Secondary SO42 Research Articles

Using sulfur isotopes to constrain the sources of sulfate in PM2.5 during the winter in Jiaozuo City

Sulfate (SO42−) is one of the main driving forces behind the explosive growth of atmospheric fine particulate matter (PM2.5), and it scatters sunlight, contributing to atmospheric cooling. Investigating the sources and transformation of SO42− is crucial for understanding PM2.5 pollution and its implications for climate change. Despite a continuous decline in PM2.5 concentrations in China since 2015, it remains unclear whether there have been changes in the sources and transformation processes of SO42−. Furthermore, previous studies that utilized isotope techniques for source identification have often overlooked the impact of isotope fractionation. This study focuses on atmospheric PM2.5 in Jiaozuo based on samples collected from December 2017 to February 2018. The analysis encompassed both the water-soluble ions and sulfur isotope compositions of SO42− (δ34S–SO42−). By employing a Bayesian isotope mixing model, quantitative analysis of SO42− sources in winter atmospheric PM2.5 in the study area was conducted. The findings revealed that the δ34S–SO42− values of winter PM2.5 ranged from −0.1‰ to 9.8‰, with an arithmetic mean of 4.9 ± 1.5‰ (n = 74). After considering sulfur isotope fractionation effects and the impact of primary SO42−, the calculated results suggested that on average, the contributions of primary SO42−, coal combustion, vehicle exhaust, biomass burning, and oil combustion to the SO42− in winter PM2.5 were 24.9 ± 15.7%, 41.2 ± 2.6%, 27.9 ± 5.3%, 4.8 ± 3.7%, and 1.2 ± 0.7%, respectively. Compared with the clean period, the contribution of the primary SO42− decreased significantly during haze episodes, while the contribution of the secondary SO42− increased. The contribution of coal combustion to the secondary SO42− increased notably by 21.9%, while the contributions from other secondary sulfate remained relatively stable. The lower δ34S values of the atmospheric PM2.5 in Jiaozuo affirm the significant impact of the anthropogenic SO2 input on the sulfur isotope composition of the atmospheric sulfate. Analysis confirmed that the decrease in the contribution of coal combustion to atmospheric SO42− in the study area, indicating the positive effects of regional air pollution control measures. Nevertheless, continued attention should be paid to the influences of coal combustion and vehicle exhaust on regional air pollution.

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.

Exploring the Influence of 34S Fractionation From Emission Sources and SO2 Atmospheric Oxidation on Sulfate Source Apportionment Based on Hourly Resolution δ34S‐SO2/SO42−

AbstractSulfate (SO42−) sources are unclear leading to the underestimation of its concentration in the model. Hourly resolution δ34S‐SO2 and δ34S‐SO42− values of three haze episodes (EP1‐EP3) were synchronously collected for the first time to quantify the influence of 34S fractionation from emission sources (e.g., coal combustion) and SO2 atmospheric oxidation on SO2/SO42− source apportionment. After considering the 34S fractionation from coal combustion and atmospheric oxidation, the reasonable and logical source contributions of SO42− were obtained, showing highly consistent with that of Positive Matrix Factorization model results. Considering the 34S fraction from atmospheric oxidation, the source apportionment of SO2/SO42− obtained by hourly resolution δ34S‐SO2 and δ34S‐SO42− can more accurately reflect the dynamic changes of emission sources. Traffic emissions (49%) and coal combustion (46%–65%) were the major contributors to SO2/SO42− in EP1 and EP2‐EP3, respectively. However, obvious deviations of coal combustion contribution were found without considering 34S fractionation from coal combustion. Especially for the northwest transmission channels in EP2‐EP3, the deviation values accounted for 17.1%–38.5% of secondary SO42−. Moreover, δ34S was considered as a more sensitive source indicator than SO2 concentration by the results comparison of 34S technique and air quality model (Nested Air Quality Prediction Model System), which can provide more reliable evidence for SO2 emission control.

