Abstract
Discrete wavelength radiance measurements from the Deep Space Climate Observatory (DSCOVR) Earth Polychromatic Imaging Camera (EPIC) allows derivation of global synoptic maps of total and tropospheric ozone columns every hour during Northern Hemisphere (NH) Summer or 2 hours during Northern Hemisphere winter. In this study, we present version 3 retrieval of Earth Polychromatic Imaging Camera ozone that covers the period from June 2015 to the present with improved geolocation, calibration, and algorithmic updates. The accuracy of total and tropospheric ozone measurements from EPIC have been evaluated using correlative satellite and ground-based total and tropospheric ozone measurements at time scales from daily averages to monthly means. The comparisons show good agreement with increased differences at high latitudes. The agreement improves if we only accept retrievals derived from the EPIC 317 nm triplet and limit solar zenith and satellite looking angles to 70°. With such filtering in place, the comparisons of EPIC total column ozone retrievals with correlative satellite and ground-based data show mean differences within ±5-7 Dobson Units (or 1.5–2.5%). The biases with other satellite instruments tend to be mostly negative in the Southern Hemisphere while there are no clear latitudinal patterns in ground-based comparisons. Evaluation of the EPIC ozone time series at different ground-based stations with the correlative ground-based and satellite instruments and ozonesondes demonstrated good consistency in capturing ozone variations at daily, weekly and monthly scales with a persistently high correlation (r2 > 0.9) for total and tropospheric columns. We examined EPIC tropospheric ozone columns by comparing with ozonesondes at 12 stations and found that differences in tropospheric column ozone are within ±2.5 DU (or ∼±10%) after removing a constant 3 DU offset at all stations between EPIC and sondes. The analysis of the time series of zonally averaged EPIC tropospheric ozone revealed a statistically significant drop of ∼2–4 DU (∼5–10%) over the entire NH in spring and summer of 2020. This drop in tropospheric ozone is partially related to the unprecedented Arctic stratospheric ozone losses in winter-spring 2019/2020 and reductions in ozone precursor pollutants due to the COVID-19 pandemic.
Highlights
The DSCOVR spacecraft carrying the EPIC instrument was successfully launched on February 11, 2015, to the Earth-Sun Lagrange-1 (L1) point at a nominal distance of 1.5 × 106 km from the Earth
We examined EPIC tropospheric ozone columns derived by subtracting MERRA-2 stratospheric ozone columns from EPIC total ozone measurements and adjusted for reduced EPIC sensitivity to the boundary layer ozone
We found that after removing a constant 3 DU offset between EPIC and sondes globally the biases in tropospheric column ozone are within ±2.5 DU
Summary
The DSCOVR spacecraft carrying the EPIC instrument was successfully launched on February 11, 2015, to the Earth-Sun Lagrange-1 (L1) point at a nominal distance of 1.5 × 106 km from the Earth. This is important for tropospheric ozone studies that are sensitive to errors caused by the presence of clouds. For comparisons with independent measurements like sondes, we need to account for the limited sensitivity of UV satellite measurements to the variability of ozone in the low troposphere including the boundary layer (BL) below ∼700 hPa. To do this, we used simulated tropospheric ozone derived from GEOS-Replay (Strode et al, 2020) constrained by the MERRA-2 meteorology through so-called replay method, whereby the analysis increments recalculated from MERRA-2 are used by the GEOS model in dynamical tendency calculations (Orbe et al, 2017).
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