Nitrogen-containing organic compounds (NOCs) may potentially contribute to aqueous secondary organic aerosols, yet the different formation of NOCs in aerosol particles and cloud droplets remains unclear. With the in-situ measurements performed at a mountain site (1690 m a.s.l.) in southern China, we investigated the formation of NOCs in the cloud droplets and the cloud-free particles, based on their mixing state information of NOCs-containing particles by single particle mass spectrometry. The relative abundance of NOCs in the cloud-free particles was significantly higher than those in cloud residual (cloud RES) particles. NOCs were highly correlated with carbonyl compounds (including glyoxalate and methylglyoxal) in the cloud-free particles, however, limited correlation was observed for cloud RES particles. Analysis of their mixing state and temporal variations highlights that NOCs was mainly formed from the carbonyl compounds and ammonium in the cloud-free particles, rather than in the cloud RES particles. The results support that the formation of NOCs from carbonyl compounds is facilitated in concentrated solutions in wet aerosols, rather than cloud droplets. In addition, we have identified the transport of biomass burning particles that facilitate the formation of NOCs, and that the observed NOCs is most likely contributed to the light absorption. These findings have implications for the evaluation of NOCs formation and their contribution to light absorption.

To investigate the seasonal characteristics in air pollution in Chengdu, a single particle aerosol mass spectrometry was used to continuously observe atmospheric fine particulate matter during one-month periods in summer and winter, respectively. The results showed that, apart from O3, the concentrations of other pollutants (CO, NO2, SO2, PM2.5 and PM10) were significantly higher in winter than in summer. All single particle aerosols were divided into seven categories: biomass burning (BB), coal combustion (CC), Dust, vehicle emission (VE), K mixed with nitrate (K-NO3), K mixed with sulfate and nitrate (K-SN), and K mixed with sulfate (K-SO4) particles. The highest contributions in both seasons were VE particles (24%). The higher contributions of K-SO4 (16%) and K-NO3 (10%) particles occurred in summer and winter, respectively, as a result of their different formation mechanisms. S-containing (K-SO4 and K-SN), VE, and BB particles caused the evolution of pollution in both seasons, and they can be considered as targets for future pollution reduction. The mixing of primary sources particles (VE, Dust, CC, and BB) with secondary components was stronger in winter than in summer. In summer, as pollution worsens, the mixing of primary sources particles with 62 [NO3]- weakened, but the mixing with 97 [HSO4]- increased. However, in winter, the mixing state of particles did not exhibit an obvious evolution rules. The potential source areas in summer were mainly distributed in the southern region of Sichuan, while in winter, besides the southern region, the contribution of the western region cannot be ignored.

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.

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.

Carbonaceous aerosols play a vital role in global climate patterns due to their potent light absorption capabilities. However, the light absorption enhancement effect (Eabs) of black carbon (BC) is still subject to great uncertainties due to factors such as the mixing state, coating material, and particle size distribution. In this study, fine particulate matter (PM2.5) samples were collected in Chengdu, a megacity in the Sichuan Basin, during the winter of 2020 and 2021. The chemical components of PM2.5 and the light absorption properties of BC were investigated. The results revealed that secondary inorganic aerosols and carbonaceous aerosols were the dominant components in PM2.5. Additionally, the aerosol filter filtration-dissolution (AFD) treatment could improve the accuracy of measuring elemental carbon (EC) through thermal/optical analysis. During winter in Chengdu, the absorption enhancement values of BC ranged between 1.56 and 2.27, depending on the absorption wavelength and the mixing state of BC and non-BC materials. The presence of internally mixed BC and non-BC materials significantly contributed to Eabs, accounting for an average of 68 % at 405 nm and 100 % at 635 nm. The thickness of the BC coating influenced Eabs, displaying an increasing-then-decreasing trend. This trend was primarily attributed to the hygroscopic growth and dehydration shrinkage of particulate matter. Nitrate, as the major component of BC coating, played a crucial role in the lensing effect and exhibited fast growth during variation in Eabs. By combining the results from PMF, we identified the secondary formation and vehicle emission as the primary contributors to Eabs. Consequently, this study can provide valuable insights into the optical parameters, which are essential for assessing the environmental quality, improving regional atmospheric conditions, and formulating effective air pollution control strategies.

Nitrophenols have received extensive attention due to their strong light-absorbing ability in the near-ultraviolet-visible region, which could be influenced by the atmospheric processes of nitrophenols. However, our knowledge and understanding of the formation and evolution of nitrophenols are still in the nascent stages. In the present study, the mixing states of four mononitrophenol particles (i.e., nitrophenol, methynitrophenol, nitrocatechol, and methoxynitrophenol), and one nitropolycyclic aromatic hydrocarbon particles (i.e., nitronaphthol (NN)) were investigated using a single-particle aerosol mass spectrometer (SPAMS) in November 2019 in Qingdao, China. The results showed, for the first time, that mononitrophenols and NN exhibit different mixing states and diurnal variations. Four mononitrophenols were internally mixed well with each other, and with organic acids, nitrates, potassium, and naphthalene. The diurnal variation in the number fraction of mononitrophenols presented two peaks at 07:00 to 09:00 and 18:00 to 20:00, and a valley at noon. Atmospheric environmental conditions, including NO2, O3, relative humidity, and temperature, can significantly influence the diurnal variation of mononitrophenols. Multiple linear regression and random forest regression models revealed that the main factors controlling the diurnal variation of mononitrophenols were photochemical reactions during the day and aqueous-phase reactions during the night. Unlike mononitrophenols, about 62-83% of NN were internally mixed with [NH4]+ and [H(NO3)2]-, but not with organic acids and potassium. The diurnal variation of NN was also different from that of mononitrophenols, generally increased from 17:00 to 10:00 and then rapidly decreaed from 11:00 to 16:00. These results imply that NN may have sources and atmospheric processes that are different from mononitrophenols. We speculate that this is mostly controlled by photochemical reactions and mixing with [NH4]+, which may influence the diurnal variation of NN in the ambient particles; however, this requires further confirmation. These findings extend our current understanding of the atmospheric formation and evolution of nitrophenols.

