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Amines and aminiums play an important role in particle formation, liquid-phase reactions, and climate change and have attracted considerable attention over the years. Here, we investigated the concentrations and compositions of aminiums in PM2.5 in 11 Chinese cities during the winter, focusing on the characteristics of aminiums during the polluted days and the key factors influencing aminium outbreak. Monomethylaminium was the dominant aminium species in most cities, except Taiyuan and Guangzhou, followed by dimethylaminium. Diethylaminium dominated the total aminiums in Taiyuan and Guangzhou. Thus, the main amine sources in Taiyuan and Guangzhou were significantly different from those in other cities. The concentrations of the total aminiums (TAs) in most cities increased significantly during the polluted days, while relatively weak aminium outbreaks during the polluted days occurred in Xi'an and Beijing. Additionally, the concentrations of TAs in Xi'an and Beijing were insignificantly correlated with those of PM2.5 and the major acidic aerosol components, while the opposite pattern was observed in nine other cities. Thus, acid–base chemistry was significantly associated with the formation of aminiums in PM2.5 in all cities, except Xi'an and Beijing. Based on the sensitivity analysis of the aminiums : ammonium ratio to ammonium changes, as well as excluding the effects of relative humidity and atmospheric oxidation, we proposed the possibility of the competitive uptake of ammonia versus amines on acidic aerosols or the displacement of aminiums by ammonia in Xi'an and Beijing (constraining aminium outbreaks). Overall, this study deepens the understanding of the spatiotemporal differences in aminium characteristic and formation in China. However, the uptake of amines on particles to form aminiums and the relevant influencing factors require further mechanistic research.

To investigate the optical characteristics and sources of brown carbon (BrC) in North China, where the atmospheric aerosol loadings are high and have severe impacts on Earth's climate system, we collected fine aerosols (PM2.5) at an urban site in Tianjin over a 1-year period. We measured the ultraviolet (UV) light absorption and excitation emission matrix (EEM) fluorescence of the water-soluble BrC (WSBrC) and the water-insoluble but methanol-soluble BrC (WI-MSBrC) in the PM2.5 using a three-dimensional fluorescence spectrometer. The average light absorption efficiency of both WSBrC (Abs365, WSBrC) and WI-MSBrC (Abs365, WI-MSBrC) at 365 nm was found to be highest in winter (10.4 ± 6.76 and 10.0 ± 5.13 Mm−1, respectively) and distinct from season to season. The averages of the fluorescence index (FI) and the biological index (BIX) of WSBrC were lower in summer than in the other seasons and opposite to that of the humification index (HIX), which implied that the secondary formation and further chemical processing of aerosols were more intensive during the summer period than in the other seasons. However, in winter, the higher HIX together with the higher FI and BIX of WI-MSBrC suggested that the BrC loading was mainly influenced by primary emissions from biomass burning and coal combustion. Based on the EEM, the types of fluorophores in WSBrC were divided into humic-like substances (HULIS), including low-oxygenated and high-oxygenated species and protein-like compounds (PLOM), though mostly PLOM in the WI-MSBrC. The direct radiation absorption by both WSBrC and WI-MSBrC in the range of 300–400 nm accounted for ∼ 40 % of that (SFEAbs, 4.97 ± 2.71 and 7.58 ± 5.75 W g−1, respectively) in the range 300–700 nm.

To better understand the origins and seasonality of atmospheric aerosols in North China, we collected fine aerosols (PM2.5) at an urban site (Nankai District, ND) and a suburban site (Haihe Education Park, HEP) in Tianjin from July 2018 to July 2019. The PM2.5 was studied for carbonaceous, nitrogenous and ionic components and stable carbon and nitrogen isotope ratios of total carbon (δ13CTC) and nitrogen (δ15NTN). On average, the mass concentrations of PM2.5, organic carbon (OC), elemental carbon (EC) and water-soluble OC (WSOC) were higher in winter than in summer at both ND and HEP. SO42-, NO3- and NH4+ were the dominant ions, and their sum accounted for 89 % of the total ionic mass at ND and 87 % at HEP. NO3- and NH4+ peaked in winter and were at their minimum in summer, whereas SO42- was higher in summer than in all the other seasons at HEP and was comparable among the seasons, although it peaked in winter at ND. δ13CTC and δ15NTN were −26.5 ‰ to −21.9 ‰ and +1.01 ‰ to +22.8 ‰ at ND and −25.5 ‰ to −22.8 ‰ and +4.91 ‰ to +18.6 ‰ at HEP. Based on seasonal variations in the measured parameters, we found that coal and biomass combustion emissions are the dominant sources of PM2.5 in autumn and winter, while terrestrial and/or marine biological emissions are important in spring and summer in the Tianjin region, North China. In addition, our results implied that the secondary formation pathways of secondary organic aerosols in autumn/winter were different from those in spring/summer; i.e., they might be driven by NO3 radicals in the former period.

Little is known about the formation processes of nitrooxy organosulfates (OSs) by nighttime chemistry. Here we characterize nitrooxy OSs at a molecular level in firework-related aerosols in urban Beijing during Chinese New Year. High-molecular-weight nitrooxy OSs with relatively low H / C and O / C ratios and high unsaturation are potentially aromatic-like nitrooxy OSs. They considerably increased during New Year's Eve, affected by the firework emissions. We find that large quantities of carboxylic-rich alicyclic molecules possibly formed by nighttime reactions. The sufficient abundance of aliphatic-like and aromatic-like nitrooxy OSs in firework-related aerosols demonstrates that anthropogenic volatile organic compounds are important precursors of urban secondary organic aerosols (SOAs). In addition, more than 98 % of those nitrooxy OSs are extremely low-volatility organic compounds that can easily partition into and consist in the particle phase and affect the volatility, hygroscopicity, and even toxicity of urban aerosols. Our study provides new insights into the formation of nitrooxy organosulfates from anthropogenic emissions through nighttime chemistry in the urban atmosphere.

