Transmission electron microscopy was used to analyze individual aerosol particles collected in Lanzhou (urban site) and its surrounding areas (rural site) in early 2023. The results revealed that from the pre-Spring Festival period to the Spring Festival period, the main pollutants at the urban site decreased significantly, while the PM2.5 and SO2 concentrations increased at the rural site. During the entire sampling period, the main particles at the urban site were organic matter (OM), secondary inorganic aerosols (SIA), and OM-SIA particles, while those at the rural site were OM, SIA, and soot particles. The degree of external mixing of single particles in both sites increased from the pre-Spring Festival period to the Spring Festival period. The proportion of the OM particles increased by 11% at the urban site, and the proportion of SIA particles increased by 24% at the rural site. During the Spring Festival, the aging of the soot particles was enhanced at the urban site and weakened at the rural site. At the urban site, the SIA particle size was more strongly correlated with the thickness of the OM coating during the pre-Spring Festival period, while the correlation was stronger at the rural site during the Spring Festival.

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 (Ox = 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.

Black carbon (BC) plays an important role in air quality and climate change, which is closely associated with its mixing state and chemical compositions. In this work the mixing state of BC-containing single particles was investigated to explore the evolution process of ambient BC particles using a single particle aerosol mass spectrometer (SPAMS) in March 2018 in Zhengzhou, China. The BC-containing particles accounted for 61.4% of total detected ambient single particles and were classified into five types including BC-nitrate (BC-N, 52.3%) as the most abundant species, followed by BC-nitrate-sulfate (BC-NS, 22.4%), BCOC (16.8%), BC-fresh (BC-F, 4.5%) and BC-sulfate particles (BC-S, 4.0%). With enhancement of the ambient nitrate concentration, the relative peak area (RPA) of nitrate in BC-N and BCNS particles both increased, yet only the number fraction (Nf) of BCN particles increased while the Nf of BC-NS particles decreased, suggesting that the enhanced mixing state of BC with nitrate was mainly due to the increase in the ambient nitrate mass concentration. In addition, the Nf of BC-N decreased from 65.3% to 28.4% as the absorbing Ångström exponents (AAE) of eBC increased from 0.75 to 1.45, which indicated the reduction of light absorption ability of aged BC particles with the enhanced formation of BC-N particles. The results of this work indicated a change in the mixing state of BC particles due to the dominance of nitrate in PM2.5, which also influenced the optical properties of aged BC particles.

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.

The rapid development of cities and economic prosperity greatly motivates the growth of vehicular exhaust particles, especially the diesel-exhausted particles from the large fleet of passenger and freight, which present profound implications on climate, air quality, and biological health (e.g., pulmonary, autoimmune and cardiovascular diseases). As important physiochemical properties of atmospheric aerosols, however, the mixing state and effective density of individual particles emitted from diesel-powered vehicles under different driving conditions and their environmental implications remain uncertain. Here, a single-particle aerosol mass spectrometer (SPAMS) was used to investigate the chemical composition and vacuum aerodynamic diameter (Dva), along with the aerodynamic diameter (Da) from an aerodynamic aerosol classifier (AAC), to determine the effective density of primary particles emitted from a light- duty diesel vehicle (LDDV) under the launching and idling engine states. Interestingly, the particle types and effective density appear to vary significantly with the engine status. A single particle type of Ca-rich particles, named Na-Ca-PAH, was predominant in the idling state, whose chemical components may be affected by the lubricants and incomplete combustion, contributing to a higher effective density (0.66 ± 0.21 g cm-3). In contrast, launching particles exhibited a lower effective density (0.34 ± 0.17 g cm-3) because of the substantial elemental carbon (EC). In addition, the effective density depends not only on the particle size but also on the chemical components with various abundances. EC and Ca play opposite roles in the effective density of LDDV emissions. Notably, a higher proportion of polycyclic aromatic hydrocarbons (PAHs) was observed in the idling particles, contributing to 78 ± 1.2%. Given the high contribution to these PAH-containing particles in the idling state, indispensable precautions should be taken at bus stops or waiting for pedestrians. This study provides more comprehensive insights into the initial characteristics of LDDV particles due to the launching and idling states, which is beneficial for improving the model results of source apportionment and understanding its environmental behavior regarding human health.

