Aerosol liquid water in PM2.5 and its roles in secondary aerosol formation at a regional site of Yangtze River Delta.

Aerosol liquid water content (ALWC) plays an important role in secondary aerosol formation. In this study, a whole year field campaign was conducted at Shanxi in north Zhejiang Province during 2021. ALWC estimated by ISORROPIA-II was then investigated to explore its characteristics and relationship with secondary aerosols. ALWC exhibited a highest value in spring (66.38 µg/m3), followed by winter (45.08 µg/m3), summer (41.64 µg/m3), and autumn (35.01 µg/m3), respectively. It was supposed that the secondary inorganic aerosols (SIA) were facilitated under higher ALWC conditions (RH > 80%), while the secondary organic species tended to form under lower ALWC levels. Higher RH (> 80%) promoted the NO3- formation via gas-particle partitioning, while SO42- was generated at a relative lower RH (> 50%). The ALWC was more sensitive to NO3- (R = 0.94) than SO42- (R = 0.90). Thus, the self-amplifying processes between the ALWC and SIA enhanced the particle mass growth. The sensitivity of ALWC and OX (NO2 + O3) to secondary organic carbon (SOC) varied in different seasons at Shanxi, more sensitive to aqueous-phase reactions (daytime R = 0.84; nighttime R = 0.54) than photochemical oxidation (daytime R = 0.23; nighttime R = 0.41) in wintertime with a high level of OX (daytime: 130-140 µg/m3; nighttime: 100-140 µg/m3). The self-amplifying process of ALWC and SIA and the aqueous-phase formation of SOC will enhance aerosol formation, contributing to air pollution and reduction of visibility.

Predicting Atmospheric Water-Soluble Organic Mass Reversibly Partitioned to Aerosol Liquid Water in the Eastern United States.

Water-soluble organic matter (WSOM) formed through aqueous processes contributes substantially to total atmospheric aerosol, however, the impact of water evaporation on particle concentrations is highly uncertain. Herein, we present a novel approach to predict the amount of evaporated organic mass induced by sample drying using multivariate polynomial regression and random forest (RF) machine learning models. The impact of particle drying on fine WSOM was monitored during three consecutive summers in Baltimore, MD (2015, 2016, and 2017). The amount of evaporated organic mass was dependent on relative humidity (RH), WSOM concentrations, isoprene concentrations, and NOx/isoprene ratios. Different models corresponding to each class were fitted (trained and tested) to data from the summers of 2015 and 2016 while model validation was performed using summer 2017 data. Using the coefficient of determination (R2) and the root-mean-square error (RMSE), it was concluded that an RF model with 100 decision trees had the best performance (R2 of 0.81) and the lowest normalized mean error (NME < 1%) leading to low model uncertainties. The relative feature importance for the RF model was calculated to be 0.55, 0.2, 0.15, and 0.1 for WSOM concentrations, RH levels, isoprene concentrations, and NOx/isoprene ratios, respectively. The machine learning model was thus used to predict summertime concentrations of evaporated organics in Yorkville, Georgia, and Centerville, Alabama in 2016 and 2013, respectively. Results presented herein have implications for measurements that rely on sample drying using a machine learning approach for the analysis and interpretation of atmospheric data sets to elucidate their complex behavior.

Comparison of Phase States of PM2.5 over Megacities, Seoul and Beijing, and Their Implications on Particle Size Distribution.

Although the particle phase state is an important property, there is scant information on it, especially, for real-world aerosols. To explore the phase state of fine mode aerosols (PM2.5) in two megacities, Seoul and Beijing, we collected PM2.5 filter samples daily from Dec 2020 to Jan 2021. Using optical microscopy combined with the poke-and-flow technique, the phase states of the bulk of PM2.5 as a function of relative humidity (RH) were determined and compared to the ambient RH ranges in the two cities. PM2.5 was found to be liquid to semisolid in Seoul but mostly semisolid to solid in Beijing. The liquid state was dominant on polluted days, while a semisolid state was dominant on clean days in Seoul. These findings can be explained by the aerosol liquid water content related to the chemical compositions of the aerosols at ambient RH; the water content of PM2.5 was much higher in Seoul than in Beijing. Furthermore, the overall phase states of PM2.5 observed in Seoul and Beijing were interrelated with the particle size distribution. The results of this study aid in a better understanding of the fundamental physical properties of aerosols and in examining how these are linked to PM2.5 in polluted urban atmospheres.

