Evaluating the Influence of Convective Downdrafts on Seasonal and Diurnal Differences in Submicrometer Aerosol Size Distributions Measured during GoAmazon2014/5

Aerosol-cloud interactions remain the source of one of the largest uncertainties in global climate models. GoAmazon 2014/5, a 2-year DOE sponsored field campaign centered on Manacapuru, Brazil, in the central Amazon, provides a unique opportunity to evaluate aerosol properties in near-natural conditions. Aerosols measured during selected days within the wet season at the Amazon Tall Tower Observatory (ATTO) show a clear bimodal size distribution with a separation of Aitken and accumulation modes suggesting an influence of in-cloud processing on the aerosol population. These Aitken mode particles are transported to the surface via precipitation induced convective downdrafts. Aitken particle size is negatively correlated with the decrease in equivalent potential temperature, due in part to the origin height of the associated convective downdraft. It has been found that the concentration of particles within the accumulation mode is lowest in the early morning and increases after sunrise. This increase is a combination of both the mixing of accumulation mode particles in the residue layer aloft down to the surface and particle growth through condensation and in-cloud processing.

A one-year period of cold pool events will be investigated in order to further evaluate the influence of convective downdrafts on aerosol size distributions. Cold pools are found by removing the diurnal and seasonal trends in surface meteorological observations and verifying the results using the Brazilian military (SIPAM) operational S-band radar in Manaus. The seasonal and diurnal patterns in aerosol size distributions for over 500 convective events identified at ATTO will be analyzed. Aerosol size distributions are expected to be influenced by the frequency and strength of the cold pools, which exhibit a small seasonal difference and a clear diurnal cycle with a maximum during the afternoon and a secondary peak in the early morning hours. In addition, the origin heights of the convective downdrafts will be calculated using radar observations and used to estimate the downdraft mass flux. These unique observations will allow for a thorough investigation into the seasonal and diurnal changes in the transport of aerosols to the surface.