Measurement report: Long emission-wavelength chromophores dominate the light absorption of brown carbon in Aerosols over Bangkok: impact from biomass burning

Abstract. Chromophores represent an important portion of light-absorbing species, i.e. brown carbon. Yet knowledge on what and how chromophores contribute to aerosol light absorption is still sparse. To address this problem, we examined soluble independent chromophores in a set of year-round aerosol samples from Bangkok. The water-soluble chromophores identified via excitation-emission matrix (EEM) spectroscopy and follow-up parallel factor analysis could be mainly assigned as humic-like substances and protein-like substances, which differed in their EEM pattern from that of the methanol-soluble fraction. The emission wavelength of chromophores in environmental samples tended to increase compared with that of the primary combustion emission, which could be attributed to secondary formation or the aging process. Fluorescent indices inferred that these light-absorbing chromophores were not significantly humified and comprised a mixture of organic matter of terrestrial and microbial origin, while these inferences exhibited a refutation with primary biomass burning and coal combustion results. A multiple linear regression analysis revealed that larger chromophores that were oxygen-rich and highly aromatic with high molecular weights, were the key contributors of light absorption, preferably at longer emission wavelength (λmax > 500 nm). Positive matrix factorization analysis further suggested that up to 60 % of these responsible chromophores originated from biomass burning emissions.