Total Water-soluble Inorganic Ions Research Articles

Assessment of Secondary Sulfate Aqueous-Phase Formation Pathways in the Tropical Island City of Haikou: A Chemical Kinetic Perspective.

Sulfate (SO42-) is an essential chemical species in atmospheric aerosols and plays an influential role in their physical-chemical characteristics. The mechanisms of secondary SO42- aerosol have been intensively studied in air-polluted cities. However, few studies have focused on cities with good air quality. One-year PM2.5 samples were collected in the tropical island city of Haikou, and water-soluble inorganic ions, as well as water-soluble Fe and Mn, were analyzed. The results showed that non-sea-salt SO42- (nss-SO42-) was the dominant species of water-soluble inorganic ions, accounting for 40-57% of the total water-soluble inorganic ions in PM2.5 in Haikou. The S(IV)+H2O2 pathway was the main formation pathway for secondary SO42- in wintertime in Haikou, contributing to 57% of secondary SO42- formation. By contrast, 54% of secondary SO42- was produced by the S(IV)+Fe×Mn pathway in summer. In spring and autumn, the S(IV)+H2O2, S(IV)+Fe×Mn, and S(IV)+NO2 pathways contributed equally to secondary SO42- formation. The ionic strength was the controlling parameter for the S(IV)+NO2 pathway, while pH was identified as a key factor that mediates the S(IV)+H2O2 and S(IV)+Fe×Mn pathways to produce secondary SO42-. This study contributes to our understanding of secondary SO42- production under low PM2.5 concentrations but high SO42- percentages.

Growth of nitrate contribution to aerosol pollution during wintertime in Xi'an, northwest China: Formation mechanism and effects of NH3

With the strengthened controls on SO2 emissions and extensive increases in motor vehicles’ exhaust, aerosol pollution shifts from sulfate-rich to nitrate-rich in recent years in Xi'an, China. To further gain insights into the factors on nitrate formation and efficiently mitigate air pollution, highly time-resolved observations of water-soluble inorganic ions (WSIIs) in PM2.5 were measured in a suburban area of Xi'an, China during wintertime. Hourly concentration of total WSIIs is 39.8 μg m−3 on average, accounting for 50.3% of PM2.5 mass. In contrast to a slight decrease in the mass fraction of SO42−, NO3− shows a significant increase of the PM2.5 contribution with the aggravation of aerosol pollution. This suggests the importance of NO3− formation to haze evolution. Furthermore, homogeneous reactions govern the formation of NO3−, while alkali metals such as calcium and sodium play an additional role in retaining NO3− in PM2.5 during clean periods. However, the heterogeneous hydrolysis reaction contributed more to NO3− formation during the pollution periods under high relative humidity. Our investigation reveals that temperature, relative humidity, oxidant, and ammonia emissions facilitate rapid NO3− formation. Using the random forest (RF) model, NO3− concentrations were successfully simulated with measured variables for the training and testing datasets (R2 > 0.95). Among these variables, CO, NH3, and NO2 were found to be the main factors affecting the NO3− concentrations. Compared with the period without vehicle restriction, the contributions of NO3− and NH4+ to PM2.5 mass decreased by 5.3% and 3.4% in traffic restriction periods, respectively. The vehicle restriction leads to the decreases of precursor gases of NO2, SO2, and NH3 by 12.8%, 5.9%, and 27.6%, respectively. The results demonstrate collaborative emission reduction of NOx and NH3 by vehicle restrictions, and using new energy vehicles (or electric vehicles) can effectively alleviate particulate matter pollution in northwest China.

Characterization of fine particulate matter water-soluble inorganic ions and estimation of aerosol acidity at three COALESCE network sites - Mysuru, Bhopal, and Mesra - in India.

