This study reveals aerosol liquid water content (ALWC) in PM2.5 ranged from 2% up to 74%, and the associated secondary inorganic fraction rose from 24% to 55%, while ambient relative humidity (RH) increased from 15% to 83% in the atmosphere over Beijing. Unexpectedly, the secondary inorganic fraction in PM2.5 increased with an increase in the ambient RH, which is a meteorological parameter independent of anthropogenic activities, indicating the presence of a feedback mechanism driven by Henry's law and thermodynamic equilibrium. During haze episodes, simultaneously elevated RH levels and anthropogenic secondary inorganic mass concentrations resulted in an abundant ALWC. The condensed water could act as an efficient medium for multiphase reactions, thereby facilitating the transformation of reactive gaseous pollutants into particles and accelerating the formation of heavy haze. ALWC was well correlated with the mass concentrations of both nitrate and sulfate, indicating both nitrate and sulfate salts play key roles in determining ALWC. Coincident with a significant reduction in SO2 emissions throughout China, nitrates will become a dominant anthropogenic inorganic salt driving ALWC. Thus, the abundance of ALWC and its effects on the aerosol chemistry and climate should be reconsidered.

An intensive field campaign was conducted in Chongqing during the summer of 2015 to explore the formation mechanisms of ozone pollution. The sources of ozone, the local production rates, and the controlling factors, as well as key species of volatile organic compounds (VOCs), were quantified by integrating a local ozone budget analysis, calculations of the relative incremental reactivity, and an empirical kinetic model approach. It was found that the potential for rapid local ozone formation exists in Chongqing. During ozone pollution episodes, the ozone production rates were found to be high at the upwind station Nan Quan, the urban station Chao Zhan, and the downwind station Jin-Yun Shan. The average local ozone production rate was 30x10(-9) V/V h(-1) and the daily integration of the produced ozone was greater than 180x10(-9) V/V. High ozone concentrations were associated with urban and downwind air masses. At most sites, the local ozone production was VOC-limited and the key species were aromatics and alkene, which originated mainly from vehicles and solvent usage. In addition, the air masses at the northwestern rural sites were NO (x) -limited and the local ozone production rates were significantly higher during the pollution episodes due to the increased NO (x) concentrations. In summary, the ozone abatement strategies of Chongqing should be focused on the mitigation of VOCs. Nevertheless, a reduction in NO (x) is also beneficial for reducing the regional ozone peak values in Chongqing and the surrounding areas.

Dinitrogen pentoxide (N2O5) is one of the basic trace gases which plays a key role in nighttime atmosphere. An intercomparison and validation of different N2O5 measurement methods is important for determining the true accuracy of these methods. Cavity ring down spectroscopy (CRDS) and cavity enhanced absorption spectrometer (CEAS) were used to measure N2O5 at the campus of the University of Chinese Academy of Sciences (UCAS) from February 21, 2016 to March 4, 2016. The detection limits were 1.6 ppt (1 sigma) at 30 s intervals for the CEAS instrument and 3.9 ppt (1 sigma) at 10 s time resolution for the CRDS instrument respectively. In this study, a comparison of the 1 min observations from the two instruments was presented. The two data sets showed a good agreement within their uncertainties, with an absolute shift of 15.6 ppt, slope of 0.94 and a correlation coefficient R-2 = 0.97. In general, the difference between the CRDS and CEAS instruments for N2O5 measurement can be explained by their combined measurement uncertainties. However, high relative humidity (> 60%) and high PM2.5 concentration (> 200 mu g/m(3)) may contribute to the discrepancies. The excellent agreement between the measurement by the CRDS and CEAS instruments demonstrates the capability of the two instruments for accurately measuring N2O5 with high sensitivity. (C) 2017 Elsevier B.V. All rights reserved.

Nocturnal reactive nitrogen compounds play an important role in regional air pollution. Here we present the measurements of dinitrogen pentoxide (N2O5) associated with nitryl chloride (ClNO2) and particulate nitrate (pNO3-) in a suburban site of Beijing in the summer of 2016. High levels of N2O5 and ClNO2 were observed in the outflow of the urban Beijing air masses, with 1-min average maxima of 937 pptv and 2900 pptv, respectively. The N2O5 uptake coefficients, γ, and ClNO2 yield, f, were experimentally determined from the observed parameters. The N2O5 uptake coefficient ranged from 0.012 to 0.055, with an average of 0.034 ± 0.018, which is in the upper range of previous field studies reported in North America and Europe but is a moderate value in the North China Plain (NCP), which reflects efficient N2O5 heterogeneous processes in Beijing. The ClNO2 yield exhibited high variability, with a range of 0.50 to unity and an average of 0.73 ± 0.25. The concentration of the nitrate radical (NO3) was calculated assuming that the thermal equilibrium between NO3 and N2O5 was maintained. In NOx-rich air masses, the oxidation of nocturnal biogenic volatile organic compounds (BVOCs) was dominated by NO3 rather than O3. The production rate of organic nitrate (ON) via NO3+BVOCs was significant, with an average of 0.10 ± 0.07 ppbv h-1. We highlight the importance of NO3 oxidation of VOCs in the formation of ON and subsequent secondary organic aerosols in summer in Beijing. 