Atmospheric PM2.5 near an Urban-Industrial Complex during Air-pollution Episodes with Various Meteorological Conditions

This study investigates the atmospheric fine particulate matter (PM2.5) issue caused by the multi-effect of complicated sources, terrain, and meteorology at Southern Taiwan. Three sampling stations represent an urban, a rural, and a coastal sites near an urban-industrial complex. The atmospheric PM2.5 were measured during a pollution episode from November to February, when the reference samples were collected in April. The sample was collected with a constant-flow sampler with the Federal Reference Method performance. After determining the PM2.5 mass, their chemical compositions of ions, metals, and carbons were analyzed for the different properties caused by the multi-factors. The chemical mass balance (CMB) model was employed to evaluate the emission contributions. Additionally, an inverse trajectory model is used to analyze the pollutant transport and support the CMB results. The air-pollution episodes occurred within the winter to spring. The PM2.5 were composed of 51-69% ions, 18-31% carbonaceous species, and 1.5-3.0% metals. The SO42-, NH4+ and NO3- contributed 92-96% of the ions. Most of the organic/elemental carbon ratios were low, suggesting more primary carbon emissions. The metal contents were minor and dominated by Fe and Zn. The CMB model indicated the PM2.5 were dominated by 24% secondary SO42-, 14.7% traffics, 8.3% petrochemical emissions, 6.8% soil dust, and 4.5% sintering plant emission. For non-episode days, the PM2.5 were contributed by 34.9% traffics, 30% the secondary SO42-, 10.3% secondary NO3- , and 6.8% soil dust. Nevertheless, the frequent sea-land breeze might lead to more powerful wind eddies and bring the primary PM2.5 and the aerosol precursors from the emission areas. Consequently, the uncontrollable meteorological changings would lead to the pollution issues at the lowly convective area. Therefore, the averaging emissions of the PM2.5 and precursors should be lowered; meanwhile, the rapid controls of the primary emissions are suggested when the high-level PM2.5 are forecasted.

Characterization and source analysis of water–soluble ions in PM2.5 at a background site in Central China

Little updated observation of background aerosols in Central China is available since the clean air actions, despite increasing aerosols pollution in this region. PM2.5 was sampled at Dongting Lake during September 2017 to August 2018 to probe the chemical characteristics of background PM2.5 in Central China. We sampled aerosols at an urban site simultaneously. Annual mean PM2.5 level reached 43.4 μg m−3, and SO42−, NO3−, and NH4+ (sulfate–nitrate–ammonium, simplified as SNA) accounted for 22.8%, 26.8%, and 15.0%, respectively. PM2.5 and most ions levels were highest during winter and lowest during summer. PM2.5 mass was driven by NO3− during cold months (October–March) or SO42− during warm months (September, April–August), suggesting that different emission control measures should be applied in different seasons in future air quality policy of this region. Both SO42− and NO3− were almost completely neutralized by NH4+. Low temperature increased the SO42− concentration and sulfur oxidation ratio. Low temperature also facilitated the partition of NO3− from gas phase into particle phase. Severe particulate pollutions occurred in cold months when the diffusion and wet scavenging of aerosols and volatilization of NH4NO3 were weak, and the NO2 concentration was high. An interesting finding was the highest NO3− concentration and conversion ratios for SNA as well as lowest SO42− concentration in Dongting Lake among major background sites, despite low levels of local gaseous precursors. Industry, secondary SO42−, and NO3− were indentified as main sources of PM2.5 mass based on PMF analysis. Concurrent occurrences of PM2.5 and SNA pollution at Dongting Lake and Wuhan sites as well as back trajectories verified the regional effect. The results will be helpful for investigating water–soluble ions and the major sources as well as geographical origins for PM2.5 at Central China.

Comparison of PM2.5 pollution between an African city and an Asian metropolis

Fine particulate matter (PM2.5) samples were collected simultaneously at urban sites in Lagos (ULG site) and in Hong Kong (TC site) for four consecutive weeks in July and August 2017, in order to investigate the potential to apply successful pollution control measures from Hong Kong to Lagos. To compare chemical characteristics and sources of PM2.5 in these two regions, organic carbon (OC), elemental carbon (EC), water soluble ions (WSIs), and elements were analyzed for the first time. It was found that total carbon and elements were much more abundant (p < 0.05) at ULG, indicating severer PM2.5 pollution in Lagos, while levels of WSIs were comparable (p ≥ 0.05) at both sites. Higher correlation coefficient (0.79) between OC and EC but lower OC/EC ratio (1.81 ± 0.18) at ULG (TC: 0.48; 3.51 ± 0.60) revealed the dominant role of primary sources in Lagos. Furthermore, examination of secondary organic carbon (SOC)/OC ratio implied that only 12 ± 8% of OC were attributable to secondary organic formation at ULG whereas 47 ± 9% at TC. Positive matrix factorization (PMF) model resolved six PM2.5 sources at each site, among which vehicular emissions contributed the most (32.2 ± 3.18%) at ULG, while secondary inorganic aerosols (including secondary SO42− and NO3−) together with regional biomass burning (36.5 ± 5.21%) dominated at TC. Sea salt source was significant at both harbor cities. For inter-comparison, the sum of vehicular emission and fugitive dust accounted for ~40.9% at ULG, triple that at TC (p < 0.01) in concentration. Severer primary PM2.5 pollution especially the street-level pollution in Lagos called for effective control measures, such as periodical upgrade of fuel and retrofits on vehicles, which have been successfully promoted in Hong Kong.