The study was carried out to understand the chemical, spatiotemporal characteristics of water-soluble inorganic ions (WSIIs), their association with PM2.5 mass, and aerosol acidity in three COALESCE (carbonaceous aerosol emissions, source apportionment, and climate impacts) network sites of India (Mesra - Eastern India, Bhopal - Central India and Mysuru - Southern India). Alternate-day 24-h integrated bulk PM2.5 samples were collected during 2019 along with on-site meteorological parameters. Annual average PM2.5 concentrations were 67 ± 46µgm-3, 54 ± 47µgm-3, and 30 ± 24µgm-3 at Mesra, Bhopal, and Mysuru, respectively. PM2.5 concentrations exceeded the annual mean (40µgm-3) recommended by the National Ambient Air Quality Standards (NAAQS) at Mesra and Bhopal. WSIIs existed in PM2.5 mass at Mesra (50.5%), Bhopal (39.6%), and Mysuru (29.2%). SO42-, NO3-, and NH4+ (SNA) were major secondary inorganic ions in total WSIIs, with an annual average of 88.4% in Mesra and 82.0% in Bhopal 78.4% in Mysuru. Low NO3-/SO42- ratios annually at Mesra (0.41), Bhopal (0.44), and Mysuru (0.24) indicated that stationary sources dominated vehicular emissions (1.0). Aerosol acidity varied from region to region and season to season depending on the presence of NH4+, the dominant counter-ion to neutralize anions. Aerosols were near-neutral or alkaline at all three sites, except during the pre-monsoon season in Mysuru. An assessment of neutralization pathways for major anions [SO42- + NO3-] suggests that they mainly existed as sulfate and nitrate salts such as ammonium sulfate ((NH4)2SO4) and ammonium bisulfate (NH4HSO4) in conjunction with ammonium nitrate (NH4NO3).

Fine particulate matter and gas emissions at different burn phases from household coal-fired heating stoves

Household coal burning contributes a large proportion of fine particulate matter (PM2.5), black carbon (BC), organic carbon (OC), and CO emissions in China. Previous lab or field investigations generally measured short-term emissions from heating coal stoves, and only covered the high power phase or the flaming combustion processes, and did not consider smoldering combustion condition, which does not provide full characterization of the actual conditions in most cases. In this study, we investigated air pollutant emissions during the different burn phases of seven types of coal burned in a typical household coal-fired heating stove in the lab with a hood sampling system. We proposed a method to estimate coal consumption at the different burn phases, and thus quantify emission factors (EFs) at different burn phases. The concentrations of CO2, CO, SO2, NOx, and PM2.5 fluctuated over 1 to 4 orders of magnitude during the whole burn cycle. The air pollutant concentration peaks, such as CO and PM2.5, were generally associated with coal addition. EFs of CO2, SO2, total water soluble inorganic ions (TWSIIs) followed the trend: high power phase > low power phase > banked fire phase. However, CO EFs followed the opposite trend, i.e., banked fire phase > low power phase > high power phase. EFs of NOx and EC in high power phase and low power phase were higher than those in banked fire phase. The variation in EFs of PM2.5 and OC in different burn phases across seven types of coal burned in this study is complicated. When using EFs from high power phase or flaming phase to represent EFs from the whole process, CO2, SO2, NOx, EC and TWSIIs emissions will be overestimated, CO emissions will be underestimated, PM2.5 and OC emissions may be overestimated or underestimated, depending on coal types.

Characteristics and Sources of Water-Soluble Inorganic Ions in PM2.5 in Urban Nanjing, China

In this study, the water-soluble inorganic ions (WSIIs) composition of fine particulate matter (PM2.5) was measured in the northern Nanjing city from 2015 to 2021. NH4+, NO3− and SO42− concentrations dominated in total WSIIs (Na+, NH4+, K+, Mg2+, Ca2+, Cl−, NO3− and SO42−), accounting for 87.8%. The nitrate with highest average concentration among all ions was 11.0 μg·m−3. Total WSIIs concentrations were higher in winter and lower in summer, with the highest levels in December (45.6 μg·m−3) and the lowest levels in August (15.1 μg·m−3). NO3−/SO42− was higher than 1, indicating the important contribution of mobile sources. The aerosols exhibited a weak acidic by the molar ratio of water-soluble anions and cations. Positive matrix factorization (PMF) analysis results showed that secondary nitrate and sulfate were the major pollution sources in December 2016 and 2020. The contribution of secondary nitrate in 2020 increased by 47.6% compared to 2016, while that of secondary sulfate decreased by 42.4%. The potential source contribution results demonstrated that for secondary aerosol concentrations, the contribution of regional transport from north of Anhui increased, while the contribution of local emissions decreased. The results from this study could contribute to the better prevention and control of regional air pollution in the future.