Measuring the chemical composition of individual atmospheric aerosol particles can provide direct evidence of their heterogeneous reactions and mixing states in the atmosphere. In this study, micro-Raman spectroscopy was used to measure the chemical composition of 1200 individual atmospheric particles in 11 samples collected in Beijing air. (NH4)2SO4, NH4NO3, various minerals, carbonaceous species (soot and organics), and NaNO3 were identified in the measured particles according to their characteristic Raman peaks. These species represented the main components of aerosol particles. In individual particles, NH4NO3 and (NH4)2SO4 either existed separately or were internally mixed. Possible reaction pathways of CaCO3 particles in the atmosphere were proposed based on the results of this study and laboratory simulations on heterogeneous reactions in the literature. CaCO3 reacted with N- and S-containing (nitrogen- and sulfur-containing) acidic gases to produce Ca(NO3)2 and CaSO4. Ca(NO3)2 further reacted with S-containing acidic gases and oxidants to produce CaSO4. Of the soot-containing particles, 23% were internal mixtures of soot and inorganic material. Of the organics-containing particles, 57% were internal mixtures of organic and inorganic materials. Micro-Raman spectroscopy directly identified functional groups and molecules in individual atmospheric particles under normal ambient conditions, rendering it a powerful tool for measuring the chemical composition of individual atmospheric particles with a diameter of ≥1.0 μm.

Recently, internal mixing states of oxalate with metals in single particles have been reported from field studies, yet the role of metals in the formation processes of oxalate remains unclear due to the diversity of chemical components and complex atmospheric environment. In this study, the mixing states of oxalate with five metals, including zinc (Zn), copper (Cu), lead (Pb), vanadium (V) and iron (Fe) were investigated in Guangzhou, China. It was found that 55% of oxalate-containing particles were internally mixed with these metals. The number fraction of oxalate in the metal-containing particles ranged from 5.4-26%, which is much higher than that in the total detected particles (4.0%), indicating significant enrichment of oxalate in the metal-containing particles. Enhanced oxalate production was found in the Fe- and V-containing particles based on distinctly higher relative peak area (RPA) ratios of oxalate to its precursors compared to the total particles, possibly due to enhanced aqueous phase reactions in the Fe- and V-containing particles. However, enrichment of oxalate in the Zn-, Pb-, and Cu-containing particles was possibly associated with complexation of gas phase oxalic acid with the metals, as indicated by the small increase in RPA ratios in these particles. On the other hand, the internal mixing of oxalate with metals was found to provide a way of efficient photolysis of oxalate-metal complexes, which led to a decrease in oxalate after sunrise in the metal-containing particles. In this study, the enhanced mixing states of oxalate with metals have revealed the important role of metals in the production and degradation of oxalate, providing insights for the evaluation of metals in the formation processes of organic aerosol in field studies, which is beneficial to the further study of air pollution in metal emission areas.

