Regional air quality can play an important role in determining whether urban ozone or PM-2.5 standards are exceeded. Interactions of nitrogen oxide species at background levels with natural organic compounds can lead to the formation of secondary aerosol products via formation of nitric acid and its subsequent reaction with ammonia to form ammonium nitrate. In addition, natural organics and less reactive organic compounds, particularly aromatic species and monoterpenes, can lead to the formation of secondary organic aerosols. Both pathways contribute to the formation of PM-2.5. Long-range transport and chemical transformation of hydrocarbons and NOx via both photochemical reactions and nighttime chemistry can yield significant regional levels of ozone and other oxidants, such as peroxyacyl nitrates. Locally generated urban plumes can interact with these regional-scale air pollutants, as well as with locally generated natural organic hydrocarbons such as isoprene and monoterpene emissions from vegetation. Recent field work at a site near Usary Mountain in the Phoenix, Arizona, area will be presented. Measurements were taken for approximately one month in May-June 1998 at this ground station above the nearby urban basin. Real-time gas-phase measurements included nitrogen dioxide, NOx, ozone, peroxyacetyl nitrate, ultraviolet-B, temperature, olefins, and nitric acid. Size-fractionated aerosol samples were also collected. Aerosol samples were examined for optical properties and for natural radionuclides (the latter as a means of estimating the lifetimes of fine aerosols). The data will be presented and discussed in terms of regional-scale and urban-scale chemical interactions and also in terms of the meteorology of the local boundary layer as it affects the daytime and nighttime levels of oxidants and fine aerosols. This work was supported by the U.S. Department of Energy, Office of Biological and Environmental Research, Atmospheric Chemistry Program
Symposium on Interdisciplinary Issues in Atmospheric Chemistry