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Discussion papers | Copyright
https://doi.org/10.5194/acp-2018-720
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 03 Sep 2018

Research article | 03 Sep 2018

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This discussion paper is a preprint. It is a manuscript under review for the journal Atmospheric Chemistry and Physics (ACP).

OMI surface UV irradiance in the continental United States: quality assessment, trend analysis, and sampling issues

Huanxin Zhang1,2, Jun Wang1,2, Lorena Castro García1,2, Yang Liu3, and Nickolay A. Krotkov4 Huanxin Zhang et al.
  • 1Department of Chemical and Biochemical Engineering, The University of Iowa, Iowa City, IA, USA
  • 2Center for Global and Regional Environmental Research, The University of Iowa, Iowa City, IA, USA
  • 3Rollins School of Public Health, Emory University, Atlanta, GA, USA
  • 4NASA Goddard Space Flight Center, Greenbelt, MD, USA

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 ~20mWm−2 over the period of 2005–2017. However, another high frequency of ~200mWm−2 occurs in OMI solar noon EDR while the ground peak values show the high frequency around 220mWm−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.

Huanxin Zhang et al.
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Short summary
The amount of surface solar UV radiation reaching the earth's surface has substantial impacts on human health and ecosystems. Here, we used satellite-based estimate and ground-based measurements of UV data to reveal the trend of surface UV radiation in U.S. during 2005–2017, assed the uncertainties in satellite-based data, and suggested the potential areas for improving satellite-based estimate of UV radiation.
The amount of surface solar UV radiation reaching the earth's surface has substantial impacts on...
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