Influence of clouds and precipitation on sub-micron aerosol number size distributions in the tropical atmosphere

AGU (American Geophysical Union) Fall Meeting 2021

Clouds and precipitation play a key role in controlling the dynamics of aerosol populations. A quantitative description of atmospheric aerosol particle abundance, variability and size is crucial to assess and predict their impact on climate, including radiative forcing and nucleation of cloud droplets. While it is known that clouds and precipitation efficiently scavenge accumulation-mode aerosols through rainout and washout, the processes through which they influence smaller aerosol particles remain unclear. The smaller particles are mainly formed by the nucleation of atmospheric vapours, but unlike many other environments, the nucleation of new particles has almost never been observed in the pristine Amazon boundary layer. Recent studies show that the rapid downward vertical transport from the free-troposphere to the boundary layer during precipitation events may be an important process to replenish small particle concentrations in this region. Here, we present a multi-year analysis of links between precipitation, clouds and in-situ aerosol number size distribution (ANSD) measurements at the Amazon Tall Tower Observatory (ATTO) (in pristine rain forest region in the central Amazon Basin at 190 m a.s.l), and at the high-altitude Atmospheric Research Station Pico Espejo (located on top of the Sierra Nevada Mountain ridge at 4765 m a.s.l in Venezuela). We characterise the variability and dependence of ANSDs on the local environment and meteorological conditions, and we compare the two tropical sites under similar meteorological and airmass history conditions. To derive the impact of cloudiness and precipitation along the airmass transport, we combine the ANSD measurements with satellite retrievals of cloud and precipitation properties (GOES-12 for cloud top temperature, and TRMM for precipitation rates) collocated along airmass backward trajectories. In ATTO, we observe a clear aerosol size threshold for which precipitation act as a source of Aitken-mode particles and as a sink for the accumulation-mode particles, whereas in Pico Espejo, no clear source effect of smaller aerosols associated with precipitation is observed.

Full poster: https://agu2021fallmeeting-agu.ipostersessions.com/default.aspx?s=D7-C1-CB-E6-9F-DE-F2-03-0C-E1-96-02-12-79-D3-CC