Air chemistry during the 1995 SOS/Nashville intensive determined from level 2 network

Air quality measurements from enhanced ground-based air monitoring stations , that were located at strategic urban, suburban, and rural (regional) loca tions, were used to determine the factors controlling excess ozone formatio n in the Nashville area. During the 6 week intensive study period, the dail y maximum hourly O-3 concentrations varied from daytime highs of less than 40 ppbv to values exceeding 120 ppbv. The higher O-3 concentrations were ob served at the urban and suburban locations, and the highest O-3 value of 13 8 ppbv was observed at the downtown Nashville station. On 12 days, one or m ore of the urban and suburban stations exceeded midday; 8-hour mean O-3 con centrations of 80 ppbv in contrast to the single occurrence at the rural st ations. The observations that the elevated O-3 concentrations occurred more frequently at the urban and suburban stations during stagnating weather co nditions suggest localized production of excess O-3. The comparison of the O-3/NOy ratios to model simulations suggest that the rural and suburban loc ations are in a NOx sensitive chemistry regime, while the urban location (d owntown Nashville) is in a chemistry regime that is sensitive to both NOx a nd VOC concentrations. The O-3 production efficiency, determined from the s lope of the O-3 versus NOz (NOy - NOx), nas found to be similar between the Dickson and Youth suburban stations and the Giles rural/regional station. The segregation of the data by SO2 concentration showed lower O-3 productio n efficiency for those measurement periods with SO2 concentrations greater than 2 ppbv. The slopes (upper limits of O-3 production efficiency) ranged from 7.7 to 10.6 for the lower SO2 concentrations and from 4.4 to 4.6 for t he higher SO2 concentrations.