Dark Brown Carbon Dominates Shortwave Absorption in Urban Emissions

YUEZHI (AUGUST) LI, Joshin Kumar, Gwan-Yeong Jung, Taveen S. Kapoor, Joseph V. Puthussery, Guodong Ren, Jordan A. Hachtel, Zezhen Cheng, Nurun Nahar Lata, Benjamin Sumlin, Chenchong Zhang, Dishit Ghumra, Ganesh Chelluboyina, Swarup China, Rohan Mishra, Rajan K. Chakrabarty, Washington University in St. Louis

     Abstract Number: 535
     Working Group: Aerosol Processes and Properties in Changing Environments in the Anthropocene

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 absorption properties render challenges in measuring and modeling its climatic impacts. Despite new evidence showing strong brown carbon absorption in the visible-infrared wavelengths, current climate models constrain brown carbon light absorption in ultraviolet-blue ranges and its sources to biomass burning, inducing large uncertainties. The increasing anthropogenic activities due to global urbanization have also led to growing sources of brown carbon particles, highlighting the urgency to understand their optical and physicochemical properties more thoroughly.

In the airmasses of Houston area, deemed the epicenter of United States’ oil industries, we observed refractory brown carbon aerosols (dark brown carbon) that contribute to ~50% of the total shortwave absorption across the visible and near-infrared wavelengths. Particle-scale measurements also show strong brown carbon absorption in the visible-infrared wavelengths, with mean imaginary refractive index of 0.106 and 0.055 at 550 and 1047 nm, respectively. Experimental compositional analyses complemented by electronic structure calculations suggest the formation and emission of these particles from anthropogenic high-temperature combustion as primary aerosols alongside black carbon. This dark-brown-black-carbon continuum in high-temperature combustion systems is controlled by the degree of sp2 hybridization of the carbon atoms. Our findings provide the first observations of dark-brown carbon in urban emissions and underscore the need for its incorporation in emission inventory and aerosol-radiation models for an improved estimate on the direct radiative forcing and associated climatic effects from urban aerosols.