A monitoring-modeling approach to SO42 − and NO3− secondary conversion ratio estimation during haze periods in Beijing, China

SO42− and NO3− are important chemical components of fine particulate matter (PM2.5), especially during haze periods. This study selected two haze episodes in Beijing, China with similar meteorological conditions. A monitoring-modeling approach was developed to estimate the secondary conversion ratios of sulfur and nitrogen based on monitored and simulated concentrations. Measurements showed that in Episode 1 (24th–25th October, 2014), the concentrations (proportions) of SO42− and NO3− reached 35.1 μg/m3 (14.9%) and 55.0 μg/m3 (22.9%), while they reached 14.4 μg/m3 (9.3%) and 59.1 μg/m3 (38.1%) in Episode 2 (26th–27th October, 2017). A modeling system was applied to apportion Beijing's SO42− and NO3− in primary and secondary SO42−/NO3− emitted from local and regional sources. Thus, secondary conversion contributions considering the local and regional level were defined. The former primarily focused on Beijing atmospheric oxidation ability and the latter mainly considered the existence form of Beijing SO42−/NO3− under the regional transport impacts. Finally, secondary oxidation ratios were estimated through combining secondary conversion contribution coefficients for simulated and monitored concentrations. At regional level, sulfur oxidation ratios in polluted (clean) days during two sampling periods were 0.57–0.72 (0.07–0.52) and 0.74–0.80 (0.08–0.61), nitrogen oxidation ratios were 0.20–0.29 (0.05–0.15) and 0.34–0.38 (0.02–0.29), indicating that atmospheric oxidation was enhanced when considering regional transport through 2014–2017. At the local level, sulfur oxidation ratios were 0.66–0.71 (0.04–0.48) in haze (clean) days, while nitrogen oxidation ratios were 0.16–0.29 (0.02–0.16). The atmospheric oxidation ability markedly increased in PM2.5 pollution days, but changed only slightly between the two periods.

Aerosols in an arid environment: The role of aerosol water content, particulate acidity, precursors, and relative humidity on secondary inorganic aerosols

Meteorological conditions, gas-phase precursors, and aerosol acidity (pH) can influence the formation of secondary inorganic aerosols (SIA) in fine particulate matter (PM2.5). Most works related to the influence of pH and gas-phase precursors on SIA have been laboratory research, but field observation research is very scarce, especially in arid environments. The relationship among SIA, pH, gas-phase precursors, and meteorological conditions are investigated in Hohhot, a major city in China with an arid environment. Secondary inorganic species, e.g., SO42−, NO3−, were typically found at low levels, reflecting the low level of secondary aerosol. It is interesting to note that the level of SO2 in Hohhot was higher than in other cities while SO42− was relatively lower than in other cities. Multiple receptor models were used to explore the contributions to the SIA and quantify the source impacts on the SIA. Annual average aerosol pH in Hohhot was 5.6 (range 1.1–8.4) which was estimated by a thermodynamic equilibrium model. Additionally, a statistical method was used to evaluate the influence of SIA sources on ambient aerosol concentrations. Aerosol water content and particulate acidity were found to be positively associated with secondary SO42−, while NO2 and RH had a significant impact on secondary NO3− in an arid atmosphere. The findings explain the relationship between gaseous precursors, relative humidity, aerosol pH and temperature in the arid city of Hohhot.

Chemical characterization and source apportionment of size-resolved particles in Hong Kong sub-urban area

