Field validation of brown carbon absorption dependence on acidity and aerosol liquid water content

Brown Carbon (BrC) is an organic component of aerosols with light-absorbing characteristics that have crucial consequences in atmospheric warming and the climate system, yet it carries significant uncertainty. This uncertainty is due to its non-static optical properties which provide a significant challenge in the measurement of the perturbation caused by them in the Earth radiation budget. The unpredictability in optical properties is because of the continuous formation of Secondary BrC and decay of existing BrC influenced by various physicochemical and meteorological factors in the ambient atmosphere. The dynamic behaviour of these chromophores can be impacted by the aerosol liquid water content (ALWC) and atmospheric acidity via influencing its atmospheric chemistry of formation and decay. The objective of this research is to investigate how the ALWC and acidity in terms of pH affect the BrC optical properties in the rarely examined Eastern part of India during extreme winters. Utilizing a thermal-optical carbon analyzer, the optical characteristics of BrC were estimated. The ISORROPIA II, thermodynamic model was employed to simulate ALWC and aerosol pH, yielding a mean pH value of 3.30 +/- 0.16 for the study duration. The study provides the first in-field evidence of a linear increase of absorption coefficient with increasing pH or decreasing aerosol acidity in the ambient atmosphere. A 39.6 Mm- 1 increase in absorption coefficient per unit increase in pH, shows that aerosol pH is one of the decisive elements influencing BrC chemistry. The results also showed the inverse relation of the absorption coefficient with ALWC. The findings indicate the sensitivity of BrC chemistry towards aerosol acidity and ALWC in the ambient atmosphere and its importance while evaluating BrC absorption.