Source and Chemistry of Hydroxymethanesulfonate (HMS) in Fairbanks, Alaska.

Fairbanks, Alaska, is a subarctic city with fine particle (PM2.5) concentrations that exceed air quality regulations in winter due to weak dispersion caused by strong atmospheric inversions, local emissions, and the unique chemistry occurring under the cold and dark conditions. Here, we report on observations from the winters of 2020 and 2021, motivated by our pilot study that showed exceptionally high concentrations of fine particle hydroxymethanesulfonate (HMS) or related sulfur(IV) species (e.g., sulfite and bisulfite). We deployed online particle-into-liquid sampler-ion chromatography (PILS-IC) in conjunction with a suite of instruments to determine HMS precursors (HCHO, SO2) and aerosol composition in general, with the goal to characterize the sources and sinks of HMS in wintertime Fairbanks. PM2.5 HMS comprised a significant fraction of PM2.5 sulfur (26-41%) and overall PM2.5 mass concentration of 2.8-6.8% during pollution episodes, substantially higher than what has been observed in other regions, likely due to the exceptionally low temperatures. HMS peaked in January, with lower concentrations in December and February, resulting from changes in precursors and meteorological conditions. Strong correlations with inorganic sulfate and organic mass during pollution events suggest that HMS is linked to processes responsible for poor air quality episodes. These findings demonstrate unique aspects of air pollution formation in cold and humid atmospheres.

Direct Measurement of Aerosol Liquid Water Content: A Case Study in Summer in Nanjing, China.

Aerosol liquid water content (ALWC) affects the mass loading, optical properties, and toxicity of aerosols. However, the measurement of ALWC is very rare due to its requirement of sophisticated instruments and its high operational costs. In this work, we improved on our previous simple, low-cost method by using a combination of one real-time fine particulate matter (PM2.5) monitor and two turbidimeters and successfully applied these for the direct measurement of ALWC in PM2.5 in Nanjing during the summer of 2023. The average ALWC during this measurement period occupied ~1/6 of the total PM2.5 mass, and this contribution was even greater with the elevation in the PM2.5 concentration. The ALWC was, as anticipated, closely related to the relative humidity (RH) and PM2.5 concentrations, but it did not always increase with the air quality index (AQI) due to the fact that polluted periods in summer were often governed by high O3 levels, not PM2.5 levels. The ALWC also had a great impact on visibility; it could decrease the visibility rapidly to hazy conditions when the dry PM2.5 was not high (~30 µg m-3) or the AQI was "good" (75~100), indicating that the air quality classified as "good" using the dry PM2.5 concentration might actually be "lightly polluted" if the ALWC is included. We also found that the air mass originating from Northeast China had the lowest PM2.5 mass concentration yet the highest ALWC values due to its high RH. Moreover, the quantification of ALWC levels can help us understand the solubility/bioavailability and thus the toxic effects of some specific components (for example, heavy metals or organics). Moreover, the influence of ALWC on air quality classifications should also be considered in the assessment of the health effects of air pollution and in public health early warning and protection.

Role of gas-particle conversion of ammonia in haze pollution under ammonia-rich environment in Northern China and prospects of effective emission reduction.

