Inorganic Ions In PM2 Research Articles

Strategic control of combustion-induced ammonia emissions: A key initiative for substantial PM2.5 reduction in Tianjin, North China Plain

Information on the temporal and spatial variations in the sources of ammonium salts (NH4+), a crucial alkaline component in PM2.5, is limited. Here, we simultaneously collected PM2.5 and gaseous ammonia (NH3) samples in both summer and winter from two sites in Tianjin: an urban site (Tianjin University, TJU) and a suburban site (Binhai New-region, BH). NH3 concentrations, the contents of major water-soluble inorganic ions in PM2.5, and the compositions of ammonium‑nitrogen isotopes (δ15N-NH4+) were measured. As a result, (NH4)2SO4 and NH4NO3 were the predominant forms of NH4+ in PM2.5 during summer and winter, respectively. However, the NH4NO3 concentrations were notably greater at TJU (6.2 ± 7.3 μg m−3) than at BH (3.8 ± 4.7 μg m−3) in summer, with no regional differences observed in winter. Both sites displayed almost half the contribution of c-NH3 (combustion-related NH3) to NH4+, differing from the finding of previous isotope-based studies. This discrepancy could be attributed to the combined effects of NHx isotope fractionation and seasonal δ15N value variations in NH3 sources. The contribution fractions of v-NH3 (volatile NH3) and c-NH3 exhibited similar patterns at both sites seasonally, probably caused by coal combustion for heating in winter and temperature fluctuations. However, the contribution fraction of c-NH3 was lower at BH than at TJU in summer but greater in winter than at TJU. In summer, NH4NO3 was unstable and limited its delivery to TJU from BH, and the high contribution of c-NH3 to NH4+ at TJU could be attributed to local vehicle emissions. In winter, the stable particulate NH4NO3 that formed from the c-NH3 in the upwind area could be transported to the downwind area, increasing the NH4+ concentration at BH. Our study provides valuable insights for devising emission mitigation strategies to alleviate the increasing burden of NH3 in the local atmosphere.

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.

Characteristics of Secondary Inorganic Ions in PM2.5 and Its Influencing Factors in Summer in Zhengzhou

Nitrate (NO3-), sulfate (SO42-), and ammonium (NH4+) are important components of PM2.5, and studying their characteristics and influencing factors is essential for the continuous improvement of air quality. A series of online instruments were used to analyze the chemical components of PM2.5 in Zhengzhou in the summer of 2020. The results showed that the average ρ(PM2.5) was (28 ±13) μg·m-3, showing a daily variation characteristic of high at night and low during the day. The main concentrations of NO3-, SO42-, and NH4+ were (7.8 ±6.7), (7.2 ±3.7), and (5.5 ±3.1) μg·m-3, accounting for 22%, 21%, and 16% in PM2.5, respectively. The proportions of NO3- (27%) and SO42- (23%) in PM2.5, respectively, increased with the increase in PM2.5 and O3 concentration. In addition, the proportions of NO3- and NH4+ increased under low wind speed, high humidity, low temperature, and rainfall conditions. Moreover, the proportion of NO3- showed a daily variation characteristic of high at night and low during the day, whereas the opposite was true for SO42-. The gas-particle partitioning process of NH4NO3 was the main factor affecting the concentrations of NO3- and NH4+ in PM2.5. Low temperature, high humidity, and high aerosol water content concentrations favored the partitioning of HNO3 and NH3 to the particulate phase. High pH also favored the partitioning of gas-phase HNO3 to NO3-; however, it was not conducive to the partition of NH3 to NH4+. These trends partially explained the increase in the concentration and proportion of NO3- in PM2.5 under different scenarios.

Analysis of concentrations of nitrate salts of PM2.5 in downwind marine atmospheres related to gas-particle partitioning and different sources in China

In order to explore the concentration characteristics and sources of major inorganic ions in marine atmospheric PM2.5, this study explores the concentration features of major inorganic ions in PM2.5 under the marine atmospheric observations of the offshore ~2000 km coastline of China in the downwind direction of the land-based sources. The observation results showed that the concentration of NO3- ranged from 0.03-10 μg m-3 with a variation of three orders of magnitude, and the main contributing species in PM2.5 was NO3- in the high concentration level observation period. The E-AIM results showed that NH3, HNO3 did not reach thermodynamic equilibrium with NH4+, NO3-, and ammonium nitrate was in volatile state, both during the day and at night. The results of the source analysis using PMF showed that there were three factors with identifiable contributions to NO3-, with Factor 1 considered to be a source of ship emissions, Factor 2 reflecting aerosols under low and medium pollution scenarios, and Factor 3 being aerosols under heavy pollution scenarios. Factor 3 was the main contributing source of NO3- concentration with a contribution of 84%, followed by Factor 2 (contribution of 15%) and the lowest contribution by Factor 1 (0.7%).

