Spectral Absorbing Capacity of Brown Carbon Aerosols Over Indo-Gangetic Plain

Organic carbon dominates in atmospheric particulate matter concentration all over the world. A part of organic carbon is known to absorb light in ultra-violet and mid visible wavelengths. Such absorbing organics are collectively called brown carbon (BrC). We present spectral BrC imaginary refractive indices of water soluble organic carbon (WSOC) and total organic carbon (OC) during the winter-spring season of 2015-16. Measurements were made from the city of Kanpur, India located inside the Indo-Gangetic Plain (IGP). Winter-spring season in the IGP is known for its high aerosol loading due to frequent wood and trash burning. Absorption and mass concentration of WSOC is measured using a combination of Particles in to Liquid (PILS), Liquid Waveguide Capillary Cell (LWCC) and Total Organic Carbon (TOC) analyzer system. Same for OC is measured using an offline method, where samples were collected over quartz filter and then analyzed in LWCC and OC-EC analyzer. Our results show that BrC in the IGP is highly absorbing when compared to other parts of the world. The WSOC shows more absorbing capacity compared to OC. Spectral nature of the refractive indices shows WSOC with a higher wavelength dependence compared to OC. Above 470 nm wavelength absorption capacity of WSOC is negligible but absorbance from OC is visible till 565 nm. Incorporating these measured values, a modeling approach is derived to identify the percentage contribution of different absorbing species to total aerosol absorption. Our results show the special characteristics of organics in IGP.