As an important precursor of secondary inorganic aerosols (SIAs), ammonia (NH3) plays a key role in fine particulate matter (PM2.5) formation. In order to investigate its impacts on haze formation in the North China Plain (NCP) during winter, NH3 concentrations were observed at a high-temporal resolution of 1 min by using the SP-DOAS in Tai'an from December 2021 to February 2022. During the observation period, the average NH3 concentration was 11.84 ± 5.9 ppbv, and it was determined as an ammonia-rich environment during different air quality conditions. Furthermore, the average concentrations of sulfate (SO42-), nitrate (NO3-) and ammonium (NH4+) were 9.54 ± 5.97 µg/m3, 19.09 ± 14.18 µg/m3 and 10.72 ± 6.53 µg/m3, respectively. Under the nitrate-dominated atmospheric environment, aerosol liquid water content (ALWC) was crucial for NH3 particle transformation during haze aggravation, and the gas-particle partitioning of ammonia played an important role in the SIAs formation. The reconstruction of the molecular composition further indicated that ammonium nitrate (NH4NO3) plays a dominant role in the increase of PM2.5 during haze events. Consequently, future efforts to mitigate fine particulate pollution in this region should focus on controlling NH4NO3 levels. In ammonia-rich environments, NO3- formation is more dependent on the concentration of nitric acid (HNO3). The sensitive analysis of TNO3 (HNO3 + NO3-) and NHX (NH3 + NH4+) reduction using the thermodynamic model suggested that the NO3- concentration decreases linearly with the reduction of TNO3. And the concentration of NO3- decreases rapidly only when NHX is reduced by 50-60 %. Reducing NOX emissions is the most effective way to alleviate nitrate pollution in this region.

Size-resolved water-soluble organic carbon and its significant contribution to aerosol liquid water.

Size-segregated aerosols collected in Beijing from 2021 to 2022 were used to investigate the contribution of organic aerosols to the aerosol liquid water content (ALWC), the influencing factors of ALWC, and the concentrations and size distribution characteristics of water-soluble organic carbon (WSOC) after clean air actions. The results showed that the concentration of WSOC in particulate matter (PM)1.8 was 3.52 ± 2.43 µg/m3 during the sampling period. Obvious changes were observed in the size distribution of WSOC after clean air actions, which may be attributed to the enhancement of atmospheric oxidation capacity and the decrease in PM concentration. The contribution of organic aerosols to the ALWC in fine PM was 18.1 % during the sampling period, which was more significant at lower particles concentration and smaller particle size ranges. The ambient relative humidity (RH) and the ratio of NO3-/SO42- had an apparent influence on ALWC. The continuous increase in the nitrate proportion significantly reduced the deliquescence point of the aerosols, making them prone to hygroscopic growth at lower RH. Analysis of the relation among nitrogen oxidation ratio (sulfur oxidation ratio), ALWC and PM1.8 mass concentrations suggests that organic matter has a significant effect on the formation of secondary inorganic aerosols in the initial phase of pollution formation and plays a crucial role in aerosol pollution formation in Beijing. These results are conducive to understanding the formation mechanism of aerosols and provide scientific data and theoretical support for the formulation of more effective emission-reduction measures.

Characteristics of secondary aerosol formation during shortened multiday reaction experiments in a smog chamber: Effects of relative humidity and ammonia.

Shortened multiday reaction experiments were conducted using the KIST chamber for atmospheric processes simulation (K-CAPS) to characterize the effects of ammonia (NH3) and relative humidity (RH) on the formation of secondary organic aerosols (SOA) due to photooxidation of a mixture of toluene and inorganic gases such as NOx, SO2, and NH3. UV lamps were repeatedly turned on for 3 h (daytime) and off for 6 h (nighttime), and precursors were injected to a reaction bag once (Multiday Initial injection, MI) or repeatedly (Multiday Cyclic injection, MC) to simulate high particulate matter episode due to foreign inflow episode and domestic stagnation episodes, respectively. As a result, the amount of SOA formed in the humid (RH 80 %) MI experiments with ammonia was approximately 1.1 times more than in the traditional single day experiment and approximately 1.6 times more than in the MC experiment, implying that aging processes including nighttime effects without additional emission of precursors during transport can produce more SOA as reactions progressed further under the experimental conditions of this study. The higher the initial RH, the more SOA was formed, with a slope increasing approximately 1.2 µg/m3 per unit RH, and the shorter run time required for SOA to increase to 30 µg/m3 (twice the WHO PM10 standard), with a slope decreasing approximately 0.3 h per unit RH, implying that more humid condition caused during long-range transport across the oceans is one of the possible reasons of high secondary aerosol formation. The SOA formation was reduced by approximately 60 % in the absence of ammonia, suggesting that ammonia reduction is needed to decrease not only secondary inorganic aerosols but also SOA. These results are useful to understand the major reason of high pollution of particulate matters by episode cases in urban areas.

