Heterogeneous Processes between Secondary Photochemical Oxidants and Aerosols

Abstract

A wide range of gaseous and particulate phase pollutants are emitted into the atmosphere by routes like external or internal combustion,evaporation,eolian and others.Once released into the atmosphere,these primary emissions are dispersed,transported and,at the same time,transformed by various chemical processes that determine their ultimate environmental fate.The secondary products of these processes are more important ones concerning their effects on human health and the quality of the environment,which represent the critical property determining the establishment of certain air standards and regulatory policies.The prevalence of tropospheric photooxidants is of international concern because of their adverse effects on human health and environment.A lot of nitrogen oxides(NOx) and volatile organic compounds(VOCs) have been emitted from natural and factitious sources,and then transform into secondary photochemical oxidants by the atmospheric oxidation reactions,contributing to atmospheric oxidation capacity and climate change patterns.The formation of secondary photochemical oxidants including ozone(O3),gaseous nitric acid(HNO3),hydrogen peroxide(H2O2) and peroxy acetyl nitrate(PAN) is driven by hydroxyl radical(OH) during the daytime whose main sources are photolysis of O3 and nitrous acid(HONO).At night,the nitrite radical(NO3) substitutes OH radical and plays the most important role in the nighttime atmospheric chemical processes.However,the more complicated situation is that aerosol particles are ubiquitous in the Earth's atmosphere.As reactive surfaces they can take part in the heterogeneous reactions with secondary photochemical oxidants.It changes the tropospheric composition and the properties of the gaseous reactants,making the atmosphere pollution even more serious.It also alters the properties of aerosols like hygroscopicity and influences the radiation balance by the direct and indirect effects.Thus,many efforts have been made to this subject,which has become one of focuses of the atmospheric chemical research currently.In order to make clear the intricate coupling effect between secondary photochemical oxidants and aerosol particles,laboratory simulation is basic.Aerosol samples produced in the laboratory or collected in the atmosphere are selected and put into the reactor such as flow tube,Knudsen cell or aerosol chamber,which react with the secondary photochemical introduced.The kinetics processes and products can be detected by the equipment involving mass spectrometry(MS),Fourier transform infrared spectrometry(FTIR),diffuse reflectance infrared Fourier transform(DRIFTS) and aerosol mass spectrometry(AMS),which are connected to the reactors.Chromatography and surface characterization techniques also can be applied to detect the heterogeneous reaction as the off-line methods.Laboratory researches about heterogeneous kinetic and mechanism are of importance to understand this coupling effect,which could contribute to comprehending the field observed events and provide the basic parameters to model forecast.Research on heterogeneous reactions of secondary photochemical oxidants is challenging.However,there are some aspects that are not clear,and more studies are needed.First,the researches mainly concentrate on the O3 and HNO3 heterogeneous reactions,while less about others.The data must be enriched through the future studies.Second,the reaction system often has isolated secondary photochemical oxidant and individual particles,which is not consistent with real atmospheric composition.It leads certain error between the results from laboratory simulation and field observation.Last but not least,no commercial equipment is applicable to this area.Researchers must establish high-precision and sensitive apparatuses to detect the transformation of reactants under atmospheric conditions.In this review,the formation of secondary photochemical oxidants is introducesd and the recent international laboratory researches and state of the art with regard to this area are concluded.In the end the main scientific problems are put forward,and the prospect is made.