Research Projects

Brown carbon (BrC), the light absorbing fraction of organic carbon (OC), exerts vital effects on climate change owing to its light absorption characteristics in the wavelength ranging from near-ultraviolet to visible light. Further, BrC can also affect the atmospheric photochemistry and secondary organic aerosol (SOA) formation processes, and therefore regional air quality by changing the photolysis rates of atmospheric radicals. However, the optical properties and formation mechanisms of BrC aerosols remain poorly understood, especially as a function of aerosol size, which also limits the estimation of their radiative forcing. The distribution of BrC and its constituents, e.g., humic-like substances (HULIS), across the aerosol size spectrum is governed by factors such as the nature of sources, including natural/anthropogenic origin, fuel type and combustion conditions, and the degree of atmospheric processing. Further, the size-resolved chemical speciation of BrC also constrains its climate-relevant optical properties to a great degree. In India, studies linking sources, atmospheric processing, molecular composition and climate properties of BrC are rather limited, while those on size-segregated aerosol are non-existent till date. In view of this, the current work plans to collect size-segregated aerosol samples from two megacities (Delhi and Kolkata) at the northwestern and eastern ends of the Indo-Gangetic Plain (IGP) corridor that are characterized by different emission sources and distinct meteorological conditions. The major objectives of this work are to understand: i) how do BrC/HULIS profiles and optical characteristics vary as a function of aerosol size over Delhi and Kolkata? ii) what is the relative importance of primary emission vs secondary formation processes in constraining size-resolved BrC/HULIS iii) what is the link between chromophoric composition and climate-relevant optical properties of size-resolved BrC/HULIS and (iv) How much is the radiative forcing efficiency of BrC aerosols in the Indian scenario? To achieve these objectives, collected samples will be analyzed for PM mass, and a variety of chemical and optical parameters including organic carbon (OC), elemental carbon (EC), 13C, water-soluble and methanol-soluble brown carbon, water-soluble organic carbon (WSOC), fluorescence intensities, functional groups, and HULIS. A wide variety of proposed chemical, optical, and isotopic measurements would provide deeper insights into the sources, composition, and optical characteristics of BrC aerosols over the two Indian megacities, and provide the first field-based estimates of size-resolved refractive index and mass absorption efficiency, which can be directly used in regional climate forcing models.