Chemical characteristics and formation mechanism of secondary inorganic aerosols: The decisive role of aerosol acidity and meteorological conditions.

In recent years, there has been a growing concern about air pollution and its impact on the air quality and human health, especially for fine particulate matter (PM2.5) and its associated secondary aerosols in urban areas. This study conducted a year-long field campaign to collect PM2.5 samples day and night in an urban area of central Taiwan. Higher PM2.5 mass concentrations were observed in winter (27.7 ± 9.7 µg/m3), followed by autumn (22.5 ± 8.3 µg/m3), spring (19.2 ± 6.4 µg/m3), and summer (11.0 ± 3.1 µg/m3). The dominant formation mechanism of secondary inorganic aerosols was heterogeneous reactions of NO3- at night and homogeneous reactions of SO42- during the day. Additionally, significant correlations were observed between aerosol liquid water content (ALWC) and NO3- during nighttime, indicating the importance of aqueous-phase NO3- formation. The role of aerosol acidity was explored and a unique alkaline condition was found in spring and summer, which showed lower PM2.5 concentrations than the neutralized condition. Under the neutralized condition, higher PM2.5 concentrations were commonly found when combining the ammonium-rich regime with molar ratios of [NO3-]/[SO42-] exceeding 1.6, suggesting the importance of reducing both NH3 and NOx. Furthermore, the results showed that reducing NH3 should be prioritized under high temperature conditions, while reducing NOx became important under low temperature conditions. Clustering of backward trajectories showed that long-range transport could enhance the formation of secondary aerosols, but local emissions emerged as the main factor driving high PM2.5 concentrations. This study provides insights for policymakers to improve air quality, suggesting that different mitigation strategies should be formulated based on meteorological variables and that using clean energy for vehicles and electricity generation is important to alleviate air pollution.

Analysis of aerosol liquid water content and its role in visibility reduction in Delhi.

Aerosols undergo significant changes due to water uptake under high RH conditions, leading to changes in physical, optical, and chemical properties. Detailed assessment and investigation are needed to understand better aerosol liquid water content (ALWC) characteristics in highly polluted regions like Delhi. Therefore, in this study, we examined the mass concentration and the factors governing the ALWC associated with PM2.5 in Delhi for two winters (Dec 2019 to Jan 2020 and Dec 2020 to Feb 2021) using the real-time measurements of NR-PM2.5 from Aerodyne aerosol chemical speciation monitor (ACSM) and the application of thermodynamic modeling (ISORROPIA II). The average NR-PM2.5 mass concentration in the 2020-2021 winter was 152 µg/m3, about 50 % higher than the average mass concentration of 102 µg/m3 in 2019-2020. Consequently, the ALWC was also 60 % higher during 2020-2021, with an average mass concentration of 150 µg/m3. ALWC increased exponentially with RH and is significant when RH > 80 %. Further, all the inorganic components of NR-PM2.5 were found to contribute significantly to ALWC uptake; however, the relative contribution varied in different RH conditions. Ammonium sulphate dominated the ALWC uptake among the inorganic components at low RH, but ammonium nitrate was the dominant contributor at high RH. The decreased chloride mass fraction in inorganics in the recent winters reduced its relative contribution to ALWC. High ALWC mass concentration during high PM2.5 and high RH leads to a significant reduction in visibility. We further validated this visibility reduction by estimating the enhanced light scattering coefficient (f(RH)) and found that the hygroscopic growth is responsible for the enhanced visibility reduction during high RH conditions (> 85 %) when light scattering efficiency increased by a factor of >3.5. Sensitivity tests of f(RH) on mass concentration of inorganic salts showed that all the salts contributed almost equally. As revealed in our study, variations in PM2.5 mass concentration and composition despite similar meteorological conditions between different winters indicate changing regional aerosol emissions. Therefore, long-term observations of ALWC and PM2.5 chemical composition are required to arrive at actionable measures and mitigation strategies. Further, the focus should be on reducing the overall inorganic mass concentrations of PM2.5 in general, decreasing the absolute ALWC, and improving visibility.

