Characterization, mixing state, and evolution of urban single particles in Xi'an (China) during wintertime haze days

Influence of ozone pollution on the mixing state and formation of oxygenated organics containing single particles

The formation and aging processes of oxygenated organic molecules (OOMs) are important for understanding the formation mechanisms of secondary organic aerosols (SOAs) in the field. In this study, we investigated the mixing states of OOM particles by identifying several oxygenated species along with the distributions of secondary organic carbon (SOC) during both clean and ozone (O3)-polluted periods in July and September of 2022 in Guangzhou, China. OOM-containing particles accounted for 57 % and 49 % of the total detected single particles in July and September, respectively. Most of the OOM particles were internally mixed with sulfate and nitrate, while elemental carbon and hydrocarbon species were absent. Despite the higher SOC/OC ratio in September (81 %) than it in July (72 %), comparative investigations of the mass spectra, diurnal patterns, and distributions of OOM particles revealed the same composition and aging states of OOMs in two O3 pollution periods. As the O3 concentration increased from the clean to the polluted periods, the ratio of SOC to OC increased along with the relative abundance of secondary OOM particles among total OOM particles. In contrast, the relative abundance of OC-type OOM particles gradually decreased, indicating the conversion of hydrocarbon species into OOMs as the SOC/OC ratio increased. Both the bulk analysis of SOC from filter measurement and the mixing states of OOM particles suggested that OOM production and degree of oxidation were higher in the O3-polluted periods than in the clean periods. These results elucidate the effects of O3 pollution on the OOM formation process and offer new perspectives for the joint investigation of SOA production based on filter sampling and single-particle measurements.

Characteristics of volatile organic compounds and secondary organic aerosol pollution in different functional areas of petrochemical industrial cities in Northwest China

The aim of this study was to better understand the characteristics of volatile organic compounds (VOCs) and secondary organic aerosol (SOA) pollution in different functional areas of petrochemical industrial cities. In Lanzhou, a typical petrochemical industrial city in Northwest China, with the use of an Integrated Atmospheric Mobile Monitoring Vehicle (IAMMV), various real-time online monitoring instruments, including a VOC monitoring instrument (TH-300B) and single-particle aerosol mass spectrometer (SPAMS), were used in combination. These instruments were employed to determine PM_2.5, VOCs and other factors at monitoring sites in Xigu (XG) and Chengguan (CG) districts in September 2020 and 2021, respectively. The results revealed that during the monitoring period, the average VOC concentrations at the XG and CG monitoring sites were 102.3 and 35.8 ppb, respectively. Benzene (45.58 %) and toluene (24.47 %) significantly contributed to the SOA formation potential at the XG site. M/P-xylene (27.88 %) and toluene (23.64 %) more notably contributed to the SOA formation potential at the CG site. The PM_2.5 mass concentration at the XG site (24.1 μg·m^-3) was similar to that at the CG site (21.2 μg·m^-3), but the proportion of particulate matter components greatly differed. The proportion of organic carbon (OC) at the XG site (19.00 %) was higher than that at the CG site (9.97 %). The number of particles containing C₂H₃O⁺ (m/z = 43) accounted for 36.96 % and 15.41 % of the total particles at the XG and CG sites, respectively. The mixing ratios of OC and hybrid carbon (OCEC) with C₂H₃O⁺ (m/z = 43) were 0.81 and 0.53, respectively, at the XG site and reached only 0.48 and 0.25, respectively, at the CG site. The secondary ageing degree of particles in XG district was high. These results could provide a reference for ambient air quality improvement and the formulation of governance measures in different functional areas of petrochemical industrial cities.

Impact of reduced anthropogenic emissions on chemical characteristics of urban aerosol by individual particle analysis

A single particle aerosol mass spectrometer was deployed in a heavily polluted area of China during a coronavirus lockdown to explore the impact of reduced anthropogenic emissions on the chemical composition, size distributions, mixing state, and secondary formation of urban aerosols. Ten particle groups were identified using an adaptive resonance network algorithm. Increased atmospheric oxidation during the lockdown period (LP) resulted in a 42.2%-54% increase in the major NaK-SN particle fraction relative to the normal period (NP). In contrast, EC-aged particles decreased from 31.5% (NP) to 23.7% (LP), possibly due to lower emissions from motor vehicles and coal combustion. The peak particle size diameter increased from 440 nm during the NP to 500 nm during LP due to secondary particle formation. High proportions of mixed ⁶²NO₃^- indicate extensive particle aging. Correlations between secondary organic (⁴³C₂H₃O⁺, oxalate) and secondary inorganic species (⁶²NO₃^-, ⁹⁷HSO₄^- and ¹⁸NH₄⁺) versus oxidants (O_x = O₃ + NO₂) and relative humidity (RH) indicate that increased atmospheric oxidation promoted the generation of secondary species, while the effects of RH were more complex. Differences between the NP and LP show that reductions in primary emissions had a remarkable impact on the aerosol particles. This study provides new insights into the effects of pollution emissions on atmospheric reactions and the specific aerosol types in urban regions.

