Sulfate, Nitrate And Ammonium Research Articles

Characteristics of sub-micron aerosols above the urban canopy in Beijing during warm seasons

To understand the characteristics of atmospheric pollution above the urban canopy in warm seasons, the characteristics of sub-micron aerosol (PM1) was studied based on high-altitude observations at the Beijing 325 m meteorological tower. The PM1 at 260 m was 34, 29 and 21 μg m−3 in May 2015, June 2015, and June 2017, respectively, indicating a reduction in PM1 pollution above the urban canopy. Meanwhile, an overall decrease was also observed in the concentrations of all PM1 chemical species (excluding Chl and BC) and organic aerosol (OA) factors. Previous instances of heavy haze in Beijing often coincided with high humidity and stagnant weather conditions. However, the heightened pollution episodes in June 2017 were accompanied by high wind speeds and low relative humidity. Compared to May 2015, the contribution of secondary components to PM1 in June 2017 was more prominent, with the total proportion of SNA (sulfate, nitrate, and ammonium) and more-oxidized oxygenated OA (MO-OOA) to PM1 increased by approximately 10 %. Secondary species of NH4NO3, (NH4)2SO4, and MO-OOA, as well as black carbon, collectively contributed the vast majority of aerosol extinction coefficient (bext), with the four species contributing a total of ≥96 % to bext at 260 m. Hydrocarbon-like OA, cooking OA, and less-oxidized oxygenated OA have undergone significant reductions, so continued emphasis on controlling local sources to reduce these three aerosol species and addressing regional sources to further mitigate overall aerosol species is imperative. In lower pollution situation, the diurnal variation of PM was smoother, and its pollution sources were more regionally uniform, which might be attributed to the reduced diversity and complexity in the physical and chemical processes in air pollution.

Chemical Characterization, Source Identification, and Health Risk Assessment of Atmospheric Fine Particulate Matter in Winter in Hangzhou Bay

PM2.5 is an important pollutant which affects air quality and human health. In this study, chemical components (water-soluble inorganic ions, organic carbons (OC), elemental carbons (EC), and elemental metals) and health effects were analyzed in wintertime in a suburban area in Hangzhou Bay. OC and SNA (sulfate, nitrate, and ammonium) contributed 76.2% to local PM2.5. NH4+ existed mainly in the form of (NH4)2SO4 and NH4NO3. Seven sources were resolved from PMF analysis, namely secondary inorganic aerosol (31.8%), vehicle exhaust (19.5%), industry mixed with coal combustion (16.3%), crustal dust (9.5%), biomass burning (9.4%), sea salt (8.7%), and the leather industry (4.8%). Potential source contribution function (PSCF) and concentration weighted trajectory (CWT) analysis were applied to study regional transport in this region. Secondary inorganic formation was enhanced from the air plume from the northwest, especially from north Jiangsu Province. The results of the health risk assessment of associated metals indicated the higher potential of Cr and Mn to cause noncarcinogenic effects in children. A significant carcinogenic risk was observed for all people of Cr emitted from the leather industry. Our results showed the chemical characterization and sources of PM2.5 in a suburban region, the health effects of which should be addressed in future policies to safeguard public health, especially in the leather industry.

Dramatic changes in Harbin aerosol during 2018–2020: the roles of open burning policy and secondary aerosol formation

Abstract. Despite the growing interest in understanding haze formation in Chinese megacities, air pollution has been largely overlooked for the Harbin–Changchun (HC) metropolitan area, located in the severe cold climate region in northeast China. In this study, we unfolded significant variations of fine particulate matter (PM2.5) in HC's central city (Harbin) during two sequential heating seasons of 2018–2019 and 2019–2020, and we explored major drivers for the observed variations. The two campaigns showed comparable organic carbon (OC) levels but quite different OC sources. The biomass burning (BB) to OC contribution decreased substantially for 2019–2020, which was attributed primarily to the transition of local policies on agricultural fires, i.e., from the “legitimate burning” policy released in 2018 to the “strict prohibition” policy in 2019. Meanwhile, the contribution of secondary OC (OCsec) increased significantly, associated with the much more frequent occurrences of high relative humidity (RH) conditions during the 2019–2020 measurement period. Similar to OCsec, the major secondary inorganic ions, i.e., sulfate, nitrate and ammonium (SNA), also exhibited RH-dependent increases. Given the considerable aerosol water contents predicted for the high-RH conditions, heterogeneous reactions were likely at play in secondary aerosol formation even in the frigid atmosphere in Harbin (e.g., with daily average temperatures down to below −20 ∘C). In brief, compared to 2018–2019, the 2019–2020 measurement period was characterized by a policy-driven decrease of biomass burning OC, a RH-related increase of OCsec and a RH-related increase of SNA, with the first two factors generally offsetting each other. In addition, we found that open burning activities were actually not eliminated by the strict prohibition policy released in 2019, based on a synthesis of air quality data and fire count results. Although not occurring during the 2019–2020 measurement period, agricultural fires broke out within a short period before crop planting in spring of 2020, and this resulted in off-the-charts air pollution for Harbin, with 1 h and 24 h PM2.5 concentrations peaking at ∼ 2350 and 900 µg/m3, respectively. This study indicates that sustainable use of crop residues remains a difficult challenge for the massive agricultural sector in northeast China.