Assessment of submicron aerosol liquid water content and mass-based growth factors in South Asian outflow over the Indian Ocean.

Aerosol-bound water, a ubiquitous and abundant component of atmospheric aerosols, has an impact on regional climate, visibility, human health, the hydrological cycle, and atmospheric chemistry. Yet, the intricate relationship between aerosol liquid water (ALWC) and chemical composition and relative humidity (RH) was not well understood. The present study explores ALWC derived from the ISORROPIA II model using real-time, high-resolution data of non-refractory submicron chemical species and meteorological parameters (temperature and RH) collected over the Indian Ocean as part of the ICARB (Integrated Campaign for Aerosols, Gases, and Radiation Budget)-2018 experiment. Results show that ALWC values over the South Eastern Arabian Sea (SEAS) were found to be higher by 4-6 times than those observed over the Equatorial Indian Ocean (EIO) due to a large decrease in aerosol loading from SEAS to EIO. ALWC peaked in the early morning hours (4:00-7:00), with greater values during the nighttime and lower values during the daytime across SEAS, which is comparable with RH variation. While the ratio of organics-to-SO42- mass fraction linearly decreased with increasing mass-based growth factors (MGFs) over EIO, such a scenario was not observed over SEAS. The latitudinal gradient of mass fraction of ALWC had shown a decrease towards EIO, consistent with organic fraction. The extinction coefficient of the dry mass of submicron particles is noticeably increased by 40 % by ALWC over SEAS and EIO. Moreover, ALWC could enhance the aerosol negative forcing by an average of 66 % (64 %) over SEAS (EIO) at the top of the atmosphere during the cruise period. These inferences imply that ALWC is the key factor in assessing the role of aerosols on atmospheric radiative forcing. Overall, the present study highlights the serious need to consider the ALWC in climate forcing simulations, particularly in moist tropical environments where their effect can be significant.

Aerosol liquid water content of PM2.5 and its influencing factors in Beijing, China.

Aerosol liquid water content (ALWC) has important influences on atmospheric radiation and aerosol chemical processes. In this work, the changes in ALWC of PM2.5 were investigated over four seasons based on hourly monitoring of inorganic water-soluble ions and their gaseous precursors using the thermodynamic model ISORROPIA II. The results showed that the ALWC concentrations exhibited pronounced seasonal (autumn > summer > spring > winter) and diurnal variation characteristics. The sensitivity tests indicated that ALWC depended strongly on TSO4 (total sulfate (gas and aerosols) expressed as equivalent H2SO4), followed by TNO3 (total nitrate (gas and aerosols) expressed as equivalent HNO3). The relatively low concentration of TCl (total chloride (gas and aerosols) expressed as equivalent HCl) limit its importance in the atmosphere. ALWC showed exponential growth features as a function RH in all four seasons. RH became the most influential factor on the variation of ALWC when RH exceeded 80% in all seasons. The seasonal average data showed that the ALWC increased from 2.92 µg·m-3 to 75.83 µg·m-3 when ambient RH increased from 30% to 90%, the associated sulfate, nitrate, and ammonium (abbreviated as SNA) mass fraction in PM2.5 rose from 0.39 to 0.58 in the atmosphere. The ALWC facilitated the formation of SNA through gas-particle conversion and partitioning. The self-amplifying processes between ALWC and SNA enhanced aerosol formation. By modeling ALWC under different seasonal atmospheric scenarios, it was found that reductions in chemical species could reduce ALWC concentrations in different degrees. Based on the current emission conditions, controlling excess NH3 emission could effectively reduce ALWC to a maximum of 45.71% in summer, indicating that NH3 control was crucial for reducing ALWC and PM2.5 concentrations under high levels of SO42- and NO3-.