Carbonaceous particles are an important chemical component of atmospheric fine particles. In this study, a single particle aerosol mass spectrometer was used to continuously measure the carbonaceous particles in Chengdu, one of the megacities most affected by haze in China, from January 22 to March 3, 2021. During the observation period, the average mass concentration of PM2.5 was 62.3 ± 37.2 μg m-3, and the emissions from mobile sources were more prominent. Carbonaceous particles accounted for 68.6% of the total particles and could be classified into 10 categories, with elemental carbon (EC) mixed with sulfate (EC-S) particles making the highest contribution (33.1%). EC particles rich in secondary components and organic carbon (OC) particles rich in secondary component exhibited different diurnal variations, suggesting different sources and mixing mechanisms. From \"excellent\" to \"polluted\" days, the contributions of EC-S, EC mixed with sulfate and nitrate (EC-SN) and OC mixed with EC (OC-EC) particles increased by 9.8%, 4.5% and 6.6%, respectively, and thus these particles are key targets for future pollution control. The potential source contribution of the southwest area was stronger than that of other areas, and the potential contribution of regional transport to EC-related particles was stronger than to OC-related particles. Most particles were highly mixed with sulfate or nitrate, and the level of secondary mixing further enhanced as pollution worsened.

Organic nitrogen (ON) compounds play a significant role in the light absorption of brown carbon and the formation of organic aerosols, however, the mixing state, secondary formation processes, and influencing factors of ON compounds are still unclear. This paper reports on the mixing state of ON-containing particles based on measurements obtained using a high-performance single particle aerosol mass spectrometer in January 2020 in Guangzhou. The ON-containing particles accounted for 21% of the total detected single particles, and the particle count and number fraction of the ON-containing particles were two times higher at night than during the day. The prominent increase in the content of ON-containing particles with the enhancement of NOx mainly occurred at night, and accompanied by high relative humidity and nitrate, which were associated with heterogeneous reactions between organics and gaseous NOx and/or NO3 radical. The synchronous decreases in ON-containing particles and the mass absorption coefficient of water-soluble extracts at 365 nm in the afternoon may be associated with photo-bleaching of the ON species in the particles. In addition, the positive matrix factorization analysis found five factors dominated the formation processes of ON particles, and the nitrate factor (33%) mainly contributed to the production of ON particles at night. The results of this study provide unique insights into the mixing states and secondary formation processes of the ON-containing particles.

To investigate the volatility of atmospheric particulates and the evolution of other particulate properties (chemical composition, particle size distribution and mixing state) with temperature, a thermodenuder coupled with a single particle aerosol mass spectrometer was used to conduct continuous observations of atmospheric fine particles in Chengdu, southwest China. Because of their complex sources and secondary reaction processes, the average mass spectra of single particles contained a variety of chemical components (including organic, inorganic and metal species). When the temperature rose from room temperature to 280°C, the relative areas of volatile and semi-volatile components decreased, while the relative areas of less or non-volatile components increased. Most (> 80%) nitrate and sulfate existed in the form of NH4NO3 and (NH4)2SO4, and their volatilization temperatures were 50-100°C and 150-280°C, respectively. The contribution of biomass burning (BB) and vehicle emission (VE) particles increased significantly at 280°C, which emphasized the important role of regional biomass burning and local motor vehicle emissions to the core of particles. With the increase in temperature, the particle size of the particles coated with volatile or semi-volatile components was reduced, and their mixing with secondary inorganic components was significantly weakened. The formation of K-nitrate (KNO3) and K-sulfate (KSO4) particles was dominated by liquid-phase processes and photochemical reactions, respectively. Reducing KNO3 and BB particles is the key to improving visibility. These new results are helpful towards better understanding the initial sources, pollution formation mechanisms and climatic effects of fine particulate matter in this megacity in southwest China.

Mixing states of aerosol particles are crucial for understanding the role of aerosols in influencing air quality and climate. However, a fundamental understanding of the complex mixing states is still lacking because most traditional analysis techniques only reveal bulk chemical and physical properties with limited surface and 3-D information. In this research, 3-D molecular imaging enabled by ToF-SIMS was used to elucidate the mixing states of PM2.5 samples obtained from a typical Beijing winter haze event. In light pollution cases, a thin organic layer covers separated inorganic particles; while in serious pollution cases, ion exchange and an organic-inorganic mixing surface on large-area particles were observed. The new results provide key 3-D molecular information of mixing states, which is highly potential for reducing uncertainty and bias in representing aerosol-cloud interactions in current Earth System Models and improving the understanding of aerosols on air quality and human health.