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Estimating Marine Aerosol Particle Volume and Number from Maritime Aerosol Network Data

A. Sayer,A. Smirnov,N. C. Hsu,L. Munchak,B. Holben

Published 2012 in Atmospheric Chemistry and Physics

ABSTRACT

As well as spectral aerosol optical depth (AOD), aerosol composition and concentration (number, volume, or mass) are of interest for a variety of applications. How- ever, remote sensing of these quantities is more difficult than for AOD, as it is more sensitive to assumptions relating to aerosol composition. This study uses spectral AOD mea- sured on Maritime Aerosol Network (MAN) cruises, with the additional constraint of a microphysical model for unpol- luted maritime aerosol based on analysis of Aerosol Robotic Network (AERONET) inversions, to estimate these quanti- ties over open ocean. When the MAN data are subset to those likely to be comprised of maritime aerosol, number and volume concentrations obtained are physically reasonable. Attempts to estimate surface concentration from columnar abundance, however, are shown to be limited by uncertain- ties in vertical distribution. Columnar AOD at 550 nm and aerosol number for unpolluted maritime cases are also com- pared with Moderate Resolution Imaging Spectroradiometer (MODIS) data, for both the present Collection 5.1 and forth- coming Collection 6. MODIS provides a best-fitting retrieval solution, as well as the average for several different solutions, with different aerosol microphysical models. The "average solution" MODIS dataset agrees more closely with MAN than the "best solution" dataset. Terra tends to retrieve lower aerosol number than MAN, and Aqua higher, linked with dif- ferences in the aerosol models commonly chosen. Collection 6 AOD is likely to agree more closely with MAN over open ocean than Collection 5.1. In situations where spectral AOD is measured accurately, and aerosol microphysical properties are reasonably well-constrained, estimates of aerosol number and volume using MAN or similar data would provide for a greater variety of potential comparisons with aerosol proper- ties derived from satellite or chemistry transport model data. However, without accurate AOD data and prior knowledge of microphysical properties, such attempts are fraught with high uncertainties.

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