Marine aerosol samples collected from the South China Sea (SCS) to the eastern Indian Ocean (EIO) during a cruise from 10 March to 26 April 2015 were studied for diacids and related compounds. In view of air mass backward trajectories, source regions, and geographical features, the cruise area was categorized into the South China Sea (SCS), the eastern Indian Ocean off the coast of western Indonesia (EIO-WI), the EIO off the coast of Sri Lanka (EIO-SL), Malacca, and the Sri Lanka docking point (SLDP). Total concentrations of diacids, oxoacids, and α-dicarbonyls were high at the SLDP, followed by the SCS and Malacca, and they were the low in the EIO-WI. In this study, oxalic acid (C2) was the dominant diacid during the cruise, followed by malonic acid (C3) in the SCS, EIO-WI, EIO-SL, and Malacca, and succinic acid (C4) was relatively more abundant than C3 diacid at the SLDP. Except for SLDP, C3∕C4 mass ratios were always greater than 1, and no significant difference was observed during the cruise. The C2∕C4 and C2∕total diacid ratios also showed similar trends. The average mass ratios of adipic acid (C6) to azelaic acid (C9) were less than unity except for in the EIO-WI; the mass ratios of phthalic acid (Ph) to azelaic acid (C9) were less than 2 except for in the SCS. The concentrations of diacids were higher when the air masses originated from terrestrial regions than when they originated from remote oceanic regions. Based on the molecular distributions of organic acids, the mass ratios, and the linear correlations of selected compounds in each area, we found that the oxidation of biogenic volatile organic compounds (BVOCs) released from the ocean surface and subsequent in situ photochemical oxidation was the main contributor to diacids, oxocarboxylic acids, and α-dicarbonyls from the SCS to the EIO. In addition, the continental outflow, which is enriched in anthropogenic VOCs and their aged products, influenced the organic aerosol loading, particularly over the SCS. Emissions from Sri Lanka terrestrial vegetation as well as fossil fuel combustion and subsequent photochemical oxidation also played a prominent role in controlling the organic aerosol loading and the molecular distribution of diacids and related compounds at the SLDP.

Firework (FW) emission has strong impacts on air quality and public health. However, little is known about the molecular composition of FW-related airborne particulate matter (PM), especially the organic fraction. Here we describe the detailed molecular composition of Beijing PM collected before, during, and after a FW event in the evening of New Year's Eve in 2012. Subgroups of CHO, CHON, and CHOS were characterized using ultrahigh-resolution Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry. These subgroups comprise a substantial fraction of aromatic-like compounds with low O∕C ratio and high degrees of unsaturation, some of which plausibly contributed to the formation of brown carbon in Beijing PM. Moreover, we found that the number concentration of sulfur-containing compounds, especially the organosulfates, increased dramatically during the FW event, whereas the number concentration of CHO and CHON doubled after the event, which was associated with multiple atmospheric aging processes including the multiphase redox chemistry driven by NOx, O3, and •OH. These findings highlight that FW emissions can lead to a sharp increase in high-molecular-weight compounds, particularly aromatic-like substances in urban particulate matter, which may affect the light absorption properties and adverse health effects of atmospheric aerosols.

In order to better understand the molecular composition and sources of organic aerosols in Tianjin, a coastal megacity in North China, ambient fine aerosol (PM2.5) samples were collected on a day/night basis from November to December 2016 and from May to June 2017. The organic molecular composition of PM2.5 components, including aliphatic lipids (n-alkanes, fatty acids, and fatty alcohols), sugar compounds, and photooxidation products from isoprene, monoterpene, β-caryophyllene, naphthalene, and toluene, was analysed using gas chromatography–mass spectrometry. Fatty acids, fatty alcohols, and saccharides were identified as the most abundant organic compound classes among all of the tracers detected in this study during both seasons. High concentrations of most organics at night in winter may be attributed to intensive residential activities such as house heating as well as the low nocturnal boundary layer height. Based on tracer methods, the contributions of the sum of primary and secondary organic carbon (POC and SOC respectively) to aerosol organic carbon (OC) were 24.8 % (daytime) and 27.6 % (night-time) in winter and 38.9 % (daytime) and 32.5 % (night-time) in summer. In detail, POC derived from fungal spores, plant debris, and biomass burning accounted for 2.78 %–31.6 % (12.4 %; please note that values displayed in parentheses in the following are average values) of OC during the daytime and 4.72 %–45.9 % (16.3 %) at night in winter, and 1.28 %–9.89 % (5.24 %) during the daytime and 2.08 %–47.2 % (10.6 %) at night in summer. Biomass-burning-derived OC was the predominant source of POC in this study, especially at night (16.0±6.88 % in winter and 9.62±8.73 % in summer). Biogenic SOC from isoprene, α-∕β-pinene, and β-caryophyllene exhibited obvious seasonal and diurnal patterns, contributing 2.23±1.27 % (2.30±1.35 % during the daytime and 2.18±1.19 % at night) and 8.60±4.02 % (8.98±3.67 % and 8.21±4.39 %) to OC in winter and summer respectively. Isoprene and α-∕β-pinene SOC were obviously elevated in summer, especially during the daytime, mainly due to strong photooxidation. Anthropogenic SOC from toluene and naphthalene oxidation showed higher contributions to OC in summer (21.0±18.5 %) than in winter (9.58±3.68 %). In summer, toluene SOC was the dominant contributor to aerosol OC, and biomass burning OC also accounted for a high contribution to OC, especially at night-time; this indicates that land/sea breezes also play an important role in the aerosol chemistry of the coastal city of Tianjin in North China.