Characterization and source analysis of water-soluble inorganic ionic species in PM2.5 during a wintertime particle pollution episode in Nanjing, China

Water-soluble inorganic ions (WSIIs) are major components of PM2.5, and play a prominent role in atmospheric acidification. In January 2019, a large-scale persistent particle pollution episode occurred in the Yangtze River Delta (YRD) region of China. High time resolution on-line monitoring of WSIIs in PM2.5 were conducted at the Atmospheric Environment Vertical Observation Station of Nanjing University by using In-situ Gas and Aerosol Compositions Monitor. From January 1 to 13, the mean concentration of WSIIs was 76.7 ± 48.2 μg·m−3. WSIIs accounted for 64.2% of PM2.5 concentration. Sulfate, nitrate and ammonium (SNA) were the major components of WSIIs, and accounted for 25.5%, 45.6% and 26.3%, respectively. The values of NO3−/SO42− increased from 1.37 to 2.86, indicating that the contribution of mobile sources to fine particle pollution was significantly enhanced than that of stationary sources. The main factors affecting the values of sulfur oxidation ratio (SOR) and nitrogen oxidation ratio (NOR) were gaseous precursor concentrations, relative humidity and air temperature. The values of SOR and NOR exceeded 0.1, implying the secondary transformation processes played critical roles during this pollution episode. To investigate the potential source areas of WSIIs in PM2.5, the Principal Component Analysis - Multiple Linear Regression (PCA-MLR) analysis and the Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) model were used. By using PCA-MLR, it was found that the main pollution sources in Nanjing were secondary transformation (58%), manmade dust (26%) and biomass combustion (16%). Based on the backward trajectory calculation, the potential source contribution function (PSCF) analysis and the concentration weighted trajectory (CWT) analysis, it was revealed that the air pollutants mainly came from the local sources in and around Nanjing. With the aggravation of pollution, the contribution of regional transport to PM2.5 concentrations decreased and the contribution of local emissions increased. Among the main components of WSIIs in Nanjing, NH4+ was mainly caused by agricultural activities, and generally came from Jiangsu, eastern Henan, southern Shandong, and northeastern Anhui. SO42− was mainly derived from coal burning in the nearby industrial areas or from the north through the long-distance transmission. NO3− mainly concentrated in the local areas, which was greatly affected by human activities. The weighted concentration weighted trajectory (WCWT) peak values of NH4+ and SO42− were about 10 μg·m−3, while that of NO3− was more than 20 μg·m−3, which further indicated that the local emission of NO3− was the main potential source. The findings can contribute to understand the behavior of WSIIs and their roles in the wintertime haze event, reveal potential source areas for the corresponding components, and enlighten beneficial suggestions for particle pollution prevention and control in high polluted areas.

Air pollutant variations in Suzhou during the 2019 novel coronavirus (COVID-19) lockdown of 2020: High time-resolution measurements of aerosol chemical compositions and source apportionment.

To control the spread of the 2019 novel coronavirus (COVID-19), China imposed rigorous restrictions, which resulted in great reductions in pollutant emissions. This study examines the characteristics of air pollutants, including PM2.5 (particles with aerodynamic diameters < 2.5 μm), gas pollutants, water-soluble ions (WSIs), black carbon (BC) and elements, as well as the source apportionment of PM2.5 in Suzhou before, during and after the Chinese New Year (CNY) holiday of 2020 (when China was under an unprecedented state of lockdown to restrict the COVID-19 outbreak). Compared to those before CNY, PM2.5, BC, SNA (sulfate, nitrate and ammonium), other ions, elements, and NO2 and CO mass concentrations decreased by 9.9%–64.0% during CNY. The lockdown policy had strong (weak) effects on the diurnal variations in aerosol chemical compositions (gas pollutants). Compared to those before CNY, source concentrations and contributions of vehicle exhaust during CNY decreased by 72.9% and 21.7%, respectively. In contrast, increased contributions from coal combustion and industry were observed during CNY, which were recorded to be 2.9 and 1.7 times higher than those before CNY, respectively. This study highlights that the lockdown policy that was imposed in Suzhou during CNY not only reduced the mass concentrations of air pollutants but also modified their diurnal variations and the source contributions of PM2.5, which revealed the complex responses of PM2.5 sources to the rare, low emissions of anthropogenic pollutants that occurred during the COVID-19 lockdown.

