Although many regulations have been enacted with the goal of improving air quality, many parts of the U.S. are still classified as "non-attainment areas" because they frequently violate federal air quality standards. Adequately monitoring the spatial distribution of pollutants both within and outside of non-attainment areas has been an ongoing challenge for regulators. Observations of near-surface pollution from space-based platforms would provide an unprecedented view of the spatial distribution of pollution and there are plans for launching satellites to broaden the view of air quality. Among the challenges in interpreting satellite observations will be to understand the expected gradients in pollution at scales smaller than the satellite footprint. NASA's DISCOVER-AQ observations offer the opportunity to examine small-scale spatial gradients for footprints similar to that expected for future satellites, and a metric for identifying those gradients and examining their size and frequency was developed.

This analysis is applied to the DISCOVER-AQ spiral profiles, which were designed to provide vertical profiles of pollution in the lower atmosphere. The spirals were approximately 5 km in diameter, meaning that the vertical gradients were sometimes influence by horizontal gradients as well. To identify instances of these horizontal gradients, spirals were divided into octants, and averages of a given species were calculated, with certain points being flagged when its value compared to adjacent octants indicated departure from a natural vertical gradient. This technique showed that a spatial heterogeneity is present in the data, but is not very pervasive, except over certain locations with persistent pollution sources.