Wind Speed Influences on Marine Aerosol Optical Depth

The Mulcahy (Mulcahy et al., 2008) power-law parameterization, derived at the coastal Atlantic station Mace Head, between clean marine aerosol optical depth (AOD) and wind speed is compared to open ocean MODIS-derived AOD versus wind speed. The reported AOD versus wind speed (U) was a function of ~U2. The open ocean MODIS-derived AOD at 550？nm and 860？nm wavelengths, while in good agreement with the general magnitude of the Mulcahy parameterization, follows a power-law with the exponent ranging from 0.72 to 2.47 for a wind speed range of 2–18？m . For the four cases examined, some MODIS cases underestimated AOD while other cases overestimated AOD relative to the Mulcahy scheme. Overall, the results from MODIS support the general power-law relationship of Mulcahy, although some linear cases were also encountered in the MODIS dataset. Deviations also arise between MODIS and Mulcahy at higher wind speeds (>15？m ), where MODIS-derived AOD returns lower values as compared to Mulcahy. The results also support the suggestion than wind generated sea spray, under moderately high winds, can rival anthropogenic pollution plumes advecting out into marine environments with wind driven AOD contributing to AOD values approaching 0.3. 1. Introduction Sea spray aerosol is one of the largest natural contributors to the global aerosol loading and thus plays an important role in the global radiative budget [1, 2]. Submicron size aerosols are especially relevant in terms of cloud condensation nuclei [3], while both sub- and supermicron sizes contribute to aerosol scattering [4] and to aerosol optical depth [5]. Both of these effects suggest that sea spray aerosol plays an important role in the global radiative budget, contributing to the aerosol climate effect [6, 7]. Mulcahy et al. [5] established that under moderate to high wind speed conditions, AOD associated with sea spray followed a power-law wind-speed dependency with an exponent of 2. At moderately high wind speeds, sea spray-derived AOD reached the order of 0.35, often exceeding AOD associated with pollution plumes over oceanic regions. The relationship of Mulcahy et al. [5] has recently been compared to model-predicted AOD from sea spray sources by Madry et al. [8]. In the latter study, they found that model-derived AOD also followed a wind speed square function, and that the formulation proposed by Mulcahy et al. [5] was highly correlated to the modeled-derived AOD values. Mulchay et al. [5] report AOD values for clean marine air which are significantly higher than those previously reported (e.g., Smirnov et al.