Optical properties closure and sources of size-resolved aerosol in Nanjing around summer harvest period

Aerosol size distribution, chemical components and their mixing state are key factors impacting the optical properties of aerosols. The researches on optical properties closure and sources for size-segregated aerosols are still scattered. A sampling campaign covering a summer harvest period was conducted in Nanjing, considering the influence of open biomass burning (BB) on optical properties of size-resolved aerosol. Two parallel FA-3 samplers were employed to collect nine size-segregated particles (0.43–10 μm), with water-soluble ions, carbonaceous components and elements analyzed. Aerosol light absorption (bap) and scattering (bsp) coefficients were synchronously online monitored. Results indicated that sulfate, nitrate and ammonium (SNA), crustal materials (CM) and organic matter (OM) were the main contributors of aerosol mass, totally accounting for 61.3%–81.4%. On hazy days, PM1.1 occupied 82.3% of the total estimated bsp by Mie model, with SNA and OM accounting for 40.9% and 16.2%, respectively. PM1.1 contributed 61.7% of the total estimated bap with EC accounting for 41.6%. Positive matrix factorization modeling indicated that vehicle emission contributed most (13.1%–52.9%) to bap especially for particles with diameter smaller than 0.43 μm. On hazy days, BB was the main source for bsp, contributing to 24.7% (for particle within 1.1–2.1 μm)-38.2% (0.65–1.1 μm), then followed by secondary inorganic aerosol, contributing to 16.3% (1.1–2.1 μm)-18.8% (0.65–1.1 μm). For the summer harvest period, the open BB impacts obviously on bsp rather than bap, promoting the haze formation. The temporal variations of aerosol externally mixed ratio and significant negative correlations between the mixed ratio and OC/EC also verified that fresh BB emissions deteriorated the visibility. This study highlights the different roles of aerosol chemical components and emission sources impacting optical properties for size-resolved aerosols, which is helpful to give more accurate visibility improvement strategies.

Real-time retrieval of aerosol chemical composition using effective density and the imaginary part of complex refractive index

Knowledge of aerosol chemical composition is essential for the research on aerosol climate effect, air quality and health issues. With the development of optical remote-sensing technology, numerous aerosol chemical composition retrieval methods based on optical properties were developed. However, the existing algorithms lack the verification with similar measurements of aerosol chemical composition. In this work, a novel algorithm based on effective density (ρe) and imaginary part (k) of the complex refractive index (CRI) at λ = 365 and 532 nm was developed to retrieve four main aerosol components, including black carbon (BC), organic matter (OM), mineral dust (MD), and sum of sulfate, nitrate and ammonium (SNA). ρe was derived by converting aerodynamic to electrical mobility diameter. CRI was retrieved from Mie code and simultaneous measurement of particle number size distribution (PNSD) and the extinction and scattering coefficients (bext and bscat). Comprehensive sensitivity tests were performed to estimate the retrieval uncertainties from the measurements and a priori inputs. Validation of the algorithm was demonstrated using a series of observations at Shouxian (China) from November 30 to December 12, 2016. The measured and retrieved aerosol chemical compositions were compared and the two results showed good agreements, proving the reliability of the method. During one clean and two pollution periods, the variation of measured aerosol physical and optical properties and retrieved chemical composition were analyzed. The aerosol chemical composition changes can be well described by the transition of air mass sources for different pollution levels. This study provides a reliable, in-situ method for better constraining the relationship between aerosol chemical compositions and optical properties. This method will help monitor the pollution event, identify aerosol sources and verify the retrievals from remote sensing data.

Chemical Characterization and Seasonality of Ambient Particles (PM2.5) in the City Centre of Addis Ababa.

