The chemical composition of layers of trace gas mixtures within the lower troposphere and their relationship to surface trace gas concentrations are investigated using airborne chemistry and meteorological measurements made over eastern Connecticut and central Massachusetts. Layers of photochemically aged material were identified by maxima above the surface stable layer in the profiles of O3, NOy, CO, aerosols, peroxyacetyl nitrate (PAN), and the ratio of highly to lesser reactive hydrocarbons (e.g., benzene and toluene). Observations suggest that strong diurnal variations in surface-ozone mixing ratios are associated with elevated layers of ozone and other trace gases. The elevated layers were also associated with strong gradients of NOx both vertically, across the mixed layer, and horizontally, between urban and rural regions, providing evidence for the dominant role of localized anthropogenic NOx emissions over North America on ozone production in urban regions. On days when elevated layers were detected, isoprene's late-morning propene-equivalent mixing ratio (a measure of nonmethane hydrocarbon reactivity with OH) was an order of magnitude greater than that of the next most reactive species up to an altitude of 650 m mean sea level. Four-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. ¿ 1998 American Geophysical Union