Interannual evolution of elemental carbon-containing particles in winter in the atmosphere of Chengdu, China

As one of the important components of PM_2.5, elemental carbon (EC) particles could quickly mix with other organic or inorganic species after entering the atmosphere, which directly affected its climate and environmental effects and impact on human health. In order to study the mixture and evolution of EC particles in the atmosphere, this study used single particle aerosol mass spectrometry to measure the EC-containing particles in three winters (2016/17, 2018/19 and 2019/20) in Chengdu, one of the most polluted megacities in China. The results showed that the average mass concentrations of PM_2.5 in the three winters were 121 ± 58 μg m^-3, 90 ± 32 μg m^-3 and 79 ± 38 μg m^-3, respectively, and the mass ratios of PM_2.5/PM₁₀ were 0.71, 0.78 and 0.78, respectively. EC-containing particles accounted for 40.7%-54.7% of all particles and could be divided into six categories: EC-dust (EC-D), fresh EC (EC_f), EC-organic carbon (EC-OC), EC-nitrate (EC-N), EC-sulfate (EC-S) and EC-sulfate-nitrate (EC-SN). From winter 2016/17 to 2019/20, the contributions of EC-N and EC-OC particles increased, while the contributions of EC-S and EC-SN particles decreased. Meanwhile, EC-N particles became increasingly important for the formation of heavy pollution. Photochemical and liquid-phase processes dominated the formation of EC-N and EC-S particles, respectively, while the dominant formation mechanism of EC-SN particles changed from photochemical to liquid-phase processes from winter 2016/17 to 2019/20. Total EC and EC-S particles in Chengdu were mainly transported from the east and southeast areas to Chengdu, while EC_f particles mainly derived from local emissions. Compared with the previous two winters, the potential source areas of the EC-containing particles in winter 2019/20 reduced significantly. These findings provide new insight into the atmospheric behavior of urban particles and are important reference for the study of EC particles in other regions in the world, especially in urban areas.

Inhalable cigarette-burning particles: Size-resolved chemical composition and mixing state

This study analyzed the size-resolved chemical compositions of cigarette-burning particles (CBPs). CBPs in the size range of 0.2-2.0 μm were characterized using a single particle mass spectrometer (SPAMS). CBPs were found to contain polycyclic aromatic hydrocarbons (PAHs), organonitrate, vinylpyridine, indene, guaiacol, methylindane, and metals such as Fe, Cr, Mn, and Cu. Fresh CBPs showed a single modal size distribution which peaked at 0.40 μm. CBPs in the size range of 0.5-0.1 μm contained more biomass burning markers (K⁺, K₂Cl⁺, and levoglucosan), sulfate, naphthalene, and methylindane than CBPs in the size range of 0.2-0.5 μm. Nicotine is favorable to uptake on large particles (>0.5 μm). Among all particles, 57% contained PAHs. Heavy metals Fe, Mn, Cr, and Cu had mixing ratios of 0.06, 0.57, 0.26, and 0.34 respectively; nicotine and guaiacol had mixing ratios of 0.26 and 0.27; and vinylpyridine, indene, and methylindane had mixing ratios of 0.54, 0.55, and 0.65 respectively. Four particle types were resolved: cigarette-burning biomass burning (CB-BB, 50.3%), CB-BB-Metals (49.3%), CB-Nicotine (0.3%), and CB-Aged (0.1%). These results improve the scientific understanding of CBPs and provide useful references for smoking exposure studies that consider the size-resolved chemical compositions and mixing states of particle-phase components. The result can also benefit the study of exposure to secondhand smoking.