Size Distributions of Water-Soluble Inorganic Ions in Atmospheric Aerosols During the Meiyu Period in the Yangtze River Delta, China

In order to investigate the chemical composition distributions and pollution characteristics of Total water-soluble inorganic ions (TWSII) in the rain period (Meiyu) in the East Asian summer monsoon season, including the impact of Meiyu on air pollution in the Yangtze River Delta, East China, the gaseous pollutant concentrations, the 9 sizes segregated particles, and water-soluble inorganic ions of aerosols were measured on the north shore of Taihu Lake from June 4 to July 5, 2016. Results show that the mass concentrations of atmospheric particulate matters (PM2.5 and PM10) and main gaseous pollutants (SO2, NO2, CO, and O3) decrease during the Meiyu period, with the largest decline in PM10 and the smallest in CO. TWSII in atmospheric particles are mainly concentrated in fine particles during the Meiyu period. The values of ρ (TWSII) for PM1.1, PM1.1–2.1, and PM2.1–10 before the Meiyu onset are generally greater than those during the Meiyu period. During the first pollution process, the ρ(TWSII) for PM1.1 and PM1.1–2.1 first increase to the peak values, and then decrease during the moderate rainfall period, when the ρ(TWSII) in PM2.1–10 increase to its maximum before the Meiyu onset. The mass concentrations for anions, cations, and total ions at different particle-size sections all exhibit bimodal distributions before and after the Meiyu onset. The mass concentration peaks at a particle size of 1.1–2.1 μm for fine particles, while at 5.8–9.0 μm (before the Meiyu onset) and 9.0–10.0 μm (during the Meiyu period) for coarse particles, respectively. The peak particle size for mass concentration of coarse particles moves toward larger sizes during the Meiyu period. The mass concentrations of SO42− at different particle-size sections show a bimodal distribution before the Meiyu onset and a multi-modal distribution during the Meiyu period. The mass concentrations of NO3− at different particle-size sections show a bimodal distribution before the Meiyu onset and a unimodal distribution during the Meiyu period. The mass concentrations of NH4+ at different particle-size sections present a bimodal distribution before and after the Meiyu onset, with the particle-size for peak concentrations distributing in 1.1–2.1 and 5.8–9.0 μm before the Meiyu onset, and 9.0–10.0 μm during the Meiyu period. The mean value of nitrogen oxidation ratio (NOR) is higher before the Meiyu onset than after, indicating that the secondary conversion of NO2 before the Meiyu onset is enhanced. The sulfur oxidation ratio (SOR) values are greater than NOR values, but the concentrations of NO2 in the same period during the Meiyu period are higher than those of SO2, which indicates that the secondary conversion of SO2 during the Meiyu period on the north bank of Taihu Lake is stronger than that of NO2. During the whole observation, the contribution of stationary sources mainly contributed to the atmospheric particulate matters during the Meiyu period. The contributions of vehicle exhaust and coal combustion to fine particles are more obviously affected by the changes in meteorological conditions during the Meiyu period, and the vehicle emissions contribute more to PM1.1–2.1 than to PM1.1.

Variations in traffic-related water-soluble inorganic ions in PM2.5 in Kanazawa, Japan, after the implementation of a new vehicle emission regulation

In this study, a three-year sampling campaign was conducted at a roadside air pollution monitoring station in Kanazawa, Japan. A total of 7 water-soluble inorganic ions (WSIIs) in ambient particulate matter (PM2.5) was determined via ion chromatography. After the implementation of a new vehicle emission regulation enacted in 2016 in Japan, the annual average concentration of the total WSIIs decreased, from 5.32 ± 3.27 μg/m3 in 2017 to 3.70 ± 2.43 μg/m3 in 2018. This decrease is mainly attributed to the reduction in the three major species of sulfate (SO42−), ammonium (NH4+), and nitrate (NO3−). The reduction in NO3− with decreasing nitrogen oxide (NOx) emphasizes the role of the new regulation. The reduction in NOx might also indirectly lead to a decrease in SO42− or directly lead to a decrease in NH4+. Through a comparison to other WSIIs emission sources, a similar composition was found to that of other traffic-related WSIIs, but the considered WSIIs are clearly different from those emitted by coal and biomass combustion sources. Via further comparison of the distribution of the [NO3−]/[SO42−] ratio values from different sources, we observed that the ratio values are close to those of WSIIs emitted by coal combustion sources, which indicates that this ratio might not be a suitable indicator for the identification of the contribution of mobile and stationary sources in Japanese cities. Overall, this study affirms the effectiveness of the new regulation in terms of WSIIs emission reduction and provides basic WSIIs data pertaining to the roadside environment in Kanazawa for the first time.

