NO₂ column amounts from ground‐based Pandora and MFDOAS spectrometers using the direct‐sun DOAS technique: Intercomparisons and application to OMI validation

Abstract

Vertical column amounts of nitrogen dioxide, C(NO2), are derived from ground‐based direct solar irradiance measurements using two new and independently developed spectrometer systems, Pandora (Goddard Space Flight Center) and MFDOAS (Washington State University). We discuss the advantages of C(NO2) retrievals based on Direct Sun ‐ Differential Optical Absorption Spectroscopy (DS‐DOAS). The C(NO2) data are presented from field campaigns using Pandora at Aristotle University (AUTH), Thessaloniki, Greece; a second field campaign involving both new instruments at Goddard Space Flight Center (GSFC), Greenbelt, Maryland; a Pandora time series from December 2006 to October 2008 at GSFC; and a MFDOAS time series for spring 2008 at Pacific Northwest National Laboratory (PNNL), Richland, Washington. Pandora and MFDOAS were compared at GFSC and found to closely agree, with both instruments having a clear‐sky precision of 0.01 DU (1 DU = 2.67 × 1016 molecules/cm2) and a nominal accuracy of 0.1 DU. The high precision is obtained from careful laboratory characterization of the spectrometers (temperature sensitivity, slit function, pixel to pixel radiometric calibration, and wavelength calibration), and from sufficient measurement averaging to reduce instrument noise. The accuracy achieved depends on laboratory‐measured absorption cross sections and on spectrometer laboratory and field calibration techniques used at each measurement site. The 0.01 DU precision is sufficient to track minute‐by‐minute changes in C(NO2) throughout each day with typical daytime values ranging from 0.2 to 2 DU. The MFDOAS instrument has better noise characteristics for a single measurement, which permits MFDOAS to operate at higher time resolution than Pandora for the same precision. Because Pandora and MFDOAS direct‐sun measurements can be made in the presence of light to moderate clouds, but with reduced precision (∼0.2 DU for moderate cloud cover), a nearly continuous record can be obtained, which is important when matching OMI overpass times for satellite data validation. Comparisons between Pandora and MFDOAS with OMI are discussed for the moderately polluted GSFC site, between Pandora and OMI at the AUTH site, and between MFDOAS and OMI at the PNNL site. Validation of OMI measured C(NO2) is essential for the scientific use of the satellite data for air quality, for atmospheric photolysis and chemistry, and for retrieval of other quantities (e.g., accurate atmospheric correction for satellite estimates of ocean reflectance and bio‐optical properties). Changes in the diurnal variability of C(NO2) with season and day of the week are presented based on the 2‐year time series at GSFC measured by the Pandora instrument.

The Dawn of Geostationary Air Quality Monitoring: Case Studies From Seoul and Los Angeles

“…Pandora spectrometers are capable of retrieving accurate and precise vertical columns of NO 2 using a direct-sun DOAS technique (Herman et al, 2009). Pandora instruments are operated continuously to retrieve a NO 2 column approximately every 90 s during daylight hours, whenever the atmospheric path between the surface and the sun is cloud-free.…”

“…In an effort to provide cost-effective methods for validating space-based UV-VIS trace gas measurements, including those from GEO, NASA, and ESA are collaborating on a global network of ground-based Pandora solar and sky-scanning spectrometers developed at NASA Goddard Space Flight Center (Herman et al, 2009). Pandora spectrometers are capable of retrieving accurate and precise vertical columns of NO 2 using a direct-sun DOAS technique (Herman et al, 2009).…”

“…To begin addressing the spatial and temporal challenges associated with GEO measurements prior to launch, NASA funded the development of the suborbital Geostationary Trace gas and Aerosol Sensor Optimization instrument (GeoTASO, Leitch et al, 2014;Nowlan et al, 2016) and has deployed it during recent field experiments that also included networks of ground-based UV-VIS solar spectrometers (Herman et al, 2009Pandora). Analogous to how the LEO observations are a transfer standard between the GEO domains, the airborne observations are a transfer standard between the spatial scales of the surface-based validation instruments (i.e., Pandora) and satellite observations.…”

Inter-comparison of NO2 column densities measured by Pandora and OMI over Seoul, Korea

“…Its field of view is 1.6˚, and the pointing precision is 0.01°. From the measured solar radiance, the trace gases total column densities are retrieved using the Differential Optical Absorption Spectroscopy (DOAS) technique (Herman et al, 2009). …”

“…Thus, it is important to validate the satellite based measurements via comparison with ground-based measurements such as Pandora (Herman et al, 2009) and MAX-DOAS (Irie et al, 2008;Lee et al, 2011).…”