The oxidation of nitric oxide to nitrogen dioxide by hydroperoxy (HO2) and organic peroxy radicals (RO2) is responsible for the chemical net ozone production in the troposphere and for the regeneration of hydroxyl radicals, the most important oxidant in the atmosphere. In Summer 2014, a field campaign was conducted in the North China Plain, where increasingly severe ozone pollution has been experienced in the last years. Chemical conditions in the campaign were representative for this area. Radical and trace gas concentrations were measured, allowing for calculating the turnover rates of gas-phase radical reactions. Therefor; the importance of heterogeneous HO(2 )uptake on aerosol could be experimentally determined. HO2 uptake could have suppressed ozone formation at that time because of the competition with gas-phase reactions that produce ozone. The successful reduction of the aerosol load in the North China Plain in the last years could have led to a significant decrease of HO2 loss on particles, so that ozone-forming reactions could have gained importance in the last years. However, the analysis of the measured radical budget in this campaign shows that HO2 aerosol uptake did not impact radical chemistry for chemical conditions in 2014. Therefore, reduced HO2 uptake on aerosol since then is likely not the reason for the increasing number of ozone pollution events in the North China Plain, contradicting conclusions made from model calculations reported in the literature.
The heterogeneous hydrolysis of dinitrogen pentoxide (N2O5) plays an important role in regulating NOx. The N2O5 uptake coefficient, c(N2O5), was determined using an iterative box model that was constrained to observational data obtained in suburban Beijing from February to March 2016. The box model determined 2289 individual c(N2O5) values that varied from <0.001 to 0.02 with an average value of 0.0046 +/- 0.0039 (and a median value of 0.0032). We found the derived winter c(N2O5) values in Beijing were relatively low as compared to values reported in previous field studies conducted during winter in Hong Kong (average value of 0.014) and the eastern U.S. coast (median value of 0.0143). In our study, field evidence of the suppression of c(N2O5) values due to pNO3 content, organics and the enhancement by aerosol liquid water content (ALWC) is in line with previous laboratory study results. Low ALWC, high pNO3 content, and particle morphology (inorganic core with an organic shell) accounted for the low c (N2O5) values in the North China Plain (NCP) during wintertime. The field-derived c(N2O5) values are well reproduced by a revised parameterization method, which includes the aerosol size distribution, ALWC, nitrate and organic coating, suggesting the feasibility of comprehensive parameterization in the NCP during wintertime. (C) 2020 Science China Press. Published by Elsevier B.V. and Science China Press. All rights reserved.
In China, a significant reduction in primary pollution has been observed due to the Clean Air Action since 2013, and ozone pollution has become increasingly prominent over the past years. Pearl River Delta (PRD) is one of the most successful regions concerning primary pollution control, while is suffering from severe ozone pollution during autumn. In this study, we present a field campaign in Shenzhen, a megacity in PRD, in October 2018 with measurements of ozone and photochemical precursors. These observational data are helpful to analyze the local ozone budget and its sensitivity to precursors with the help of an observation-based model (RACM2-LIM1). The observed ozone concentration was up to 121 ppbv during a photochemical episode from 1 to 8 October, when intensive ozone formation up to tens of ppbv/h was found. Ozone vertical measurement indicates the fast ozone production is happening throughout the planetary boundary layer (PBL), which is an important source of morning ozone increase resulting in ozone pollution. An explicit case study is performed to reveal the diurnal feature of instantaneous ozone production rate (P(O-x)) and accumulative P(O-x) based on the O-3-NOx-VOC sensitivity, ROx radical primary production rate (P (ROx)), and L-N/Q for three cases including ozone pollution and attainment periods. Results show that nitrogen oxides (NOx = NO + NO2) reduction have positive and negative impact on local ozone production from one pollution episode to the other, which indicates the complexity of O-3-precursors sensitivity and difficulty to control ozone pollution in Shenzhen. Finally, comparison among measurements in other campaigns provides additional evidence on local ozone production sensitivity on NOx and anthropogenic volatile organic compounds (AVOCs) with respect to a temporal and spatial change. The
Formaldehyde (HCHO) is one of the most important intermediate products of atmospheric photochemical reactions and is also a radical source that promotes ozone formation. Given its high solubility, HCHO is likely to exist in particulate form. In this work, gaseous HCHO (HCHOg) and particulate HCHO (HCHOp) were separated and collected by a rotating wet annular denude (RWAD) and an aerosol growth chamber-coil aerosol cooler (AC). The collected HCHO from the RWAD and AC are measured by two online Hantzsch method-based formaldehyde analyzers. The comprehensive campaign was held in the Yangtze River Delta of China from 15 May to 18 June 2018, which is during the harvest season. Several biomass burning events were identified by using acetonitrile as a tracer. During the period influenced by biomass burning, the mixing ratios of HCHOg and HCHOp were respectively 122% and 231% higher than those during other time periods. The enhancement ratio of HCHOg to acetonitrile obtained from this work generally agrees with those from the existing literature. Biomass burning contributed 14.8% to HCHOg, but the abundant freshly discharged precursors it emitted greatly promoted the secondary production of HCHOg. We suggest that the high concentration of HCHOp during the biomass burning period was from uptake of HCHOg by aerosols during their transportation; the liquid state particles are conducive to HCHOg uptake. High relative humidity, a low particle rebound fraction f, as well as low temperatures may result in higher uptake coefficient values.