The different sensitivities of aerosol optical properties to particle concentration, humidity, and hygroscopicity between the surface level and the upper boundary layer in Guangzhou, China.

This study investigates the impact of aerosol liquid water content (ALWC) and related factors, i.e., relative humidity (RH), aerosol mass concentration (PM2.5), and aerosol hygroscopicity, on aerosol optical properties, based on field measurements made in the Pearl River Delta (PRD) region of China at the surface (1 November 2019 to 21 January 2020) and in the upper boundary layer (the 532-m Guangzhou tower from 1 February to 21 March 2020). In general, temporal variations in the ambient aerosol backscattering coefficient (ßp) and ALWC followed each other. However, the surface ßp and 532-m ßp had generally opposite diurnal variation patterns, caused by dramatic differences in PM2.5 and ambient RH between the surface and the upper boundary layer. The ambient 532-m RH was systematically higher than the surface RH, with the latter having a much pronounced diurnal cycle than the former. The surface PM2.5 concentration was systematically higher than the PM2.5 concentration at 532 m, and their diurnal cycle patterns were overall opposite. These dramatic differences reveal that the atmospheric variables, i.e., ambient RH and the PM2.5 concentration in the upper boundary layer, cannot be directly represented by the same variables at the surface. Vertical variability should be considered. Clear differences in the sensitivities of aerosol light scattering to ambient RH, PM2.5, and aerosol hygroscopicity between the two levels were found and examined. Aerosol chemical composition played a minor role in causing the differences between the two levels. In particular, ßp was more sensitive to PM2.5 at the surface level but more to the ambient RH in the upper boundary layer. The larger contribution of aerosol loading to the variability in ßp at the surface implies that local emission controls can decrease ßp and further improve atmospheric visibility effectively at the surface during winter in the PRD region.

Atmospheric sulfate formation in the Seoul Metropolitan Area during spring/summer: Effect of trace metal ions.

Despite the effort to control SO2 emission, sulfate is still one of the major inorganic components of PM2.5 in urban area. Moreover, there is still a lack of understanding of the sulfate formation mechanism via SO2 oxidation under various ambient conditions. In this study, we focus on sulfate formation during a haze pollution episode in the spring/summertime of 2016 in Seoul Metropolitan Area (SMA). During the pollution episode, PM2.5 mass concentration exceeded over 60 µg m-3, and sulfate accounted for about 25% of the total PM2.5 mass concentration. A sharp increase of sulfur oxidation ratio (SOR) values along with aerosol liquid water content (AWC) under humid conditions could be ascribed to an apparent contribution of aqueous-phase oxidation of SO2 of sulfate formation during the pollution period. Comparisons of SOR values with four representative oxidants for the aqueous-phase oxidation (i.e., NO2, H2O2, O3, and TMIs) indicated that TMIs concentration, especially for Mn (II), showed the best positive correlation. Furthermore, for calculating the sulfate production rate, the contribution of TMIs concentration was found to be dominant within the pH range in SMA (2.1-3.0), which was determined by the chemical composition and derived AWC. These results imply that not only the SO2 emission but also other chemical components (e.g., TMI and nitrate) would play a critical combined role in sulfate formation under urban haze condition.

Retrieval of aerosol liquid water content from high spectral resolution lidar.