Ambient air pollution is a growing public health concern in major African cities, including Addis Ababa (Ethiopia), where little information is available on fine particulate matter (PM2.5, with aerodynamic diameter <2.5 µm) pollution. This paper aims to characterize annual PM2.5, including bulk composition and seasonal patterns, in Addis Ababa. We collected 24-h PM2.5 samples in the central city every 6 days from November 2015 to November 2016. The mean (±SD) daily PM2.5 concentration was 53.8 (±25.0) µg/m3, with 90% of sampled days exceeding the World Health Organization’s guidelines. Principal components were organic matter (OM, 44.5%), elemental carbon (EC, 25.4%), soil dust (13.5%), and SNA (sulfate, nitrate, and ammonium ions, 8.2%). Higher PM2.5 concentrations were observed during the heavy rain season, while crustal dust concentrations ranged from 2.9 to 37.6%, with higher levels during dry months. Meteorological variables, vehicle emissions, biomass fuels, unpaved roads, and construction activity contribute to poor air quality. Compared to the Air Quality Index (AQI), 31% and 36% of observed days were unhealthy for everyone and unhealthy for sensitive groups, respectively. We recommend adopting effective prevention strategies and pursuing research on vehicle emissions, biomass burning, and dust control to curb air pollution in the city.

Chemical Characteristics of Major Inorganic Ions in PM2.5 Based on Year-Long Observations in Guiyang, Southwest China—Implications for Formation Pathways and the Influences of Regional Transport

Sulfate, nitrate and ammonium (SNA) are the dominant components of water-soluble ions (WSIs) in PM2.5, which are of great significance for understanding the sources and transformation mechanisms of PM2.5. In this study, daily PM2.5 samples were collected from September 2017 to August 2018 within the Guiyang urban area and the concentrations of the major WSIs in the PM2.5 samples were characterized. The results showed that the average concentration of SNA (SO42−, NO3−, NH4+) was 15.01 ± 9.35 μg m−3, accounting for 81.05% (48.71–93.76%) of the total WSIs and 45.33% (14.25–82.43%) of the PM2.5 and their possible chemical composition in PM2.5 was (NH4)2SO4 and NH4NO3. The highest SOR (sulfur oxidation ratio) was found in summer, which was mainly due to the higher temperature and O3 concentrations, while the lowest NOR (nitrogen oxidation ratio) found in summer may ascribe to the volatilization of nitrates being accelerated at higher temperature. Furthermore, the nitrate formation was more obvious in NH4+-rich environments so reducing NH3 emissions could effectively control the formation of nitrate. The results of the trajectory cluster analysis suggested that air pollutants can be easily enriched over short air mass trajectories from local emission sources, affecting the chemical composition of PM2.5.

Secondary inorganic aerosol during heating season in a megacity in Northeast China: Evidence for heterogeneous chemistry in severe cold climate region

The characteristics of secondary inorganic aerosol including sulfate, nitrate and ammonium (SNA) were investigated during a six-month long heating season in the Harbin-Changchun metropolitan area, i.e., China’s only national-level city cluster located in the severe cold climate region. The contribution of SNA to fine particulate matter (PM2.5) tended to decrease with increasing PM2.5 concentration, opposite to the trend repeatedly observed during winter in Beijing. Heterogeneous sulfate formation was still evident when the daily average temperature was as low as below −10 °C, with the preconditions of high relative humidity (RH; above ∼80%) and high nitrogen dioxide (above ∼60 μg/m3). Both the sulfur oxidation ratio (SOR) and nitrogen oxidation ratio (NOR) were enhanced at high RH, reaching ∼0.3. However, the high RH conditions were not commonly seen during the heating season, which should be responsible for the overall lack of linkage between the SNA contribution and PM2.5 temporal variation.

MICS-Asia III: multi-model comparison and evaluation of aerosol over East Asia

Abstract. A total of 14 chemical transport models (CTMs) participated in the first topic of the Model Inter-Comparison Study for Asia (MICS-Asia) phase III. These model results are compared with each other and an extensive set of measurements, aiming to evaluate the current CTMs' ability in simulating aerosol concentrations, to document the similarities and differences among model performance, and to reveal the characteristics of aerosol components in large cities over East Asia. In general, these CTMs can well reproduce the spatial–temporal distributions of aerosols in East Asia during the year 2010. The multi-model ensemble mean (MMEM) shows better performance than most single-model predictions, with correlation coefficients (between MMEM and measurements) ranging from 0.65 (nitrate, NO3-) to 0.83 (PM2.5). The concentrations of black carbon (BC), sulfate (SO42-), and PM10 are underestimated by MMEM, with normalized mean biases (NMBs) of −17.0 %, −19.1 %, and −32.6 %, respectively. Positive biases are simulated for NO3- (NMB = 4.9 %), ammonium (NH4+) (NMB = 14.0 %), and PM2.5 (NMB = 4.4 %). In comparison with the statistics calculated from MICS-Asia phase II, frequent updates of chemical mechanisms in CTMs during recent years make the intermodel variability of simulated aerosol concentrations smaller, and better performance can be found in reproducing the temporal variations of observations. However, a large variation (about a factor of 2) in the ratios of SNA (sulfate, nitrate, and ammonium) to PM2.5 is calculated among participant models. A more intense secondary formation of SO42- is simulated by Community Multi-scale Air Quality (CMAQ) models, because of the higher SOR (sulfur oxidation ratio) than other models (0.51 versus 0.39). The NOR (nitric oxidation ratio) calculated by all CTMs has larger values (∼0.20) than the observations, indicating that overmuch NO3- is simulated by current models. NH3-limited condition (the mole ratio of ammonium to sulfate and nitrate is smaller than 1) can be successfully reproduced by all participant models, which indicates that a small reduction in ammonia may improve the air quality. A large coefficient of variation (CV &gt; 1.0) is calculated for simulated coarse particles, especially over arid and semi-arid regions, which means that current CTMs have difficulty producing similar dust emissions by using different dust schemes. According to the simulation results of MMEM in six large Asian cities, different air-pollution control plans should be taken due to their different major air pollutants in different seasons. The MICS-Asia project gives an opportunity to discuss the similarities and differences of simulation results among CTMs in East Asian applications. In order to acquire a better understanding of aerosol properties and their impacts, more experiments should be designed to reduce the diversities among air quality models.

