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. Surface full-sky erythemal dose rate (EDR) from the Ozone MonitoringInstrument (OMI) at both satellite overpass time and local noon time isevaluated against ground measurements at 31 sites from the US Department ofAgriculture's (USDA) UV-B Monitoring and Research Program (UVMRP) over the period of 2005–2017. We find that both OMIoverpass and solar noon time EDR are highly correlated with the measuredcounterparts (with a linear correlation coefficient of 0.90 and 0.88,respectively). Although the comparison statistics are improved with a longertime window (0.5–1.0 h) for pairing surface and OMI measurements, both OMIoverpass and local noon time EDRs have 7 % overestimation that is largerthan 6 % uncertainty in the ground measurements and show different levelsof dependence on solar zenith angle (SZA) and to lesser extent on cloud opticaldepth. The ratio of EDR between local noon and OMI overpass time is often(95 % in frequency) larger than 1 with a mean of 1.18 in the OMI product;in contrast, the same ratio from surface observation is normally distributed with 22 %of the times less than 1 and a mean of 1.38. This contrast inpart reflects the deficiency in the OMI surface UV algorithm that assumesconstant atmospheric conditions between overpass and noon time. Theprobability density functions (PDFs) for both OMI and ground measurements ofnoontime EDR are in statistically significant agreement, showing dual peaksat ∼20  and ∼200  mW m −2 , respectively; the latter is lowerthan 220 mW m −2 , the value at which the PDF of daily EDR from ground measurementspeaks, and this difference indicates that the largest EDR value for a givenday may not often occur at local noon. Lastly, statistically significantpositive trends of EDR are found in the northeastern US in OMI data, butopposite trends are found within ground-based data (regardless of samplingfor either noontime or daily averages). While positive trends areconsistently found between OMI and surface data for EDR over the southernGreat Plains (Texas and Oklahoma), their values are within the uncertainty ofground measurements. Overall, no scientifically sound trends can be foundamong OMI data for aerosol total and absorbing optical depth, cloud opticaldepth and total ozone to explain coherently the surface UV trends revealedeither by OMI or ground-based estimates; these data also cannot reconcile trenddifferences between the two estimates (of EDR from OMI and surface observations). Future geostationary satellites withbetter spatiotemporal resolution data should help overcome spatiotemporalsampling issues inherent in OMI data products and therefore improve theestimates of surface UV flux and EDR from space.

atmospheric chemistry and physics

Surface erythemal UV irradiance in the continental United States derived from ground-based and OMI observations: quality assessment, trend analysis and sampling issues