Aerosol liquid water content (ALWC) has significant effects on aerosol optical properties, radiative forcing, and the development of severe pollution events. In this study, the vertical distribution and temporal evolution of ALWC were determined through linear particle depolarization measured by a high spectral resolution lidar (HSRL) from December 9 to 12, 2020. Near-surface ALWC datasets retrieved by HSRL were validated by measurements from a three-wavelength humidified nephelometer. The ALWC datasets derived by two methods were highly correlated (R = 0.94, N = 192), illustrating the feasibility of retrieving the ALWC by HSRL. A positive correlation between the ALWC and the enhancement of aerosol scattering coefficient F calculated by the scattering coefficient at 525 nm measured in dry and ambient states proves the reliability of the ALWC obtained from HSRL. However, previous research has implied that fine mode particles dominating the total aerosol loading are required to precisely retrieve the ALWC, while the uncertainty of ALWC data will be large when the particle depolarization ratio is larger than 0.07. When it is less than 0.07, the ALWC derived from HSRL has high precision. By analyzing the aerosol property measurements (e.g., PM2.5, PM10, particle depolarization ratio, and scattering coefficient) near the surface, we found that ALWC contributes greatly to the deterioration of visibility. The variability of optical parameters in the vertical direction showed that ALWC significantly promotes the enhancement of aerosol extinction coefficients. Moreover, high ALWC significantly increases the scattering capacity of aerosols, leading to an enhanced cooling effect on the climate system.

Sources, species and secondary formation of atmospheric aerosols and gaseous precursors in the suburb of Kitakyushu, Japan.

The simultaneous assessment of source apportionment and secondary formation processes was comprehensively studied in a suburban area located on the western edge of Japan by combining year-round daily observation using a filter-pack method with model calculations. Secondary formation was the most important pollution source, accounting for ca. 45% (23% (secondary sulfates) + 22% (secondary nitrates)) of the sources of total atmospheric aerosol mass. For the secondary aerosol composition at this suburban site in western Japan, the secondary sulfates were mainly derived from volcanic eruptions (Sakurajima volcano and/or Aso volcano), the oxidation of SO2 from industrial combustion, ship emissions in the Kyushu area, and long-distance transportation from several coastal cities in Eastern China. Multiple regression results further revealed that the secondary sulfate formation process was significantly influenced by and related to HNO3, HCl, and the relative humidity (RH) (p < 0.01). While the potential pollution source region of secondary nitrates was located in the northwest region of the sampling site, where air masses pass through Mongolia and Northern China, the formation mechanism of secondary nitrates was more complicated, with the important driving factors being Ox, NO2, NH3, HCl, temperature (T), and RH. In addition, if the presence of atmospheric HNO3 was ignored, the nitrogen oxidation rate (NOR) would be significantly underestimated, especially at relative humidity levels less than 60% and temperatures greater than 16 °C. The results of this study clearly demonstrate the source contribution and characteristics of secondary aerosols in the suburban area of western Japan and can be adopted as the important basis to mitigate particle pollution.

Important contribution of N2O5 hydrolysis to the daytime nitrate in Xi'an, China during haze periods: Isotopic analysis and WRF-Chem model simulation.

Nitrate, as one of the major components of tropospheric aerosols, plays a crucial role in winter haze formation. While, the formation mechanism of the high production of nitrate in Chinese megacities is still not fully understood. To quantify the contributions of major formation pathways to nitrate, airborne particles in Xi'an, inland China during the winter of 2017 were measured and analyzed for the water-soluble ions and stable nitrogen/oxygen isotope compositions of nitrate in PM2.5, followed by a WRF-Chem model simulation. The oxygen isotopic results indicated that N2O5 hydrolysis was an important formation pathway for the daytime nitrate in the haze episodes. The model simulation further revealed that N2O5 hydrolysis contribution increased from 8.2% to 20.5% of the total nitrate over 14:00-16:00 p.m., clearly showing that N2O5 formation followed by a heterogeneous hydrolysis to nitrate can effectively proceed in daytime under the abundantly co-existing O3, NO2 and NH3 conditions.

Enhanced aqueous-phase formation of secondary organic aerosols due to the regional biomass burning over North China Plain.

