Experimental Study On The Formation Of Nitrogen-Containing Compounds From Ammonia And Toluene Secondary Organic Aerosols

Monoaromatic hydrocarbons derived from motor vehicle exhaust and other anthropogenic sources are the main volatile organic compounds in the atmosphere.They can react with atmospheric oxidants such as OH radicals under solar radiation to form monoaromatic secondary organic aerosol?SOA?.SOA particles are an important component of atmospheric fine particulate matter(PM_2.5),which can also cause photochemical smog and haze in certain conditions,and have a vital impact on climate change,atmospheric visibility and human health.Ammonia?NH3?is the main alkaline pollutant gas present in the atmosphere.In addition to acid-base neutralization with acid gases such as H₂SO₄ and HNO₃ in the atmosphere to produce inorganic ammonium salts,it can also interact with organic substances such as aldehydes and carboxylic acids to form nitrogen-containing organic compounds,thereby affecting the formation and chemistry components of SOA.Previous studies focused on photochemical reaction of biogenic SOA and NH₃.However,the reaction of monoaromatic SOA and NH₃ in anthropogenic SOA has not been reported.On this basis,this thesis takes anthropogenic toluene SOA as the research object,and explores the reaction mechanism and influencing factors of nitrogen-containing compounds formed by NH₃ and toluene SOA,which will help to understand the formation of atmospheric brown carbon.Firstly,a smoke chamber device for studying photooxidation of toluene with SOA was built,and the following experimental results were obtained by using the device:In the photooxidation system of OH radical and toluene,the reaction products have UV absorption at 205 nm and 270 nm under NH₃ conditions and strong fluorescence efficiency compared with the condition of absence of NH₃.There are absorption peaks of C=O,C-O,O-H,C=N,C-N,and N-H bonds in the infrared spectrum of reaction products.Also,their aerosol laser time-of-flight mass spectra contain characteristic fragment peaks of m/z 18?NH₄⁺?,44?COO^-?and 67?C₃H₃N₂⁺?,41?C₂H₂N⁺?,28?CH₂N⁺?.These spectral information indicate that the photooxidation of toluene with NH₃ forms organic ammonium and imidazole nitrogen-containing organic products.Compared to the case of without NH₃,the center particle size and mass concentration of toluene SOA particles with NH3 measured by scanning mobile particle size spectrometer increased by 50%and 25%,respectively.NH₃ can significantly promote the formation of toluene SOA.The concentration of nitrogen-containing products such as organic acid ammonium and imidazoles increases with the increasing of ultraviolet light irradiation time and the concentration of toluene,NH₃ and O₃.However,when the concentration of O₃ reaches a certain value,the high concentration of OH radical generated by photolysis of O₃ can oxidize the photooxidation product of toluene into more volatile compounds,which is not conducive to the formation of nitrogen-containing organic compounds.The increase of water molecules reduces the concentration of OH radicals generated by photolysis of O3,which leads to the decrease of the concentration of organic acid ammonium and imidazole products with the increasing of relative humidity.The inorganic seed aerosol has a large surface area and can serve as the center for the condensation of photooxidation gaseous products of toluene,thereby promoting the formation of nitrogen-containing organic products.The order of promotion is calcium chloride>sodium silicate>ammonium sulfate>sodium chloride>ammonium nitrate.The concentration of organic acid ammonium and imidazole products increases with the increasing of calcium chloride seed aerosol concentration and acidity,but decreases with the increase of relative humidity.However,when the calcium chloride seed aerosol is alkaline,OH^-would undergo the acid-base neutralization reaction with the condensed organic acid and inhibit the hydration of the dialdehyde compound to form tetraol product,thus being disadvantageous for the formation of the nitrogen-containing organic product.