Vertically Resolved Measurements of Nighttime Radical Reservoirs; in Los Angeles and Their Contribution to the Urban Radical Budget

Photolabile nighttime radical reservoirs, such as nitrous acid (HONO) and nitryl chloride (ClO2), contribute to the oxidizing potential of the atmosphere, particularly in early morning. We present the first vertically resolved measurements of ClO2, together with vertically resolved measurements of HONO. These measurements were acquired during the California Nexus (CalNex) campaign in the Los Angeles basin in spring 2010. Average profiles of ClO2 exhibited no significant dependence on height within the boundary layer and residual layer, although individual vertical profiles did show variability. By contrast, nitrous acid was strongly enhanced near the ground surface with much smaller concentrations aloft. These observations are consistent with a ClO2 source from aerosol uptake of N2O5 throughout the boundary layer and a HONO source from dry deposition of NO2 to the ground surface and subsequent chemical conversion. At ground level, daytime radical formation calculated from nighttime-accumulated HONO and ClO2 was approximately equal. Incorporating the different vertical distributions by integrating through the boundary and residual layers demonstrated that nighttime-accumulated ClO2 produced nine times as many radicals as nighttime-accumulated HONO. A comprehensive radical budget at ground level demonstrated that nighttime radical reservoirs accounted for 8% of total radicals formed and that they were the dominant radical source between sunrise and 09:00 Pacific daylight time (PDT). These data show that vertical gradients of radical precursors should be taken into account in radical budgets, particularly with respect to HONO.