Characteristics of Aerosol and Effect of Aerosol-Radiation-Feedback in Handan, an Industrialized and Polluted City in China in Haze Episodes

In order to investigate the chemical characteristics of aerosol pollution including PM1 and PM2.5 in Handan, the offline sampling campaign was conducted and the concentrations of total water-soluble inorganic ions (TWSI), carbonaceous components (OC and EC) were analyzed. The average concentrations were 88.5 μg/m3 for PM1 and 122 μg/m3 for PM2.5, and the corresponding ratios of PM1 versus PM2.5 on non-pollution, mild-moderate pollution and heavy pollution were 0.67, 0.70 and 0.77, respectively. TWSI and OC accounted for 43.2% and 15.4% in PM1, 41.8% and 16.0% in PM2.5. Secondary components in PM2.5 and PM1 increased with heavy pollution, SNA (SO42−, NO3− and NH4+) was enriched in PM1 but SOC (Secondary Organic Carbon) was more enriched in PM1–2.5. Furthermore, for evaluating the effect of aerosol feedback the WRF-Chem model was applied to identify the aerosol-radiation interaction of aerosol feedback influence on the PM2.5 concentration and various meteorological factors in Handan. The results indicated that the aerosol radiative effects will result in an average 32.62%(36.18 W/m2) decrease in downward short wave flux at ground surface (SWDOWN), an average 17.52% (39.15 m) and 0.16% (0.44 K) decrease in planetary boundary layer height(PBLH) and surface temperature (T2). The wind speed at 10 m (WS) and relative humidity (RH) will be increased by about 4.16%(0.11 m/s) and 1.89% (0.78%), respectively.

Effect of forest floor fuel moisture content on chemical components of PM2.5 emitted during combustion

The moisture content of forest floor fuels changes continuously with the influence of environmental factors; thus it has an important impact on the concentration and chemical composition of particulate matter emitted during forest fire. However, most previous studies quantify emissions of particulate matter and constituents using dry samples. In this study, we use a self-designed semi closed combustion simulator to quantify emission of total carbon (TC), organic carbon (OC), elemental carbon (EC) and water-soluble ions in fine particulate matter (PM2.5) using fuels of four tree species that differ in moisture content (0, 10, 20 and 30%). The results showed that the emissions of TC, OC and EC and total water-soluble inorganic ions increased significantly (<0.05) with increasing moisture content of fuels, and fuels of coniferous species emitted significantly more pollutants than fuels of broadleaved species. Similarly combustion of leaf samples emitted more carbonaceous components and water-soluble ions than combustion of branches. K+, NH4+ and Cl− were the main components of water-soluble ionic species, and emissions of K+, Ca2+, Na+, Mg2+, NH4+, Cl−, Br−, NO3−, NO2−, SO42− increased with increasing moisture content of fuels. Fuel moisture content had a great impact on the inorganic salt composition in the particulate matter emitted during combustion. The findings have an important implication on the use of prescribed early fire as management tools as the moisture content of the fuels early during the dry season is still high.

Particles liquid water and acidity determine formation of secondary inorganic ions in Urumqi, NW China

Secondary atmospheric pollutants are an important component of the particles found in Urumqi city, China, Further, secondary inorganic aerosols, in particular, increase significantly during the winter heating period. In this study, samples collected from October 2017 to August 2018 from Urumqi were analyzed. Results showed that the average concentration of water-soluble inorganic ions (WSIs) in the heating period (86.9 ± 31.1 μg·m−3) was significantly higher than that in the non-heating period (14.1 ± 8.67 μg·m−3). SNA (SO42−, NO3−, NH4+) accounted for 90.7% of the total WSIs (TWSIs) in the heating period. Simulations revealed that the in situ pH (3.1) was higher in the heating period than that in non-heating period (0.66) when they were calculated by the aerosol thermodynamic model. The particles were found to be acidity during the entire period. The liquid water content of the particulates (LWC) was found to higher in the heating period (55.5 ± 51.8 μg·m−3) than the non-heating period (21.3 ± 40.3 μg·m−3). The average nitrogen oxidation rate (NOR) and sulfur oxidation rate (SOR) were 0.16 and 0.36, respectively, indicating a significant secondary formation of nitrate and sulfate in the atmosphere of Urumqi. Further analysis indicated that sulfate was significantly affected by the LWC and acidity of the particulates; nitrate was produced under ammonia-rich conditions in Urumqi, and the LWC facilitated the production of nitrate. This study provides information about the formation of secondary sulfate and nitrate aerosols during different periods in suburban Urumqi.

