Impacts of some co-dissolved inorganics on in-cloud photochemistry of aqueous brown carbon

摘要

This experimental study explored the impacts of some codissolved inorganic ions (in particular, sulfate, nitrate and chloride) on the photo-bleaching kinetics of the aqueous solutions of brown carbon (BrC) extracted from rice-straw smoldering aerosol. All the aqueous BrC solutions showed two distinct phases of decreasing mass absorption coefficient (MAC) with irradiation time, where the first phase (0-1h) was 2.4-5.3 times faster than the second phase (1-3h). Interestingly, the photo-bleaching process was slowed down for both phases when SO42-(0-1h: 0.196 +/- 0.02h(-1); 1-3h: 0.045 +/- 0.01 h(-1)), NO3- (0-1h: 0.158 +/- 0.004 h(-1); 1-3h: 0.066 +/- 0.007 h(-1)) and Cl-(0-1h: 0.169 +/- 0.007 h(-1); 1-3h: 0.032 +/- 0.006 h(-1)) salt solutions were added separately to the BrC extracts (0-1h: 0.27 +/- 0.04 h(-1); 1-3h: 0.067 +/- 0.01 h(-1)). Alternatively, the corresponding lifetimes of the light-absorbing organic species were enhanced in the salt-added BrC solutions. In addition, the magnitudes of the fluorescence quantum yields were measured to be of the order of 10(-3) - 10(-2), implying that most of the absorbed solar energy would convert into heat rather than fluorescing back into the atmosphere. The findings of this study are suggestive of possible increase in heating of BrC containing atmospheric aqueous bodies like cloud-water, resulting in water evaporation and hence reduced precipitation (semi-direct effect).