Size-resolved particulate matter (PM) samples were collected with a 10-stage Micro-Orifice Uniform Deposit Impactor (MOUDI) at a sub-urban site (Tung Chung) in Hong Kong for four non-consecutive months representing four seasons from 2011 to 2012. Major chemical components were water-soluble anions (i.e., Cl−, NO3−, and SO42−), cations (i.e., NH4+, Na+, K+, and Ca2+), organic and elemental carbon and elements. Both chemical mass closure and positive matrix factorization (PMF) were employed to understand the chemical composition, resolve particle size modes, and evaluate the PM sources. Tri-modal size distributions were found for PM mass and major chemical components (e.g., SO42−, NH4+, and OC). Mass median aerodynamic diameters (MMADs) with similar standard deviations (1.32<σ<1.42) were 0.4, 0.7 and 3.8μm, consistent with condensation, droplet and coarse modes. A bi-modal distribution peaking at condensation and droplet modes was found for EC, with a single mode peaking at 3.8μm for Cl−. Besides secondary SO42−, carbonaceous aerosol dominated the condensation mode with 27% by engine exhaust and 18–19% each by residual oil combustion (shipping) and coal/biomass burning. Secondary SO42− is also the most dominant component in the droplet mode, accounting for 23% of PM mass, followed by an industrial source (19%). Engine exhaust, secondary NO3−, and sea salt each accounted for 13–15% of PM mass. Sea salt and soil are the dominated sources in the coarse mode, accounting for ~80% of coarse mass.

Characteristics of water-soluble inorganic species in PM10 and PM2.5 at two coastal sites during spring in Korea

PM10 and PM2.5 were collected during spring at East (“Gangneung”) and West (“Taean”) coastal sites of the Korean peninsula to investigate chemical characteristics and likely formation routes of their water-soluble inorganic species. The Gangneung site is inland, about 4.5–5.0 km from the East Sea; the Taean site is about 200 m from the coastline and about 400 km from eastern China. The total water-soluble ionic species contributions to PM10 and PM2.5 were respectively 28.8 and 37.4% at the Gangneung site and 46.8 and 53.1% at the Taean site. Concentrations of SO42−, NH4+, and NO3− in both PM10 and PM2.5 were found to be two times higher at the Taean site than at the Gangneung site. The potential source contribution function (PSCF) maps indicate that the enhancement of PM10 and its secondary aerosol species concentrations at two sites were primarily the result of atmospheric processing during long-range transport from the polluted regions of eastern China. Also upwind sources (“the capital region of Korea”) and local emissions influenced the concentration levels of secondary inorganic species at Gangneung.SO42− events, defined by PM10SO42− concentrations exceeding the average by one standard deviation, were identified at the two sites – six at Gangneung and four at Taean – to investigate possible mechanisms for the formation of SO42− and NO3−. High SO42− and high NO3− at the Gangneung site were strongly associated with either high RH (89–94%) and low wind speed or high O3 (62–103 ppb), suggesting that either gas-phase oxidation or aqueous phase oxidation played a critical role in the enhanced SO42− production. On the other hand, at the Taean site the association was with both high RH (76–92%) and high O3 (53–79 ppb), indicating that these conditions trigger aqueous-phase and gas-phase reactions to produce secondary SO42− and NO3− particles. Also long-range transport of air masses could be one possible factor for enhanced SO42− and NO3− concentrations during the events at the two coastal sites, as evidenced by PSCF maps.

Source identification of water-soluble organic aerosols at a roadway site using a positive matrix factorization analysis

Daily PM2.5 measurements were carried out at a local roadway every sixth day from May 2011 to August 2013 to obtain seasonal quantitative information on the primary and secondary sources of two water-soluble organic carbon (WSOC) fractions. Filter samples were analyzed for OC, elemental carbon (EC), WSOC, hydrophilic and hydrophobic WSOC fractions (WSOCHPI and WSOCHPO), and ionic species. An XAD solid phase extraction method and a total organic carbon analyzer were used to isolate the two WSOC fractions and determine their amounts, respectively.The WSOC/OC and WSOCHPI/WSOC ratios were 0.62±0.13 and 0.47±0.14, respectively. Similar seasonal profiles in EC, OC, and WSOC concentrations were observed, with higher concentrations occurring in the cold season and lower concentrations in the warm season. However, opposite results were obtained in WSOC/OC and WSOCHPI/WSOC ratios, with the higher in the warm season and the lower in the cold season. Correlation analyses indicated that two WSOC fractions in winter were likely attributed to secondary formation processes, biomass burning (BB), and traffic emissions, while WSOCHPI observed in other seasons were associated with secondary formation processes similar to those of oxalate and secondary inorganic species.A positive matrix factorization (PMF) model was employed to investigate the sources of two WSOC fractions. PMF indicated that concentrations of WSOC fractions were affected by five sources: secondary NO3− related, secondary SO42− and oxalate related, traffic emissions, BB emissions, and sea-salt. Throughout the study period, secondary organic aerosols were estimated to be the most dominant contributor of WSOC fractions, with higher contributions occurring in the warm seasons. The contribution of secondary aerosol formation processes (NO3− related+SO42− and oxalate related) to WSOCHPI and WSOCHPO was on an average 56.2% (45.0–73.8%) and 47.7% (39.6–52.1%), respectively. The seasonal average contribution of WSOCHPI and WSOCHPO attributed to BB was 19.0% (14.3–25.3%) and 14.8% (7.2–19.5%), respectively, with higher fractions occurring in the fall and winter. Traffic sources contributed to WSOCHPI and WSOCHPO from 4.2 to 21.0% (an average of 11.6%) and from 7.9 to 32.3% (an average of 19.9%), respectively, with higher fractions in the fall and winter compared with the other seasons. During the study period, for an episode associated with high local O3 level (~110ppbv) and high WSOCHPI/WSOC (0.80), secondary formation processes contributed 67.1% to WSOCHPI, and 72.6% to WSOCHPO, respectively. However, for an episode associated with local and severe regional haze pollutions, contributions of secondary formation processes to WSOC fractions were observed to be low (32.4–43.1%), while traffic and BB emissions contributed 16.8% and 24.3% to WSOCHPI, respectively, and 18.3% and 18.7% to WSOCHPO, respectively. The PMF results suggest that the contribution of traffic emissions to concentrations of two WSOC fractions cannot be neglected at the studied roadway site.

