Evaluating current satellite capability to observe diurnal change in nitrogen oxides in preparation for geostationary satellite missions

Abstract

This study characterizes the degree to which current polar-orbiting satellites can evaluate the daytime change in NO2 vertical column density (VCD) in urban, suburban, and rural areas. We examine these issues by considering the diurnal cycle of NO2 over the United States, using the large NO2 monitoring network supported by states, tribes, and the US Environmental Protection Agency (EPA). Through this analysis, we identify the potential opportunities and limitations of current space-based NO2 data in capturing diurnal change. Ground-based monitoring data from the US EPA are compared with satellite retrievals of NO2 from the KNMI Tropospheric Emissions Monitoring Internet Service (TEMIS) for two instruments: GOME-2 with a mid-morning overpass, and OMI with an early afternoon overpass. Satellite data show evidence of higher morning NO2 in the vicinity of large urban areas. Both satellites and ground monitors show ~1.5-2x greater NO2 abundance between morning and afternoon in urban areas. Despite differences in horizontal resolution and overpass time, the two satellite retrievals show similar agreement with ground-based NO2 measurements. When analyzed on a pixel-by-pixel basis, we find evidence for spatial structure in the diurnal change in NO2 between city center and surrounding areas in Southern California. Wider analysis of urban-suburban structure in diurnal NO2 change is hindered by resolution differences in the two satellite instruments, which have the potential to create data artefacts. This study highlights the value of future geostationary instruments to provide comparable satellite retrievals for NO2 over the course of a day, and research needs related to the effective utilization of NO2 satellite data for air quality applications.