Dark Brown Carbon Dominates Aerosol Light Absorption in Urban Anthropogenic Emissions
YUEZHI (AUGUST) LI, Taveen Kapoor, Joseph V. Puthussery, Gregory W. Vandergrift, Zezhen Cheng, Guodong Ren, Joshin Kumar, Nurun Nahar Lata, Benjamin Sumlin, Felipe Rivera-Adorno, Alexander Laskin, Rohan Mishra, Swarup China, Rajan K. Chakrabarty, Washington University in St. Louis
Abstract Number: 487
Working Group: Coast to Coast Campaigns on Aerosols, Clouds, Chemistry, and Air Quality
Abstract
Brown carbon aerosols are important climate forcers that alter the Earth’s radiative budget by absorbing and scattering incident solar radiation. However, brown carbon’s highly variable physicochemical and optical properties render large uncertainties in measuring and modeling its climatic impacts. The growing anthropogenic sources due to global urbanization are potential emitters of brown carbon particles, highlighting the urgency to understand their properties more thoroughly.
In this study, we present the bulk and particle-scale absorption, morphology, volatility, and chemical composition of ambient aerosols in La Porte, Texas, an urban coastal city influenced by mixed anthropogenic emission sources such as traffic and petrochemical industries. Our measurements show significant brown carbon absorption at near-infrared wavelengths, accounting for 53.6 ± 16.6% (0.97 ± 0.55 Mm-1) at 721 nm and 37.8 ± 18.8% (0.34 ± 0.31 Mm-1) at 1047 nm. This is attributed to the dominance of strongly absorbing dark brown carbon comprised of extremely low volatility organic compounds, contributing to 20.1 ± 9.5% (0.28 ± 0.06 μg/m3) of the total carbonaceous aerosol mass. Particle-scale measurements on sampled dark brown carbon particles also show strong absorption across the ultraviolet-visible-infrared spectrum, with mean imaginary refractive indices of 0.106 at 550 nm and 0.055 at 1047 nm. These particles, unlike tar balls, exhibit non-spherical morphology, but their water insolubility, low volatility, and thermal stability are consistent with reported dark brown carbon properties. The observed dark brown carbon particles are only partially soluble in common organic solvents, with the soluble fraction predominantly comprised of aged (O/C > 0.5) organosulfates. Our findings reveal possible underestimation on brown carbon absorption in urban environments in past studies assuming negligible near-infrared brown carbon absorption. Given the sheer number of cities like La Porte globally, a robust parameterization of anthropogenic dark brown carbon can significantly improve climate models’ estimation on aerosol radiative forcing and associated warming effects.