UV aerosol absorption experiment (2002-04): 1. UV-MFRSR calibration and performance at GSFC

Abstract

To reduce uncertainties in cloud-free sky UV radiative transfer models of the atmosphere, we conducted an aerosol UV absorption closure experiment where a shadow-band radiometer (a UV-MFRSR, USDA UVB Monitoring and Research Network) and a rotating group of four sun-sky CIMEL radiometers (part of NASA AERONET network) were run side-by-side continuously for 17 months at NASA/GSFC site in Greenbelt, MD. This paper describes a UV-MFRSR on-site calibration method based on AERONET CIMEL measurements of aerosol extinction optical thickness (τₑₓₜ) by direct a sun technique and Langley Mauna Loa calibrations. The AERONET τₑₓₜ was interpolated or extrapolated to the UV-MFRSR wavelengths and measurement intervals and used as input to the spectral band model along with column ozone and surface pressure measurements to estimate zero air mass voltages, V₀ in each UV-MFRSR channel. The method does not require stability of τₑₓₜ and allows independent V₀ estimations for every individual 3-min UV-MFRSR measurement. Daily <V₀> estimates were obtained for cloud-free conditions and compared with the on-site Langley plot technique. On the clearest stable days both techniques agree within 1%. Uncertainties in τₑₓₜ measurements were estimated using co-located identical CIMEL instruments and considered as part of combined V₀ error budget. Such comparisons provide an independent check of both instrument’s calibration, and allow relative tracking of the UV-MFRSR diffuser changes (from surface contamination), by repeating the comparisons on clear days. Using such comparisons we found relatively good daily UV-MFRSR <V₀> reproducibility in summer (+/-2% in <V₀(368)>), but larger than expected <V₀> relative changes in fall-winter seasons. The changes include systematic day-to-day <V₀> decline for extended periods (with ~0.15%/day V₀ decline rate) alternating with step jump changes after major precipitation periods (rain or snow). The <V₀> changes were highly correlated in all UV-MFRSR channels, and possibly result from diffuser contamination and cleanings. Daily <V₀> values were used to calculate τₑₓₜ for 3-minute UV-MFRSR measurements. These results compared well with interpolated AERONET τₑₓₜ measurements (at 368nm daily rms differences in τₑₓₜ were within 0.01 (1σ) for τₑₓₜ< 0.4 and within 0.02(1σ) for τₑₓₜ<1.2). The advantage of the shadowband technique is that the <V₀> calibration obtained for direct-sun voltage can be applied to diffuse and total voltages to obtain total and diffuse atmospheric transmittances. These transmittances, in combination with accurate τₑₓₜ data, provide an essential foundation for the aerosol column absorption retrievals described in the second part of this paper.