Characteristics and Generation Mechanism of Secondary Inorganic Ions in PM2.5 in Changsha-Zhuzhou-Xiangtan City Group

Secondary inorganic ions, the main components of atmospheric PM2.5, are a dominant contributor to haze formation. The detailed characteristics and main generation mechanism of secondary inorganic ions in PM2.5 are still unclear in the Changsha-Zhuzhou-Xiangtan City Group, which is suffering from severe haze pollution, particularly in the autumn and winter seasons. For our study, we collected PM2.5 samples in November 2020 and January 2021 from four urban sites in the Changsha-Zhuzhou-Xiangtan City Group. Secondary inorganic components such as SO42-, NO3-, and NH4+ in PM2.5 were quantified. The average values(μg·m-3) of ρ(SO42-), ρ(NO3-), and ρ(NH4+) in autumn and winter were(5.2±2.5) and(7.9±4.8),(4.1±2.2) and(7.2±4.2), and(17.1±10.5) and(7.8±5.2), respectively. During the heavy haze pollution events in winter, the sum of ρ(SO42-), ρ(NO3-), and ρ(NH4+)(SNA) contributed 72.7% to the growth of PM2.5 mass concentration, and ρ(NO3-) accounted for 41.2%. This result suggested that the generation of NO3- was the key factor leading to the formation of winter haze pollution. In the polluted stage, high aerosol water content(AWC) promoted the rapid secondary generation of SNA, whereas adverse meteorological conditions also led to the accumulation of pollutants. The values of sulfur oxidation rate(SOR) and nitrogen oxidation rate(NOR) were still high in the dissipation stage. It indicated that the PM2.5 concentration fell due to the reduction in primary emissions and favorable weather conditions in dissipation, instead of the weakening of secondary generation of SNA. Compared to that in autumn, the higher AWC concentration, pH value, and lower temperature in winter were the main factors for the higher ρ(NO3-)/ρ(PM2.5) and NOR values in the Changsha-Zhuzhou-Xiangtan City Group. At the same time, the heterogeneous reaction was the main generation pathway of NO3-, when the AWC concentration was high in winter. Affected by aerosol pH value and generation rate, the liquid-phase oxidation reactions of H2O2 and SO2 were the main generation pathways of SO42- in autumn and winter in the Changsha-Zhuzhou-Xiangtan City Group. Compared to that in autumn, the higher AWC was more conducive to forming SO42-, which led to higher SOR in winter.

Temporal Variation and Potential Sources of Water-Soluble Inorganic Ions in PM2.5 in Two Sites of Mexico City

This study presents the characterization and source apportionment of water-soluble inorganic ions (WSII), contained in particulate matter with an aerodynamic diameter equal to or less than 2.5 μm (PM2.5), performed using the positive matrix factorization model (PMF). PM2.5 were collected in Mexico City from two sites: at Merced (MER), which is a residential location with commercial activities, and at Metropolitan Autonomous University (UAM), which is located in an industrial area. The monitoring campaign was carried out across three seasons named Hot Dry (HD) (March–June), Rain (RA) (July–October), and Cold Dry (CD) (November-February). PM2.5 concentration behavior in both sites was similar, following the order: CD > HD > RA. The UAM site exhibited higher concentrations of PM2.5, of the five cations (Na+, Mg2+, Ca2+, K+ and NH4+), and of the four anions (Cl−, SO42−, NO3− and PO43−) than MER, since the UAM site is surrounded by several industrial zones. PM2.5 average concentrations for UAM and MER were 28.4 ± 11.2 and 20.7 ± 8.4 μg m−3, respectively. The ratio of cation equivalent to anion equivalent (CE/AC) showed that aerosol pH is acidic, which was confirmed by direct pH measurements. The sulfur oxidation rate (SOR) was 20 times larger than the nitrogen oxidation rate (NOR). Additionally, SO42− was the most abundant ion during the whole year, especially during the CD season with 5.13 ± 2.5 μg m−3 and 4.9 ± 3.6 μg m−3 for UAM and MER, respectively, when solar radiation displayed a high intensity. On the opposite side, the conversion of NO2 to NO3−, respectively, was low. The air mass backward trajectories were modeled using the National Oceanic and Atmospheric Administration (NOAA-HYSPLIT), which allowed us to know that differences in the mass trajectories during the days with higher concentrations were due to an effect of air recirculation, which favored PM2.5 accumulation and resuspension. On the other hand, on the days with less PM2.5, good air dispersion was observed. The main sources identified with the PMF model were secondary aerosol, vehicular, industrial crustal, and biomass burning for UAM, while for MER they were vehicular, secondary aerosol, and crustal.

