Evolution of light absorption properties during photochemical aging of straw open burning aerosols

Straw burning comprises more than 30% of all types of burned biomass in Asia, while the estimation of the emitted aerosols' direct radiative forcing effect suffers from large uncertainties, especially when atmospheric aging processes are considered. In this study, the light absorption properties of primary and aged straw burning aerosols in open fire were characterized at 7 wavelengths ranging from 370 nm to 950 nm in a chamber. The primary rice, corn and wheat straw burning bulk aerosols together had a mass absorption efficiency (MAE) of 2.43 +/- 1.36 m(2) g(-1) at 520 nm and an absorption Angstrom exponent (AAE) of 1.93 +/- 0.71, while the primary sorghum straw burning bulk aerosols were characterized by a relatively lower MAE of 0.95 +/- 0.54 m(2) g(-1) and a higher AAE of 4.80 +/- 0.68. Both the MAE and AAE of primary aerosols can be well parameterized by the (PM-BC)/BC ratio (in wt.). The MAE of black carbon (BC) increased by 11-190% during photoreactions equivalent to 16-60 h of atmospheric of aging, which was positively correlated with the (PM-BC)/(BC) ratio. The MAE of organic aerosols first slightly increased or leveled off, and then decreased. Specifically, at 370 nm, the first growth/plateau stage lasted until OH exposure reached 0.47-1.29 x 10(11) molecule cm(-3) s, and the following period exhibited decay rates of 1.0-2.8 x 10(-12) cm(3) molecule(-1) s(-1) against the OH radical, corresponding to half-lives of 46-134 h in a typical ambient condition. During photoreactions, competition among the lensing effect, growth/bleach of organic chromophores, and particle mass and size growth complicated the evolution of the direct radiative forcing effect. It is concluded that rice and corn straw burning aerosols maintained a warming effect after aging, while the cooling effect of fresh sorghum straw burning aerosols increased with aging.

Automated summary

Straw burning accounts for more than 30% of burned biomass in Asia, with large uncertainties in estimating the direct radiative forcing effect of emitted aerosols, especially considering atmospheric aging processes. This study characterized light absorption properties of primary and aged straw burning aerosols, finding variations in the mass absorption efficiency and absorption Angstrom exponent during aging.