Measuring aerosol UV absorption optical thickness by combining use of shadowband and almucantar techniques

We report final results of an aerosol UV absorption closure experiment where a UV-shadow-band radiometer (UV-MFRSR, USDA UVB Monitoring and Research Network) and 4 rotating sun-sky radiometers (CIMEL, NASA AERONET network) were run side-by-side continuously for 17 months at NASA/GSFC site in Greenbelt, MD. The aerosol extinction optical thickness τext, was measured by the CIMEL direct-sun technique in the visible and at two UV wavelengths 340 and 380 nm. These results were used for UV-MFRSR daily on-site calibration and 3-min measurements of τext at 325nm, 332nm and 368nm. The τext measurements were used as input to the radiative transfer model along with AERONET retrievals of the column-integrated particle size distribution (PSD)to infer an effective imaginary part of the UV aerosol refractive index, k, by fitting MFRSR measured voltage ratios. Using all cases for cloud-free days, we derive diurnal and seasonal dependence of the aerosol absorption optical thickness, τabs with an uncertainty 0.01-0.02. At our site τabs follows pronounced seasonal dependence with maximum values ~0.07 at 368nm (~0.15 at 325nm) occurring in summer hazy conditions and <0.02 in winter-fall seasons, when aerosol loadings are small. Inferred values of k allow calculation of the single scattering albedo, ω, in UVA and comparisons with AERONET almucantar ω440 retrievals at 440nm. Overall, ω was slightly lower in UV than in the visible: case average =0.93 compared to =0.95. However, the differences ( ~0.02, rms difference ~0.016) are smaller than uncertainties of both retrievals (δω~0.03). Low values are consistent with higher values for imaginary refractive index, k: ~0.01 compare to ~0.006. However, mean differences in k (~0.004) were only slightly larger than AERONET retrieval uncertainty δk ~0.00327. We also found that ω decreases with decrease in τext, suggesting different aerosol composition in summer and winter months. So far, our results do not allow explaining the causes of apparent larger aerosol absorption in UV. Continuing co-located measurements at GFSC is important to improve the comparison statistics, but conducting aerosol absorption measurements at different sites with varying conditions is also desirable.