Changes in Secondary Inorganic Ions in PM2.5 at Different Pollution Stages Before and After COVID-19 Control

To investigate the change characteristics of secondary inorganic ions in PM2.5 at different pollution stages before and after COVID-19, the online monitoring of winter meteorological and atmospheric pollutant concentrations in Zhengzhou from December 15, 2019 to February 15, 2020 was conducted using a high-resolution (1 h) online instrument. This study analyzed the causes of the haze process of COVID-19, the diurnal variation characteristics of air pollutants, and the distribution characteristics of air pollutants at different stages of haze.The results showed that Zhengzhou was mainly controlled by the high-pressure ridge during the haze process, and the weather situation was stable, which was conducive to the accumulation of air pollutants. SNA was the main component of water-soluble ions, accounting for more than 90%. Home isolation measures during COVID-19 had different impacts on the distribution characteristics of air pollutants in different haze stages. After COVID-19, the concentration of PM2.5 in the clean, occurrence, and dissipation stages increased compared with that before COVID-19 but significantly decreased in the development stage. The home isolation policy significantly reduced the high value of PM2.5. The concentrations of NO2, SO2, NH3, and CO were the highest in the haze development stage, showing a trend of first increasing and then decreasing. The concentration of O3 was lowest in the pre-COVID-19 development stage but highest in the post-COVID-19 development stage. The linear correlation between[NH4+]/[SO42-] and[NO3-]/[SO42-] at different time periods before and after COVID-19 was strong, indicating that the home isolation policy of COVID-19 did not change the generation mode of NO3-, and the corresponding reaction was always the main generation mode of NO3-. The correlation between[excess-NH4+] and[NO3-] was high in different periods before COVID-19, and NO3- generation was related to the increase in NH3 or NH4+ in the process of PM2.5 pollution in Zhengzhou.

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.

Characterization and Formation Mechanism of Water-soluble Inorganic Ions in PM2.5 and PM10 in Summer in the Urban Agglomeration of the Ili River Valley