We present the in-situ measurements in Chengdu, a major city in south west of China, in September 2016. The concentrations of ozone and its precursor were measured at four sites. Although the campaign was conducted in early autumn, up to 100 ppbv (parts per billion by volume) daily maximum ozone was often observed at all sites. The observed ozone concentrations showed good agreement at all sites, which implied that ozone pollution is a regional issue in Chengdu. To better understand the ozone formation in Chengdu, an observation based model is used in this study to calculate the ROx radical concentrations (ROx = OH + HO2 + RO2) and ozone production rate (P(O3)). The model predicts OH daily maximum is in the range of 4–8 × 106 molecules cm−3 , and HO2 and RO2 are in the range of 3–6 × 108 molecules cm−3 . The modelled radical concentrations show a distinct difference between ozone pollution and attainment period. The relative incremental reactivity (RIR) results demonstrate that anthropogenic VOCs reduction is the most efficient way to mitigate ozone pollution at all sites, of which alkenes dominate >50% of the ozone production. Empirical kinetic modelling approach shows that three out of four sites are under the VOC-limited regime, while Pengzhou is in a transition regime due to the local petrochemical industry. The ozone budget analysis showed that the local ozone production driven by the photochemical process is important to the accumulation of ozone concentrations.

Particulate nitrate (pNO3-) is an important component of secondary aerosols in urban areas. Therefore, it is critical to explore its formation mechanism to assist with the planning of haze abatement strategies. Here we report vertical measurement of NOx and O3 by in-situ instruments on a movable carriage on a tower during a winter heavy-haze episode (December 18 to 20, 2016) in urban Beijing, China. Based on the box model simulation at different height, we found that pNO3- formation via N2O5 heterogeneous uptake was negligible at ground level due to N2O5 concentration of near zero controlling by high NO emission and NO concentration. In contrast, the contribution from N2O5 uptake was large at high altitudes (e.g., > 150 m), which was supported by the low total oxidant (NO2 + O3) level at high altitudes than that at ground level. Modeling results show the specific case that the nighttime integrated production of pNO3- for the high-altitude air mass above urban Beijing was estimated to be 50 μg m-3 and enhanced the surface-layer pNO3- the next morning by 28 μg m-3 through vertical mixing. Sensitivity tests suggested that the nocturnal NOx loss by NO3-N2O5 chemistry was maximized once the N2O5 uptake coefficient was over 2×10-3 on polluted days with Sa was 3000 μm2 cm-3 in wintertime. The case study provided a chance to highlight that pNO3- formation via N2O5 heterogeneous hydrolysis may be an important source of the particulate nitrate in the urban airshed during wintertime.

The first wintertime in-situ measurements of hydroxyl (OH), hydroperoxy (HO2) and organic peroxy (RO2) radicals (ROx=OH+HO2+RO2) in combination with observations of total reactivity of OH radicals, kOH in Beijing are presented. The field campaign “Beijing winter finE particle STudy – Oxidation, Nucleation and light Extinctions” (BEST-ONE) was conducted at the suburban site Huairou near Beijing from January to March 2016. It aimed to understand oxidative capacity during wintertime and to elucidate the secondary pollutants formation mechanism in the North China Plain (NCP). OH radical concentrations at noontime ranged from 2.4×106cm−3 in severely polluted air (kOH~27s−1) to 3.6×106cm−3 in relatively clean air (kOH~5s−1). These values are nearly two-fold larger than OH concentrations observed in previous winter campaign in Birmingham, Tokyo, and New York City. During this campaign, the total primary production rate of ROx radicals was dominated by the photolysis of nitrous acid accounting for 46% of the identified primary production pathways for ROxradicals. Other important radical sources were alkene ozonolysis (28%) and photolysis of oxygenated organic compounds (24%). A box model was used to simulate the OH, HO2 and RO2 concentrations based on the observations of their long-lived precursors. The model was capable of reproducing the observed diurnal variation of the OH and peroxy radicals during clean days with a factor of 1.5. However, it largely underestimated HO2 and RO2 concentrations by factors up to 5 during pollution episodes. The HO2 and RO2 observed-to-modeled ratios increased with increasing NO concentrations, indicating a deficit in our understanding of the gas-phase chemistry in the high NOxregime. The OH concentrations observed in the presence of large OH reactivities indicate that atmospheric trace gas oxidation by photochemical processes can be highly effective even during wintertime, thereby facilitating the vigorous formation of secondary pollutants.

