Assimilation of satellite NO2 observations at high spatial resolution

Other literature type English OPEN
Liu, Xueling ; Mizzi, Arthur P. ; Anderson, Jeffrey L. ; Fung, Inez ; Cohen, Ronald C. (2016)

Observations of trace gases from space based instruments offer the opportunity to constrain chemical and weather forecast and reanalysis models using the tools of data assimilation. To date, attempts at assimilation of nitrogen dioxide (NO<sub>2</sub>) satellite remote sensing have focused on updating emissions and concentrations. These initial efforts evaluated updates at length scales of ~&thinsp;100&thinsp;km using once a day measurements from satellites with ground pixels of 13&thinsp;km&thinsp;×&thinsp;24&thinsp;km or larger. In the boundary layer, NO<sub>2</sub> has a lifetime on the order of five hours and corresponding 1/e concentration variations near urban and point sources occur on spatial scales on the order of 50&ndash;75&thinsp;km. Accurate observations and modeling of these variations require spatial resolution of order 4&thinsp;km. In addition, because of the short lifetime, NO<sub>2</sub> variations are more strongly coupled to short time scale meteorological parameters than longer lived chemicals such as CO or CO<sub>2</sub>. In the next few years, we anticipate the launch of several instruments with ~&thinsp;3&thinsp;km spatial resolution. In addition, some of these instruments will be in geostationary orbits and thus have hourly revisit times. In anticipation of these instruments, we investigate the potential of high space and time resolution column measurements to serve as constraints on urban NO<sub>x</sub> emissions using a geostationary observation simulator coupled to a data assimilation system. We find that constraints on emissions are strongest in regions with high emissions and are most effective when coupled to hourly assimilation of meteorological observations. We find that errors in the meteorological fields result in unrecoverable biases in the updated emissions confirming a conjecture that simultaneous meteorology and chemical assimilation is essential to accurate description of the emissions and chemistry.
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