Photochemical ageing drives the browning of urban outflows through secondary brown carbon production

Abstract. Brown carbon (BrC) absorbs solar radiation, thereby partially offsetting the cooling effect of aerosols on the climate. However, its contribution to climate forcing remains poorly constrained and is currently not explicitly included in the overall radiative forcing estimates of the IPCC. In this work, we quantify the contributions of BrC to light absorption in urban outflows from Paris using aircraft observations. We observe a progressive "browning" of the plume over 6 hours of transport time, characterised by a significant increase in the relative contribution of BrC to aerosol light absorption. This optical shift is strongly correlated with photochemical ageing as evidenced by the increase in the organic aerosols-to-BC mass ratio and the simultaneous depletion of aromatic volatile organic precursors. The corresponding rise in the Absorption Angström Exponent from 1.1 to 2.6 provides further evidence that secondary organic aerosol formation is the primary driver of enhanced light absorption at 450 nm in the aged plume. Our observations indicate that BrC is photochemically produced during transport at a rate of ~0.25 h^-1 over 6 hours of atmospheric processing under the conditions examined in this study. For these cases, the findings suggest that urban outflows are not merely diluted during transport but undergo significant chemical transformations that alter their radiative impact. Accounting for these time-dependent optical changes provides a way to refine how climate models simulate radiative impacts of large metropolitan areas to climate change.