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Biomass burning is one of the key sources of urban aerosols in the North China Plain, especially during winter, when the impact of secondary organic aerosols (SOAs) formed from biogenic volatile organic compounds (BVOCs) is generally considered to be minor. However, little is known about the influence of biogenic SOA loading on the molecular composition of wintertime organic aerosols. Here, we investigated the water-soluble organic compounds in fine particulate matter (PM 2.5) from urban Tianjin by ultrahigh-resolution Fourier transform ion cyclotron resonanc mass spectrometry (FT-ICR MS). Our results show that most of the CHO and CHON compounds are derived from biomass burning which are poor in oxygen and contain aromatic rings that probably contribute to light-absorbing brown carbon (BrC) chromophores. Under moderate to high SOA-loading conditions, the nocturnal chemistry is more efficient than photooxidation to generate secondary CHO and CHON compounds with high oxygen content. Under low SOA loading, secondary CHO and CHON compounds with low oxygen content are mainly formed by photochemistry. Secondary CHO compounds are mainly derived from oxidation of monoterpenes. However, nocturnal chemistry may be more productive to sesquiterpene-derived CHON compounds. In contrast, the number- and intensity-weight of S-containing groups (CHOS and CHONS) increased significantly with the increase of biogenic SOA loading, which agrees with the fact that a majority of the S-containing groups are identified as organosulfates (OSs) and nitrooxy–organosulfates (nitrooxy–OSs) that are derived from the oxidation of BVOCs. Terpenes may be potential major contributors to organosulfates and nitrooxy–organosulfates. While the nocturnal chemistry is more beneficial to the formation of organosulfates and nitrooxy–organosulfates under low SOA loading. The SOA loading is an important factor that is associated with the oxidation degree, nitrate group content and chemodiversity of nitrooxy-organosulfates. Furthermore, our study suggests that the hydrolysis of nitrooxy-organosulfates is a possible pathway for the formation of organosulfates. [ABSTRACT FROM AUTHOR]

Brown carbon (BrC) aerosols exert vital impacts on climate change and atmospheric photochemistry due to their light absorption in the wavelength range from near-ultraviolet (UV) to visible light. However, the optical properties and formation mechanisms of ambient BrC remain poorly understood, limiting the estimation of their radiative forcing. In the present study, fine particles (PM 2.5) were collected during 2016–2017 on a day/night basis over urban Tianjin, a megacity in northern China. Light absorption and fluorescence properties of water extracts of PM 2.5 were investigated to obtain seasonal and diurnal patterns of atmospheric water-soluble BrC. There were obvious seasonal, but no evident diurnal, variations in the light absorption properties of BrC. In winter, BrC showed much stronger light-absorbing ability, with a mass absorption efficiency at 365 nm (MAE 365) in winter (1.54±0.33 m 2 gC -1) that was 1.8 times larger than MAE 365 in summer (0.84±0.22 m 2 gC -1). Direct radiative effects by BrC absorption relative to black carbon in the UV range were 54.3±16.9 % and 44.6±13.9 % in winter and summer, respectively. In addition, five fluorescent components in BrC, including three humic-like fluorophores and two protein-like fluorophores were identified with excitation–emission matrix fluorescence spectrometry and parallel factor (PARAFAC) analysis. The less oxygenated components contributed more to winter and nighttime samples, while more oxygenated components increased in summer and daytime samples. The higher humification index (HIX), together with lower biological index (BIX) and fluorescence index (FI), suggests that the chemical compositions of BrC were associated with a high aromaticity degree in summer and daytime due to photobleaching. Fluorescent properties indicate that wintertime BrC were predominantly affected by primary emissions and fresh secondary organic aerosol (SOA), while summer ones were more influenced by aging processes. Results of source apportionments using organic molecular compositions of the same set of aerosols reveal that fossil fuel combustion and aging processes, primary bioaerosol emission, biomass burning, and biogenic and anthropogenic SOA formation were the main sources of BrC. Biomass burning contributed much more to BrC in winter and at nighttime, while biogenic SOA contributed more in summer and during the daytime. In particular, our study highlights that primary bioaerosol emission is an important source of BrC in urban Tianjin in summer. [ABSTRACT FROM AUTHOR]

In order to better understand the molecular composition and sources of organic aerosols in Tianjin, a coastal megacity in North China, ambient fine aerosol (PM 2.5) samples were collected on a day/night basis from November to December 2016 and from May to June 2017. The organic molecular composition of PM 2.5 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. [ABSTRACT FROM AUTHOR]

Air quality in Beijing-Tianjin-Hebei and its surrounding regions has been significantly improved during the Three-Year Action Plan for Winning the Battle Against Air Pollution from 2018 to 2020. The PM 2.5 concentration can be significantly influenced by meteorological conditions, and the effectiveness of anthropogenic emissions reduction on PM 2.5 over the North China Plain remains unclear. In this study, the Nested Air Quality Model System (NAQPMS) and observations were used to quantitatively examine the contribution of meteorological and emission variations to autumn-winter (November to February) PM 2.5 concentrations in Beijing and its surrounding regions from 2018 to 2020. The model reproduced the temporal evolution of atmospheric pollutants and components well. The results showed that the observed mean PM 2.5 concentrations during the winter were reduced by 15% and 29% in 2019 and 2020, respectively, compared with that in 2018. In 2019, the meteorological conditions over the Beijing-Tianjin-Hebei (BTH) region were poor, causing an increase of PM 2.5 by 7.6% in BTH and 11% in Beijing; therefore, the control of air pollutant emission compensated for the unfavorable influence of meteorology and emission reduction was the decisive factor for PM 2.5 reduction; In 2020, the meteorological conditions were relatively favorable, and the changes in meteorological factors and emission reduction led to a decrease in PM 2.5 concentration by 18.6% and 10.5%, respectively, compared to 2018. Emission reduction also contributed to decrease in nitrate, ammonium, organic matter, element carbon and precursors. This work confirmed the obvious environmental benefit of pollution control measures from 2018 to 2020. There was obvious regional heterogeneity in the effect of meteorology and the impact of adverse meteorological conditions should be considered during the formulation of air pollution control measures. The results can serve as a reference for the formulation of air quality objectives or the evaluation of the environmental effect of pollution control schemes. • The effect of winter meteorology and control measures in 2018 to 2020 was assessed. • Observed winter PM 2.5 reduced by 15% and 29% in 2019 and 2020, respectively, compared to 2018. • Unfavorable meteorology in 2019 partially counteracted the emission control effects. • Emission mitigation played an important role in air quality improvement. [ABSTRACT FROM AUTHOR]