The simultaneous observation and analysis of atmospheric particles on a regional scale is an important approach to developing control strategies for air pollution. To study the spatial distribution characteristics of particulate matter and water-soluble inorganic ions in the Ili Valley Urban agglomeration, PM2.5 and PM10 samples were synchronously collected from July 19 to July 29, 2021 in Yining City and the surrounding three counties, and then nine types of water-soluble inorganic ions (WSIIs) were analyzed. The spatial distribution characteristics, existence form of WSIIs, and influencing factors were discussed in depth. The results showed that the average ρ(PM2.5) and ρ(PM10) in the Ili River Valley urban agglomeration in summer were 23 μg·m-3 and 59 μg·m-3, respectively. The emission of local industrial and mobile sources in Yining City was higher than that of the surrounding three counties, resulting in the highest ρ(PM2.5) in the region (25 μg·m-3). Due to the influence of dust sources and topography, the ρ(PM10) in Yining county was the highest in the region (63 μg·m-3). Huocheng county is located upwind of the region, and these favorable diffusion conditions resulted in the lowest ρ(PM2.5) and ρ(PM10) (20 μg·m-3 and 49 μg·m-3, respectively). The concentrations of WSIIs in PM2.5 and PM10 ranged from 28.2%-29.9% and 16.0%-20.2%, respectively. The four main ions (SO42-, NO3-, NH4+, and Ca2+) accounted for approximately 90% of WSIIs mass concentrations. The concentration order of the four main ions in PM2.5 was SO42->Ca2+>NH4+>NO3- and SO42->Ca2+>NO3->NH4+ in PM10. The results of correlation analysis showed that the similar SO42- concentrations in the four cities were mainly caused by regional transport. Ca2+ was the highest-concentration ion in PM10 of Yining City and Qapqal Xibe Autonomous county, and the proportion of Ca2+ was significantly higher than that in most cities in China, which reflected that the cities in the core area of the Ili Valley were greatly affected by the dust sources. The ratios of n(NO3-)/n(SO42-) in PM2.5 and PM10 were 0.78 and 0.76, respectively, indicating that the influence of stationary sources was greater than that of mobile sources. The ratio of n(NO3-)/n(SO42-) in Yining City>Huocheng county>Yining county>Qapqal Xibe Autonomous county, which was consistent with the motor vehicle populations of the four cities, reflecting that Yining City was affected by motor vehicle sources more than the surrounding three counties. The secondary components mainly existed in the form of (NH4)2SO4, NH4HSO4, and NH4NO3. There was excess ammonia after the reaction between NH4+ and SO42- in each city. NH4NO3 mainly existed in Yining City, which was mainly related to high NO2 in Yining City. The NOR of the four cities were 0.03-0.10 and 0.03-0.16 in PM2.5 and PM10, respectively, and the secondary transformation of NO3- was weak due to the influence of high temperatures in summer. The SOR were 0.21-0.41 and 0.23-0.44, respectively. The SOR of Qapqal Xibe Autonomous county was the highest due to the relatively high humidity, whereas the SOR of Huocheng county was higher than that of the three sites in Yining City due to the influence of regional transportation. The formation mechanisms showed that SO42- in Qapqal Xibe Autonomous county and Yining City were mainly produced by the heterogeneous reaction, and in Yining county it was mainly formed via the homogeneous reaction. However, the formation mechanism in Huocheng county was complex and was affected by both homogeneous and heterogeneous reactions.

Pollution Characteristics of Water-Soluble Inorganic Ions in PM2.5 from a Mountainous City in Southwest China

In order to explore the characteristics of water-soluble inorganic ions (WSIIs) in the atmosphere of Wanzhou, a small mountainous city in Chongqing, four representative seasonal PM2.5 samples and gaseous precursors (SO2 and NO2) were collected from April 2016 to January 2017. The WSIIs (including Cl−, NO3−, SO42−, Na+, NH4 +, K+, Mg2+, and Ca2+) were analyzed by ion chromatography. During the sampling period, daily PM2.5 concentration varied from 3.47 to 156.30 μg·m−3, with an average value of 33.38 μg·m−3, which was lower than the second-level annual limit of NAAQS-China. WSIIs accounted for 55.6% of PM2.5, and 83.1% of them were secondary inorganic ions (SNA, including SO42−, NO3−, and NH4+). The seasonal variations of PM2.5 and WSIIs were similar, with the minimum in summer and the maximum in winter. PM2.5 samples were the most alkaline in summer, weakly alkaline in spring and winter, and close to neutral in fall. The annual average ratio of NO3−/SO42− was 0.54, indicating predominant stationary sources for SNA in Wanzhou. NO3−, SO42−, and NH4+ mainly existed in the form of (NH4)2SO4 and NH4NO3. The results of the principal component analysis (PCA) showed that the major sources of WSIIs in Wanzhou were the mixture of secondary inorganic aerosols, coal combustion, automobile exhaust (49.53%), dust (23.16%), and agriculture activities (9.68%). The results of the backward trajectory analysis showed that aerosol pollution in Wanzhou was mainly caused by local emissions. The enhanced formation of SNA through homogeneous and heterogeneous reactions contributed to the winter PM2.5 pollution event in Wanzhou.

Characteristics of Water-Soluble Inorganic Ions in PM2.5 in Typical Urban Areas of Beijing, China.

