Aerosol Size Distribution Modification by Interaction with Fog or Clouds Observed by AERONET

The modification of aerosol optical properties due to the interaction with fog or clouds is examined from measurements made by sun/sky radiometers at several AERONET sites. Almucantar retrievals, which utilize measured sky radiances and spectral aerosol optical depth, are examined for cases where fog and/or clouds have recently dissipated or advected away. Retrieved volume size distributions for cases identified as aerosol modified by cloud show very large median fine mode radius (~0.25-0.42 microns), which often show a bimodal sub-micron distribution. This bimodal accumulation mode distribution may be due to one mode (the larger one) from cloud-processed aerosol and the other from interstitial aerosol, or possibly from two different aerosol species (differing chemical composition) with differing hygroscopic growth factors. The size of the fine mode particles from AERONET retrieved for these cases exceeds the size of sub-micron sized particles retrieved for nearly all other aerosol types, suggesting significant modification of aerosols within the fog or cloud environment. The almucantar retrievals are analyzed from the Kanpur site in the Indo-Gangetic Plain in India (fog in January), Arica on the northern coast of Chile (stratocumulus), Fresno, CA in the San Joaquin Valley (fog in winter), and several other sites including Beijing with aerosol observations made when clouds dissipate or gaps occur in cloud cover. Aerosols of this type and large size range may also be formed in partly cloudy conditions and may contribute to the ‘shoulder’ of larger size particles in regions where sulfate aerosol is a significant component. In-situ measured aerosol size distributions made during fog events are compared to the AERONET retrievals.