Characteristics of Water-soluble Inorganic Ions in Aerosol and Precipitation and their Scavenging Ratios in an Urban Environment in Southwest China

Daily fine particulate matter (PM2.5) and precipitation samples were collected simultaneously at an urban site in southwest China in four segregated months in 2015 for measuring major water-soluble inorganic ions (WSIIs). Online hourly concentrations of PM10 and PM2.5 were also monitored, which showed annual mean concentrations of 67.8 and 41.6 µg m–3, respectively. PM2.5 showed the highest concentration in winter and lowest in summer. The annual mean concentration of the total WSIIs was 20.3 µg m–3, accounting for about 48.7% of PM2.5. Among the total WSIIs in ambient PM2.5, SO42– was the predominant component (49.7%), followed by NH4+ (24.1%) and NO3– (21.4%). NH4+ and SO42– were the two most abundant ions in precipitation, followed by Ca2+ and NO3–. Seasonal patterns of the major inorganic ions in precipitation were similar to those in PM2.5, with the highest concentration in winter and lowest in summer. The mean scavenging ratios were 454, 445, 364, 456, and 394 for SO42–, NO3–, NH4+, Cl–, and K+, and 116, 353, and 18 for gas SO2, HNO3, and NH3, respectively. The higher scavenging ratios of particulate ions than their gaseous precursors suggest the higher contributions of particles than gases to the total wet deposition.

Chemical composition, water content and size distribution of aerosols during different development stages of regional haze episodes over the North China Plain

Aerosol size distribution and chemical composition are found to have significant effects on its hygroscopicity, acidity/alkalinity and light extinction. Heavy haze pollution occurred frequently in the wintertime of NCP, with long duration time and large impact area, which had important influences on air quality and human health. However, the study concerning the formation mechanism and physicochemical characteristics of aerosols in different haze stages have been rarely carried out. In this study, aerosol size distribution in the range of 10 nm - 10 μm, water soluble inorganic ions (WSIIs), PM (PM2.5, PM10), trace gases, organic carbon (OC), elemental carbon (EC) and meteorological elements were derived during a regional haze pollution episode from Nov. 9 to 16, 2018. The aerosol water content and pH were further calculated using the ISORROPIA model. We divided the whole observation period into segments of clean days, fog processes, and haze processes, including three stages of I: accumulation, II: growth, and III: explosion, and a dry haze (D-haze) process based on the PM2.5 concentration, visibility and RH. Given the shift of particle size to larger segment ascribed to the ageing process during fog/haze process, the size distribution peak in aerosol number concentration was located at 100 nm in the fog/haze episode and was 70 nm larger than that on clean days. The concentration descended significantly in stage III, peaking at 160 nm, suggesting a strong aging process of aerosols. The PM2.5 increased sharply under high RH, static weather conditions and strengthening oxidation that favored liquid and heterogeneous reactions. The nitrate concentration was found to account for 20.5% (D-haze) - 29.0% (fog) of the total water soluble inorganic ions (WSIIs) during the fog/haze process, while sulfate constituted only 13.8% (stage III) - 21.3% (stage I), revealing dominant nitrate pollution. Hence, nitrate was considered to originate mainly from photochemical and heterogeneous reactions in haze episodes and was generated only by heterogeneous reactions. Higher (lower) aerosol water content made aerosols more acidic (alkaline) under similar chemical compositions. Therefore, the chemical reactions may differ under dry and wet haze pollution. Sources of carbonaceous aerosols changed in different haze stages with descending (enhancing) contributions from coal combustion (vehicle exhaust).

Characteristics of Water-soluble Inorganic Ions in PM2.5 in Beijing During 2017-2018