Status and chemical characteristics of ambient PM2.5 pollutions in China: a review

The ambient fine particulate matter is a considerable hazard to human health and the surrounding environment of the majority of Chinese cities. This article reviews the status of air pollution, especially PM2.5, in 21 cities of China, on the basis of their status, chemical characteristics, and regulations data collected from the published literature. The observed results show Zhengzhou, Yulin, Jinan, Qingdao, and Changchun as significantly polluted cities where the annual mean concentration of PM2.5 was noted to be greater than 120 µg m−3. However, some cities such as Xiamen, Hong Kong, Shenzhen, and Jinchang reported average annual PM2.5 concentrations less than 40 µg m−3. In general, the results of spatial distribution reported that the cities of the east, north, and northeast China are highly polluted. According to the average mass of PM2.5 in maximum cities of China, the sum of sulfate, nitrate and ammonium (SNA) and organic matter (OM) contributed over 40 and 35%, respectively. The higher amount of SNA and OM in PM2.5 results from heavy traffic or vehicle emission and burning solid fuel utilized in most part of China. A proposed systemic approach to address the PM2.5 in China can improve the quality of ambient atmosphere.

Characteristics of size-resolved atmospheric inorganic and carbonaceous aerosols in urban Shanghai

Size-segregated aerosol particles were collected with a 10-stage Micro-Orifice Uniform Deposit Impactor (MOUDI) at an urban site in Shanghai, China for four non-consecutive months representing four seasons from 2015 to 2016. Chemical composition, including water-soluble ions as well as organic carbon (OC), elemental carbon (EC) and secondary organic carbon (SOC) of size-resolved (0.056–18 μm) atmospheric aerosols in four seasons and in different polluted cases were studied. The size distributions of sulfate, nitrate and ammonium (SNA) and carbonaceous aerosol (OC, EC and SOC) were discussed and the potential sources of PM1.8-associated secondary species (SO42−, NO3−, SNA and SOC) in different seasons were identified by potential source contribution function (PSCF) model. Results showed that atmospheric ultrafine and fine particle pollution in Shanghai were very serious during the study period. Most of the water-soluble ions tended to be enriched in fine particles, especially being abundant in the droplet mode in polluted cases. Compared with sulfate, size distributions of nitrate and ammonium presented more significant seasonal variations and showed distinctive characteristics in polluted days. Abundant nitrate was concentrated in fine particles in cold seasons (spring and winter), whereas it was enriched in coarse mode during summer and autumn. The droplet mode sulfate with high concentration did not result in the aggravation of air pollution, while the nucleation mode sulfate may have made a great contribution to the air pollution in urban Shanghai. It was also found that the formation of air pollution in urban Shanghai had a significant link with nitrate and ammonium, especially with nitrate and ammonium in condensation mode and droplet mode, and the contribution of sulfate to the pollution formation in Shanghai would somehow be surpassed by the increasing nitrate and ammonium. OC and EC concentrations from spring to winter were found to be 11.10, 7.10, 12.30, 20.16, and 3.73, 2.84, 4.63, 7.10 μg m−3, respectively, distinctly presenting the summer minima and winter maxima in this study. The maximum OC/EC was in the droplet mode and the minimum was in the nucleation mode for both clean and polluted days. The great contribution of SOC to OC in droplet mode and the occurrence of PM pollution necessarily had an important bearing on the SOC formation in droplet mode particles. Particle acidity may play a key role in secondary organic aerosol formation and the particles with the size of 0.056–0.1 μm was the most sensitive particles to acid catalysis in SOA formation. The similar PSCF results of PM1.8-associated SOC to those of SO42−, NO3− and SNA indicated possible connections between the formation of SOC and secondary inorganic species in PM.

