Abstract. We analyzed measurements of ozone (O3) and its precursors made at rural/suburban sites downwind of four large Chinese cities – Beijing, Shanghai, Guangzhou and Lanzhou, to elucidate their pollution characteristics, regional transport, in situ production, and impacts of heterogeneous processes. The same measurement techniques and observation-based model were used to minimize uncertainties in comparison of the results due to difference in methodologies. All four cities suffered from serious O3 pollution but showed different precursor distributions. The model-calculated in situ O3 production rates were compared with the observed change rates to infer the relative contributions of on-site photochemistry and transport. At the rural site of Beijing, export of the well-processed urban plumes contributed to the extremely high O3 levels (up to an hourly value of 286 ppbv), while the O3 pollution observed at suburban sites of Shanghai, Guangzhou and Lanzhou was dominated by intense in-situ production. The O3 production was in a VOCs-limited regime in both Shanghai and Guangzhou, and a NOx-controlled regime in Lanzhou. The key VOC precursors are aromatics and alkenes in Shanghai, and aromatics in Guangzhou. The potential impacts on O3 production of several heterogeneous processes, namely, hydrolysis of dinitrogen pentoxide (N2O5), uptake of hydro peroxy radical (HO2) on particles and surface reactions of NO2 forming nitrous acid (HONO), were assessed. The analyses indicate the varying and considerable impacts of these processes in different areas of China depending on the atmospheric abundances of aerosol and NOx, and suggest the urgent need to better understand these processes and represent them in photochemical models.

<p><strong>Abstract.</strong> Nitryl chloride (ClNO<sub>2</sub>) is a dominant source of chlorine radical in polluted environment and can significantly affect the atmospheric oxidative chemistry. However, the abundance of ClNO<sub>2</sub> and its exact role are not fully understood under different environmental conditions. During the summer of 2014, we deployed a chemical ionization mass spectrometer to measure ClNO<sub>2</sub> and dinitrogen pentoxide (N<sub>2</sub>O<sub>5</sub>) at a rural site in the polluted North China Plain. Elevated mixing ratios of ClNO<sub>2</sub> (> 350 pptv) were observed at most of the nights with low levels of N<sub>2</sub>O<sub>5</sub> (< 200 pptv). The highest ClNO<sub>2</sub> mixing ratio of 2070 pptv (1-min average) was observed in a plume from megacity (Tianjin) and was characterized with faster N<sub>2</sub>O<sub>5</sub> heterogeneous loss rate and ClNO<sub>2</sub> production rate compared to average condition. The abundant ClNO<sub>2</sub> concentration kept increasing even after sunrise and reached a peak 4 hours later. Such highly sustained ClNO<sub>2</sub> peaks after sunrise are discrepant from the previously observed typical diurnal pattern. Meteorological and chemical analysis show that the sustained ClNO<sub>2</sub> morning peaks are caused by significant ClNO<sub>2</sub> production in the residual layer at night followed by downward mixing after break-up of the nocturnal inversion layer in the morning. We estimated that ~ 1.7–4.0 ppbv of ClNO<sub>2</sub> would exist in the residual layer in order to maintain the observed morning ClNO<sub>2</sub> peaks at the surface site. Observation-based box model analysis show that photolysis of ClNO<sub>2</sub> produced chlorine radical with a rate up to 1.12 ppbv h<sup>−1</sup>, accounting for 10–30 % of primary RO<sub><i>x</i></sub> production in the morning hours. The perturbation in total radical production leads to an increase of integrated daytime net ozone production by 3 % (4.3 ppbv) on average, and with a larger increase of 13 % (11 ppbv) in megacity outflow that was characterized with higher ClNO<sub>2</sub> and relatively lower OVOC to NMHC ratio.</p>

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Abstract. The chlorine atom (Cl·) has a high potential to perturb atmospheric photochemistry by oxidizing volatile organic compounds (VOCs), but the exact role it plays in the polluted troposphere remains unclear. The Master Chemical Mechanism (MCM) is a near-explicit mechanism that has been widely applied in the atmospheric chemistry research. While it addresses comprehensively the chemistry initiated by the OH, O3 and NO3 radicals, its representation of the Cl· chemistry is incomplete as it only considers the reactions for alkanes. In this paper, we develop a more comprehensive Cl· chemistry module that can be directly incorporated within the MCM framework. A suite of 205 chemical reactions describes the Cl·-initiated degradation of alkenes, aromatics, alkynes, aldehydes, ketones, alcohols, and some organic acids and nitrates, along with the inorganic chemistry involving Cl· and its precursors. To demonstrate the potential influence of the new chemistry module, it was incorporated into a MCM box model to evaluate the impacts of nitryl chloride (ClNO2), a product of nocturnal halogen activation by nitrogen oxides (NOX), on the following day's atmospheric photochemistry. With constraints of recent observations collected at a coastal site in Hong Kong, southern China, the modeling analyses suggest that the Cl· produced from ClNO2 photolysis may substantially enhance the atmospheric oxidative capacity, VOC oxidation and O3 formation, particularly in the early morning period. The results demonstrate the critical need for photochemical models to include more detailed chlorine chemistry in order to better understand the atmospheric photochemistry in polluted environments subject to intense emissions of NOX, VOCs and chlorine-containing constituents.

Abstract. Heterogeneous processes on dust particles are important for understanding the chemistry and radiative balance of the atmosphere. This paper investigates an intense Asian dust storm episode observed at Mount Heng (1269 m a.s.l.) in southern China on 24-26 April 2009. A set of aerosol and trace gas data collected during the study was analyzed to investigate their chemical evolution and heterogeneous photochemistry as the dust traveled to southern China. Results show that the mineral dust arriving at Mt. Heng experienced significant modifications during transport, with large enrichments in secondary species (sulfate, nitrate, and ammonium) compared with the dust composition collected at an upwind mountain top site (Mount Hua). A photochemical age "clock" (−Log 10 (NO x /NO y )) was employed to quantify the atmospheric processing time. The result indicates an obvious increase in the abundance of secondary water-soluble ions in dust particles with the air mass atmospheric processing time. Based on the observations, a 4-stage evolution process is proposed for carbonate-containing Asian dust, starting from fresh dust to particles coated with hydrophilic and acidic materials. Daytime-enhanced nitrite formation on the dust particles was also observed, which indicates the recent laboratory result of the TiO 2 photocatalysis of NO 2 as a potential source of nitrite and nitrous acid.