To explore the characteristics of water-soluble inorganic ions (WSIIs) in PM2.5 during the process of continuous improvement of air quality in Beijing in recent years, a continuous collection of PM2.5 sample campaign was conducted in Beijing from 2017 to 2018. The PM2.5 mass concentration and WSIIs were then determined. The results showed that the average concentration of PM2.5 in Beijing was (77.1±52.1) μg ·m-3, with the highest and lowest values during spring [(102.9±69.1) μg ·m-3]and summer [(54.7±19.9) μg ·m-3], respectively. The average concentration of WSIIs was (31.7±30.1) μg ·m-3, accounting for 41.1% of the PM2.5 mass, and the seasonal contributions were: autumn (45.9%) > summer (41.9%) > spring (39.9%) ≥ winter (39.2%). SNA was an important component of the WSIIs that accounted for 86.0%, 89.5%, 74.6%, and 73.0% of the total WSIIs during spring, summer, autumn, and winter, respectively. With an increase in temperature, the concentration of NO3- increased initially and then decreased, while the concentration of SO42- increased. When the relative humidity was less than 90%, the concentrations of both NO3- and SO42- increased with an increase in relative humidity. With the aggravation of pollution, the overall contribution of WSIIs in PM2.5 increased significantly, and the evolution characteristics of different ions were different. Among them, the concentration and contribution of NO3- continued to increase, while the contributions of SO42- and the ions from dust (Mg2+, Ca2+, and Na+) decreased. During the observation period, the primary sources of WSIIs were secondary conversion, combustion source, and dust. The control of coal combustion and motor vehicles is critical to reduce the emission of WSIIs. The backward trajectory analysis showed that the air masses from the south and west of Beijing corresponded to the high PM2.5 concentration and proportion of WSIIs, and the contribution of secondary ions was significant. However, the concentrations and proportions of the air masses from the northwest and north were relatively low, but the contribution of Ca2+ was high.

Long-term variability of inorganic ions in TSP at a remote background site in Japan (Wajima) from 2005 to 2015

Eleven years (2005–2015) of data from long-term monitoring at a Japanese remote background site in Wajima, were analyzed to investigate temporal trends and sources. Water-soluble inorganic ions (WSIIs) were analyzed for aerosol chemical composition. The total WSIIs concentration was 7.93 ± 3.93 μg/m3, accounting for 42.3% of TSP mass, ranged from 11.4 to 93.9%. SO42− is the most abundant ion, contributing a total WSII mass from 18.0 to 79.8%, and non-sea-salt (nss-) SO42− contributed from 63.6% to 99.6% of total SO42−, which was related to human activities on the Asian continent and the effects of marine precursors in spring and summer, respectively. NO3− and NH4+ contribute 6.3 and 7.4% of the total WSIIs and were affected by long-range transport and local sources as well. The increasing trend of Na+ and Cl− indicates the increased influence of sea salt, which is caused by more frequent strong winds. K+ is mainly produced from biomass burning with a stable seasonal variation, Ca2+ as the characteristic ion of dust has the highest concentration in spring. Mg2+ comes from minerals and marine sources during spring and summer, respectively. This work describes in detail the annual change trend of the WSIIs of atmospheric particles in the Wajima area, seasonal characteristics, and source contributions, provide a comprehensive understanding of long-term variation in atmospheric particulate.

Diurnal and seasonal variations in water-soluble inorganic ions and nitrate dual isotopes of PM2.5: Implications for source apportionment and formation processes of urban aerosol nitrate

Recently, increasing urban aerosol NO3− deposition have aroused widespread attention. To reveal its sources and formation processes, the water-soluble inorganic ions (WSIIs) and NO3− dual isotopes (i.e., δ15N and δ18O) were measured in fine particles (i.e., PM2.5), which were collected during rush hours in the morning and afternoon, and non-rush hours at noon, discretely in winter and in summer at urban Guiyang, southwest China. Results showed the sulfate, nitrate, and ammonium together accounted for more than 90% of the total WSIIs in both seasons. Both NO3− and its dual isotopes were distinctly higher in winter than in summer. While only NO3− and δ15N were observed significantly higher during rush hours than the non-rush hours in summer, which could be potentially controlled by the temperature, relative humidity and ambient NH3 contents. The gradual but insignificant increase of δ18O during from morning to afternoon were ascribed to the decreasing contributions of •OH oxidation pathway. By using the Bayesian isotope mixing model, we revealed the coal combustion and biomass burning accounted for the predominant NOx sources in contributing to the particulate NO3− in winter, followed by vehicular and soil biogenic emissions. Whereas in summer, biomass burning played the most important role, since the coal combustion proportion sharply decreased. Moreover, the particle NO3− source contributions showed minor difference during diurnal periods in winter, while the difference was larger in summer, especially for that during between morning rush and noon non-rush hours, which were likely due to the changes of NOx emissions (e.g. from vehicular and soil biogenic source), as well as the meteorological conditions.

