Transformations
of biomass burning brown carbon aerosols (BB-BrC)
over their diurnal lifecycle are currently not well studied. In this
study, the aging of BB tar proxy aerosols processed by NO
3
•
under dark conditions followed by the photochemical
OH
•
reaction and photolysis were investigated in
tandem flow reactors. The results show that O
3
oxidation
in the dark diminishes light absorption of wood tar aerosols, resulting
in higher particle single-scattering albedo (SSA). NO
3
•
reactions augment the mass absorption coefficient
(MAC) of the aerosols by a factor of 2–3 by forming secondary
chromophores, such as nitroaromatic compounds (NACs) and organonitrates.
Subsequent OH
•
oxidation and direct photolysis both
decompose the organic nitrates (ONs, representing bulk functionalities
of NACs and organonitrates) in the NO
3
•
-aged wood tar aerosols, thus decreasing particle absorption. Moreover,
NACs degrade faster than organonitrates by photochemical aging. The
NO
3
•
-aged wood tar aerosols are more
susceptible to photolysis than to OH
•
reactions.
The photolysis lifetimes for the ONs and for the absorbance of the
NO
3
•
-aged aerosols are on the order of
hours under typical solar irradiation, while the absorption and ON
lifetimes toward OH
•
oxidation are substantially
longer. Overall, nighttime aging via NO
3
•
reactions increases the light absorption of wood tar aerosols and
shortens their absorption lifetime under daytime conditions.
Abstract. Agricultural residues are among the most abundant biomass burned globally, especially in China. However, there is little information on primary emissions and photochemical evolution of agricultural residue burning. In this study, indoor chamber experiments were conducted to investigate primary emissions from open burning of rice, corn and wheat straws and their photochemical aging as well. Emission factors of NO x , NH 3 , SO 2 , 67 non-methane hydrocarbons (NMHCs), particulate matter (PM), organic aerosol (OA) and black carbon (BC) under ambient dilution conditions were determined. Olefins accounted for > 50 % of the total speciated NMHCs emission (2.47 to 5.04 g kg −1 ), indicating high ozone formation potential of straw burning emissions. Emission factors of PM (3.73 to 6.36 g kg −1 ) and primary organic carbon (POC, 2.05 to 4.11 gC kg −1 ), measured at dilution ratios of 1300 to 4000, were lower than those reported in previous studies at low dilution ratios, probably due to the evaporation of semi-volatile organic compounds under high dilution conditions. After photochemical aging with an OH exposure range of (1.97-4.97) × 10 10 molecule cm −3 s in the chamber, large amounts of secondary organic aerosol (SOA) were produced with OA mass enhancement ratios (the mass ratio of total OA to primary OA) of 2.4-7.6. The 20 known precursors could only explain 5.0-27.3 % of the observed SOA mass, suggesting that the major precursors of SOA formed from open straw burning remain unidentified. Aerosol mass spectrometry (AMS) signaled that the aged OA contained less hydrocarbons but more oxygen-and nitrogencontaining compounds than primary OA, and carbon oxidation state (OS c ) calculated with AMS resolved O / C and H / C ratios increased linearly (p < 0.001) with OH exposure with quite similar slopes.
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