Characteristics and potential exposure risks of environmentally persistent free radicals in PM2.5 in the three gorges reservoir area, Southwestern China

Environmentally persistent free radicals (EPFRs) are a novel class of hazardous substances that can exist stably in airborne particles for a period ranging from days to weeks and are potentially toxic to human health. Electron paramagnetic resonance spectroscopy (EPR) was used to characterize particulate EPFRs in Wanzhou in the Three Gorges Reservoir area in 2017. During the whole of 2017, the average concentration of particulate EPFRs was 7.0 × 10¹³ ± 1.7 × 10¹³ spins/m³. The seasonal concentration of EPFRs in PM_2.5 showed a trend of autumn > winter > spring > summer. The maxima and minima of EPFRs occurred in spring with concentrations of 2.1 × 10¹⁴ spins/m³ and 9.4 × 10¹² spins/m³ respectively. The EPFRs in PM_2.5 were mainly carbon-centered radicals with adjacent oxygen atoms. Significant positive correlations were found between EPFRs and SO₄^2-, NO₃^- and NH₄⁺ (r > 0.55, n = 111), indicating that EPFRs are associated with secondary sources. The atmospheric processing of particles from coal combustion, traffic, and agriculture were important sources of EPFRs. They were also particularly well correlated with K⁺ and Cl^- in winter, suggesting that EPFRs may also be derived from wintertime biomass burning emissions. The amount of inhalable EPFRs in Wanzhou was equivalent to the range of 2.3-6.8 cigarettes per capita per day. This study provides evidence of the potential health risks of EPFRs in PM_2.5, and references for air pollution control in the Three Gorges Reservoir area.

The characteristics and mixing states of PM2.5 during a winter dust storm in Ningbo of the Yangtze River Delta, China

Dust particulates play an essential role for the nucleation, hygroscopicity and also contribute to aerosol mass. We investigated the chemical composition, size distribution and mixing states of PM_2.5 using a single-particle aerosol mass spectrometer (SPAMS), Monitor for AeRosols and Gases (MARGA), and off-line membrane sampling from 2018.1.24 to 2018.2.20 at a coastal supersite in Ningbo, a port city in Yangtze River Delta, China. During the study campaign, the eastern part of China had experienced a wide range of cooling, sandstorm, and snowfall processes. The entire sampling campaign was categorized into five sub-periods based on the levels of PM_2.5 and the ratios of PM_2.5/PM₁₀, namely clean (T1), heavy pollution (T2), light pollution (T3), dust (sandstorm) (T4) and cleaning pollution (T5) period. After comparing the average mass spectrum for each period, it shows that the primary ions, such as Ca²⁺and SiO₃^-, rarely coexist with each other within a single particle, but secondary ions generally coexist with these primary ions. Furthermore, the coexistence of each two different ions within a particle does not show distinct variation for the whole study periods. All these suggest that the absorption and partitioning of gaseous contaminants into the surface of primary aerosol through heterogeneous reactions are the major pathways of aging and growth of aerosol; and the merging of particles through collisions usually is insignificant. Although the absolute concentrations of nitrate and sulfate all increased with the PM_2.5 concentrations, the relative equivalent concentrations of NO₃^- and SO₄^2- displayed opposite trends; the relative contribution of sulfate decreased and that of nitrate increased as the increase of pollution. During the dust period, the relative equivalent concentrations of calcium and/or potassium ions in PM_2.5 are significantly higher. This study provided deep insights about the mixing states and characteristics of particulate after long-range transport and a visualization tool for aerosol study.

Characteristics, evolution, and regional differences of biomass burning particles in the Sichuan Basin, China

The Sichuan Basin has experienced serious air pollution from fine particulate matter (PM_2.5) in the past few years with biomass burning has been identified as a major source of PM_2.5 in this region. We used single particle aerosol mass spectrometer to investigate the characteristics of biomass burning particles in three interacting cities representing different types of urban environment in the Sichuan Basin. A total of 739,794, 279,610, and 380,636 biomass burning particles were detected at Ya'an, Guang'an, and Chengdu, which represented 42%, 69%, and 61%, respectively, of the total number of particles. We analyzed the chemical composition, transportation, and evolution of biomass burning particles. The contribution of K-elemental carbon and K-secondary inorganic particles was highest in Ya'an (36%) and Guang'an (47%), respectively, reflecting the important role of fresh biomass burning particles and long-distance transport in these two cities. Air masses originating from different directions corresponded to different levels of PM_2.5 and the contributions of polluted clusters increased significantly on polluted days. Fresh and secondary inorganic biomass burning particles increased pollution at Ya'an and Guang'an, respectively, but dominated different stages of pollution in Chengdu. K-nitrate particles were formed by photochemical reactions, whereas K-sulfate particles were formed by both photochemical and liquid-phase reactions. Investigation of the degree of particle aging showed that there were more fresh particles at Ya'an and more aged particles at Guang'an. These results are useful in helping our understanding of the characteristics of biomass burning particles and evaluating their role in PM_2.5 pollution in the Sichuan Basin.

