FORMATION MECHANISMS AND CHEMICAL CHARACTERISTICS OF ELEVATED PHOTOCHEMICAL LAYERS OVER THE NORTHEAST UNITED-STATES

The chemical composition of layers of trace gas mixtures within the lo wer troposphere and their relationship to surface trace gas concentrat ions are investigated using airborne chemistry and meteorological meas urements made over eastern Connecticut and central Massachusetts. Laye rs of photochemically aged material were identified by maxima above th e surface stable layer in the profiles of O-3, NOy, CO, aerosols, pero xyacetyl nitrate (PAN), and the ratio of highly to lesser reactive hyd rocarbons (e.g., benzene and toluene). Observations suggest that stron g diurnal variations in surface-ozone mixing ratios are associated wit h elevated layers of ozone and other trace gases. The elevated layers were also associated with strong gradients of NOx both vertically, acr oss the mixed layer, and horizontally, between urban and rural regions , providing evidence for the dominant role of localized anthropogenic NO, emissions over North America on ozone production in urban regions. On days when elevated layers were detected, isoprene's late-morning p ropene-equivalent mixing ratio (a measure of nonmethane hydrocarbon re activity with OH) was an order of magnitude greater than that of the n ext most reactive species up to an altitude of 650 m mean sea level. F our-dimensional data assimilation was used within a mesoscale model to study the formation mechanism and history of these layers, with a key result being that no unique ''age'' or source region could generally be attributed to these layers as a consequence of turbulent mixing and transport leading to their formation.