Humic-like substances in residential biofuel use emissions from the northwestern Himalayan region, India

Indoor biofuel use for cooking/heating purposes is potentially a significant source of fine-mode aerosols in the Indian Himalayan region, with important implications for ambient atmospheric processes. Incomplete combustion of biofuels leads to emission of absorbing aerosols such as elemental carbon (EC), brown carbon (BrC) and humic-like substances (HULIS), which can affect climate via direct as well as indirect forcing.  However, profiles of these aerosols in indoor microenvironments are poorly studied, especially from the Indian Himalayan region. Here, we report size-segregated light absorption properties (absorption coefficient; b_{abs_aq}) of aqueous brown carbon (BrC_aq) for cow dung cake (CDC), firewood (WD), charcoal (CC), kerosene (KS) and liquified petroleum gas (LPG) combustion in indoor settings from two northwest Himalayan states (Uttarakhand and Himachal Pradesh). We further characterize BrC composition using excitation emission matrix (EEM) fluorescence spectroscopy coupled with parallel factor (PARAFAC) analysis. Size-segregated samples (>2.5 µm, 2.5-1 µm, 1-0.5 µm, 0.5-0.25 µm and <0.25 µm) were collected during indoor cooking activities which showed PM levels exceeding Indian ambient PM_2.5 standard by 8-460 times, with CDC showing the highest PM concentration in all size ranges. Size distribution profile of the fuels showed trimodal distributions with one peak in the Aitken nuclei mode (0.05 μm for CDC, WD and LPG; 0.06 μm for CC and KS) and the other two in the accumulation mode (0.22 and 2.03 μm for CDC, 0.21 μm and 2.1 μm for WD, 0.24 and 1.91 μm for KS, and 0.17 and 0.83 μm for CC). PM size distribution across kitchen and fuel types was characterized by more than 50% concentration in the Greenfield Gap region (0.1-2 μm)., CDC showed highest b_{abs_365} for all size ranges combined (2245±357 Mm^-1) compared to other fuels while LPG showed the lowest (190±46 Mm^-1). BrC_aq in the lowest size fraction showed greater b_abs compared to other size ranges in all fuels except KS. High values of b_abs ratio (>5) for300-400 nm (b_{abs_300_400}) to 400-500 nm (b_{abs_400_500}) wavelength range suggested the presence of HULIS in CDC and WD samples while other fuels exhibited values below 2.6. The lower size fractions (<0.5 µm) of CDC and WD were characterized by higher b_{abs_300_400}/b_{abs_400_500} values (6.9±1.6 for CDC and 7.4±2.8 for WD) than upper size ranges (3.2±0.8 for CDC and 2.3±0.5 for WD), indicating a dominance of HULIS components in finer aerosol. Upon analysis of EEMs coupled with PARAGAC, three major chromophoric categories in BrC_aq of CDC and WD samples were observed: two types of HULIS components with varying degree of conjugation and one type of protein-like substances (PRLIS). HULIS was the dominant chromophore type in lower size fractions (<0.5 µm) of CDC and WD samples (70±4% for CDC and 61±6% for WD) while for upper size fractions its contribution was relatively lower (50±10% for CDC and 36±9% for WD). Finally, BrC_aq from CDC and WD emissions exhibited direct climate forcing potential equivalent to EC (relative radiative forcing (RRF): 91-98%), confirming that indoor biofuel emissions are indeed significant sources of climate forcing agents in the Indian Himalayan context.