Change in characteristics of water-soluble and water-insoluble brown carbon aerosols during a large-scale biomass burning

Light-absorbing organic aerosol (brown carbon (BrC)) can significantly affect Earth’s radiation budget and hydrological cycle. Biomass burning (BB) is among the major sources of atmospheric BrC. In this study, day/night pair (10-h integrated) of ambient PM₂.₅ were sampled every day before (defined as T1, n = 21), during (T2, n = 36), and after (T3, n = 8) a large-scale paddy-residue burning during October–November over Patiala (30.2° N, 76.3° E, 250 m amsl), a site located in the northwestern Indo-Gangetic Plain (IGP). PM₂.₅ concentration varied from ~ 90 to 500 μg m⁻³ (average ± 1σ standard deviation 230 ± 114) with the average values of 154 ± 57, 271 ± 122, and 156 ± 18 μg m⁻³ during T1, T2, and T3 periods, respectively, indicating the influence of BB emissions on ambient air quality. The absorption coefficient of BrC (bₐbₛ) is calculated from the high-resolution absorption spectra of water-soluble and methanol-soluble organic carbon measured at 300 to 700 nm, and that at 365 nm (bₐbₛ_₃₆₅) is used as a general measure of BrC. The bₐbₛ_₃₆₅_Wₐₜₑᵣ and bₐbₛ_₃₆₅_Mₑₜₕₐₙₒₗ ranged ~ 2 to 112 Mm⁻¹ (avg 37 ± 27) and ~ 3 to 457 Mm⁻¹ (avg 121 ± 108), respectively, suggesting a considerable presence of water-insoluble BrC. Contrasting differences were also observed in the daytime and nighttime values of bₐbₛ_₃₆₅_Wₐₜₑᵣ and bₐbₛ_₃₆₅_Mₑₜₕₐₙₒₗ. Further, the levoglucosan showed a strong correlation with K⁺ (slope = 0.89 ± 0.06, R = 0.92) during the T2 period. We propose that this slope (~ 0.9) can be used as a typical characteristics of the emissions from paddy-residue burning over the IGP. Absorption Ångström exponent (AAE) showed a clear day/night variability during the T2 period, and lower AAEMₑₜₕₐₙₒₗ compared to AAEWₐₜₑᵣ throughout the sampling period. Further at 365 nm, average relative atmospheric radiative forcing (RRF) for BrCWₐₜₑᵣ is estimated to be ~ 17%, whereas that of BrCMₑₜₕₐₙₒₗ ~ 62% with respect to elemental carbon, suggesting that BrC radiative forcing could be largely underestimated by studies those use BrCWₐₜₑᵣ only as a surrogate of total BrC.