the sources of uncertainties quite well captures the observed deviations between different instruments in the size distribution measurements. Finally, based on present understanding of the characteristics of a number of systems in which gas-to-particle conversion takes place, and of the instrumental limitations, we suggest guidelines for selecting suitable instruments for various applications.

SO2 concentration decreased rapidly in recent years in China due to the implementation of strict control policies by the government. Particulate sulfate (pSO(4)(2-)) and gaseous H2SO4 (SA) are two major products of SO2 and they play important roles in the haze formation and new particle formation (NPF), respectively. We examined the change in pSO(4)(2-) and SA concentrations in response to reduced SO2 concentration using long-term measurement data in Beijing. Simulations from the Community Multiscale Air Quality model with a 2-D Volatility Basis Set (CMAQ/2D-VBS) were used for comparison. From 2013 to 2018, SO2 concentration in Beijing decreased by similar to 81% (from 9.1 ppb to 1.7 ppb). pSO(4)(2-) concentration in submicrometer particles decreased by similar to 60% from 2012-2013 (monthly average of similar to 10 mu g.m(-3)) to 2018-2019 (monthly average of similar to 4 mu g.m(-3)). Accordingly, the fraction of pSO(4)(2-) in these particles decreased from20-30% to b10%. Increased sulfur oxidation ratio was observed both in the measurements and the CMAQ/2D-VBS simulations. Despite the reduction in SO2 concentration, there was no obvious decrease in SA concentration based on data from several measuring periods from 2008 to 2019. This was supported by the increased SA:SO2 ratio with reduced SO2 concentration and condensation sink. NPF frequency in Beijing between 2004 and 2019 remains relatively constant. This constant NPF frequency is consistent with the relatively stable SA concentration in Beijing, while different from some other cities where NPF frequency was reported to decrease with decreased SO2 concentrations. (C) 2020 Elsevier B.V. All rights reserved.

Core sampling method (extracting a portion of a flow from the core of the flow) will reduce diffusional losses of highly diffusive species (e.g., aerosol nanoparticles, ions, and gases) when transporting them through a sampling tube. Revealing parameters governing the sampling efficiency of a core sampling system, ηsam, helps to design the apparatus and to optimize its performance. In this study, we report an analytical solution for quantifying the ηsam by solving the convection diffusion equation of laminar flow field. The analytical results were experimentally evaluated using 1–5 nm tungsten oxide nanoparticles. ηsam is governed by a dimensionless loss parameter and the transport-to-sample flow ratio. Theoretically predicted values for ηsam agree with experimental results, e.g., the relative deviation is within 5% when the value for the loss parameter is less than 0.1. The core sampling method is recommended to work at the loss parameter less than 0.1 such that ηsam is equal or close to the maximum value of unity and is also insensitive to variations in sampling conditions. In this study, how to apply the findings in designing and optimizing a core sampling system was discussed. A core sampling apparatus was then designed and experimentally evaluated. Its sampling efficiency was shown to be significantly higher than those of a tee, a cross fitting, and a Y fitting when the same sampling conditions were used. Copyright © 2019 American Association for Aerosol Research

New particle formation (NPF) and the subsequent particle growth occur frequently in various atmospheric environments. Significant influence of transport on aerosol size distributions is commonly observed, especially for non-regional NPF events. With certain assumptions and approximations, a population balance method is proposed to examine the influence of transport on the temporal evolution of aerosol size distributions during NPF events. The method is derived from the aerosol general dynamic equation in the continuous form. Meteorological information (e.g., wind speed, wind direction, water vapor mixing ratio) was used to complement the analysis. The NPF events observed in South-East Tibet, Fukue Island, and urban Beijing were analyzed using the proposed method. Significant contribution of transport to the observed aerosol size distributions is found during the NPF events in both South-East Tibet and Fukue Island. The changes in the contribution of transport is in good correlation with the changes in wind speed and direction. This correlation indicates that local mountain and valley breezes govern the observed new particles at the South-East Tibet site. Most NPF events observed at Fukue Island are closely related to the long-range transport of aerosols and gaseous precursors due to the movement of air masses. Regional NPF events are typically observed in urban Beijing and the contribution of transport to the observed aerosol size distributions is negligible compared to condensational growth and coagulation scavenging. In relatively clean atmospheric environment, the proposed method can be used to characterize the contribution of transport to particles in the size range from ~ 10 nm to ~ 50 nm. However, during intense NPF events in relatively polluted atmosphere, the estimated contribution of transport is sensitive to the uncertainties in condensational growth and coagulation scavenging due to the dominance of their corresponding terms in the population balance equation.

