威海市夏季臭氧及大气氧化性特征分析

The characteristics of ozone and atmospheric oxidation capacity analysis in summer in Weihai

威海市属于我国北方典型沿海城市,近年来的持续观测发现夏季易出现臭氧(O_(3))污染。以2023年夏季观测数据为约束,利用零维盒子模式分析威海市夏季O_(3)及大气氧化性特征,给出O_(3)污染防治的建议。2023年5月30日—6月30日,威海市污染天的首要污染物是O_(3),烷烃是挥发性有机物(VOCs)中浓度占比最高的组分,烯烃则是活性占比最高的组分。从清洁日到污染日烯烃活性增强最显著,O_(3)的化学生成和大气氧化性在污染日均有增加,促进了二次污染过程的发生。威海夏季大气氧化性水平较强,O_(3)对前体物的敏感区处于氮氧化物(NOx)和VOCs的过渡区,O_(3)污染防控需结合NOx和VOCs进行协同防控。O_(3)对前体物烯烃减排的变化最敏感,因此其污染防控过程需结合VOCs反应活性开展。基于威海市夏季观测数据分析O_(3)及大气氧化性特征,从O_(3)敏感性区域变化及VOCs反应活性角度为O_(3)污染防治策略提供了思路。

Background,aim,and scope Ozone(O_(3))pollution has emerged as a significant concern for air quality in China,particularly as atmospheric fine particulate matter concentrations have declined in recent years.The complex formation of O_(3) is influenced by nitrogen oxides(NOx)and volatile organic compounds(VOCs),as well as by meteorological conditions and regional transport.While numerous urban studies have probed O_(3) formation mechanisms,coastal cities like Weihai,which are inf luenced by both anthropogenic emissions and the marine environment,may exhibit distinct O_(3) formation pathways compared to inland urban areas.This study aims to identify the primary controlling factors and key precursors for O_(3) formation in such coastal city.Materials and methods The observational data were integrated with a chemistry box model to analyze O_(3) characteristics and atmospheric oxidation capacity(AOC)in Weihai.This study also evaluated the production and loss rates of O_(3),the relative increment reactivity(RIR)of major VOCs in O_(3) production,and explored O_(3) mitigation strategies through simulations of NOx and VOCs reduction.Results During the summer pollution episodes,O_(3) emerged as the predominant pollutant.The concentration of alkanes increased from 6.1 nmol∙mol-1 to 9.1 nmol∙mol-1,accounting for the majority of the∙OH reactivity,with a substantial contribution exceeding 60%.In contrast,aromatics and alkenes contributed 18.2%and 17.3%,respectively.The overall reactivity of VOCs with the∙OH rose from 3.9 s-1 during clean days to 4.6 s-1 on polluted days.Concurrently,the net production rate of O_(3) increased from 6.0 nmol·mol-1·h-1 in clean days to 7.4 nmol∙mol-1∙h-1 under polluted days.The AOC during the daytime O_(3) pollution period reached 6.0×10-17 mol∙cm^(-3)∙s-1,surpassing the levels observed during clean days,which were 4.5×10-17 mol∙cm^(-3)∙s-1.The investigation of relative increase reactivity(RIR)analysis showed that NOx was negative between 06:00 and 10:00,while the RIR of NOx became positive after 10:00.In contrast,the RIR of VOCs remained positive throughout the day.Discussion The reactions of VOCs with∙OH were identified as the principal contributors to the AOC during daytime.This suggests that the heightened reactivity of diverse VOCs with∙OH may be an important factor inf luencing O_(3) production during pollution episodes.Analysis of∙OH reactivity revealed that alkenes and aromatics were the most significant contributors to O_(3) formation,with their reactivity toward∙OH escalating from clean to polluted conditions.Furthermore,the O_(3) production rate escalated during polluted days,predominantly attributed to the enhanced AOC.Elevated AOC levels in the summer,coupled with O_(3) existing in a transition sensitivity regime,imply that concurrent reduction of both NOx and VOCs could be beneficial for mitigating ozone pollution.Emission reduction simulations for various types of VOCs demonstrated that O_(3) was most sensitive to variations in alkene emissions,succeeded by aromatics and alkanes.This finding aligns with the previous analysis of the reactivity of VOCs with∙OH,reinforcing the notion that the formulation of O_(3) reduction strategies should prioritize the reduction of emissions based on the distinct reactivity profiles of VOCs with∙OH.This approach is more strategic than a blanket reduction of total VOCs concentrations.Additionally,the RIR value of NOx indicated that NOx emission abatement would be particularly beneficial for mitigating O_(3) levels in the afternoon when the photochemistry was the strongest in the daytime.Conclusions During the summer,O_(3) emerged as the primary pollutant on polluted days in Weihai,predominantly due to the intense local photochemical activity.Effective mitigation of O_(3) production could be achieved through the daytime control of VOCs and CO emissions,as the impact of NOx reduction on O_(3) formation was relatively minor in comparison to VOCs.Moreover,the control of O_(3) was found to be particularly sensitive to the reduction of alkene emissions.Consequently,mitigation strategies should be tailored based on a comprehensive assessment of O_(3) precursors,taking into account their varying contributions to O_(3) formation.Recommendations and perspectives O_(3) formation in urban areas is generally governed by VOCs emissions,with recent regulatory efforts primarily aimed at controlling VOCs emissions.This approach contrasts with the VOCs and NOx joint control strategy for O_(3) production observed in Weihai.The atmospheric environment of coastal cities,including factors such as relative humidity,temperature,and dilution conditions,differs from that of inland cities,and typically exhibits lower pollutant concentrations.These differences may account for the distinct dominant factors inf luencing O_(3) pollution in coastal versus inland urban areas.The findings of this study suggest that the mitigation of O_(3) pollution should be formulated based on the local meteorological conditions and emission profiles to develop effective reduction strategies.