Net radiative effects of dust in the tropical North Atlantic based on integrated satellite observations and in situ measurements

In this study, we integrate recent in situ measurements with satellite retrievals of dust physical and radiative properties to quantify dust direct radiative effects on shortwave (SW) and longwave (LW) radiation (denoted as DREinline-formula_SW and DREinline-formula_LW, respectively) in the tropical North Atlantic during the summer months from 2007 to 2010. Through linear regression of the CERES-measured top-of-atmosphere (TOA) flux versus satellite aerosol optical depth (AOD) retrievals, we estimate the instantaneous DREinline-formula_SW efficiency at the TOA to be inline-formula $M4inlinescrollmathml-\mathrm{normal\; 49.7}\pm \mathrm{normal\; 7.1}$ 58pt10ptsvg-formulamathimga94bbb0d8cb5fdcbdd5b1f2762c588ec acp-18-11303-2018-ie00001.svg58pt10ptacp-18-11303-2018-ie00001.png  W minline-formula⁻² AODinline-formula⁻¹ and inline-formula $M7inlinescrollmathml-\mathrm{normal\; 36.5}\pm \mathrm{normal\; 4.8}$ 58pt10ptsvg-formulamathimg21b020b4081686d3ee9d8906d0032ec7 acp-18-11303-2018-ie00002.svg58pt10ptacp-18-11303-2018-ie00002.png  W minline-formula⁻² AODinline-formula⁻¹ based on AOD from MODIS and CALIOP, respectively. We then perform various sensitivity studies based on recent measurements of dust particle size distribution (PSD), refractive index, and particle shape distribution to determine how the dust microphysical and optical properties affect DRE estimates and its agreement with the above-mentioned satellite-derived DREs. Our analysis shows that a good agreement with the observation-based estimates of instantaneous DREinline-formula_SW and DREinline-formula_LW can be achieved through a combination of recently observed PSD with substantial presence of coarse particles, a less absorptive SW refractive index, and spheroid shapes. Based on this optimal combination of dust physical properties we further estimate the diurnal mean dust DREinline-formula_SW in the region of inline-formula−10 W minline-formula⁻² at TOA and inline-formula−26 W minline-formula⁻² at the surface, respectively, of which inline-formula∼ 30 % is canceled out by the positive DREinline-formula_LW. This yields a net DRE of about inline-formula−6.9 and inline-formula−18.3 W minline-formula⁻² at TOA and the surface, respectively. Our study suggests that the LW flux contains useful information on dust particle size, which could be used together with SW observations to achieve a more holistic understanding of the dust radiative effect.