西安市大气臭氧污染光化学特征与敏感性分析

Analysis of the photochemical characteristics and sensitivity of ozone pollution in Xi’an

汾渭平原是我国综合交通枢纽,也是重要的产业基地,但2017~2019年间臭氧均值浓度达到184μg/m;.西安市作为汾渭平原上的特大型城市,自2018年以来臭氧年均浓度始终超过国家二级标准.2019年夏季在西安市选择一段典型臭氧污染过程,将其分为起始、加重和清除3个阶段,利用基于观测模型(obserbation based model,OBM)对各阶段的臭氧污染特征与敏感性进行分析.结果发现,在污染加重阶段出现强辐射、高温、低湿和低压等有利于臭氧产生的气象条件,并且该阶段的高温和强辐射提高了异戊二烯与含氧挥发性有机物(oxygenated volatile organic compounds,OVOCs)的占比.敏感性分析表明浐灞站点处于过渡区,中国科学院地球环境研究所(简称"地环所")站点处于挥发性有机物(volatile organic compounds,VOCs)控制区,秦岭站点处于NO;控制区,且3个站点在污染过程的3个阶段中臭氧控制区类型未发生变化.将源解析方法与敏感性分析相结合,发现臭氧生成在起始阶段对各个排放源的敏感性要高于加重阶段;在臭氧污染加重阶段的局地臭氧产生速率要显著高于起始和清除阶段,浐灞、地环所和秦岭臭氧产生速率最大值分别为17.7、33.9和14.9 ppbv/h.综上,对于持续时间较长的臭氧污染过程,其臭氧浓度控制策略应该适当将时间提前到起始阶段,此时臭氧对前体物的敏感性更强,控制前体物浓度对降低臭氧生成的效率最高.

The Fenwei Plain is an important agricultural production base and transportation hub in China with a residential population of over 55 million.With rapid industrialization and urbanization,this area has experienced serious photochemical pollution in the summer season.In this study,a field campaign was conducted in summer 2019 with measurements of ozone and photochemical precursors in Xi’an,a megacity in the Fenwei Plain.An explicit case was performed to explore O;-NO;VOCs sensitivity and the local ozone budget using an observation-based model.Results revealed that precursors significantly promoted ozone production.The positive matrix factorization(PMF)model combined with the relative incremental reactivity(RIR)method was applied to identify the key VOC sources that affect ozone formation.An ozone pollution episode lasting 11 d was selected and divided into three periods:Initial(July 8–11),polluted(July12–15),and sweep(July 16–18)periods.It was found that strong solar radiation intensity,high temperature,low relative humidity,and low pressure prevailed during the polluted period,which favored ozone production.The maximum O;concentrations at the upwind CB site and urban DHS site were found to be 125 ppbv during the polluted period,whereas at the QL site,the maximum O;concentrations were much lower(98 ppbv).The maximum local ozone production rate(P(Ox))of the three sites during the polluted period was significantly higher than that in the initial and sweep periods.The P(Ox)was 17.7,33.9,and 14.9 ppbv/h for the CB,DHS,and QL sites,respectively.The in situ Oxbudget demonstrated that ozone pollution at the CB and DHS sites was mainly attributed to local photochemical production,whereas the regional transportation effect could not be ignored at the QL site.The O;-NO;-VOCs sensitivity analysis revealed that ozone production regimes were transitional,VOCs-limited,and NO;-limited at the CB,DHS,and QL sites,respectively,and the regimes stayed unchanged for all the three sites during the entire ozone pollution episode.However,during the initial period at the DHS site,NO;reduction led to a negative effect on ozone control.The ozone formation regime obtained from the emission rate box model was consistent with that of the RIR method.From the proportion of grouped VOCs,the alkane and OVOCs were the dominant species in the mixing ratio(78%–86%),whereas the contribution of aromatics to OFP(16%–24%)was second only to that of OVOCs.Finally,by combining the PMF model and RIR method,ozone production was found to be more sensitive to precursors reduction in the initial period than in the polluted period at the three sites,which indicated that pollutants control measures for ozone pollution mitigation will be more effective in the initial than in polluted periods.Therefore,during an ozone pollution episode,the time to take action for ozone pollution control is not during the polluted period when the ozone concentrations peak,but should be brought forward,when controlling the concentration of precursors has the highest efficiency in reducing ozone production.