This study reveals the impact of biomass burning (BB) on secondary organic aerosols (SOA) formation in the North China Plain (NCP). Filter samples were analyzed for secondary inorganic aerosols (SIA), oxalic acid (C2) and related aqueous-phase SOA compounds (aqSOA), stable carbon isotope composition of C2 (δ13C(C2)) and aerosol liquid water content (ALWC). Based on the PM2.5 loadings, BB tracer concentrations, wildfire spots and air-mass back trajectories, we distinguished two episodes from the whole campaign, Episode I and Episode II, which were characteristic of regional and local BB, respectively. The abundances of PM2.5 and organic matter in the two events were comparable, but concentrations and fractions of SIA, aqSOA during Episode I were much higher than those during Episode II, along with heavier δ13C(C2), suggesting an enhanced aqSOA formation in the earlier period. We found that the enhancement of aqSOA formation during Episode I was caused by an increased ALWC, which was mainly driven by SIA during the regional BB event. Our work showed that intensive burning of crop residue in East Asia can sharply enhance aqSOA production on a large scale, which may have a significant impact on the regional climate and human health.

Molecular characteristics and stable carbon isotope compositions of dicarboxylic acids and related compounds in the urban atmosphere of the North China Plain: Implications for aqueous phase formation of SOA during the haze periods.

In the past five years, Chinese government has promulgated stringent measures to mitigate air pollution. However, PM2.5 levels in the China North Plain (NCP), which is one of the regions with the heaviest air pollution in the world, are still far beyond the World Health Organization (WHO) standard. To improve our understanding on the sources and formation mechanisms of haze in the NCP, PM2.5 samples were collected during the winter of 2017 on a day/night basis at the urban site of Liaocheng, which is one of the most polluted cities in the NCP. The samples were determined for molecular distributions and stable carbon isotope compositions of dicarboxylic acids and their precursors (ketocarboxylic acids and α-dicarbonyls), levoglucosan, elemental carbon (EC), organic carbon (OC) and water-soluble organic carbon (WSOC). Our results showed that oxalic acid (C2) is the dominant dicarboxylic acid, followed by succinic acid (C4) and malonic acid (C3), and glyoxylic acid (ωC2) is the most abundant ketocarboxylic acids. Concentrations of C2, glyoxal (Gly) and methylglyoxal (mGly) presented robust correlations with levoglucosan, suggesting that biomass burning is a significant source of PM2.5 in the NCP. Moreover, C2 and Gly and mGly linearly correlated with SO42-, relative humidity (RH), aerosol liquid water content (LWC) as well as particle in-situ pH (pHis), indicating that aqueous-phase oxidation is the major formation pathway of these SOA, and is driven by acid-catalyzed oxidation. Concentrations and relative abundances of secondary species including SNA (SO42-, NO3- and NH4+), dicarboxylic acids, and aerosol LWC in PM2.5 are much higher in the haze periods than in the clean periods, suggesting that aqueous reaction is a vital role in the haze formation. In comparison with those in the clean periods, stable carbon isotopic compositions (δ13C) of major dicarboxylic acids and related SOA and the mass ratios of C2/diacids, C2/Gly and C2/mGly are higher in the haze periods, indicating that haze particles were more aged and enriched in secondary species.

Variations in submicron aerosol liquid water content and the contribution of chemical components during heavy aerosol pollution episodes in winter in Beijing.

The aerosol liquid water content (ALWC) of submicron particles (PM1) was calculated in this work by three methods based on the aerosol physical and chemical properties measurement campaigns in winter in Beijing, including (a) the PM1 volume difference between the ambient and dry states by applying the particle number size distribution and particle hygroscopicity measurement; (b) the thermodynamic equilibrium model (ISORROPIA II) based on the chemical composition; and (c) the κ-Köhler theory of chemical composition with a volume mixing scheme. The three methods agreed well with reasonable uncertainties. The ALWC showed an exponential trend depending on the relative humidity (RH), and an abundant ALWC was also favored by the high PM1 mass loading. The contribution of different chemical component to the ALWC was evaluated by the κ-Köhler method, which revealed that during the measurement, the inorganics and organics could contribute to ~80% and ~20%, respectively, under ambient RH conditions, with the largest contributor of ammonium nitrate. When the RH was above 85%, the mass concentration of ALWC was comparable to, or even larger than, that of the dry PM1.