Abstract
Abstract. We present a case study of Aitken and accumulation mode aerosol observed downwind of the anvil of a deep tropical thunderstorm. The measurements were made by condensation nuclei counters flown on the Egrett high-altitude aircraft from Darwin during the ACTIVE campaign, in monsoon conditions producing widespread convection over land and ocean. Maximum measured concentrations of aerosol with diameter greater than 10 nm were 25 000 cm−3 (STP). By calculating back-trajectories from the observations, and projecting onto infrared satellite images, the time since the air exited cloud was estimated. In this way a time scale of about 3 hours was derived for the Aitken aerosol concentration to reach its peak. We examine the hypothesis that the growth in aerosol concentrations can be explained by production of sulphuric acid from SO2 followed by particle nucleation and coagulation. Estimates of the sulphuric acid production rate show that the observations are only consistent with this hypothesis if the particles coagulate to sizes >10 nm much more quickly than is suggested by current theory. Alternatively, other condensible gases (possibly organic) drive the growth of aerosol particles in the TTL.
Highlights
1.1 Aerosol population in the Tropical Tropopause LayerThis paper presents a case study of aerosol measurements in the Tropical Tropopause Layer (TTL) and estimates the time scale for the growth of new aerosol to Aitken mode sizes (≥10 nm)
As there was no biomass burning in the Darwin area during the active monsoon in January, it is very unlikely that the SO2 concentration on 23 January 2006 exceeded that in December, and 100 pptv is taken as the maximum conceivable SO2 concentration during the flight considered here
Much higher Aitken mode aerosol concentrations were found in this detrained outflow than in the boundary layer or in the anvil cloud itself
Summary
1.1 Aerosol population in the Tropical Tropopause LayerThis paper presents a case study of aerosol measurements in the Tropical Tropopause Layer (TTL) and estimates the time scale for the growth of new aerosol to Aitken mode sizes (≥10 nm). The inactive monsoon which immediately followed (25 January to 3 February 2006) was free of deep convection, allowing background aerosol concentrations in the TTL to be measured. For the first CPC (> 10 nm) the median concentration was ∼ 4500 cm−3 (STP) with maxima < 6000 cm−3 (STP); for the second the corresponding values were 100 and 160 cm−3 (STP) These background concentrations allow us to place the measurements that follow, taken in convective conditions, in context. As there was no biomass burning in the Darwin area during the active monsoon in January, it is very unlikely that the SO2 concentration on 23 January 2006 exceeded that in December, and 100 pptv is taken as the maximum conceivable SO2 concentration during the flight considered here
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