Following the implementation of “coal-to-gas conversion” policy in the Haidian District of Beijing during summer, the present comparative study was performed employing 41 PM2.5 samples as precursors to analyze the characteristics of water-soluble inorganic ions. The concentrations of water-soluble inorganic ions in PM2.5 were analyzed by ion chromatography, and the occurrence form of ions was characterized via time-of-flight secondary ion mass spectrometry (TOF-SIMS). Results revealed that the daily average mass concentration of PM2.5 in Beijing during the sampling period was 94.28 ± 52.49 μg/m3. As compared to the winter of 2016, the average daily PM2.5 concentration in Beijing decreased by 29 μg/m3 in 2017 (28.2% decrease), with a remarkable decline in the number of days with pollution. During the pollution period, the concentrations of NO3–, SO42–, and NH4+ were significantly higher in PM2.5 as compared to the cleaning period. The ratio of the concentrations of [NO3–]/[SO42–] was greater than 1, and the contribution from mobile sources was relatively large, indicating that the implementation of the “coal-to-gas conversion” policy in Beijing has led to the reduction of SO42– emissions from fixed sources, such as coal. Furthermore, TOF-SIMS analysis results showed that NH4+ tended to exist in the form of molecular ammonium sulfate or ammonium hydrogen sulfate during the period of pollution.

Carbonaceous aerosol and inorganic ions of PM2.5 in Yangon and Mandalay of Myanmar: Seasonal and spatial variations in composition and sources

To characterize the seasonal and spatial patterns of the concentration, composition and sources of the atmospheric fine particles in Myanmar, 72 p.m.2.5 samples were collected during 2016 and 2017 at two urban sites in Yangon and Mandalay, representative coastal and inland cities in Myanmar, with the concentrations of organic carbon (OC), elemental carbon (EC), water-soluble organic carbon (WSOC) and the water-soluble ions being measured. Significantly higher average concentration of PM2.5 was found in Mandalay (147 μg m−3) during the sampling periods than that in Yangon (77 μg m−3). High contribution of organic matter (averaged at 48% and 37% by mass in winter and summer) and low contribution of secondary ions (<15% except that in summer Yangon) were found in both Yangon and Mandalay. Higher contribution of mineral dust to PM2.5 was found in Mandalay compared to Yangon. Meanwhile, higher average concentrations of SO42− and NH4+ in Yangon indicated a higher contribution from coal usage in Yangon. Correlation analysis and the compositional change during the rainfall event (2017/4/14–16 in Yangon) indicated that NO3− was mainly associated with Ca2+, and the high temperature and low humidity during the dry season in Myanmar inhibited the formation of NH4NO3. Regional transport of pollutants should be the important cause of the different seasonal variations for the measured species in Yangon and Mandalay besides the seasonal differences in emission sources. Positive matrix factorization (PMF) analysis showed that vehicle emissions, biomass burning, mineral dust, secondary formation and sea salt were the main sources of PM2.5 in Myanmar.

Decade-long trends in chemical component properties of PM2.5 in Beijing, China (2011−2020)

A 10-year-long measurement of water-soluble inorganic ions in PM2.5 was made in Beijing from June 2011 to December 2020, to investigate the interannual trends of chemical characteristics of PM2.5 and to provide insights into the future prevention and control of PM2.5 pollution. From 2011 to 2020, with the implementation of strict air pollution control strategies, significant changes of PM2.5 have been observed in Beijing, with NO3−, SO42− and NH4+ decreasing at rates of 5.10, 8.80 and 7.64% yr−1 respectively. The percentages of NO3− and SO42− under elevated pollution levels were investigated. When PM2.5 values fell in the range of 0–400 μg m−3, NO3−/ SO42− values were mostly higher than 1 and showed upward trends from 2011 to 2020. However, under extremely heavy haze conditions, SO42− dominated PM2.5 formation. This result was closely related to the change characteristics of the oxidation ratio of sulfate (SOR), the oxidation ratio of nitrate (NOR) and gaseous precursors under different pollution levels. The change characteristics of NOR and SOR under elevated PM2.5 levels indicated that the aqueous phase oxidation was the key process driving SO42− formation; while as for NO3−, in addition to the availability of NH4+, the atmospheric oxidation capacity made crucial roles. The analysis of typical haze episodes during the past decade indicated that the emission reduction of gaseous pollutants, especially SO2, made great contributions to the improved PM2.5 air quality in Beijing. We highlighted that future efforts should focus on enhanced reduction of NO2 emission and control of atmospheric oxidation capacity to further reduce particulate nitrate formation.