During the EXPLORE-YRD campaign (EXPeriment on the eLucidation of the atmospheric Oxidation capacity and aerosol foRmation, and their Effects in Yangtze River Delta) in May June 2018, we measured N2O5, NO2, O-3 and relevant parameters at a regional site in Taizhou, Jiangsu Province. The nocturnal average NO3 production rate was 1.01 +/- 0.47 ppbvh(-1), but the mixing ratio of N2O5 was low, with a maximum of 220 pptv in 1 min, suggesting rapid loss of NO3 and N2O5. The nocturnal steady-state lifetime of N2O5 was 43 + 52 s on average, which may be attributed to the elevated monoterpene and fast N2O5 uptake. VOCs (mainly monoterpenes) dominated daily NO3 loss with the percentage of 36.4% and N2O5 uptake accounted for 14.4%, when taking NO + NO3 and NO3 photolysis into consideration. We demonstrated that the nonnegligible daytime NO3 oxidation of monoterpene in YRD region, which contributes to the daytime formation of organic nitrate and secondary organic aerosol. The daily average NOx consumption rate via rapid NO3 reaction reached 0.63 ppbvh(-1), corresponding to 57.3% NOx loss in comparison with the OH oxidation pathway at this site, highlighting the key role of NO3 and N2O5 in NOx removal and subsequent photochemistry in the YRD region.
As has been the case in North America and western Europe, the SO2 emissions have substantially reduced in the North China Plain (NCP) in recent years. Differential rates of reduction in SO2 and NOx concentrations result in the frequent occurrence of particulate matter pollution dominated by nitrate (pNO(3)(-)) over the NCR. In this study, we observed a polluted episode with the particulate nitrate mass fraction in nonrefractory PM1 (NR-PM1) being up to 44 % during wintertime in Beijing. Based on this typical pNO(3)(-)-dominated haze event, the linkage between aerosol water uptake and pNO(3)(-) enhancement, further impacting on visibility degradation, has been investigated based on field observations and theoretical calculations. During haze development, as ambient relative humidity (RH) increased from similar to 10 % to 70 %, the aerosol particle liquid water increased from similar to 1 mu g m(-3) at the beginning to similar to 75 mu g m(-3) in the fully developed haze period. The aerosol liquid water further increased the aerosol surface area and volume, enhancing the condensational loss of N2O5 over particles. From the beginning to the fully developed haze, the condensational loss of N2O5 increased by a factor of 20 when only considering aerosol surface area and volume of dry particles, while increasing by a factor of 25 when considering extra surface area and volume due to water uptake. Furthermore, aerosol liquid water favored the thermodynamic equilibrium of HNO3 in the particle phase under the supersaturated HNO3 and NH3 in the atmosphere. All the above results demonstrated that pNO(3)(-) is enhanced by aerosol water uptake with elevated ambient RH during haze development, in turn facilitating the aerosol take-up of water due to the hygroscopicity of particulate nitrate salt. Such mutual promotion between aerosol particle liquid water and particulate nitrate enhancement can rapidly degrade air quality and halve visibility within 1 d. Reduction of nitrogen-containing gaseous precursors, e.g., by control of traffic emissions, is essential in mitigating severe haze events in the NCP.
Heterogeneous reactivity of N2O5 on aerosols is a critical parameter in assessing NOx fate, nitrate production, and particulate chloride activation. Accurate measurement of its uptake coefficient (gamma N2O5) and representation in air quality models are challenging, especially in the polluted environment. With an in situ aerosol flow-tube system, the gamma N2O5 was directly measured on ambient aerosols at two rural sites in northern and southern China. The results were analyzed together with the gamma N2O5 derived from previous field studies in China to obtain a holistic picture of gamma N2O5 uptake and the influencing factors under various climatic and chemical conditions. The field-derived or measured gamma N2O5 was generally promoted by the aerosol water content and suppressed by particle nitrate. Significant discrepancies were found between the measured gamma N2O5 and that estimated from laboratory-determined parameterizations. An observation-based empirical parameterization was derived in the present work, which better reproduced the mean value and variability of the observed gamma N2O5. Incorporating this new parameterization into a regional air quality model (WRF-CMAQ) has improved the simulation of N2O5, nitrogen oxides, and secondary nitrate in the polluted regions of China.
Particulate nitrate (pNO3–) has often been found to be the major component of fine particles in urban air-sheds in China, the United States, and Europe during winter haze episodes in recent years. However, there is a lack of knowledge regarding the experimentally determined contribution of different chemical pathways to the formation of pNO3–. Here, for the first time, we combine ground and tall-tower observations to quantify the chemical formation of pNO3– using observationally constrained model approach based on direct observations of OH and N2O5 for the urban air-shed. We find that the gas-phase oxidation pathway (OH+NO2) during the daytime is the dominant channel over the nocturnal uptake of N2O5 during pollution episodes, with percentages of 74% in urban areas and 76% in suburban areas. This is quite different from previous studies in some regions of the US, in which the uptake of N2O5 was concluded to account for a larger contribution in winter. These results indicate that the driving factor of nitrate pollution in Beijing and different regions of the US is different, as are the mitigation strategies for particulate nitrate.