Aircraft observations of aerosols, O3 and NOy in a nighttime urban plume

Nighttime measurements of aerosol surface area, O3, NOy and moisture were made downwind of Portland, Oregon, as part of a study to characterize the chemistry in a nocturnal urban plume. Air parcels sampled within the urban plume soon after sunset had positive correlations between O3, relative humidity, NOy and aerosol number density. However, the air parcels sampled within the urban plume just before dawn had O3 mixing ratios that were highly anti-correlated with aerosol number density, NOy and relative humidity. Back-trajectories from a mesoscale model show that both the post-sunset and pre-dawn parcels came from a common maritime source to the northwest of Portland. The pre-dawn parcels with strong anti-correlations passed directly over Portland in contrast to the other parcels that were found to pass west of Portland. Several gas-phase mechanisms and a heterogeneous mechanism involving the loss of O3 to the aerosol surface, are examined to explain the observed depletion in O3 within the pre-dawn parcels that had passed over Portland.