The seasonal characterization and source analysis of water-soluble inorganic ions in PM2.5 in Fuxin, northeast China

In Fuxin, a few existing studies have illustrated the pollution characteristics of water-soluble ions (WSIIs) in PM2.5 in four seasons. To track the particulate pollution in Fuxin, sample filters of PM2.5 in the four seasons of spring, summer, autumn and winter in 2019 were collected. For the first time, the concentration and seasonal variation characteristics of PM2.5 and WSIIs were tested by gravimeter method and ion chromatography. Characteristics of WSIIs and their existence sources and forms were explored by ion balance calculation, correlation analysis and principal component analysis. The annual average concentrations of PM2.5 and WSIIs were (33.23 ± 13.03) μg·m−3 and (22.86 ± 9.62) μg·m−3, respectively. The seasonal WSIIs concentration followed the order: spring (29.98 μg·m−3) > autumn (28.85 μg·m−3) > winter (21.32 μg·m−3) > summer (11.28 μg·m−3). In WSIIs, 81.41% of them were secondary inorganic ions (SO4 2−, NO3 −, and NH4 +), contributed about 68.80% to PM2.5, and the chemical forms were mainly (NH4)2SO4 and NH4NO3. In this study, it has been revealed that PM2.5 samples exhibited acid properties in spring and autumn while, displayed alkalinity in summer and winter. The average value of NO3 −/SO4 2− was 0.67, so from these data it can be illustrated that fixed pollution sources were found to be dominant over mobile sources in Fuxin. The main identified sources of WSIIs in four seasons in Fuxin were sea salt particles and secondary particles, respectively. These results showed that the geographical factors of Bohai Bay and secondary transformation had a significant impact on the concentration of WSIIs of PM2.5 in Fuxin.

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.

Local emissions and secondary pollutants cause severe PM2.5 elevation in urban air at the south edge of the North China Plain: Results from winter haze of 2017–2018 at a mega city

Severe haze occurrence in the north of the North China Plain (NCP) is recognized as a consequence of the regional transport of pollutants initially from the south and then the rapid formation of secondary pollutants in the local air. However, the origin of pollutants causing haze in the southern NCP has not yet been elucidated even through careful data observation. Based on the contents of water-soluble inorganic ions in PM2.5 samples collected during two severe haze episodes in Zhengzhou, a mega city located on the southern edge of the NCP, we estimated the contributions of local primary emissions and secondary pollutants to haze occurrence. On average, Na+, K+, and Ca2+ mainly originated from anthropogenic sources, and their anthropogenic fractions had proportions of 97.5%, 93.9%, and 76.5% in their respective total mass. Anions Cl− and SO42− substantially originated from not only produced substantially via secondary formation but also from primary emissions, and their primary proportions in their respective total mass were 51.1% and 30.8%. In contrast, NH4+ and NO3− were dominated by secondary formation. The increase in PM2.5 was mainly caused by the formation of secondary inorganic (29.1%) and organic species (57.2%) and the primary anthropogenic emissions (12.5%). These results indicated that the haze at the southern edge of the NCP was mainly caused by pollutants in the local areas. Compared to the haze in the northern NCP, the haze in the southern NCP edge had a higher PM2.5 mass concentration and a higher proportion of secondary species, but a lower proportion of primary species, indicating the high heterogeneity of winter haze over the NCP.

Emission characteristics of water-soluble ions in PM2.5 released by forest fuel combustion in Great Xing'an Mountains, Inner Mongolia, China.

