Characteristics And Formation Mechanisms Of Alkyl Nitrates In Typical Areas Of North China

Alkyl nitrates are an important class of reactive nitrogen compounds in the atmosphere and play a key role in the tropospheric chemistry,regional air quality and climatic change.Based on the field observations,the historical data was analyzed with the aid of many statistical tools and modeling techniques.Through analyzing the observational data of field campaigns carried out inBeijing and the Yellow River Delta,we investigated the pollution characteristics,sources and formation mechanisms of alkyl nitrates in typical atmospheric environment in north China.The spatial and temporal variations of alkyl nitrates in Beijing and in Yellow River Delta were investigated in this study.Compared with the suburban site,the influence of anthropogenic pollution was greater at the urban site,however,the mixing ratios of alkyl nitrates were comparable between both sites.For suburban site,we found that there was a large difference of the mixing ratios of alkyl nitrates between 2005 and 2008 but a small difference of parent hydrocarbons.In terms of diurnal variations,the peak of the mixing ratios of alkyl nitrates appeared at noon or in the afternoon both at the city site and the suburban site,when was the most intense photochemical reaction time in a day.Compared with the urban site,the peak of concentration at suburban site slightly delayed.In Yellow River Delta,alkyl nitrates showed a seasonal pattern with higher concentrations in summer and lower concentrations in winter.In addition,we found the highest concentration of alkyl nitrates appeared at noon or in the afternoon(12:00-15:00).What’s more,another peak of alkyl nitrates appeared at 19:00 in Yellow River Delta because of the influence of biomass burning.This conclusion can provide theoretical support for the research of related subjects.The mixing ratios of C4-C5 alkyl nitrates in the source area of Yellow River Delta are higher than those of the rural site.This study also explained the relationship between alkyl nitrates and ozone.Alkyl nitrates have a strong correlation with ozone in Beijing and Yellow River Delta,which can be explained by the fact that approximately 25-45 ppbv and 37-41 ppbv of O3 could be formed for every 100 pptv of alkyl nitrates in Beijing in Yellow River Delta.The comparison showed that under conditions of the same production of alkyl nitrates,the cleaner the area was,the more ozone was generated.The contribution of alkyl nitrates to reactive nitrogen oxides(NOy)in the atmosphere of different sites and seasons was also analyzed.Generally speaking,the contribution of alkyl nitrates to NOy is less than 10%in Beijing.Besides,this study found that the contribution of alkyl nitrates to NOy in the relatively clean Yellow River delta region was higher than that of the heavily polluted Beijing area.This study further explained the sources of alkyl nitrates.In addition to second formation of photochemical reactions,the formation of alkyl nitrates are also affected by vehicle exhaust emissions,combustion of fossil fuels,biomass burning,and biogenic emissions and petrochemical processes.The formation of alkyl nitrates was mainly influenced by vehicle exhaust emissions and biogenic emissions in Beijing,and influenced by petrochemical processes and biomass burning in Yellow River Delta.This study illustrated the photochemical mechanism of alkyl nitrates in the atmosphere.The observed C4-C5 alkyl nitrates have single source,while the sources of short-chain(C1-C3)alkyl nitrates were more complex both in Beijing and Yellow River Delta.The source of C4-C5 alkyl nitrates can be well explained by the photochemical-degradation of n-butane and n-pentane.While biogenic sources,long-chain alkanes,alkenes and halogenated hydrocarbons have great influence on short-chain alkyl nitrates in addition to the oxidation of methane,ethane and propane.The pollution characteristics and formation mechanisms of alkyl nitrates in typical atmospheric environment in north China were systematically analyzed in this study.It can provide theoretical support for further understanding of formation mechanisms of photochemical smog,and also provide reference for the control of photochemical pollution.