An Examination of the 6:00 a.m.–9:00 a.m. Measurements of Ozone Precursors in the New York City Metropolitan Area

Abstract

In recent years, ambient measurements of hourly ozone precursor concentrations, namely speciated and total nonmethane organic compounds (NMOCs), have become available through the Photochemical Assessment Monitoring Stations (PAMS) program. Prior to this, NMOCs were measured in the central business district using a canister to obtain the 3-hr integrated sample for the 6:00 a.m.–9:00 a.m. period. Such sampling had been carried out annually for nearly a decade at three locations in the New York City metropolitan area. The intent of these measurements, along with measurements of the other ozone precursor, NO_x, was to provide an understanding of ozone formation and the emissions loading and mix in the urban area. The analysis of NMOC and NO_x measurements shows a downward trend in the case of NMOC.

In addition, we compared the canister-based NMOC concentrations with data obtained from the PAMS program for the 6:00 a.m.–9:00 a.m. period. Analysis of the NMOC concentrations reveals poor spatial correlation between the various monitors, reflecting the effect of localized emissions. This suggests that NMOC measurements made at a single location cannot be viewed as representative of the entire region. On the other hand, correlations were found to be higher among the NO_x monitors, indicating the commonality of emission sources and associated physical and chemical processes. Also, the magnitude of the average total NMOC measured at the continuous sampling locations (PAMS) was found to be ~50% lower than the measurements reported from the canister-based measurements. However, such large differences were not evident in the case of the NO_x concentrations. It is therefore important to note that, while the NMOC to NO_x ratio is spatially variant, it is also dependent upon the method by which the total NMOC are measured in an urban area. Furthermore, the ratios have been found to be correlated with neither the daily maximum ozone nor the daily average ozone concentrations measured within the urban region.On the other hand,correlations were found to be higher among the NO_xmonitors, indicating the commonality of emission sources and associated physical and chemical processes. Also, the magnitude of the average total NMOC measured at the continuous sampling locations (PAMS) was found to be ~50% lower than the measurements reported from the canister-based measurements. However, such large differences were not evident in the case of the NO_x concentrations. It is therefore important to note that,while the NMOC to NO_x ratio is spatially variant, it is also dependent upon the method by which the total NMOC are measured in an urban area. Furthermore, the ratios have been found to be correlated with neither the daily maximum ozone nor the daily average ozone concentrations measured within the urban region. Hence, the 6:00 a.m.–9:00 a.m. ratio of NMOC to NO_x concentrations cannot be used as the sole criteria to determine whether an urban area is NO_x- or VOC-limited.