Mixing of Asian dust with pollution aerosol and the transformation of aerosol components during the dust storm over China in spring 2007

An intensive spring aerosol sampling campaign over northwestern and northern China and a megacity in eastern China was conducted in the spring of 2007 to investigate the mixing of Asian dust with pollution aerosol during its long‐range transport. On the basis of the results of the three sites near dust source regions (Tazhong, Yulin, and Duolun) and a metropolitan city (Shanghai), three dust sources, i.e., the western high‐Ca dust in the Taklimakan Desert, the northwestern high‐Ca dust and the northeastern low‐Ca dust in Mongolia Gobi, were identified on the basis of the air mass trajectories and the elemental tracer analysis (e.g., Ca/Al, SO42−/S, Ca2+/Ca, and Na+/Na). The western dust was least polluted in comparison to the other two dust sources. The results evidently indicated that the dust could have already mixed with pollution aerosol even in near dust source regions. The concentrations of As, Cd, Cu, Pb, Zn, and S were elevated several times at all sites during dust days, showing the entrainment of pollution elements by dust. The secondary SO42− was observed to show much higher concentration due to the heterogeneous reaction on the alkaline dust during dust storm, while the concentrations of NO3− and NH4+ decreased owing to the dilution of the local pollution by the invaded dust. The western dust contained relatively low anthropogenic aerosols, and it mainly derived from the Taklimakan Desert, a paleomarine source. The northwestern dust had a considerable chemical reactivity and mixing with sulfur precursors emitted from the coal mines on the pathway of the long‐range transport of dust. The northeastern dust reached Shanghai with high acidity, and it became the mixed aerosol with the interaction among dust, local pollutants, and sea salts. Comparison of the speciation of the water‐soluble ions on both nondust and dust days at all sites illustrated the evolution of major ion species from different dust sources during the long‐range transport of dust. The mixing mechanisms of the dust with the pollution aerosol on the local, medium‐range, and long‐range scale revealed from this study would improve the understanding of the impacts of Asian dust on the regional/global climate change.

Source Characterization and Apportionment of PM10 in Panzhihua, China

ABSTRACTA total of 258 particulate matter (PM10) filter samples and 69 source samples applicable to receptor model source apportionment were collected and chemically analyzed from February to August 2007 in Panzhihua, China. Contributive sources were identified and the chemical profiles were reported for resuspended dust, paved and unpaved road dust, coal-fired power plant exhaust, emissions from coking plants and other industrial sources in Panzhihua. All samples were analyzed for 19 elements (Na-Pb), two ions (NO3– and SO42–) and organic and total carbon. Elevated abundances of geological components (Al, Si, Ca and Fe) from fugitive dust materials and elements (Ti, Cr, Mn, Cu and Zn) from special industry plants were found in the profiles. The contributions to the ambient PM10 levels at six sites in three seasons (spring, summer and winter) were estimated using a chemical mass balance receptor model. The concentration of PM10 was high (150 μg/m3) on winter days and low in summer and spring (133 and 129 μg/m3, respectively). Apportionment results indicate that coal combustion ash, iron and steel industry dust, vehicle exhaust and secondary SO42– were major contributors, accounting for about 70% of PM10. More attention should be paid to particulate matter emitted by iron and steel manufacturing facilities in view of high contribution and potentially toxic metals.