Ambient Particles (PM10, PM2.5 and PM1.0) and PM2.5 Chemical Components in Western Yangtze River Delta (YRD): An Overview of Data from 1-year Online Continuous Monitoring at Nanjing

Understanding particle chemical composition is important in source apportionment and health impact assessment. Since 2012, a Complex Air Pollution Research Station located on the top roof of a 22-floor building (~80 m) in Jiangsu Provincial Academy of Environmental Science was established as a platform to characterize components and temporal variations of hazy and air transport in the region, so as to provide scientific supports on Joint Prevention and Control of Regional Air Pollution control in the Yangtze River Delta. Here, based on 1-year online measurement, we present a preliminary result on occurrence of ambient particles and chemical components, and reconstructed light extinction coefficients based on measured species. In 2015, the annual average concentration of PM2.5 was 68 ± 40 µg/m3, comprising up to nearly 75% of the total PM10. The submicron PM1.0 had an annual average concentration of 48 ± 28 µg/m3 and contributed about 70% of the total PM2.5 mass. Secondary inorganic composition including sulfate, nitrate and ammonium (SNA) was the major fraction of PM2.5, accounting for ~70% on an annual average. The estimated light extinction of ambient particle averaged at 422 ± 256 Mm−1, of which 412 ± 257 Mm−1 was induced by fine PM2.5. SNA contributed largely to the light extinction induced by fine PM2.5 (about 72%), followed by organic matter (18%). During the high-pollution episodes, the mass fraction of SNA increased notably, calling for necessary controls on key gaseous precursors.

Formation and Size Distribution of the Secondary Aerosol Inorganic Ions in Different Intensity of Haze in Qingdao, China

Size-segregated atmospheric aerosol samples were collected from September 2015 to February 2016 at a coastal site in Qingdao, and the concentrations of major water-soluble inorganic ions were analyzed by ion chromatography. Characteristics and variation of size distribution of secondary inorganic components in aerosol were discussed, as well as the formation process and influencing factors of SNA(sulfate, nitrate and ammonium). The results indicated that the concentrations of NO3-, SO42-, NH4+, NO2-, Cl- in the aerosols were in the range of 10.32-193.46, 4.42-74.05, 2.21-57.75, 0.05-2.22 and 1.35-17.39 μg·m-3 respectively. And the mass concentration of SNA increased with the intensity of haze pollution. The concentrations of NO3- on the slight, mild, moderate and severe haze days were 55%, 77%, 240% and 537% higher than that on non-haze days respectively, while concentrations of SO42- increased by 4.7%, 35%, 77% and 262% respectively, and concentrations of NH4+ increased by 72%, 83%, 201% and 526% respectively. The contribution of these water-soluble ions to PM2.5 showed that the proportion of sum of NO3-, SO42-, NH4+, NO2- and Cl- to PM2.5 was in range of 62.03%-80.93%. The proportion of ion to PM2.5 decreased in the order of NO3- > SO42- > NH4+ > Cl- > NO2-. With the enhancement of haze pollution, the proportion of NO3- in PM2.5 increased from 29.53% to 45.54%. The correlation analysis showed that NO3- and SO42- in the fine particle were significantly correlated with gaseous precursors NO2 and SO2, and also showed good correlations with relative humidity, visibility, wind speed and other weather conditions. These results indicated that the formation of SNA in fine particles was one of the main reasons for visibility decrease and the formation of air pollution in haze days. Meanwhile, high concentration of gaseous precursors, high relative humidity and low wind speed were the important influencing factors of haze formation. Except for slight haze days, SOR and NOR in the haze days were higher than those on the non-haze days, and increased significantly with the intensifying of haze, especially for 0.43-0.65 and 0.65-1.1 μm particle size. Conversion rates of nitrogen and sulfur in severe haze days were 1.5 times that in non-haze days, which showed nitrate and sulfate in these fine mode were mainly from gas-to-particle conversion. NO3-, SO42-, NH4+ and NO2- increased in haze significantly, which mainly existed in the fine particles. The cloud process played a more important role on haze days. While on non-haze day, cloud process and the heterogeneous reaction were both the main factors. And the highest proportion of fine mode concentration to total one was observed for NO3-(79.4%) and SO42-(74.4%) on severe haze days respectively. NO3- showed a bimodal distribution with peaks in the size-bin of 0.43~0.65 μm and 3.3-4.7 μm on non-haze, slight, mild haze days, and the fine peak moved to 0.65-1.1 μm on moderate haze days, however the bimodal distribution changed to unimodal distribution with peak at 0.65-1.1 μm on severe hazy days. SO42- showed a bimodal distribution with peaks at 0.43-0.65 μm and 2.1-3.3 μm in the non-haze weather, while the size distribution changed to unimodal distribution on hazy days. But the peak sizes were different in different intensity of haze, with peak at 0.43-0.65 μm on mild and slight haze and 0.65-1.1 μm in moderate and severe haze days. NH4+ showed a single peak distribution in the fine mode, with the peak in the particle size of 0.43-0.65 μm on slight and non-hazy days, and 0.65-1.1 μm on mild, moderate and severe hazy days. Therefore, haze has a great influence on the size distribution of SNA.

