Stratosphere–troposphere separation of nitrogen dioxide columns from the TEMPO geostationary satellite instrument
Authors: Geddes Jeffrey A. , Martin Randall V. , Bucsela Eric J. , McLinden Chris A. , Cunningham Daniel J. M. 
Published: November 2018

Abstract. Separating the stratospheric and tropospheric contributions in satellite
retrievals of atmospheric NO2 column abundance is a crucial step in the
interpretation and application of the satellite observations. A variety of
stratosphere–troposphere separation algorithms have been developed for
sun-synchronous instruments in low Earth orbit (LEO) that benefit from global
coverage, including broad clean regions with negligible tropospheric NO2
compared to stratospheric NO2. These global sun-synchronous algorithms
need to be evaluated and refined for forthcoming geostationary instruments
focused on continental regions, which lack this global context and require
hourly estimates of the stratospheric column. Here we develop and assess a
spatial filtering algorithm for the upcoming TEMPO geostationary instrument
that will target North America. Developments include using independent
satellite observations to identify likely locations of tropospheric
enhancements, using independent LEO observations for spatial context,
consideration of diurnally varying partial fields of regard, and a filter
based on stratospheric to tropospheric air mass factor ratios. We test the
algorithm with LEO observations from the OMI instrument with an afternoon
overpass, and from the GOME-2 instrument with a morning overpass. We compare our TEMPO field of regard algorithm against an identical global
algorithm to investigate the penalty resulting from the limited spatial
coverage in geostationary orbit, and find excellent agreement in the
estimated mean daily tropospheric NO2 column densities (R2=0.999, slope=1.009 for July and R2=0.998, slope=0.999 for
January). The algorithm performs well even when only small parts of the
continent are observed by TEMPO. The algorithm is challenged the most by
east coast morning retrievals in the wintertime (e.g., R2=0.995,
slope=1.038 at 14:00 UTC). We find independent global LEO observations (corrected for
time of day) provide important context near the field-of-regard edges. We
also test the performance of the TEMPO algorithm without these supporting
global observations. Most of the continent is unaffected (R2=0.924
and slope=0.973 for July and R2=0.996 and slope=1.008 for
January), with 90 % of the pixels having differences of less than ±0.2×1015 molecules cm−2 between the TEMPO tropospheric NO2
column density and the global algorithm. For near-real-time retrieval, even
a climatological estimate of the stratospheric NO2 surrounding the
field of regard would improve this agreement. In general, the additional
penalty of a limited field of regard from TEMPO introduces no more error
than normally expected in most global stratosphere–troposphere separation
algorithms. Overall, we conclude that hourly near-real-time
stratosphere–troposphere separation for the retrieval of NO2
tropospheric column densities by the TEMPO geostationary instrument is both
feasible and robust, regardless of the diurnally varying limited field of
regard.