Inland transported sea spray aerosol (SSA) particles along with multiphase reactions are essential to drive the regional circulation of nitrogen, sulfur and halogen species in the atmosphere. Specially, the physicochemical properties of SSA will be significantly affected by the displacement reaction of chloride. However, the role of organic species and the mixing state on the chloride depletion of SSA during long-range inland transport remains unclear. Hence, a single particle aerosol mass spectrometer (SPAMS) was employed to investigate the particle size and chemical composition of individual SSA particles over inland southern China during the East Asian summer monsoon. Based on the variation of chemical composition, SSA particles were clustered into SSA-Aged, SSA-Bio and SSA-Ca. SSA-Aged was regarded as the aged Na-rich SSA particles. In comparison to the SSA-Aged, SSA-Bio involved some extra organic species associated with biological origin (i.e., organic nitrogen and phosphate). Each type occupies for approximately 50% of total detected SSA particles. Besides, SSA-Ca may relate to organic shell of Na-rich SSA particles, which is negligible (~3%). Tight correlation between Na and diverse organic acids was exhibited for the SSA-Aged (r2 = 0.52, p < 0.01) and SSA-Bio (r2 = 0.61, p < 0.01), reflecting the impact of organic acids to the chloride displacement during inland transport SSA particles. The chloride depletion occupied by organic acids is estimated to be up to 34%. It is noted that distinctly different degree of chloride depletion was observed between SSA-Aged and SSA-Bio. It is more likely to be attributed to the associated organic coatings for the SSA-Bio particles, which inhibits the displacement reactions between acids and chloride. As revealed from the mixing state of SSA-Bio, defined hourly mean peak area ratio of Cl / Na increases with the increasing phosphate and organic nitrogen. This finding provides additional basis for the improvement of modeling simulations in chlorine circulation and a comprehensive understanding of the effects of organics on chloride depletion of SSA particles.

The effect of the mixing state of black carbon (BC) on light absorption is of enduring interest due to its close connection to regional/global climate. Herein, we present concurrent measurements of both BC absorption enhancement (Eabs) and the chemical mixing state in southern China. Eabs was obtained by simultaneous measuring the light absorption coefficient using an aethalometer before and after being heated. The observed Eabs was categorized into non- (Eabs ≤ 1.0), slight (1.0 < Eabs ≤ 1.2), and higher (Eabs > 1.2) enhancement groups, and it was compared to the mixing state of elemental carbon (EC) particles detected by a single particle aerosol mass spectrometer (SPAMS). The individual EC-containing particles were classified into four types, including EC with sodium and potassium ion peaks (NaK-EC), long EC cluster ions (Cn+/-, n ≥ 6) with sulfate (EC-Sul1), short EC cluster ions (Cn+/-, n < 6) with sulfate (EC-Sul2), and EC with OC and sulfate (ECOC-Sul). NaK-EC and EC-Sul2 are the dominant EC types. Slight enhancement group is mainly explained by the photochemical production of ammonium sulfate and organics on EC-Sul2 during afternoon hours. In contrast, the higher Eabs is primarily attributed to the enhanced mixing of ammonium chloride with NaK-EC during morning hours, without photochemistry. The characterization of source emissions indicates that NaK-EC is likely from coal combustion and is associated with a relatively higher amount of ammonium chloride. To our knowledge, this is the first report to state that EC particles associated with ammonium chloride have a relatively higher Eabs.

Many field studies have investigated the formation mechanisms of organic aerosol (OA) based on bulk analysis, yet the source and formation process of individual organic particles may be quite different due to the diversity of chemical composition and mixing state in single particles. Here we present the observation results of chemical composition and mixing state of carbonaceous single particles at an urban site in Guangzhou. The carbonaceous particles accounted for 74.6% of the total detected single particles, and were grouped into four types including elemental carbon-aged (EC-aged), elemental and organic carbon (ECOC), organic carbon-rich (OC-rich) and secondary ions-rich (SEC) particles. The formation of EC-aged particles was closely associated with the absorption of organics onto fresh EC particles from primary sources, and the further enrichment of organics in EC-aged particles resulted in the production of ECOC particles. In the daytime OC-rich and SEC particles were mainly produced from the photochemical reactions, while in the nighttime their sharp increases were found along with the enrichment of nitrate and organic nitrogen fragments, suggesting the heterogeneous formation of nitrate and organic nitrogen in OC-rich and SEC particles. The production rates of carbonaceous particles were also investigated in an episodic event, and the EC-aged particles showed the highest production rate compared to the other carbonaceous particles both in the daytime and nighttime, suggesting a significant role of EC in the formation and aging process of carbonaceous particles. The results from this work have revealed different formation processes and production rates of carbonaceous particles due to their diversity in mixing state, providing further insights into the formation mechanisms of OA in field studies.