Wintertime Molecular and Optical Properties of Carbonaceous Aerosols in a Semi-Arid Environment

Key-words: Carbonaceous aerosol, WSOC, Molecular-level characterization, radiative-forcing, Semi-arid region

Carbonaceous aerosols are a major component of atmospheric fine particulate matter and play a crucial role in climate forcing due to their light-absorbing properties. Their influence on wintertime radiative forcing is particularly pronounced, while the source-specific absorption and molecular properties remain poorly understood in semi-arid regions¹. To address this knowledge gap, we conducted a 24-hr three-week PM_2.5 winter campaign in a semi-arid urban region of northwestern India. We performed measurement of BC concentration using multi-wavelength AE-33, filter-based elemental carbon-organic carbon (EC–OC) analysis, water-soluble organic carbon (WSOC) quantification and brown carbon (BrC) optical characterization, and molecular characterization of isolated HULIS fractions (HULIS-A at pH=2 and HULIS-N at pH=7) using UHPLC-ESI-ToF-MS.

eBC remained consistently elevated, with most daily mean values above 15 µg/m³ and episodic peaks exceeding 26 µg/m³, indicating sustained wintertime loading. Fossil-fuel derived BC (BC_ff) dominated throughout the day, with early-morning (12.61 µg/m³) and evening (15.52 µg/m³) peaks, 2.84–15.52 µg/m³, while biomass-burning BC (BC_bb) showed characteristic morning and late-evening enhancements of 8–11 µg/m³, consistent with residential biomass or wood burning in colder periods. Further, we observed WSOC/OC ratio of 0.75–0.80 with average WSOC levels of 35 µg/m³. A strong near-UV absorption, with SUV₂₅₄ (MAC₂₅₄ ≈ 5.0–5.2 m²/g) and MAC₂₅₄ > 5 m²/g, along with a high AAE (4.04) confirmed the presence of strongly light-absorbing BrC from biomass-burning precursors and secondary processing^2,3.

The molecular characterization of HULIS showed that CHO- and CHON-rich ions were present not only in the semi-volatile oxygenated organic aerosol (SVOOA) domain but also in regions associated with biomass-burning organic aerosol (BBOA). This indicates the simultaneous presence of both primary BB products and secondarily aged organics. HULIS-A exhibited stronger near-UV absorption, confirming its dominant contribution to wintertime chromophores. In DBE–N_C space, HULIS-A displayed a much broader distribution compared to HULIS-N, extending into regions characteristic of unsaturated aromatics, phenolic and nitro-aromatic BrC precursors, and cata-PAH-like structures, all of which are known carriers of strong near-UV absorption.

Together, the optical measurements (MAC spectra, UV–Vis absorption) and high-resolution molecular analysis indicate that wintertime WSOC at this semi-arid site is strongly enriched in both primary BBOA-linked chromophores and secondary OOA-derived oxygenated species. Among these, HULIS-A emerges as the principal carrier of light-absorbing organic matter, driving enhanced shortwave absorption during winter.

References:

(1)        Laskin, A.; Laskin, J.; Nizkorodov, S. A. Chemistry of Atmospheric Brown Carbon. Chem. Rev. 2015, 115 (10), 4335–4382. https://doi.org/10.1021/cr5006167.

(2)        Weishaar, J. L.; Aiken, G. R.; Bergamaschi, B. A.; Fram, M. S.; Fujii, R.; Mopper, K. Evaluation of Specific Ultraviolet Absorbance as an Indicator of the Chemical Composition and Reactivity of Dissolved Organic Carbon. Environ. Sci. Technol. 2003, 37 (20), 4702–4708. https://doi.org/10.1021/es030360x.

(3)        Hecobian, A.; Zhang, X.; Zheng, M.; Frank, N.; Edgerton, E. S.; Weber, R. J. Water-Soluble Organic Aerosol Material and the Light-Absorption Characteristics of Aqueous Extracts Measured over the Southeastern United States. Atmospheric Chem. Phys. 2010, 10 (13), 5965–5977. https://doi.org/10.5194/acp-10-5965-2010.