Characterization of aerosol particles during the most polluted season (winter) in urban Chengdu (China) by single-particle analysis

Chengdu, the capital city of Sichuan Province, is one of the most polluted cities in China. We used single-particle aerosol mass spectrometer to monitor particulate matter pollution in an urban area of Chengdu from December 9, 2015 to January 4, 2016 to determine the characteristics of air pollution during the winter months. The mass concentrations of particulate matter were high during the whole observation period, with mean values for PM_2.5 and PM₁₀ of 101 ± 60 and 162 ± 99 μg m^-3, respectively. The particles were clustered into nine distinct particle types: dust (3%), potassium-elemental carbon (KEC) (24%), organic carbon (OC) (12%), combined OC and EC (OCEC) (6%), K-organic nitrogen (KCN) (10%), K-nitrate (KNO₃) (12%), K-sulfate (KSO₄) (18%), K-sulfate and nitrate (KSN) (12%), and metal (3%) particles. Analysis on different types of day showed that: (1) from "excellent" (days with PM_2.5 lower than 35 μg m^-3) to "light pollution" (PM_2.5 between 75 and 115 μg m^-3) days, local/regional combustion was the major contributor, whereas the aggravation of pollution from light pollution to "heavy pollution" (PM_2.5 higher than 150 μg m^-3) days was mainly determined by the combined effect of local/regional combustion and long-distance transport; (2) as the air quality deteriorated, the mixing of sulfate and nitrate in particles increased sharply, especially sulfate; and (3) the relative aerosols acidity increased from excellent to light pollution days, while decreased significantly from light pollution to heavy pollution days. Backward trajectory analysis showed that there were significant differences in PM_2.5 concentrations and particle compositions between clusters of trajectories, which affected the level and evolution of PM_2.5 pollution in Chengdu. These results give a deeper understanding of PM_2.5 pollution in Chengdu and the Sichuan Basin.

Characteristics of single atmospheric particles in a heavily polluted urban area of China: size distributions and mixing states

To investigate the chemical composition, size distribution, and mixing state of aerosol particles on heavy pollution days, single-particle aerosol mass spectrometry was conducted during 9-26 October 2015 in Xi'an, China. The measured particles were classified into six major categories: biomass burning (BB) particles, K-secondary particles, elemental carbon (EC)-related particles, metal-containing particles, dust, and organic carbon (OC) particles. BB and EC-related particles were the dominant types during the study period and mainly originated from biomass burning, vehicle emissions, and coal combustion. According to the ambient air quality index, two typical episodes were defined: clean days (CDs) and polluted days (PDs). Accumulation of BB particles and EC-related particles was the main reason for the pollution in Xi'an. Most types of particle size were larger on PDs than CDs. Each particle type was mixed with secondary species to different degrees on CDs and PDs, indicating that atmospheric aging occurred. The mixing state results demonstrated that the primary tracers were oxidized or vanished and that the amount of secondary species was increased on PDs. This study provides valuable information and a dataset to help control air pollution in the urban areas of Xi'an. Graphical abstract.

Composition, mixing state, and size distribution of single submicron particles during pollution episodes in a coastal city in southeast China

Size-resolved particle composition, size distribution, and mixing state were characterized at the single-particle level during two air pollution episodes during 12-25 January, 2016 in a coastal city in southeast China. The two pollution episodes occurred under distinct meteorological conditions (i.e., different wind speeds, relative humidity, and backward trajectories); thus, they were assigned to stagnation and transport episodes, respectively. Single-particle data, obtained from single-particle aerosol mass spectrometry (SPAMS), showed that carbonaceous particles were the predominant particles during the whole study period, accounting for more than 60% of the total particles. However, the number fractions of carbonaceous particles and nitrate-containing particles significantly increased in the stagnation episode, while the number fractions of sulfate- and ammonium-containing particles both increased in the transport episode compared to the levels over the whole study period. The potassium-rich (K-rich) particle class was more abundant and more strongly mixed with sulfate in the transport episode, which indicates the impact of biomass burning emissions and the subsequent aging process by acquiring sulfate during transport. The particle classes (e.g., carbonaceous and K-rich classes) had a broader size distribution during the pollution episodes than during the clean episode. The diameters of the size distribution peak for all particle classes (except for dust class) were observed to be larger in the transport episode than in the stagnation episode. This suggests that the particles underwent an extensive aging process through the addition of sulfate and ammonium during transport, leading to the growth of particles.

