Seasonal variation of total column formaldehyde, nitrogen dioxide, and ozone over various Pandora spectrometer sites with a comparison of OMI and diurnally varying DSCOVR-EPIC satellite data

Abstract

Observations of trace gases, such as O3, HCHO, and NO2, and their seasonal dependence can be made using satellite and ground-based data from the Ozone Monitoring Instrument (OMI) satellite and Pandora ground-based instruments. Both operate with spectrometers that have similar characteristics in wavelength range and spectral resolution that enable them to retrieve total column amounts of formaldehyde (TCHCHO) and nitrogen dioxide (TCNO2) and total column ozone (TCO). The polar orbiting OMI observes at 13:30 ± 0:25 LST (local solar time) plus an occasional second side-scan point 90 min later at mid-latitudes. The ground-based Pandora spectrometer system observes the direct sun all day, with a temporal resolution of 2 min. At most sites, the Pandora data show a strong seasonal dependence for TCO and TCHCHO and less seasonal dependence for TCNO2. Use of a low-pass filter LOWESS(3-month) can reveal the seasonal dependence of TCNO2 for both OMI and Pandora at mid-latitude sites usually correlated with seasonal heating using natural gas or oil. Compared to Pandora, OMI underestimates the amount of NO2 air pollution that occurs during most days, as the OMI TCNO2 retrieval occurs around 13:30 ± 0:25 LST, which tends to be near the frequent minimum of the daily TCNO2 time series. Even when the Pandora data are restricted to between 13:00 and 14:00 LST, OMI retrieves less TCNO2 than Pandora over urban sites because of OMI’s large field of view. The seasonal behavior of TCHCHO is mostly caused by the release of HCHO precursors from plant growth and emissions from lakes that peak in the summer, as observed by Pandora and OMI. Long-term averages show that OMI TCHCHO usually has the same seasonal dependence but differs in magnitude from the amount measured by Pandora and is frequently larger. Comparisons of OMI total column NO2 and HCHO with Pandora daily time series show both agreement and disagreement at various sites and for different days, with the Pandora results frequently being larger. For ozone, daily time-dependent comparisons of OMI TCO with those retrieved by Pandora show good agreement in most cases. Additional diurnal comparisons are shown of Pandora TCO with hourly retrievals during a day from the EPIC (Earth Polychromatic Imaging Camera) spacecraft instrument orbiting the Earth–Sun Lagrange point L1