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

Abstract

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 3.5ppb near the surface to 0.2ppb at 2400m.