Study On The Effect Of Water On Atmospheric Toluene Oxidation Products

In recent years,air pollution incidents have occurred frequently,and secondary organic aerosols(SOA)have attracted widespread attention as one of the important organic components of atmospheric fine particles,mainly from the oxidation reactions of volatile organic compounds(VOCs)in the atmosphere.SOA in the atmosphere are mainly formed in different phases.In the gas phase,photo-oxidation reactions of VOCs in the atmosphere generate semi-volatile organic compounds and low-volatile organic compounds,and the products condense to form new particles or contribute to the growth of the existing particles via adsorption or absorption,and these SOA are called gas phase SOA;In the aqueous phase including cloud drops,fog drops,rain drops or wet aerosols,VOCs will dissolve in water,and produce organic products with lower volatility via aqueous phase oxidation reactions.After the water in the cloud droplets evaporates,the remaining organic compounds forms aerosols called aqueous SOA.Toluene is an important anthropogenic volatile organic compound in urban areas,and is one of the major contributors to secondary organic aerosols and ozone in the atmosphere.Studies have shown that gaseous water and liquid water can affect the yield and composition of gas phase toluene SOA.However,the conclusions of different studies are inconsistent.In this study,a quartz flow tube reactor and a quartz static reactor device were used,combined with instruments such as High Performance Liquid Chromatography(HPLC),Ion Chromatography(IC),Organic Carbon/Elemental Carbon analyzer(OC/EC),and desktop scanning electron microscope.The present study simulated the oxidation reaction of toluene and OH radical in the gas phase in the presence of water vapor and in the liquid water,respectively,and quantitatively measured some of gas phase products,particulate phase products and water phase products generated by gas phase reaction and water phase reaction.The mechanism of how gas water and liquid water affect the toluene oxidation was deduced.The main research contents and conclusions of this paper are as follows:(1)Gas-phase oxidation of toluene and OH radicals.A flow tube reactor was used to study the effects of different relative humidity on the oxidation products of toluene in the presence of NO_x.The carbonyl compounds,organic acids and benzene compounds in the oxidation products of toluene were determined.The results show that gaseous water promotes the formation of gas-phase small molecule carbonyl compounds and organic acids,which may be the result of the combined effect of gas-phase and water-phase reactions.In addition,gaseous water has an effect not only on the ratio of the reaction pathway,but also on the yield and degree of oxidation of the products.The proportion of cresol and benzaldehyde pathways increased under high relative humidity conditions,and this change is likely to cause changes in particulate phase organic species and yield through gas-particle distribution.As the relative humidity increases,the relative specific gravity of the organic acid salt in the particulate phase product increases,and the degree of oxidation of the particulate phase product increases with the increase of the relative humidity.(2)Aqueous oxidation of toluene with OH radicals.A quartz static reactor was used to study the oxidation of toluene and OH radicals in liquid water.The yield of some products with temperature and acidity was investigated,including important aqueous SOA precursors such as glyoxal and methyl glyoxal.This study found that under the action of liquid water,the effects of temperature and acidity on various products in the aqueous phase reaction of toluene are different.Compared with the low temperature environment(T=10?),the high temperature environment(T=25?)promotes the increase of reacted toluene and the increase of products.However,under different acidity environments,the yields of glyoxal and methyl glyoxal decreased with decreasing pH,while formaldehyde and acetaldehyde exhibit the opposite behavior.The same yield trend indicates that the main product glyoxal and acetylformaldehyde are likely to come from the same production pathway and are different from formaldehyde and acetaldehyde.The studies of the effects and roles of water on the oxidation and oxidation products in the atmosphere are conducted in this paper.It provides important experimental evidence on the effect of water on the oxidation products of atmospheric toluene in the gas and aqueous phases.