Understanding the emission factors of fine particulate matter (PM2.5) released by forest fuel combustion is important for revealing the impacts of forest fire on atmosphere and ecosystem. Water-soluble ions are important components of fine particulate matter, with great significance to the formation of particulate matter. A self-designed biomass combustion system was used to simulate the combustion of three components (trunks, branches, barks) and their surface dead fuel (litter, semi-humus, humus) of five tree species (Quercus mongolica, Betula platyphylla, Larix gmelinii, Betula dahurica, Populus davidiana) and branches of three shrub species (Corylus heterophylla, Lespedeza bicolor, Rhododendron dauricum) in Great Xing'an Mountains in Inner Mongolia. The water-soluble ion emission factors (Na+, NH4+, K+, Mg2+, Ca2+, F-, Cl-, NO3-, NO2-, SO42-) in PM2.5 under two combustion conditions (smoldering and flaming) were measured by ISC1100 ion chromatograph. The results showed that for the water-soluble ion detected in PM2.5 from combustion of all types of materials, K+, Cl- and Na+ were the main components in smoldering, while K+, Cl- and SO42- were the main components in flaming. There was significant difference in the total amount of water-soluble ions in PM2.5 from the same type of material under different combustion conditions. During the smoldering period, the emission factor of water-soluble inorganic ions in PM2.5 of shrub branches was higher than that of flaming. The cation to anion ratio in PM2.5 was 1.26 for all trees, 1.12 for surface dead fuel of trees, and 2.0 for branch of shrub, indicating that the particulate matter was alkaline. Forest fires in Great Xing'an Mountains could not result in ecosystem acidification by releasing water-soluble ions.

Dry Deposition of PM2.5 Nitrate in a Forest according to Vertical Profile Measurements

The atmospheric nitrogen compounds can serve as a nutrient; however, its excess deposition has harmful effects on terrestrial ecosystems due to acidification and eutrophication. There are still large uncertainties concerning the dry deposition process of PM2.5 nitrate in forests, even though this process affects the accuracy of chemical transport model simulations. To better understand this process, we conducted vertical profile measurements of inorganic ions in PM2.5 and SO2 above and within a forest canopy in the Field Museum Tamakyuryo site in suburban Tokyo with a particular focus on the processes observed under both daytime and nighttime and both leafy and leafless conditions. We performed two observations during leafy periods (July 21–August 1, 2015, and September 27–October 11, 2016) and one observation during a leafless period (February 23–29, 2016). To obtain daytime and nighttime vertical profiles, we set filter holders at 4 or 5 heights on an observation tower in the forest and changed the filters for each daytime and nighttime. For the PM2.5, the vertical gradients of NO3− concentration were larger than those of SO42− during both the daytime and nighttime for all observational periods, particularly during the leafy periods. In addition, the decreasing rate of NO3− in the PM2.5 within the canopy was larger than that of SO2 for some observational periods. In the daytime, the air temperature was higher near the canopy surface during the leafy period and near the ground surface during the leafless period. As also suggested by past studies, the large gradients of NO3− in the PM2.5 during the leafy period were likely caused by the volatilization of NH4NO3 near the deposition surfaces due to the higher temperature in the daytime and the lower concentration of HNO3 caused by its fast removal during both the daytime and nighttime.

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.

Changes in ammonia and its effects on PM2.5 chemical property in three winter seasons in Beijing, China

NH3, SO2, NOx and the inorganic ions of PM2.5 in winter 2009, 2014 and 2016 were examined to investigate the change in NH3 and aerosol chemistry in Beijing, China. NH3 concentrations showed an increase by 59% on average, in contrast to the decrease of SO2 by 63% from winter 2009 to 2016. The mean mass ratio of NH3/NHx was 0.83 ± 0.12 in 2016, which is higher than those obtained in 2009 and 2014, implying more NHx remaining as free NH3 in 2016 winter. Our findings suggest that vehicles exhaust emissions are an important NH3 source in urban central atmosphere of Beijing in winter. Despite the observed NOx presenting declining trends from 2014 to 2016, nitrate concentrations even exhibited a significant increasing trend, which may be largely attributable to high NH3 levels. An in-depth analysis of measured NH3 and aerosol species in a heavy pollution episode in December 2016, combined with the acidity predicted by ISORROPIA II model demonstrated abundant NH3 most of the time in air, where NH3 is not only a precursor for NH4+ but also effect the neutralization of SO42− and NO3− in PM2.5. With high RH and low photochemical activity, elevated NO3− concentration was attributed to an enhanced heterogeneous conversion of NOx to HNO3 to form NH4NO3 in pollution transport stage. The decrease in NOx from high level and the increase in NH3, with peaks of SO42− occurring were observed in pollution cumulative stage. The aqueous-phase oxidation of SO2 by NO2 to sulfate might play an important role with high pH values. Our results suggested that the simultaneous control of NH3 emissions in conjunction with SO2 and NOx emissions would be more effective in reducing particulate matter PM2.5 formation.