Origin and properties of soluble brown carbon in freshly emitted and aged ambient aerosols over an urban site in India.

This work investigates the absorption properties of soluble brown carbon (BrC), extracted in methanol and water, from ambient aerosol (PM) samples, collected over an urban background site in Mumbai, India. The diurnal variability was investigated in samples collected in the morning (7-11 a.m.) and afternoon (12-4 p.m.) periods. Absorption properties of BrC (in the 300-600-nm wavelength range) were measured in filter extracts of water-soluble organic carbon (WSOC) and methanol-soluble organic carbon (MSOC). WSOC and MSOC accounted for on average 52% and 77%, respectively, of the measured OC, potentially indicating unextracted BrC and rendering these values the lower bound. Compared with afternoon samples, the morning samples of MSOC and WSOC had increased BrC concentrations and absorption coefficients (b365; 40%-65%). The correlation between b365 and EC, ns-K, and NO in the morning samples indicated contributions from primary sources, including both biomass and vehicular sources. The decreased b365 in the afternoon samples was partly explained by mixing layer dilution, accompanied by a reduction in the concentrations of primary aerosol constituents. Furthermore, in the afternoon samples, HNMR spectroscopy revealed the presence of more oxidized functional groups and significantly higher OC/EC and WSOC/OC ratios, indicating the greater aging of afternoon aerosol. The MAC (mgC) for both WSOC and MSOC extracts decreased significantly by 20%-34% in the afternoon samples compared with the morning samples, indicating degradation in the absorption properties of the particles and potentially a change in the constituent BrC chromophores.