Origins of chemical pollution derived from Mid-Atlantic aircraft profiles using a clustering technique

Upwind sources of NOx and SO2 play a crucial role in the amount of O3 and aerosols in the lower troposphere in the Mid-Atlantic US. This paper describes a novel method of clustering trace gas and aerosol profiles allowing for the quantification of the relationship between point sources and pollution levels. This improves our understanding of pollution origins and has the potential for prediction of episodes of poor air quality. A hierarchical clustering method was used to classify distinct chemical and meteorological events from over 200 aircraft vertical profiles in the lower troposphere. Profile measurements included O3, SO2, CO and particle scattering from June to August 1997–2003, in the Mid-Atlantic US (mostly in Maryland, Pennsylvania and Virginia). The clustering technique could discriminate distinct profile shapes including measurements made during the 2002 Canadian forest fires. Forty-eight-hour back trajectories were run for each profile and the integrated NOx and SO2 point source emissions encountered by each trajectory were calculated using data from the EPA Clean Air Market Division’s emissions database. There was a strong correlation between integrated NOx emissions and O3 profiles, indicating that O3 profiles are strongly influenced by and can be predicted with point source emissions. There is a prevalent concentration of SO2 over the eastern US with mixing ratios decreasing smoothly from about 3.5 ppb near the surface to 0.2 ppb at 2400 m.