Ozone Pollution Characteristics,Formation Mechanism,and Prevention Strategies In A Valley Petrochemical Industrial City,Northwestern China

In recent years,the problem of ozone(O₃)pollution in China has become increasingly evident,and O₃ has become an important factor limiting the improvement of ambient air quality in China.O₃ is a secondary gaseous pollutant with a complex formation mechanism and a highly non-linear relationship with precursors.Since the1970s,O₃ pollution has become a prominent environmental problem in Lanzhou due to factors such as petrochemical industry,complex terrain and unfavorable weather conditions.However,there is still a lack of in-depth understanding of O₃formation mechanisms and pollution control in petrochemical industrial areas with complex terrain,and there is an urgent need for basic scientific research to be carried out to guide the accurate control of O₃ pollution in Lanzhou.In this context,this study analyzed the spatial and temporal variation characteristics of O₃ and its precursors in Lanzhou using ground monitoring data,and introduced the morning ozone peak(MOP)phenomenon in Lanzhou.To study the formation mechanism of O₃ pollution in Lanzhou,a high-resolution anthropogenic emission inventory of Lanzhou(HEI-LZ16)was established.Then,the WRF-Chem model was utilized to design sensitivity tests to study the effects of complex topography on O₃ and analyze the causes of MOP phenomenon.Further,based on the observed data of trace gases and meteorological variables,an observation-based model(OBM)was employed to investigate the mechanism of O₃ generation.Finally,based on multiple approaches,a strategy for O₃ pollution control in Lanzhou from the perspective of VOCs species was proposed.The major conclusions are as follows:1.From 2014-2018,the maximum daily 8-hour mean 90th percentile concentration(MDA8-90)of O₃ in Lanzhou increased year by year,and then slowly decreased from 2018-2021,with the maximum O₃ MDA8-90 occurring in 2018(168.2?g/m~3).In the summer of 2014-2021,the number of days with MOP days in the petrochemical industrial zone accounted for 27.7%of the total number of days,and the percentage of MOP days among O₃ exceedance days was 43.5%.The O₃MDA8concentrations on MOP and NMOP days were 162.9±54.1 and 130.6±46.2?g/m~3,respectively.From the perspective of controlling O₃ pollution,controlling the occurrence of MOP phenomenon can significantly reduce the O₃ exceedance days at the LH site.Correlation analysis and significance test results showed that strong radiation,high temperature and low humidity were favorable meteorological conditions for the occurrence of MOP phenomenon,while the morning concentrations of VOCs and NO₂ were higher on MOP days than on NMOP days.2.The total anthropogenic emissions of SO₂,NO_x,CO,NH₃,VOCs,PM₁₀,PM_2.5,BC and OC in Lanzhou in 2016 were 25642,53998,319003,10475,35289,49250,19822,2476 and 1482 t/a,respectively.Fossil fuel combustion made the largest contributions to SO₂ and CO(84.1%and 45.9%,respectively),mobile sources for NO_xemissions(54.0%),agricultural sources for NH₃ emissions(89.3%),petrochemicals for VOCs emissions(25.3%),road dust for PM₁₀ and PM_2.5 emissions(56.4%and 33.9%,respectively),and biomass combustion for BC and OC(59.7%and 78.9%,respectively).3.During the day,the decrease in downward solar surface radiation(SR)under the shading of the mountains showed a significant correlation with the decrease in O₃concentration(R=0.62,p<0.01).At night,mountain winds carried high O₃concentrations from the mountains into the valley,accompanied by a weakening of the NO titration effect,leading to an increase in nighttime O₃ concentrations.Although the complex topography limited the transport and dispersion of O₃(reduced PBLH and wind speed),the steep mountains blocked the solar radiation reaching the city,and ultimately,the complex topography still led to a decrease of 6.8?g/m~3(95%CI:9.7,3.8)O₃ concentrations in the urban area of Lanzhou during the summertime daytime.4.In the petrochemical industrial area of Lanzhou,the contributions of photochemical reactions and vertical mixing and transport of O₃ to MOP formation were 60.2%and 39.8%,respectively.the morning NO_x cycle and RO_x cycle rates were approximately four times higher on MOP days than on NMOP days,and the HO₂+NO cycle and RO₂+NO cycle contributed equally to O₃ generation.The reaction of VOCs with O₃ was the most important primary source of OH radicals in the petrochemical industrial area of Lanzhou,and the photolysis of OVOCs was the most important primary source of HO₂ and RO₂ radicals.The daytime atmospheric oxidation was dominated by OH radicals,and the nighttime atmospheric oxidation was dominated by NO₃ radicals.5.After 2018,the O₃ formation in the petrochemical industrial area of Lanzhou was transferred from NO_x-limited regime to transition regime or VOCs-limited regime.In summer 2019,morning O₃ production was in the VOCs-limited regime,afternoon O₃ production was in the transition regime.Multiple approaches have shown that reducing alkene concentrations,especially C4-C5 alkenes(trans-2-butene,cis-2-butene,1,3-butadiene,trans-2-pentene,cis-2-pentene,and isoprene),was the most effective strategy for controlling O₃ pollution in the petrochemical industrial area of Lanzhou,while reducing NO_x and non-alkene concentrations had little effect on mitigating O₃pollution.