Aerosol and trace‐gas measurements in the Darwin area during the wet season

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Abstract
The composition of the planetary boundary layer in regions of deep tropical convection has a profound impact on the Tropical Tropopause Layer (TTL). The Aerosol and Chemical Transport in tropIcal conVEction (ACTIVE) aircraft campaign was conducted from November 2005 to February 2006 from Darwin, Australia, to characterize the influence of both monsoonal and localized land‐based deep convection on the composition of the TTL. This paper summarizes the composition of the potential inflow to such convection in terms of aerosol particle size and composition, carbon monoxide, and ozone, as measured in the lowest 4 km of the atmosphere by the NERC Dornier‐228 aircraft during 28 flights in different meteorological regimes over the course of ACTIVE. Six contrasting periods are identified in the boundary layer background as a result of the prevailing meteorology and sources of pollution. The campaign began with a relatively polluted and variable biomass burning season in November, followed by a transition to the monsoon season through December with much less burning. A clean maritime flow dominated the wet‐active, and dry‐inactive, monsoon period in January; it was followed by a monsoon break period in February, with a return to continental flow and a more premonsoon background state. Deep convective systems, capable of transporting boundary layer air to the TTL, were observed daily outside of the monsoon periods. The chemical composition of submicron aerosols in the premonsoon periods was dominated by a mix of fresh and aged organic material with significant black carbon, well‐correlated with carbon monoxide indicating a common burning source, while marine aerosol during the monsoon changed markedly between the wet and dry phases. High concentrations of coarse‐mode aerosols were also observed in the monsoon: the clean, marine air masses and high surface winds imply that sea salt may be the dominant aerosol type under these conditions. The climatology presented here will provide a valuable data set for model simulation of chemical and aerosol transport by deep convection in the Darwin region.

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