The predominating role of aerosol Fuchs surface area, AFuchs, in determining the occurrence of new particle formation (NPF) events in Beijing was elucidated in this study. Analysis was based on a field campaign from March 12th to April 6th, 2016, in Beijing, during which aerosol size distributions down to ~ 1 nm and sulfuric acid concentration were simultaneously monitored. The 26 days were classified into 11 typical NPF days, 2 undefined days, and 13 non-event days. A dimensionless factor, LΓ, characterizing the relative ratio of the coagulation scavenging rate over the condensational growth rate and predicting whether or not a NPF event would occur (Kuang et al., 2010), was applied. The three parameters determining LΓ are sulfuric acid concentration, the growth enhancement factor characterizing contribution of other gaseous precursors to particle growth, Γ, and AFuchs. Different from other atmospheric environment such as in Boulder and Hyytiälä, the variations of daily maximum sulfuric acid concentration and Γ in Beijing are in a narrow range with geometric standard deviations of 1.40 and 1.31, respectively. Positive correlation was found between estimated new particle formation rate, J1.5, and sulfuric acid concentration with a mean fitted exponent of 2.4. However, sulfuric acid concentration on NPF days is not significantly higher than that on non-event days. Instead, AFuchs varies greatly among days in Beijing with a geometric standard deviation of 2.56, while it is relatively stable at other locations such as Tecamac, Atlanta, and Boulder. Good correlation was found between AFuchs and LΓ in Beijing (R2 = 0.88). It appears that the abundance of gaseous precursors such as sulfuric acid in Beijing is high enough to have nucleation, however, it is AFuchs that determines the occurrence of NPF event in Beijing. 10 in 11 NPF events occurred when AFuchs is smaller than 200 μm2/cm3, and the NPF event was suppressed due to coagulation scavenging when AFuchs is larger than 200 μm2/cm3. Measured AFuchs is in good correlation with PM2.5 mass concentration (R2 = 0.85) since AFuchs in Beijing is mainly determined by particles in the size range of 50–500 nm that also contribute to PM2.5 mass concentration.

Due to profound impact on climate and human health, air quality has attracted attention from all levels of the civil society. The key step in the provision of required tools for the society to tackle the complex air quality problem is to characterize it in a comprehensive manner with a long-term perspective. Here, we describe a continuous and comprehensive observation station and its accompanying state-of-the-art instrumentation that was established to investigate the complex urban atmospheric environment in a rapidly developing Chinese Megacity. The station, located in downtown Beijing, aims to study air quality by identifying the major atmospheric pollutants and key processes determining their formation and loss mechanisms. A few hundreds of parameters are continuously measured with the state-of-the-art instruments, including trace gas concentrations, aerosol particle size distributions, and mass concentrations, covering aerosol particle chemical composition from molecules to micrometer-sized aerosol particles. This produced long-term, comprehensive big data with around 1×1011 bytes per year. In this paper, we provide an overview on the facilities of the station, the instrumentation used, the workflow of continuous observations and examples of results from 2018 to 2019 and a basis for establishing a modern long-term, comprehensive atmospheric urban observation station in other megacities.

Due to profound impact on climate and human health, air quality has attracted attention from all levels of the civil society. The key step in the provision of required tools for the society to tackle the complex air quality problem is to characterize it in a comprehensive manner with a long-term perspective. Here, we describe a continuous and comprehensive observation station and its accompanying state-of-the-art instrumentation that was established to investigate the complex urban atmospheric environment in a rapidly developing Chinese Megacity. The station, located in downtown Beijing, aims to study air quality by identifying the major atmospheric pollutants and key processes determining their formation and loss mechanisms. A few hundreds of parameters are continuously measured with the state-of-the-art instruments, including trace gas concentrations, aerosol particle size distributions, and mass concentrations, covering aerosol particle chemical composition from molecules to micrometer-sized aerosol particles. This produced long-term, comprehensive big data with around 1×1011 bytes per year. In this paper, we provide an overview on the facilities of the station, the instrumentation used, the workflow of continuous observations and examples of results from 2018 to 2019 and a basis for establishing a modern long-term, comprehensive atmospheric urban observation station in other megacities.