The Concentrations, Formations, Relationships and Modeling of Sulfate, Nitrate and Ammonium (SNA) Aerosols over China

Sulfate, nitrate and ammonium (SNA) are the dominant composition of secondary aerosols in the atmosphere and have a significant impact on public health, atmospheric chemistry process and climate. In this study, to evaluate SNA pollution in China, a first nationwide investigation derived from almost all published data in the field measurement before 2012 was carried out. The results show that SNA levels in China are about 3–5 times higher than those in USA and Europe. SNA account for 34.2 ± 10.9% in PM2.5 and 28.2 ± 8.5% in PM10. The highest SNA concentrations occur in urban areas of northern China. SNA all have peaks in winter, but the nadirs are in spring for sulfate and ammonium and in summer for nitrate. SOR (sulfur oxidation ratio) and NOR (nitrogen oxidation ratio) values show that the formations of sulfate and nitrate are distinct in different regions and seasons. The low average NO3–/SO42– ratio (0.43 ± 0.26) indicates that the stationary emissions from coal combustion remain the main sources. There is a good relationship between (2[SO42–] + [NO3–]) and [NH4+] with near 1 slope, signifying that (NH4)2SO4 and NH4NO3 are the predominant forms which SNA exist in particles in China. Based on the comprehensive observational data in China, the simulation for SNA aerosols by GISS in CMIP5 were evaluated.

Effect of current emission abatement strategies on air quality improvement in China: A case study of Baotou, a typical industrial city in Inner Mongolia

The national Air Pollution Prevention and Control Action Plan required significant decreases in PM2.5 levels over China. To explore more effective emission abatement strategies in industrial cities, a case study was conducted in Baotou to evaluate the current national control measures. The total emissions of SO2, NOX, PM2.5 and NMVOC (non-methane volatile organic compounds) in Baotou were 211.2Gg, 156.1Gg, 28.8Gg, and 48.5Gg, respectively in 2013, and they would experience a reduction of 30.4%, 26.6%, 15.1%, and 8.7%, respectively in 2017 and 39.0%, 32.0%, 24.4%, and 12.9%, respectively in 2020. The SO2, NOX and PM2.5 emissions from the industrial sector would experience a greater decrease, with reductions of 37%, 32.7 and 24.3%, respectively. From 2013 to 2020, the concentrations of SO2, NO2, and PM2.5 are expected to decline by approximately 30%, 10% and 14.5%, respectively. The reduction rate of SNA (sulfate, nitrate and ammonium) concentrations was significantly higher than that of PM2.5 in 2017, implying that the current key strategy toward controlling air pollutants from the industrial sector is more powerful for SNA. Although air pollution control measures implemented in the industrial sector could greatly reduce total emissions, constraining the emissions from lower sources such as residential coal combustion would be more effective in decreasing the concentration of PM2.5 from 2017 to 2020. These results suggest that even for a typical industrial city, the reduction of PM2.5 concentrations not only requires decreases in emissions from the industrial sector, but also from the low emission sources. The seasonal variation in sulfate concentration also showed that emission from coal-burning is the key factor to control during the heating season.

