Sensitivity Analysis Of Ozone Formation Using Design Of Experiment Methodology And Identification Of Key VOCs Species

Tropospheric ozone（O₃）is a secondary pollutant generated by the photochemical reaction of the precursors NO_x（nitric oxide and nitrogen dioxide）,volatile organic compounds（VOCs）,and CO（carbon monoxide）under sunlight.O₃formation exhibits a very complex non-linear relationship with precursors.In mainland China,after the Air Pollution Prevention and Action Plan was implemented in 2013,the concentration of primary pollutants such as nitrogen oxides has decreased significantly,but O₃pollution in most urban areas hasn’t been improved.And there are significant regional differences in the response of O₃to precursors’abatement due to the different concentrations and species of pollutants in various regions.At present,the most popular methods for establishing relationships between O₃and its precursors have limitations in terms of efficiency and accuracy,and in order to efficiently develop reasonable pollution control measures,it is necessary to improve the shortcomings of the methods.In this context,based on the box model simulation this study assessed the influence of multiple precursors and their interactions on O₃formation,quantified the O₃responses to its key precursors and investigated the effect of different VOCs species’percentage on the O₃isopleths,providing an new approach for efficient sensitivity analysis of O₃generation.During a O₃multi-day pollution period,the key VOCs species were identified by a detailed analysis of peroxyl radicals’（RO₂/HO₂）generation mechanism based an observation-based model（OBM）that provided scientific supports for the development of VOCs control strategies.The major conclusions are as follows:（1）With reference to the precursors’concentrations in a typical petrochemical industrial city（Lanzhou）and a coal chemical industrial city（Wuhai）,firstly the contributions’rank of each precursor to the daily maximum ozone formation concentration(O_3max)in VOC-limited regime by the fractional factorial design methodology was that NO>O₃>olefins>olefins interaction with NO>olefins interaction with NO₂>NO₂>NO interaction with NO₂>O₃interaction with NO>aromatics>OVOCs interaction with HONO>CO>aromatics interaction with NO₂>alkanes>olefins interaction with O₃>aromatics interaction with NO.The negative contributions of olefins and aromatics interacting with NO_xto O_3maxindicated that only by reducing the concentrations of VOCs and NO_xsimultaneously could mitigate the contribution of NO_xreduction to O₃formation.Secondly the response surface methodology was used to quantify the contribution of olefins to O₃formation and when the radio ofφ（olefin）to[φ（NO_x）-15]was less than 0.91 as well the value of theφ（olefin）less than 35 ppb,olefin went far towards generating O₃.Finally Mixture design was used to explore the effect of different VOCs species’percentage on the O₃isopleths,and the results showed that the O₃peak concentration in the O₃isopleths increased when the percentage of olefins increased and the one-order term coefficient of olefins was 4.56.When olefins content is fixed,the O₃peak concentration was less influenced by the change of the proportion of alkanes,aromatics and OVOCs and their one-order term coefficients were 2.46,2.63 and 2.10 respectively.In summary the combination of the fractional factorial design,response surface and mixture design methodologies can effectively investigate the impact of different precursors on ozone concentration in ambient air.（2）According to the differences of peroxyl radicals（RO₂/HO₂）source pathways,they were divided into primary and secondary RO₂/HO₂.At Chem model simulations for multi-day ozone pollution period from 2020.08.25 to 2020.09.15 showed that the average daily contributions of secondary RO₂/HO₂to NO+RO₂/HO₂→NO₂reaction rate was 76.92%.CH₃O₂and CH₃CO₃were the dominant secondary RO₂that converted NO to NO₂,contributing more than 35%to the NO+RO₂（secondary）reaction rate.A case study of 2020.09.06,the key VOCs species were identified as:propylene,dichloroethylene,cis-2-butene,and 2-pentene by analyzing chemical budget of CH₃O₂,CH₃CO₃,and secondary HO₂.