Molecular chlorine (Cl-2) and nitryl chloride (GINO(2)) concentrations were measured using chemical ionization mass spectrometry at a rural site over the North China Plain during June 2014. High levels of daytime Cl-2 up to similar to 450 pptv were observed. The average diurnal Cl-2 mixing ratios showed a maximum around noon at pptv. ClNO2 exhibited a strong diurnal variation with early morning maxima reaching ppbv levels and afternoon minima sustained above 60 pptv. A moderate correlation (R-2 = 0.31) between Cl-2 and sulfur dioxide was observed, perhaps indicating a role for power plant emissions in the generation of the observed chlorine. We also observed a strong correlation (R-2 = 0.83) between daytime (10:00-20:00) Cl-2 and ClNO2, which implies that both of them were formed from a similar mechanism. In addition, Cl-2 production is likely associated with a photochemical mechanism as Cl-2 concentrations varied with ozone (O-3) levels. The impact of Cl-2 and ClNO2 as Cl atom sources is investigated using a photochemical box model. We estimated that the produced Cl atoms oxidized slightly more alkanes than OH radicals and enhanced the daily concentrations of peroxy radicals by 15% and the O-3 production rate by 19%.

Heterogeneous reactions of mineral dust aerosol with trace gases in the atmosphere could directly and indirectly affect tropospheric oxidation capacity, in addition to aerosol composition and physicochemical properties. In this article we provide a comprehensive and critical review of laboratory studies of heterogeneous uptake of OH, NO3, O-3, and their directly related species as well (including HO2, H2O2, HCHO, HONO, and N2O5) by mineral dust particles. The atmospheric importance of heterogeneous uptake as sinks for these species is assessed (i) by comparing their lifetimes with respect to heterogeneous reactions with mineral dust to lifetimes with respect to other major loss processes and (ii) by discussing relevant field and modeling studies. We have also outlined major open questions and challenges in laboratory studies of heterogeneous uptake by mineral dust and discussed research strategies to address them in order to better understand the effects of heterogeneous reactions with mineral dust on tropospheric oxidation capacity.

The heterogeneous hydrolysis of dinitrogen pentoxide (N2O5) is important to understanding the formation of particulate nitrate (pNO(3)(-)). Measurements of N2O5 in the surface layer taken at an urban site in Beijing are presented here. N2O5 was observed with large day-to-day variability. High N2O5 concentrations were determined during pollution episodes with the co-presence of large aerosol loads. The maximum value was 1.3 ppbv (5 s average), associated with an air mass characterized by a high level of O-3. N2O5 uptake coefficients were estimated to be in the range of 0.025-0.072 using the steady-state lifetime method. As a consequence, the nocturnal pNO(3)(-) formation potential by N2O5 heterogeneous uptake was calculated to be 24-85 mu g m(-3) per night and, on average, 57 mu g m(-3) during days with pollution. This was comparable to or even higher than that formed by the partitioning of HNO3. The results highlight that N2O5 heterogeneous hydrolysis is vital in pNO(3)(-) formation in Beijing.

A chemical box model was used to study nitrate radical (NO3), dinitrogen pentoxide (N2O5) and nitryl chloride (C1NO(2)) in a rural site during the Campaign of Air Quality Research in Beijing 2006 (CAREBeijing-2006). The model was based on regional atmospheric chemistry mechanism version 2 (RACM(2)) with the heterogeneous uptake of N2O5 and the simplified chloride radical (C1) chemistry mechanism. A high production rate of NO3 with a mean value of 0.8 ppbv/h and low mixing ratios of NO3 and N2O5 (peak values of 17 pptv and 480 pptv, respectively) existed in this site. Budget analysis showed that NO emission suppressed the NO3 chemistry at the surface layer, the reaction of NO3 with VOCs made a similar contribution to NO3 loss as N2O5 heterogeneous uptake. The NO3 chemistry was predominantly controlled by isoprene, and NO3 oxidation produced organic nitrate with a mean value of 0.06 ppbv/h during nighttime. The organic nitrate production initiated by NO3 was equal to that initiated by OH, implying the importance of nighttime chemistry for secondary organic aerosol (SOA) formation. We confirmed that the N2O5 heterogeneous reaction accounted for nighttime particle NO3 enhancement, with a large day to day variability, and made less of a contribution to NOx loss compared to that of OH reacting with NO2. Additionally, abundant C1NO(2), up to 5.0 ppbv, was formed by N2O5 heterogeneous uptake. C1NO(2) was sustained at a high level until noon in spite of the gradually increasing photolysis of C1NO(2) after sunrise. Chlorine activation caused by N2O5 heterogeneous uptake increased primary ROx formation by 5% and accounted for 8% of the net ozone production enhancement in the morning.