Causes And Source Apportionment Of Ozone Pollution In Coal Chemical Industrial City Over The Valley Region Of Western China

In recent years,ozone pollution has become a key problem that restricts the continuous improvement of air quality in China.Although the ozone pollution has been preliminarily controlled in 2020,further efforts are still needed to fundamentally solve the problem.Clarifying the emission characteristics and dynamic trends of air pollutants,quantifying the contribution of local emissions,regional transport and meteorological conditions to ozone,exploring formation mechanism of ozone pollution and identifying key chemical reactions and species are the basis for objectively understanding the ozone pollution status and formulating scientific pollution control policies and measures.Taking Wuhai,a typical coal chemical industry city in Western China,as study area,this study first analyzed the spatial and temporal distribution characteristics of ozone,constructed the high-resolution emission inventory of Wuhai（HEI-WH18）,and dynamically updated and evaluated it.Secondly,the synergistic effects of various air pollutants on the meteorological fields at the junction of Mongolian Plateau,Loess Plateau and Tibetan Plateau in summer were discussed using Weather Research and Forecasting Model（WRF）and Weather Research and Forecasting Model Coupled to Chemistry（WRF-Chem）models,and the potential mechanism of the effects was explained.The influence of meteorological fields on the ozone formation was quantified and the meteorological factors and its combinations which had significant impact on ozone were screened out using the geographical detector model.Thirdly,the important role of chemical process in ozone pollution was determined through a series of typical ozone pollution processes analysis,and the main formation and consumption reactions of ozone were classified and quantified based on the results of the integrated reaction rate module.The contribution of different emission regions and sources to ozone concentration and its chemical formation was discussed combined with brute force method.Finally,the formation,transformation and termination reactions of hydroxyl radicals（OH）,peroxy hydroxyl radical（HO₂）and peroxyalkyl radical（RO₂）in different function areas during the pollution process are classified and discussed to further clarify the key pathways and species in photochemical reaction.This study lays a foundation for the case database of research on ozone pollution formation mechanism in Wuhai,and provides reference methods for exploring the causes of ozone pollution in coal chemical industrial cities with complex topography.The main conclusions of this study are as follows:（1）The ozone concentration increased first and then decreased from 2015 to2020 in Wuhai.The ozone pollution is relatively serious from May to August,and the ozone concentration is highest from 13:00 to 16:00.HEI-WH18 could capture the variation and magnitude of ozone and its precursors better compared with the MEIC,which is suitable for the O₃simulation and source apportionment in summer.Haibowan region was a high-value area of ozone during the daytime,and the three industrial parks were low-value areas of ozone and high-value areas of NO₂during the daytime and nighttime.The spatial distribution characteristics of CO were consistent with the spontaneous combustion of coal and coal Gangue source.（2）The synergistic effects of various air pollutants led to negative and positive variation patterns of meteorological fields.In the positive variation pattern,the solar radiation flux reaching the ground and the outgoing longwave radiation flux at the top of the atmosphere were increased by approximately 2%～8%and 1%～4%,respectively,the surface was warmed by 0.2～0.6 K,planetary boundary layer height increased by3%～12%,and relative humidity decreased by 1～4%.The positive variation pattern was driven by the evaporation of clouds due to the synergistic semidirect effects of air pollutants,which was more significant in the junction region.The negative variation pattern was caused by the direct absorption and scattering of solar radiation by air pollutants.The average change of ozone caused by meteorological conditions during the day was-1.22%～-17.51%.The interaction between wind direction and NO₂,wind direction and meteorological factors such as temperature,air pressure and relative humidity had great influence on the ozone concentration.The change of relative humidity and its interaction with change of air pressure and temperature may be the main reason for the change of ozone.（3）Chemical process generally consumed the surface ozone and promoted ozone formation at the middle and upper boundary layers,which was transmitted to the ground or upper layers through vertical mixing process."Completely new increase"was the main pathways of ozone direct chemical formation.The reaction between ozone and NO contributed significantly to ozone consumption in the surface,especially in Hainan industrial park,because the surface NO concentration was higher than that at the middle and upper layers,and the ozone consumption was dominant by photolysis at the middle and upper layers.From the perspective of regional sources contribution,the sources in Northwest China had the greatest contribution to the average ozone concentration,followed by the sources in North China,with average daytime contribution of 26.27%and 7.60%,respectively.The average contribution of local emissions to ozone during the daytime and nighttime was 48.09%and 51.83%respectively.Both the local emissions and regional emissions in Northwest China contributed to ozone consumption in the chemical reactions at surface.Among the local sources,the mobile sources had the highest chemical consumption rate of surface ozone,which may be due to emission of mobile sources was near the surface.（4）From the perspective of free radicals formation,the formation rates of newly increased OH,HO₂and RO₂radicals were lower than the cyclic formation rate of these radicals,and the reactions involving free radicals play an important role in the ozone pollution formation and maintenance.The newly increased OH radical was produced by"ozone photolysis"and"ozone reacts with directly emitted VOCs",and"OH reacts with directly emitted VOCs"and"photolysis of directly emitted VOCs"contributed greatly to the newly increased HO₂and RO₂radical.The main VOCs model species were acrylic（C₃H₆）、formaldehyde（CH₂O）、acetaldehyde（CH₃CHO）、lumped alkanes（C>3,BIGALK）、lumped alkenes（C>3,BIGENE）,etc.CH₂O and CH₃CHO are mainly emitted from synthetic resin production,mobile source and spontaneous combustion of coal and coal gangue in industrial processes.BIGALK and BIGENE are mainly emitted from biomass combustion,coking,fine chemicals industry and manufacture of non-metallic products.C₃H₆is mainly emitted by the manufacture of non-metallic products.（5）"Reaction of HO₂and NO"and"reaction of RO₂and NO"contributed greatly to the cyclic OH,HO₂and RO₂radical formation.NO plays an important role in the mutual transformation of RO₂and the circulation of RO₂→HO₂→OH.As the NO concentration in Hainan industrial park was higher than that in other three areas,the contribution rate of the above pathways in Hainan industrial park was significantly higher than that in urban and Qianlishan industrial park.RO₂,HO₂and ozone competed for NO,which promoted the ozone accumulation.Hainan industrial park was low-value area of ozone due to high NO concentration and more significant titration effect,followed by Wuda industrial park.The urban area and Qianlishan industrial park were more prone to occur ozone pollution,as the NO concentration in there was lower than that in Hainan and Wuda industrial park.