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

Abstract

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, τₐ₆ₛ with an uncertainty 0.01­-0.02. At our site τₐ₆ₛ 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 <ω₃₆₈>=0.93 compared to <ω₄₄₀>=0.95. However, the differences (<ω₄₄₀ - ω₃₆₈> ~0.02, rms difference ~0.016) are smaller than uncertainties of both retrievals (δω~0.03). Low <ω₃₆₈> values are consistent with higher values for imaginary refractive index, k: <k₃₆₈> ~0.01 compare to <k₄₄₀> ~0.006. However, mean differences in k (<k₃₆₈-k₄₄₀>~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.