Satellite remote sensing methods for estimating clear Sky shortwave Top of atmosphere fluxes used for aerosol studies over the global oceans

a r t i c l e i n f o The difference between the top of atmosphere shortwave clear sky (cloud and aerosol free, SWCLR) and aerosol sky radiative fluxes is known as direct radiative effect (DRE) for all aerosols or Direct Climate Forcing (DCF) for anthropogenic aerosols. There are several methods for calculating SWCLR including satellite-based methods and radiative transfer approaches. Since uncertainties in SWCLR can propagate into errors in DRE or DCF, we assess the SWCLR estimates over the global oceans using three approaches and quantify the differences among these methods both as a function of space and season. Our results indicate that the more commonly used intercept (73.4 ± 3.6) and radiative transfer methods (74.7 ± 4.0 Wm − 2) are in close agreement to within ± 1.3 Wm − 2. Values of SWCLR are provided as a function of space and season that can be used by other studies that require such values or as a source of validation. We further recommend that research studies report the methods and assumptions used to estimate SWCLR to facilitate easier intercomparisons among methods. Tropospheric aerosols are usually defined as solid or liquid particles suspended in air that can be produced from both natural and anthropogenic sources. Examples of natural sources include sea salt aerosols over the global oceans and mineral dust from arid deserts while anthropogenic sources include smoke from agricultural burning and fossil fuel combustion. Although sub-groupings of anthropogenic aerosols do exist, they are in general characterized as sulfate (SU), black carbon (BC), and particulate organic matter (POM). Aerosol particles are usually categorized based on size including nucleation mode (0.001–0.1 μm diameter), accumulation mode (0.1–1 μm diameter), and coarse mode (N 1 μm diameter). Much of the anthropogenic aerosols are found in the accumulation mode, whereas mechanically produced aerosols such as dust and sea salt are predominantly coarse mode aerosols. Considering that nearly 70% of the earth is ocean, it is not surprising that the total median sea salt aerosol source strength is the largest (6000 Tg year − 1) when compared to dust (1600 Tg year − 1) and BC, POM, and SU put together (300 Tg year − 1). However, in terms of median mass loading, dust has the highest mass (20 Tg) when compared to sea salt (6 Tg) with SU, BC, and POM totaling 4 Tg (CCSP 2009). Aerosols have a wide range …