Highly Time Resolved Characteristics of Light Absorbing Water-soluble Organics over New Delhi

Spectral characteristics of BrC were studied over the New Delhi, the capital of India, during winter period (January-February, 2018). The BrC absorption spectra and water-soluble organic carbon (WSOC) were measured every 4 min using the particle-into-liquid sampler (PILS) coupled with liquid waveguide capillary cell (LWCC, 2 m path length) and total organic carbon (TOC) analyser. In addition, black carbon (BC), carbon monoxide (CO) and nitrogen oxides (NO_X = NO + NO₂) were also measured using aethalometer, CO and NO_X analysers, respectively. Light absorption by WSOC at 365 nm (b_{abs_365 nm}), a proxy for water-soluble BrC, was well correlated with WSOC (slope = 1.21 ± 0.006, r² = 0.73, n = 11534), and exhibited strong diurnal and day-to-day variability throughout the study period. The b_{abs_365} showed high values during 00:00 to 10:00 hrs, attributable to primary emissions in the night hours as the similar trends were shown by BC at 370 nm and 450 nm, and also by CO (markers of primary emissions). Further, the variability in b_{abs_365}/CO ratio also suggested that secondary BrC was significant during 00:00 to 12:00 hrs, which reduced during early evening rush hours. Further, the average mass absorption efficiency values (MAE) of BrC during January and February were 1.24 ± 0.55 and 1.05 ± 0.35, respectively, at 365 nm and reduced significantly at higher wavelengths. The BrC at 365 nm is known to be associated with humic-like substances (HULIS) and that at wavelengths above 400 nm infers the presence of different nitro aromatics. Further, the absorption angstrom exponent (AAE) values during January (7.2) and February (5.9) reflect the wavelength dependency of BrC. Higher AAE during January indicate that biomass burning was the major emission source. Further, significant diurnal variability was observed in b_{abs_450}/b_{abs_365} andb_{abs_490}/b_{abs_365} ratios, which followed the similar diurnal pattern as shown by NO_X, suggesting the formation of nitrogenous secondary BrC. In January, BrC spectra showed a dual hump with first peak at 300 nm (likely due to HULIS) and second at 500 nm (likely due to secondary nitro aromatics). The second hump disappeared during day hours and overall BrC absorption at shorter wavelengths also decreased, which was ascribed to photo-bleaching/volatilization.