Study Of Pollution Characteristics Of Atmospheric Peroxides In Urban Area And Suburb Of Beijing And Their Contributions To Sulfate Formation

Peroxides are important photochemical oxidants apart from ozone（O₃）in the atmosphere,including hydrogen peroxide（H₂O₂）and kinds of organic peroxides.Peroxides play very important roles in tropospheric chemistry:firstly,peroxides are center substance of the cycle of free radicals（OH,HO₂,CH₃OO）,and act as the sources and sinks of free radicals.Peroxides have important effects on photochemical smog formation.In addition,in the liquid phase,especially when pH is<4.5,H₂O₂ is the main oxidant of SO₂ and involved in the formation of acid rain and secondary aerosols.This paper investigated and modeled peroxide concentrations during the spring,summer,autumn of 2015 and 2016 in Beijing,and peroxide concentrations in the suburb of Beijing were monitored in October,2016.At the same time,peroxides in rainwater were also analyzed,and the relationship between hydrogen peroxide and sulfate in rainwater and fine particles were explored.Results are summarized as follows:（1）During the monitoring period,the concentration ranges of H₂O₂ and MHP were 0.01～5.74 and 0.00～0.98 ppbv,respectively.H₂O₂ concentrations were higher in summer than that in spring and autumn,and the average concentration was about that of spring and autumn～5 times.In sunny days,the concentrations of H₂O₂ increased from 7:00,and reached the highest value at 15:00 and then decreased.The daily variation of MHP concentration was not as significant as H₂O₂,and MHP peaks occurred during 16:00 and 18:00.The increase in the concentrations of peroxides in the evening may be due to the deterioration of the diffusion conditions or the new generation pathways,which need more data to support.There was a significant underestimation of modeled H₂O₂,and the simulated values were lower than observed values of 2 orders of magnitude.However,simulation results were relatively good in July 2016.（2）The concentrations of gaseous peroxides are not only closely related to meteorological factors（temperature,humidity,solar radiation,etc.）,but also affected by pollutants,such as O₃,SO₂,NO_x,CO and hydrocarbons.Results of correlation analysis showed that H₂O₂ was positively related to temperature and solar radiation,but negatively related to humidity.As for pollutants,H₂O₂ and O₃ showed significant positive correlations,whereas no clear correlations were figured out between H₂O₂ and SO₂ or NO_x.The correlation of MHP and other environmental factors were significant than H₂O₂ in the whole.The results of Principal Component Analysis showed that H₂O₂ is mainly affected by T and RH during the five monitoring periods,and it is also closely related to O₃.H₂O₂ is also closely related to NO_x besides above factors in summer,which is consistent with the correlation analysis.MHP is mainly related to SR and CO in summer,while the correlations of MHP and other variables are poor in spring and autumn.（3）The average values of H₂O₂ were 0.80±0.51 and 0.75±0.50 ppbv,meantime,average values of MHP were 0.06±0.04,0.06±0.06 ppbv,respectively,in urban area and suburb of Beijing in October 2016.Peroxide concentrations decreased in rainfall process,especially H₂O₂.The concentrations of the MHP on haze days were lower than in rainfall process,and H₂O₂ concentrations on haze days were still high.It can be concluded that H₂O₂ concentrations were quickly removed in liquid phase,but less sensitive to solar radiation compared with MHP.The diurnal variations of H₂O₂ were similar day two sampling sites.MHP did not show clear variations due to lack of data and bad weather condition.During the monitoring period,there were sometimes high concentrations of H₂O₂ in the morning（7:00-9:00）,which can be explained by that was that there was a new net generation of H₂O₂ at night,and the removal of H₂O₂ was inhibited due to no light.（4）H₂O₂ concentrations in rainwater ranged from 0.14 to 118.79μmol/L.Rainfall and aqueous H₂O₂ concentration showed negative correlation in the middle and early,late of rain,respectively.Two scenarios were considered:（a）concentrations of gaseous SO₂ were actual monitoring values;（b）SO₂ was sufficient to react with H₂O₂.The rates of H₂O₂ oxidation to generate sulfate were 0.65±0.41 and 17.10±16.91 mmol/L in two scenarios.The dataset of rain process was small,so the results require further validation.（5）The sulfate generation rate in PM_2.5 was 0.3×10^-12～565×10^-12 mol m^-3 h^-1,which was equal to～0.06μg/m³,while the actual increase was-5.62～7.87μg/m³,By comparing with the actual sulfate increase,The contribution of H₂O₂ liquid phase oxidation to sulfate increase was-9.3%to 9.3%.The results of comparison with the remaining oxidants（O₃ and NO₂）showed that the oxidation rate was essentially O₃>H₂O₂>NO₂.During the monitoring period,SO₄^2- hourly increase oxided by O₃,NO₂,were～0.3μg/m³ and～0.045μg/m³,respectively,and the contributions were-34.2%～26.2%and-17.5%～7.6%.O₃ and H₂O₂ concentrations are low in winter haze periods due to weak sunlight,while NO₂ concentrations are higher in winter than in other seasons.So NO₂ oxidation rate is large,and NO₂ may be the reason of high PM_2.5 sulfate contents in winter haze periods.