Analysis of a summertime PM2.5 and haze episode in the mid-Atlantic region.

Observations of the mass and chemical composition of particles less than 2.5 microm in aerodynamic diameter (PM2.5), light extinction, and meteorology in the urban Baltimore-Washington corridor during July 1999 and July 2000 are presented and analyzed to study summertime haze formation in the mid-Atlantic region. The mass fraction of ammoniated sulfate (SO4(2-)) and carbonaceous material in PM2.5 were each approximately 50% for cleaner air (PM2.5< 10 microg/m3) but changed to approximately 60% and approximately 20%, respectively, for more polluted air (PM2.5>30 microg/m3). This signifies the role of SO4(2-) in haze formation. Comparisons of data from this study with the Interagency Monitoring of Protected Visual Environments network suggest that SO4(2-) is more regional than carbonaceous material and originates in part from upwind source regions. The light extinction coefficient is well correlated to PM2.5 mass plus water associated with inorganic salt, leading to a mass extinction efficiency of 7.6 +/- 1.7 m2/g for hydrated aerosol. The most serious haze episode occurring between July 15 and 19, 1999, was characterized by westerly transport and recirculation slowing removal of pollutants. At the peak of this episode, 1-hr PM2.5 concentration reached approximately 45 microg/m3, visual range dropped to approximately 5 km, and aerosol water likely contributed to approximately 40% of the light extinction coefficient.