Clouds as Sources and Sinks in the Life Cycle of the Pristine Aerosol

December 2, 2020 3:00pm to 4:00pm
Zoom Seminar



Speaker: Meinrat O. Andreae

Institution(s): Max Planck Institute for Chemistry & Scripps Institution of Oceanography


We are conducting long-term measurements at two sites that experience extended periods of near-pristine conditions, one in Central Siberia (ZOTTO), the other in the Central Amazon Basin (ATTO). At both sites, we find the classical type of nucleation events (“bananas”) to be exceedingly rare, in contrast to sites that experience more significant anthropogenic influence. This calls in question the source of new particles in the regions represented by our remote sites.

Observations during the ACRIDICON-CHUVA aircraft campaign over the Amazon Basin showed high aerosol concentrations in the upper troposphere (UT), with number concentrations (normalized to standard conditions) often exceeding those in the boundary layer (BL) by one or two orders of magnitude.

Aerosol enhancements in the UT were consistently observed on all flights, using several aerosol metrics, including condensation nuclei (CN), cloud condensation nuclei (CCN), and chemical species mass concentrations. These UT aerosols were different in their composition and size distribution from the aerosol in the BL, making convective transport of particles unlikely as a source. The regions in the immediate outflow of deep convective clouds were depleted in aerosol particles, confirming the convective clouds as sinks of atmospheric aerosols. In contrast, dramatically enhanced small (<90 nm diameter) aerosol number concentrations were found in UT regions that had experienced outflow from deep convection in the preceding 24-48 hours. We also found elevated concentrations of larger (>90 nm) particles in the UT, which consisted mostly of organic matter and nitrate and were very effective CCN.

Our findings suggest that aerosol production takes place in the UT from volatile material brought up by deep convection, which is converted to condensable species in the UT. Subsequently, downward mixing and transport of upper tropospheric aerosol may be a source of particles to the BL, where they increase in size by the condensation of biogenic volatile organic carbon (BVOC) oxidation products. This may be an important source of aerosol particles in the remote and pristine troposphere, where aerosol nucleation and new particle formation is not commonly observed. We propose that this may have been the dominant process supplying secondary aerosols in the pristine atmosphere, making clouds the dominant control of both removal and production of atmospheric particles.

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