What is column integrated aerosol remote sending telling us about cloud condensation nuclei?

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Description

Since aerosol measurements from space were first made, there has been a desire to use them as proxy estimates of the concentration of cloud condensation nuclei (CCN) in the boundary layer. A number of studies have shown that concentration of aerosol particles in the accumulation mode is a good proxy for CCN concentration (N_CCN). However, spaceborne aerosol measurements typically use the column integrated optical depth aerosol optical depth (AOD) as a proxy for CCN. There are several reasons why N_CCN is not uniquely predicted from AOD: (a) hygroscopic growth may vary from location to location and with cloud conditions, (b) the dry aerosol size can vary in time and space, (c) the depth of the layer in which most of the scattering takes place can vary. In this study, we use a variety of different observational and modeling results to assess these potentially complicating factors in the estimation of CCN concentration from space.

The following three areas are explore various aspects of the problem:

1. We use a large number of rawinsonde launches and collocated cloud property information from the Clouds, Aerosol and Precipitation in the Marine Boundary Layer (CAP-MBL) ARM Mobile Facility (AMF) deployment on Graciosa Island in the Azores to quantify the hygroscopic growth contribution to AOD and explore its variability and relationship with cloud cover and other factors.
2. We use data from the VOCALS Regional Experiment to assess the geographical variability in the factors controlling AOD from a polluted region to a clean region. We show how small increases in aerosol particle size and the depth of the boundary layer both have a major impact on the offshore gradient in AOD. This is such that these variations almost cancel out the impact on AOD of an almost threefold decrease in accumulation mode and CCN concentration moving offshore.
3. We use WRF-Chem simulations over the southeastern Pacific (VOCALS region) to explore the correlation between aerosol properties in clear regions and cloud microphysical properties in adjacent cloudy regions to explore the impact of spatial separation of aerosol and cloud properties on the correlations between these variables.