Observation of dark brown carbon in urban aerosols and its contribution to surface dimming

Abstract. Dark brown carbon (d-BrC) aerosols are commonly associated with biomass burning and wildfire emissions, yet their occurrence and climatic impacts in urban environments remain elusive. Southeast Texas – a hub for petrochemical industries at the intersection of continental and marine air masses – is strongly influenced by aerosol-induced surface dimming and extreme meteorological events. Here, the in-situ photoacoustic measurements reveal that over 80 % of aerosol light absorption at blue and near-infrared wavelengths is due to non-black carbon (BC) aerosols. The computer-controlled scanning electron microscopy and particle-scale electron energy loss spectroscopy demonstrates the presence of refractory d-BrC in the coastal urban atmosphere as an absorption contributor. The diurnal pattern of BC and non-BC absorption suggests association of the non-BC aerosols such as d-BrC particles with local non-biomass burning combustion sources, potentially including traffic, flares, and industrial activities. Observationally constrained radiative-transfer calculations show that d-BrC contributes 40 % of daytime-mean surface dimming and 50 % of the total top-of-atmosphere radiative forcing attributable to light-absorbing carbon. Such radiative changes that influence local meteorology may also impact ~25 % of the global population residing in coastal urban regions like Houston. Our findings demonstrate the presence and climatic importance of d-BrC in a coastal urban environment in addition to wildfire plumes, highlighting the need for incorporation of d-BrC in aerosol-climate models.