Urban-to-downwind variations in aerosol composition and optical properties in the Paris urban plume from coordinated airborne and ground-based observations

Aerosols play a critical role in Earth’s climate, but substantial evolution in their physicochemical properties after emission introduces uncertainties in predicting their climate impacts. Observational constraints on how aging modifies aerosol properties remain limited. Here, we investigate the effects of ~2–6 h of aging on aerosol physicochemical properties using coordinated airborne and ground-based measurements in Paris and its downwind regions. Urban plumes contributed modestly to particle number concentrations in the 80–200 nm size range and resulted in a moderate enhancement of submicron particle (PM1) mass relative to out-plume background levels. Organic aerosol (Org) dominated PM1 mass both near the urban source and downwind. Aircraft observations showed enhanced Org and non-refractory PM1 relative to excess CO (CO above surrounding background) in downwind plumes, indicating net secondary organic aerosol production during aging. Aerosol optical properties evolved concurrently. Downwind plume-average single-scattering albedo (SSA) at 450 and 630 nm was higher than near-source values. Consistently, the complex refractive index shifted from lower real (~1.35–1.40) and higher imaginary (~0.03–0.08) components near source to higher real (~1.45–1.50) and lower imaginary (~0.015–0.02) components downwind. The absorption Ångström exponent also increased, indicating a greater fractional contribution of brown carbon to light absorption. These results demonstrate that urban plume aging alters aerosol composition and optical properties and highlight the need to represent evolving aerosol characteristics in atmospheric models and remote-sensing retrievals.