Abstract. Nitro-aromatic compounds (NACs) were measured hourly at a rural site in China during wintertime to monitor the changes due to local and regional impacts of biomass burning (BB). Concurrent and continuous measurements of the concentrations of 16 NACs in the gas and particle phases were performed with a time-of-flight chemical ionization mass spectrometer (CIMS) equipped with a Filter Inlet for Gases and AEROsols (FIGAERO) unit using iodide as the reagent ion. NACs accounted for <2 % of the mass concentration of organic matter (OM) and total particulate matter (PM), but the total particle mass concentrations of these compounds can reach as high as 1000 ng m−3 (299 ng m−3 avg), suggesting that they may contribute significantly to the radiative forcing effects of atmospheric particles. Levels of gas-phase NACs were highest during the daytime (15:00–16:00 local time, LT), with a smaller night-time peak around 20:00 LT. Box-model simulations showed that this occurred because the rate of NAC production from gas-phase sources exceeded the rate of loss, which occurred mainly via the OH reaction and to a lesser degree via photolysis. Data gathered during extended periods with high contributions from primary BB sources (resulting in 40 %–60 % increases in NAC concentrations) were used to characterize individual NACs with respect to gas–particle partitioning and the contributions of regional secondary processes (i.e. photochemical smog). On days without extensive BB, secondary formation was the dominant source of NACs, and NAC levels correlated strongly with the ambient ozone concentration. Analyses of individual NACs in the regionally aged plumes sampled on these days allowed precursors such as phenol and catechol to be linked to their NAC derivatives (i.e. nitrophenol and nitrocatechol). Correlation analysis using the high time resolution data and box-model simulation results constrained the relationships between these compounds and demonstrated the contribution of secondary formation processes. Furthermore, 13 of 16 NACS were classified according to primary or secondary formation process. Primary emission was the dominant source (accounting for 60 %–70 % of the measured concentrations) of 5 of the 16 studied NACs, but secondary formation was also a significant source. Photochemical smog thus has important effects on brown carbon levels even during wintertime periods dominated by primary air pollution in rural China.

Water-soluble organic aerosol (WSOA) constitutes a large fraction of organic aerosol (OA) and plays an important role in affecting aerosol hygroscopicity and cloud condensation nuclei formation. Here, we characterized WSOA in fine particles that were collected at ground level and at 260 m on a meteorological tower in Beijing in winter using offline aerosol mass spectrometry (AMS) and compared them with the simultaneous real-time online AMS measurements. Our results showed large increases in WSOA from nonheating season to heating season by 43% and 53% at ground level and at 260 m, respectively. WSOA was highly oxidized in winter with a higher oxygen-to-carbon ratio (O/C = 0.69) at 260 m than at ground level (0.63). On average, WSOA contributed 47% and 52% to the total OA at ground level and at 260 m, respectively. Positive matrix factorization analysis further showed that primary OA was the dominant source of WSOA (52−61%) with coal combustion (CCOA) and biomass burning OA (BBOA) being two major contributors. The vertical differences in water-soluble CCOA and BBOA, however, were very different with ubiquitously higher CCOA at 260 m than at ground level. Comparatively, more than 50% of secondary OA in winter was found to be water-insoluble, highlighting the influences of fossil emissions. The observed high water-solubilities of CCOA (42−53%) and BBOA (61−78%) may have significant implications in severe haze formation in winter in Beijing through a positive feedback mechanism between substantial increases in primary emissions during heating season and the subsequent changes in aerosol liquid water content, radiative forcing, and boundary layer, which is not yet considered in chemical-transport models.

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Abstract. 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.

Biomass burning (BB) aerosols, primary biological aerosol particles, and secondary organic aerosols, are critical components of particulate matter in the atmosphere. In this study, saccharides, lignin/resin acids, biogenic secondary organic aerosols tracers, aromatic acids, and hydroxyacids/polyacids were measured in total suspended particles collected from April 2012 to March 2013 at an urban site in Beijing using gas chromatography/mass spectrometry. The concentration of levoglucosan was elevated during 18-20 June. The high ratio of levoglucosan to mannosan and low ratio of galactosan to mannosan in June demonstrated the influence of wheat straw combustion in the North China Plain via long-rang transport. Sucrose, glucose, and fructose were more abundant in April, the spring bloom season. The maximum trehalose content (298 ng/m 3 ) occurred during a dust storm (27-29 April) with the highest stable carbon isotope ratio (δ 13 C, À21.2‰) of total carbon. Fungal spore tracers (arabitol and mannitol) peaked in summer. The largest concentrations of the oxidization products of isoprene and α/β-pinene also occurred in summer. However, the β-caryophyllene tracer was much more abundant on long-lasting haze days in winter and was positively related to levoglucosan. The contributions of biogenic primary sources (plant debris, fungal spores, and BB) to organic carbon were estimated to be in range of 2.64-35.2% (11.8%), with BB being the predominant source. Biogenic secondary sources (isoprene, α-pinene, and β-caryophyllene) and anthropogenic secondary source (naphthalene) accounted for 0.23-10.9% (3.36%) of organic carbon with large contributions in summer and winter, respectively.

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