Understanding the influence of local and regional sources on the temporal and spatial variability of pollutants in urban environments

Urban and industrial areas are major sources of greenhouse gases and key precursor gases for tropospheric ozone (O₃) and associated oxidants. Real time, fast response mobile measurements of multiple trace gases and particulates were carried out in 1998 and 1999 in Manchester, NH and Boston, MA, two urban areas in the northeastern United States. We have studied the temporal and spatial variability of pollutants, including greenhouse gases (e.g. nitrous oxide (N₂O), carbon dioxide (CO₂), carbon monoxide (CO), and methane (CH₄)), and smog precursors (nitric oxide (NO), nitrogen dioxide (NO₂)) along with ozone (O₃), and CO₂ and fine particles as tracers for combustion sources. The deployment of our mobile system has advantages over fixed point measurement sites. By measuring the trace gases over city sectors, we have gathered information about their spatial distribution and how this affects air quality over the urban area. A fixed site measurement may be dominated by a single local source and the data consequently skewed. The variability of trace gases recorded by our fast response (1 Hz) instruments is also compared to typical reports from slower measurements at urban air monitoring stations. The data from these stations are currently used to set environmental regulations and policies. However, they usually report averaged trace gas levels for time spans of about 1 hour and at only a few locations. Our fast response, mobile data indicate that there is much spatial and temporal variability in the trace gas levels. The range of pollutant concentrations concealed in the time averaged values is important to the understanding and accurate modeling of urban air quality.

In this paper we will present results of studies to understand the contribution of both point and diffuse emission sources to measurements of pollutant precursors and secondary pollutants throughout an urban area. The simultaneous measurement of multiple trace gases and fine particles allows us to study the correlation of these species and provides information about their sources. We use a simple model to describe the impact of nitrogen oxide distributions on ozone formation in the urban area. Correlation of elevated CO₂ with elevated NO_x is used to identify local combustion sources. The impact of automobiles on the distributions of NO, NO₂ and O₃ can thus be quantified. We use this and other modeling and analysis techniques to describe the variability and influence of pollutants in urban areas.