Work

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

Year: 2019

Type: article

Abstract: . Surface full-sky erythemal dose rate (EDR) from Ozone Monitoring Instrument (OMI) at both satellite overpass time and local noon time are evaluated against ground measurements at 31 sites from USDA UV-B Monitoring and Research Program over the period of 2005–2017. We find that both OMI overpass time and local solar noon EDR are highly correlated with the measured counterparts (R = 0.88). Although the comparison statistics are improved with longer time window used for pairing surface and OMI measurements, OMI data overall has ~ 4 % underestimate for overpass EDR while ~ 8 % overestimate for the solar noon time EDR. The biases are analyzed regarding the spatial and temporal data collocation, the effects of solar zenith angle (SZA), clouds and the assumption of constant atmospheric conditions. The difference between OMI overpass EDR and ground observation shows some moderate dependence on SZA and the bias could be up to −30 % with SZA greater than ~ 65°. In addition, the ratio of EDR between solar noon to overpass time is often (95 % in frequency) larger than 1 from OMI products; in contrast, this ratio from ground observation is shown to be normally distributed around 1. This contrast suggests that the current OMI surface UV algorithm would not fully represent the real atmosphere with the assumption of a constant atmospheric profile between noon and satellite overpass times. The viability of surface UV in terms of peak UV frequency is also studied. Both OMI Noon_FS and ground peak EDR show a high frequency of occurrence of ~ 20 mW m−2 over the period of 2005–2017. However, another high frequency of ~ 200 mW m−2 occurs in OMI solar noon EDR while the ground peak values show the high frequency around 220 mW m−2, implying that the OMI solar noon time may not always represent the peak daily UV values. Lastly, OMI full-sky solar noon EDR shows statistically significant positive trends in parts of the northeastern U.S., the Ohio River Valley region and California. However, the UV trends estimated from ground-based network using two sampling methods (one corresponds to the OMI noon time and one averages all the data in a day) show significant negative trends in the Northeast and the Ohio River Valley region, which is consistent with the increase of absorption aerosol optical depth as revealed by OMI aerosol product in these regions. No statistically-significant trend can be found for OMI columnar O3 or cloud optical depth. The future surface UV data estimated with better spatial and temporal resolution obtained from geostationary satellites would help resolve these discrepancies found in the biases and estimated surface UV trends.

Source: Atmospheric chemistry and physics

DOI: 10.5194/acp-19-2165-2019