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

Nighttime measurements of aerosol surface area, O-3, 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 O-3, relative humidity, NOy and aerosol number density. However, the air parcels sampled within the urban plume just before dawn had O-3 mixing ratios that were highly anti-correlated with aerosol number density, NOy and relative humidity. Bank-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 O-3 to the aerosol surface, are examined to explain the observed depletion in O-3 within the pre-dawn parcels that had passed over Portland. Published by Elsevier Science Ltd.