Single particle characterization of summertime particles in Xi'an (China)

Urban particles in Xi'an during summertime were investigated using a single particle aerosol mass spectrometer (SPAMS). Twelve major particle types were resolved, including EC-Sul-Nit (-Sul stands for sulfate, -Nit for nitrate, and 25% in number fraction), EC(6%), EC-Nit (12%) and, EC-Sul (8%), mixed Elemental and Organic Carbon-Sul-Nit (9%), ECOC-Sul (8%), K-Nit (12%), OC (8%), NaK-Nit (5%), Fe-Nit (5%), Ca-Nit (1%), and Other (1%). Among these particle types, chemical composition, mixing state, and wind-dependent analyses were conducted to investigate their originations and sources. During summertime, traffic-related particles were up to 83% in the SPAMS dataset. Two major originations of urban particles were identified, including the local aging and short-distance transport mainly from the southeast. Size-resolved relative acidity ((sulfate + nitrate) / ammonium) analysis suggested that urban particles were more acidic with an aerodynamic diameter < 0.8 μm. In diurnal cycle, the strongest relative aerosol acidity occurred between 7:00 and 9:00 in the morning when relative humidity was between 60 and 70%, and the weakest acidity occurred from 13:00-15:00. Among all major particle types, OC and K-Nit had stronger relative aerosol acidity than other types. Mixing state analysis indicated that the organic semi-volatile vapor is favorable to condense on the OC-related particles (OC, ECOC-Sul, and ECOC-Sul-Nit) as evidence that primary organic aerosol (POA) would enhance the secondary organic aerosol (SOA) formation via gas-to-particle phase partitioning when SOA and POA are miscible. Oxalate also tends to be observed in the droplet of OC-related particles. In addition, the enrichment of oxalate was observed in Fe-Nit particles. This study would be useful to understand the characterization, mixing state, source, origination, and processing of urban particles during summertime in Xi'an as well as the urban areas in the Guanzhong Basin.

Chemical nature of PM2.5 and PM10 in Xi'an, China: Insights into primary emissions and secondary particle formation

In Xi'an, a city that frequently experiences serious PM pollution in northern China, 1476 PM₁₀ and 1464 PM_2.5 valid daily filter samples were collected at six sites from December 2014 to November 2015 and analyzed for 29 species. The annual mean PM₁₀ and PM_2.5 concentrations were 149.4 ± 93.1, 108.0 ± 70.9 μg/m³, respectively. Organic carbon (OC) is the predominant PM_2.5 component while crustal material predominated in PM₁₀. Sulfate concentrations, which was the largest component in Xi'an PM in previous studies, were lower than nitrate. Winter sulfate, OC, and elemental carbon (EC) have decreased since 2003, while nitrate remained constant in recent years and the ratio of NO₃^-/SO₄^2- increased from 0.4 in 2006 to 1.3 in 2014. This result suggests that the motor vehicle contribution to PM has increased relative to coal-fired power plant emissions over the past decade. The mass fractions of crustal material, sulfate, and EC in PM_2.5 decreased as the PM_2.5 concentrations increased from "clean" days (<50 μg/m³) to the highest values, while nitrate significantly increased. Despite forming through secondary reactions, the high concentrations of SOC and SO₄^2- in winter are attributed to primary emissions and particularly to residential heating and cooking with coal. Primary SOC and SO₄^2- accounted for 33% and 42% of their total PM_2.5 concentrations in winter, respectively. Therefore, control measures applied to these primary sources can substantially improve air quality.

Source characterization of urban particles from meat smoking activities in Chongqing, China using single particle aerosol mass spectrometry

A Single Particle Aerosol Mass Spectrometer (SPAMS) was deployed in the urban area of Chongqing to characterize the particles present during a severe particulate pollution event that occurred in winter 2014-2015. The measurements were made at a time when residents engaged in traditional outdoor meat smoking activities to preserve meat before the Chinese Spring Festival. The measurement period was predominantly characterized by stagnant weather conditions, highly elevated levels of PM_2.5, and low visibility. Eleven major single particle types were identified, with over 92.5% of the particles attributed to biomass burning emissions. Most of the particle types showed appreciable signs of aging in the stagnant air conditions. To simulate the meat smoking activities, a series of controlled smoldering experiments was conducted using freshly cut pine and cypress branches, both with and without wood logs. SPAMS data obtained from these experiments revealed a number of biomass burning particle types, including an elemental and organic carbon (ECOC) type that proved to be the most suitable marker for meat smoking activities. The traditional activity of making preserved meat in southwestern China is shown here to be a major source of particulate pollution. Improved measures to reduce emissions from the smoking of meat should be introduced to improve air quality in regions where smoking meat activity prevails.