Online Measurement of PM2.5 at an Air Monitoring Supersite in Yangtze River Delta: Temporal Variation and Source Identification

To comprehensively explore the transport of air pollutants, one-year continuous online observation of PM2.5 was conducted from 1 April 2015 to 31 March 2016 at Dianshan Lake, a suburban junction at the central of Yangtze River Delta. The chemical species of PM2.5 samples mainly focused on Organic carbon (OC), Elemental carbon (EC) and Water-Soluble Inorganic Ions (WSIIs). The annual average of PM2.5 concentration was 59.8 ± 31.7 µg·m−3, 1.7 times higher than the Chinese National Ambient Air Quality Standards (CNAAQS) (35 µg·m−3). SNA (SO42−, NO3− and NH4+) was the most dominated species of PM2.5 total WSIIs, accounting for 51% of PM2.5. PM2.5 and all of its chemical species shared the same seasonal variations with higher concentration in winter and spring, lower in autumn and summer. The higher NO3−/EC and NOR occurred in winter suggested that intensive secondary formation of nitrate contributed to the higher levels of PM2.5. Cluster analysis based on 72-h backward air trajectory showed that the air mass cluster from nearby inland cities, including Zhejiang, Anhui and Jiangxi Provinces contributed mostly to the total trajectories. Furtherly, potential source contribution function (PSCF) analysis revealed that local sources, namely the emissions in the Yangtze River, were the primary sources. During haze pollution, NO3− was the most important fraction of PM2.5 and the heterogeneous formation of nitrate became conspicuous. All the results suggested that the anthropogenic emissions (such as traffic exhaust) was responsible for the relatively high level of PM2.5 at this monitoring station.

Day–night variation and size distribution of water-soluble inorganic ions in particulate matter in Ulsan, South Korea

In the present study, 11 size classes of particulate matter were collected from a semi-rural site in the industrial city of Ulsan, South Korea in 2019 to investigate the size distribution and day–night variation of water-soluble inorganic ions (WSIIs). Approximately 70% of the detected WSIIs were found in fine particles, with Na+, Ca2+, Cl−, and NO3− dominant (~70%) in coarse particles and SO42−, NO3−, and NH4+ (SNA) dominant (~70%) in fine particles. Monthly variation in total WSIIs was observed, with the highest average concentration found in April for both coarse (2.71 μg/m3) and fine particles (5.75 μg/m3). Given that this month is characterized by prevailing southeasterly winds, these high levels may represent the adverse effects of industrial activity. Coefficients of divergence for individual WSIIs exhibited large variability, which is indicative of significant day–night differences in meteorology, chemistry, and source contributions. The size distributions of Na+, K+, F−, Cl−, NO3−, and SO42− were bimodal, while Mg2+, Ca2+, and NH4+ were unimodal. Peaks were generally found at 1.8–5.6 μm and 0.18–0.56 μm for coarse and fine particles, respectively. By plotting the linear relationship between the mole charge ratios of SNA, we found that coarse NO3− particles were mainly the result of heterogeneous reactions between HNO3 and sea salts or crustal species, while the homogeneous reaction of HNO3 and NH3 played a crucial role in the formation of fine NO3− particles. In conclusion, though day–night variation in WSIIs was apparent, especially in SNA, the formation pathways for both coarse and fine particles were similar for day and night. Based on the verification of the formation mechanisms for WSIIs at this semi-rural site, the influence of industrial activity on the secondary formation of PM in urban and industrial areas can be investigated further.

Heterogeneous formation of particulate nitrate under ammonium-rich regimes during the high-PM&amp;lt;sub&amp;gt;2.5&amp;lt;/sub&amp;gt; events in Nanjing, China

Abstract. Particulate nitrate (NO3-) not only influences regional climates but also contributes to the acidification of terrestrial and aquatic ecosystems. In 2016 and 2017, four intensive online measurements of water-soluble ions in PM2.5 were conducted in Nanjing City in order to investigate the potential formation mechanisms of particulate nitrate. During the sampling periods, NO3- was the predominant species, accounting approximately for 35 % of the total water-soluble inorganic ions, followed by SO42- (33 %) and NH4+ (24 %). Significant enhancements of nitrate aerosols in terms of both absolute concentrations and relative abundances suggested that NO3- was a major contributing species to high-PM2.5 events (hourly PM2.5≥150 µg m−3). High NO3- concentrations mainly occurred under NH4+-rich conditions, implying that the formation of nitrate aerosols in Nanjing involved NH3. During the high-PM2.5 events, the nitrogen conversion ratios (Fn) were positively correlated with the aerosol liquid water content (ALWC; R&gt;0.72 and p&lt;0.05). Meanwhile, increasing NO3- concentrations regularly coincided with increasing ALWC and decreasing Ox (Ox=O3+NO2). These results suggested that the heterogeneous reaction was probably a major mechanism of nitrate formation during the high-PM2.5 events. Moreover, the average production rate of NO3- by heterogeneous processes was estimated to be 12.6 % h−1 (4.1 µg m−3 h−1), which was much higher than that (2.5 % h−1; 0.8 µg m−3 h−1) of gas-phase reactions. This can also explain the abrupt increases in nitrate concentrations during the high-PM2.5 events. Utilizing the ISORROPIA II model, we found that nitrate aerosol formation in Nanjing during the high-PM2.5 events was HNO3 limited. This indicated that control of NOx emissions will be able to efficiently reduce airborne particulate nitrate concentrations and improve the air quality in this industrial city.