Role of secondary aerosols in haze formation in summer in the Megacity Beijing

A field experiment from 18 August to 8 September 2006 in Beijing, China, was carried out. A hazy day was defined as visibility<l0km and RH (relative humidity)<90%. Four haze episodes, which accounted for ~60% of the time during the whole campaign, were characterized by increases of SNA (sulfate, nitrate, and ammonium) and SOA (secondary organic aerosol) concentrations. The average values with standard deviation of SO42−, NO3−, NH4+ and SOA were 49.8 (±31.6), 31.4 (±22.3), 25.8 (±16.6) and 8.9 (±4.1)μg/m3, respectively, during the haze episodes, which were 4.3, 3.4, 4.1, and 1.7 times those in the non-haze days. The SO42−, NO3−, NH4+, and SOA accounted for 15.8%, 8.8%, 7.3%, and 6.0% of the total mass concentration of PM10 during the non-haze days. The respective contributions of SNA species to PM10 rose to about 27.2%, 15.9%, and 13.9% during the haze days, while the contributions of SOA maintained the same level with a slight decrease to about 4.9%. The observed mass concentrations of SNA and SOA increased with the increase of PM10 mass concentration, however, the rate of increase of SNA was much faster than that of the SOA. The SOR (sulfur oxidation ratio) and NOR (nitrogen oxidation ratio) increased from non-haze days to hazy days, and increased with the increase of RH. High concentrations of aerosols and water vapor favored the conversion of SO2 to SO42− and NO2 to NO3−, which accelerated the accumulation of the aerosols and resulted in the formation of haze in Beijing.

Characteristics of secondary inorganic aerosol and sulfate species in size-fractionated aerosol particles in Shanghai

Sulfate, nitrate and ammonium (SNA) are the dominant species in secondary inorganic aerosol, and are considered an important factor in regional haze formation. Size-fractionated aerosol particles for a whole year were collected to study the size distribution of SNA as well as their chemical species in Shanghai. SNA mainly accumulated in fine particles and the highest average ratio of SNA to particulate matter (PM) was observed to be 47% in the fine size fraction (0.49–0.95 μm). Higher sulfur oxidation ratio and nitrogen oxidation ratio values were observed in PM of fine size less than 0.95 μm. Ion balance calculations indicated that more secondary sulfate and nitrate would be generated in PM of fine size (0.49–0.95 μm). Sulfur K-edge X-ray absorption near-edge structure (XANES) spectra of typical samples were analyzed. Results revealed that sulfur mainly existed as sulfate with a proportion (atomic basis) more than 73% in all size of PM and even higher at 90% in fine particles. Sulfate mainly existed as (NH4)2SO4 and gypsum in PM of Shanghai. Compared to non-haze days, a dramatic increase of (NH4)2SO4 content was found in fine particles on haze days only, which suggested the promoting impact of (NH4)2SO4 on haze formation. According to the result of air mass backward trajectory analysis, more (NH4)2SO4 would be generated during the periods of air mass stagnation. Based on XANES, analysis of sulfate species in size-fractionated aerosol particles can be an effective way to evaluate the impact of sulfate aerosols on regional haze formation.

Characteristics of Aerosol Optical Properties and Their Chemical Apportionments during CAREBeijing 2006

Field campaigns monitoring the aerosol optical properties and chemical components of PM_(10) were carried out in Beijing in 2006 summer. The average light extinction coefficient b_(ext), dry aerosol scattering coefficient b_(sp) and aerosol absorption coefficient b_(ap) were 895.0 ± 820.8 Mm^(-1), 364.0 ± 324.3 Mm^(-1) and 57.8 ± 31.1 Mm^(-1), respectively. b_(ext), b_(sp) and bap had the similar increasing trend during the formation process of haze. Pronounced diurnal cycles were observed for ω_(550) (aerosol single scattering albedo at 550 nm), b_(sp), b_(ap) and b_(ext). The dry b_(sp) was elevated during the daytime with a maximum mean value of 475.8 Mm^(-1) (LST 06:00). b_(ext), PM_(2.5) mass concentration and PM_(2.5)/PM_(10) ratio increased at night due to continuous emissions of pollutants to the lower nocturnal boundary layer, and decreased during the daytime due to convective mixing. b_(ap) increased at night, and decreased during the daytime and reached the minimum (37 Mm^(-1)) at LST 16:00. The single scattering albedo reached its maximum (0.87) at LST 11:00. This trend was consistent with the SNA (sulfate, nitrate, and ammonium)/PM_(10) ratio and was contrary to the BC (black carbon)/PM_(10) ratio, which demonstrated that secondary pollution largely influenced the scattering ability of aerosols. Ammonium sulfate, ammonium nitrate, organic mass, elemental carbon and coarse mass contributed 26.5%, 15.2%, 21.8%, 16.1% and 20.4% to the total extinction coefficient during clean days, and 44.6%, 22.3%, 13.6%, 10.8% and 8.7% during hazy days. The fractional contributions of ammonium sulfate and ammonium nitrate were significantly higher during the hazy time than those during the clean days. While the fractional contributions of organic mass, elemental carbon and coarse mass were lower during the haze time than those during the clean days.