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The multiannual global mean of aerosol optical depth at 550 nm (AOD(550))over land is similar to 0.19, and that over oceans is similar to 0.13. About 45% of the Earth surface shows AOD550 smaller than 0.1. There is a need for measurement techniques that are optimized to measure aerosol optical properties under low AOD conditions. We present an inherently calibrated retrieval (i.e., no need for radiance calibration) to simultaneously measure AOD and the aerosol phase function parameter, g, based on measurements of azimuth distributions of the Raman scattering probability (RSP), the near-absolute rotational Raman scattering (RRS) intensity. We employ radiative transfer model simulations to show that for solar azimuth RSP measurements at solar elevation and solar zenith angle (SZA) smaller than 80 degrees, RSP is insensitive to the vertical distribution of aerosols and maximally sensitive to changes in AOD and g under near-molecular scattering conditions. The University of Colorado two-dimensional Multi-AXis Differential Optical Absorption Spectroscopy (CU 2-D-MAX-DOAS) instrument was deployed as part of the Two Column Aerosol Project (TCAP) at Cape Cod, MA, during the summer of 2012 to measure direct sun spectra and RSP from scattered light spectra at solar relative azimuth angles (SRAAs) between 5 and 170 degrees. During two case study days with (1) high aerosol load (17 July, 0.3 < AOD(430) < 0.6) and (2) near-molecular scattering conditions (22 July, AOD(430) < 0.13) we compare RSP-based retrievals of AOD(430) and g with data from a co-located CIMEL sun photometer, Multi-Filter Rotating Shadowband Radiometer (MFRSR), and an airborne High Spectral Resolution Lidar (HSRL-2). The average difference (relative to DOAS) for AOD(430) is + 0.012 +/- 0.023 (CIMEL), -0.012 +/- 0.024 (MFRSR), -0.011 +/- 0.014 (HSRL-2), and +0.023 +/- 0.013 (CIMELAOD - MFRSRAOD) and yields the following expressions for correlations between different instruments: DOAS(AOD) = -(0.019 +/- 0.006) + (1.03 +/- 0.02) X CIMELAOD (R-2 = 0.98), DOAS(AOD) = -(0.006 +/- 0.005) +. 1.08 +/- 0.02) x MFRSRAOD (R-2 = 0.98), and CIMELAOD = (0.013 +/- 0.004) + (1.05 +/- 0.01) x MFRSRAOD (R-2 = 0.99). The average g measured by DOAS on both days was 0.66 +/- 0.03, with a difference of 0.014 +/- 0.05 compared to CIMEL. Active steps to minimize the error in the RSP help to reduce the uncertainty in retrievals of AOD and g. As AOD decreases and SZA increases, the RSP signal-to-noise ratio increases. At AOD(430) similar to 0.4 and 0.10 the absolute AOD errors are similar to 0.014 and 0.003 at 70 degrees SZA and 0.02 and 0.004 at 35 degrees SZA. Inherently calibrated, precise AOD and g measurements are useful to better characterize the aerosol direct effect in urban polluted and remote pristine environments.

http://dx.doi.org/10.5194/amt-9-3893-2016

Journal Article

NOAA Cooperative Institutes
Office of Oceanic and Atmospheric Research (OAR)