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) locations, were used to determine the factors controlling excess ozone formation in the Nashville area. During the 6 week intensive study period, the daily 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 observed at the urban and suburban locations, and the highest O-3 value of 138 ppbv was observed at the downtown Nashville station. On 12 days, one or more of the urban and suburban stations exceeded midday; 8-hour mean O-3 concentrations of 80 ppbv in contrast to the single occurrence at the rural stations. The observations that the elevated O-3 concentrations occurred more frequently at the urban and suburban stations during stagnating weather conditions suggest localized production of excess O-3. The comparison of the O-3/NOy ratios to model simulations suggest that the rural and suburban locations are in a NOx sensitive chemistry regime, while the urban location (downtown Nashville) is in a chemistry regime that is sensitive to both NOx and VOC concentrations. The O-3 production efficiency, determined from the slope 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 production 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 the higher SO2 concentrations.