Photochemical Aging of PM_2.5 Nitroaromatic Compounds From Solid Fuel Combustion Enhanced Light Absorption and Oxidation Potential

This study investigates primary emissions and photochemical aging evolution mechanisms of PM_2.5 Nitroaromatic compounds (NACs) released from solid fuels through conducting field combustion experiments. The emission factors (EFs) of NACs primary emissions are much higher from biomass burning (BB) than coal combustion (CC). The EFs of measured NACs species mostly increased during the aging process except some species in CC. Photochemical aging significantly enhanced the absorbance of ∑NACs, but through different mechanisms between BB and CC. In BB, increasing production of 4-nitrocatechol (4NC), 4-nitroguaiacol (4NG), and 4-methyl-5-nitrocatechol (4M5NC) through nitrogen addition reactions during aging process enhanced the NACs light absorption. In CC, increasing 4NC and 4-methyl-2,6-dinitrophenol from nitrogen addition reactions as well as indirect photolysis of 4NG and 4M5NC with ·OH to form organic intermediates both increase light absorption. Furthermore, 4M5NC and 4NG enhanced PM_2.5 oxidation potential substantially through auto-biotoxicity and generation of facilitating ¹O₂ in BB and decomposing into nitro-quinones in CC.