Diagnosing horizontal gradients in aircraft soundings over the Baltimore metropolitan area during DISCOVER-AQ

During July 2011, NASA's DISCOVER-AQ project conducted flights over the Baltimore metropolitan area. The intent of the project is articulated by its acronym, Deriving Information on Surface Conditions from COlumn and VERtically Resolved Observations Relevant to Air Quality. By collecting concurrent observations at the surface and from aircraft overhead using in situ instruments and remote sensors, the project seeks to characterize the challenge satellites face in separating conditions at the surface from the overlying atmosphere. Detailed information on the composition and vertical structure of the lower atmosphere was collected using NASA's P-3B aircraft, resulting in 254 soundings across 14 different days and 6 surface sites. Soundings were conducted in a spiral pattern generally extending from 1,000 feet above the ground to 10,000 feet, which was almost always high enough to reach the lower free troposphere. While the purpose of these soundings was to document vertical structure, the footprint of the spiral pattern (~5 km in diameter) raises the possibility of significant horizontal structure as well. We introduce a method for diagnosing these horizontal gradients and apply it to the DISCOVER-AQ observations. Specific examples as well as overall statistics on the presence and magnitude of horizontal gradients in the aircraft soundings will be presented. These horizontal gradients have their own significance in that the spiral footprint is comparable to fine resolution models as well as current and future satellite observations.