First-principles Studies On The Photolysis Of Atmospheric Organic Nitrogen Oxides

Nitrous acid（HONO）is an important trace gas in the heavily polluted atmosphere,which acts an indispensable role in the formation of atmospheric oxidizing species（OH·）,and attracts extensive attentions.However,the source of HONO is still not clear.Therefore,the exploration of the source of HONO has become an important topic in both theoretical and experimental aspects.In recent years,it has been found that the formation of HONO is closely related to the photochemistry.HONO has been found in some light-absorbing substances,such as PM2.5 particulate matter,nitrophenol,nitrate and other compounds.However,due to the complexity of the system studied in the experiment,it is difficult to explain the mechanism of HONO formation from the molecular point of view.Nitrophenols and organic nitrates are two main components in atmospheric secondary organic aerosols,and have the ability to absorb light,which is of great significance for the study of aerosol aging.In this thesis,we systematically study the photochemical mechanism of these two types of organic nitrogen oxides in gas and aqueous environment using quantum mechanical（QM）and chemical kinetics methodolgies.The main contents in the thesis including the following aspects:1.Proposed the gas-phase photochemical mechanism of water-insoluble（WI）nitrophenols in atmosphere,and estimated the rate of HONO formation from WI-nitrophenols.In this thesis,taking 2-nitrophenol as an example,spectral properties and photolysis mechanism of WI-nitrophenol are studied,and the role of water is also compared.The results show that this type of nitrophenols have a high efficiency in absorbing sunlights in the wavelength range of 300～400 nm,and OH·is the major product via photolysis,and HONO is the minor product.2.Proposed the gas and aqueous phase photolysis mechanism of water-soluble（WS）nitrophenols in atmosphere,and estimated the rate of HONO from WS-nitrophenols.In this thesis,taking 4-nitrophenol as an example,spectral properties and photolysis mechanism of WS-nitrophenol in atmosphere are studied,and the role of water is also compared.The results show that this type of nitrophenols also has a high efficiency of absorbing sunlight in the wavelength range of 300～400 nm,and can undergo photolysis reaction in both gas phase and aqueous phase,in which NO is the main product.The photolysis of 4-nitrophenol in the solution also produces HONO,where the generation of HONO undergoes intermolecular excited-state hydrogen transfer rather than direct molecular reaction,but is still a minor product.By comparing2-nitrophenol with 4-nitrophenol,it is found that gas-phase photolysis of WI-nitrophenols is a more important source of HONO.The photolysis rate constants of HONO are 5.73×10^-5and 5.25×10^-6s^-1,respectively.3.Proposed the gas and aqueous phase photolysis mechanism of organic nitrates in atmosphere.Organic nitrates are easily to generate dimers in aqueous solution,so monomers in gas and dimers in solution are the main forms of organic nitrates in atmosphere.In this thesis,taking methyl nitrate as an example,the photolysis of organic nitrates in gas and solution was studied.The results show that organic nitrates have a weak light-absorbing ability.In both gas and solution,photolysis of organic nitrates mainly produces NO₂,while HONO is hardly formed（about～1 ppt concentration）.This also shows that the photolysis of organic nitrates hardly produces HONO in atmosphere,whether it is a monomer or dimer.These results also confirm the experimental observations on the photolysis of organic nitrates.Meanwhile,we also find that methyl nitrates substituted with a hydroxyl group at theαposition has a similar formation rate of HONO to methyl nitrates without any substitution,and had higher HONO formation rates than methyl nitrates substituted with other functional groups.