The characteristics of ozone and atmospheric oxidation capacity analysis in summer in Weihai
Background,aim,and scope Ozone(O3)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 O3 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 O3 formation mechanisms,coastal cities like Weihai,which are influenced by both anthropogenic emissions and the marine environment,may exhibit distinct O3 formation pathways compared to inland urban areas.This study aims to identify the primary controlling factors and key precursors for O3 formation in such coastal city.Materials and methods The observational data were integrated with a chemistry box model to analyze O3 characteristics and atmospheric oxidation capacity(AOC)in Weihai.This study also evaluated the production and loss rates of O3,the relative increment reactivity(RIR)of major VOCs in O3 production,and explored O3 mitigation strategies through simulations of NOx and VOCs reduction.Results During the summer pollution episodes,O3 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 O3 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 O3 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 influencing O3 production during pollution episodes.Analysis of OH reactivity revealed that alkenes and aromatics were the most significant contributors to O3 formation,with their reactivity toward OH escalating from clean to polluted conditions.Furthermore,the O3 production rate escalated during polluted days,predominantly attributed to the enhanced AOC.Elevated AOC levels in the summer,coupled with O3 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 O3 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 O3 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 O3 levels in the afternoon when the photochemistry was the strongest in the daytime.Conclusions During the summer,O3 emerged as the primary pollutant on polluted days in Weihai,predominantly due to the intense local photochemical activity.Effective mitigation of O3 production could be achieved through the daytime control of VOCs and CO emissions,as the impact of NOx reduction on O3 formation was relatively minor in comparison to VOCs.Moreover,the control of O3 was found to be particularly sensitive to the reduction of alkene emissions.Consequently,mitigation strategies should be tailored based on a comprehensive assessment of O3 precursors,taking into account their varying contributions to O3 formation.Recommendations and perspectives O3 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 O3 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 influencing O3 pollution in coastal versus inland urban areas.The findings of this study suggest that the mitigation of O3 pollution should be formulated based on the local meteorological conditions and emission profiles to develop effective reduction strategies.
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