NO₂对二甲苯光氧化生成SOA及其吸光度影响

NO₂ is an important pollutant in the urban atmosphere and significantly affect the formation of tropospheric secondary organic aerosol (SOA). In this work the reaction process of photooxidation of xylene was studied by using a laboratory smog chamber, focusing on the influence of NO₂ on the mass concentration, optical properties and chemical composition of the SOA. The results showed that along with an increase in concentration of NO₂ from 0 to 900×10^-9, mass concentration of the xylene SOA increased by about twice, suggesting a promoting effect of NO₂ on the SOA formation. We found that the mass concentration of o-xylene SOA was greater than that of p-xylene SOA, because the ortho substitution group tends to form aldehydes, which favors the oligomerization and thus promotes the SOA formation. Moreover, the increase in NO₂ concentration can also enhance the SOA mass absorption coefficients (MAC). Due to the different chemical structures of the two precursors, nitrogen-containing organic compounds (NOC) produced from o-xylene is of stronger light absorption. The online aerosol mass spectrometer (AMS) results further showed that the dominant components of o-xylene SOA and p-xylene SOA are organic acids with a lower H/C ratio for the o-xylene SOA, suggesting that the ortho-substituents are easier to be oxidized. However, the differences in chemical compositions between o-xylene SOA and p-xylene SOA was gradually diminished along with an increase in NO₂ concentration.