Chemical Characteristics and Source Apportionment of Water-Soluble Ions in Atmosphere Aerosols over the East China Sea Island During Winter and Summer

A total of 70 total suspended particulate (TSP) samples were collected from December 2017 to February 2018 and June to August 2018 in Shengsi Islet, East China Sea. In this study, the mass concentrations of water-soluble ions in the TSP (including Na+, K+, NH4+, Mg2+, Ca2+, Cl-, SO42-, NO3-, and MSA) samplers were determined by ion chromatography. The chemical characteristics, seasonal differences, and main sources of water-soluble ions in this background aerosol site were investigated by a multiple-technique analysis combining a HYSPLIT model, correlation analysis of water-soluble ions, and primary component analysis. The results showed that the average mass concentrations of TSP and the main water-soluble inorganic ions (WSIIs) were both high in winter and low in summer; the average mass concentration of total WSIIs in winter was (26.5±16.3) μg·m-3, and in summer was (8.8±3.8) μg·m-3. Secondary inorganic ions (NO3-, SO42-, and NH4+) are the most important ionic components in TSP, which accounted for 86.2% and 74.9% of TWSIIs in winter and summer, respectively. Meanwhile, the study site was affected by seasonal temperature change, long-distance transmission, and summer biogenic sulfates. The mass concentration of nitrate was highest in winter, and the mass concentration of sulfate was highest in summer. Anthropogenic sources were the main source of nss-SO42- in atmospheric aerosols. The analysis of sulfate sources showed that contributions of biogenic sulfates to nss-SO42- were 28.1% and 5.9% in summer and winter, respectively. The results of principal component analysis indicated that the main sources of aerosol chemical composition were marine and anthropogenic sources in summer and winter, respectively. In winter, Cl- showed a certain degree of enrichment due to the influence of human activities, and the average value of the enrichment factor was 38.5%.

Water-Soluble Inorganic Ions in Fine Particulate Emission During Forest Fires in Chinese Boreal and Subtropical Forests: An Indoor Experiment

Understanding of the characteristics of water-soluble inorganic ions (WSI) in fine particulate matter (PM2.5) emitted during forest fires has paramount importance due to their potential effect on ecosystem acidification. Thus, we investigated the emission factors (EFs) of ten most common WSI from combustion of leaves and branches of ten dominant tree species in Chinese boreal and sub-tropical forests under smoldering and flaming combustion stages using a self-designed combustion unit. The results showed that EF of PM2.5 was three times higher for the boreal (6.83 ± 0.67 g/kg) than the subtropical forest (1.97 ± 0.34 g/kg), and coniferous species emitted 1.5 times more PM2.5 (5.35 ± 0.64 g/kg) than broadleaved species (3.45 ± 0.37 g/kg). EF of total WSI was 1.27 ± 0.08 g/kg for the boreal and 1.08 ± 0.07 g/kg for the subtropical forest and 1.28 ± 0.09 and 1.07 ± 0.06 g/kg for broadleaved and coniferous species, respectively. Individual ionic species also varied significantly between forest types and species within forest types, and K+ and Cl− were the dominant ionic species in PM2.5, accounting for 25% and 30% for the boreal forest and 23% and 27% for the subtropical forest, respectively. Emissions of NO2− and SO42− were the lowest, accounting for 3% and 5% for the boreal forest and 4% for each of the subtropical forests, respectively. Combustion of leaves emitted significantly more ionic species (1.29 ± 0.05g/kg) than branches (1.05 ± 0.07 g/kg), and smoldering consistently emitted more ionic species (1.49 ± 0.09 g/kg) than flaming combustion (0.88 ± 0.03 g/kg). The cation to anion ratio was ≥1.0, suggesting that the particulate matter is neutral to alkalescent. As a whole, our findings demonstrate that forest fire in these regions may not contribute to ecosystem acidification despite the emission of a considerable amount of WSI during forest fires.