The objective of this paper is to demonstrate an approach to characterize the spatial variability in ambient air concentrations using mobile platform measurements. This approach may be useful for air toxics assessments in Environmental Justice applications, epidemiological studies, and environmental health risk assessments. In this study, we developed and applied a method to characterize air toxics concentrations in urban areas using results of the recently conducted field study in Wilmington, DE. Mobile measurements were collected over a 4- 4-km area of downtown Wilmington for three components: formaldehyde (representative of volatile organic compounds and also photochemically reactive pollutants), aerosol size distribution (representing fine particulate matter), and water-soluble hexavalent chromium (representative of toxic metals). These measurements were used to construct spatial and temporal distributions of air toxics in the area that show a very strong temporal variability, both diurnally and seasonally. An analysis of spatial variability indicates that all pollutants varied significantly by location, which suggests potential impact of local sources. From the comparison with measurements at the central monitoring site, we conclude that formaldehyde and fine particulates show a positive correlation with temperature, which could also be the reason that photochemically generated formaldehyde and fine particulates over the study area correlate well with the fine particulate matter measured at the central site.
Recent public debate has focused on urban air quality, where near-source impacts are large, especially from traffic on major roadways.1,2 Our growing understanding of the health risks for populations near major roadways and major point emission sources emphasizes the critical need to study the highly localized impacts by mapping pollutant concentration gradients in the area. Characterizing the spatial variability of air pollutants in an urban setting is critical for improved air toxics exposure assessments.3,4 Current approaches for characterizing ambient air toxics concentrations rely on developing a detailed emissions inventory and applying a dispersion model such as AERMOD. Previous modeling studies have shown large gradients in ambient concentrations of toxic pollutants in urban areas can arise from specific stationary or mobile sources.5 However, these findings can not be verified with the widely spaced monitoring networks that currently exist in most urban areas.
To address this need, new experimental approaches are needed to provide a quick assessment of air pollutant concentration gradients, especially those arising from vehicle emission impacts. Among the most powerful approaches is the use of a mobile platform equipped with state-of-the-art, real-time, and near-real-time monitoring instruments that provide the necessary time resolution to identify high concentrations and sharp spatial gradients in an urban area.
Mobile platforms can provide a unique capability that, heretofore, has not been available with stationary monitor measurements. In addition, mobile platform measurements can establish the location where concentrations attain high levels (“hot spots”) that may not be possible to find without using a large number of fixed monitors. Traditional modeling approaches are uncertain about the locations of these hot spots because of shortcomings of the emissions inventory (e.g., sources not reported in the emissions inventory) or limitations in source characterization input for the model (e.g., inability to characterize the release as a stack or fugitive source). This is especially important for air toxics assessments in Environmental Justice applications, for environmental health risk assessments, and to support epidemiological studies, in which resolving fine scale in air toxics concentrations is critical.