Ozone Pollution Characteristics And Formation Mechanism In A Typical Tropical Seaside City

With China’s air pollution prevention and control pushing forward,the nationwide concentrations of PM2.5 have dropped significantly in recent years.However,the concentrations of ozone(O3)show an increasing trend especially in summer and autumn.O3 has replaced PM2.5 as the primary pollutant in many urban areas.Haikou City is located in the north of Hainan Island,which is a typical tropical seaside city with high temperature and strong radiation all year round.The above meteorological conditions can accelerate the rate of atmospheric photochemical reactions,and at the same time enhance the evaporative emissions of VOCs as well as the emissions of VOCs from biological sources,which in turn affect the O3 concentration levels.O3 has become an important pollutant affecting the air quality in Haikou.Accurate understanding of O3 pollution characteristics and causes can develop effective O3 pollution prevention and control strategies.This study conducted an observation experiment on O3 and its precursors at an urban site in Haikou,Hainan Province,from June to October in 2019 and 2020.The O3 pollution characteristics were analyzed comprehensively;the O3-NOx-VOCs sensitivities and key precursors were determined,the control strategies for O3 pollution were carried out,and the key VOCs species and their key sources were analyzed.The 90th percentile of MDA8 O3 concentration in Haikou City from June to October 2020(118 μg/m3)was lower than that of the same period in 2019(134 μg/m3),and O3 pollution has been alleviated,but the percentage of O3 as the leading pollutant has increased,and O3 was gradually replacing particulate matter as the most important pollutant affecting ambient air quality in Haikou City.The trend of monthly average values shows that the high values of O3 concentration were mainly concentrated in September to October,and there was a seasonal change characteristic of low in summer and high in autumn.Pearson correlation coefficient analysis showed that the correlation between MDA8 O3 and other pollutants increased significantly in autumn,except for NO2 and TVOCs(Total VOCs),the correlation between MDA8 O3 and NO2 did not change significantly,while the Pearson correlation coefficient with TVOCs decreased to-0.04 with no significant correlation.The observed data of O3 precursors in Haikou City from June to October in 2019 and 2020 showed that the concentration of each pollutant decreased during the observation period from June to October 2020 compared to June to October 2019.The analysis of NOx and VOCs concentrations during O3 pollution from June to October in 2019 and 2020 found that NO,NO2 and VOCs concentrations on the O3 pollution days were much higher than the attainment days,and the increase in pollutant concentrations on the O3 pollution days provided sufficient reactants for atmospheric photochemical reactions to further promote O3 generation,and the concentrations of the above pollutants changed more significantly during the morning and evening peak hours of residents’ travel,and the increase in motor vehicle pollutant emissions during the morning and evening peak hours of residents’travel was an intrinsic factor causing O3 pollution in Haikou City.In this study,the relationship between meteorological conditions and O3 concentration was analyzed.It was found that the O3 concentration in Haikou showed a significant negative correlation with relative humidity and a poor correlation with temperature,and the O3 concentration was prone to high values when the wind direction was northeast.The HYSPLIT model was used to analyze the backward 24 h air mass trajectories at target locations during O3 pollution hours,and the results showed that O3 in Haikou was influenced by regional transport,with pollutants mainly coming from the northeastern part of Haikou City.O3-NOx-VOCs sensitivity in Haikou City was in the VOCs and NOx transitional regime,and the most sensitive precursors in various months were different.O3 formation in September was most sensitive to anthropogenic VOCs,but NOx in October.The relative changes of O3 concentration under different reduction ratios of VOCs and NOx indicated that the reduction ratio of VOCs to NOx should be 1:1-4:1 to control O3 pollution effectively in the future in Haikou City.To further investigate the effects of different VOCs species and sources on O3 pollution in Haikou,this study calculated the differences in the contributions of different VOCs components to O3 generation based on OFP,and found that olefins OFP values were the largest in June to October 2019,followed by alkanes,while alkanes OFP values were the largest in June to October 2020,followed by olefins.By species,isoprene,ethylene,toluene,m-p-xylene,nbutane,methylcyclopentane,1-butene and propylene are the key precursors of O3,and priority control of the above-mentioned species with high potential for O3 generation can mitigate O3 pollution in the urban area of Haikou.The source analysis of VOCs from June to October 2019 and 2020 using PMF shows that the main VOCs sources are both motor vehicle emissions(gasoline vehicle emissions and diesel vehicle emissions)and industrial emissions,and further combined with the OFP analysis shows that the OFP of motor vehicle emissions VOCs is the largest among anthropogenic sources,with gasoline vehicle emissions and diesel vehicle emissions being 19.7%and In addition,the contribution of plant sources is also not negligible,with an OFP contribution of 39.0%.The results of this thesis will help to improve the understanding of the characteristics of O3 pollution in urban areas of Haikou,provide scientific basis for the formulation of O3 pollution prevention and control strategies in Haikou,and also help to explore the characteristics and causes of urban O3 pollution in tropical areas,which is of significance to the study of O3 formation mechanism in the field of atmospheric photochemistry in China.