Science Inventory

Four Decades of United States Mobile Source Pollutants: Spatial–Temporal Trends Assessed by Ground-Based Monitors, Air Quality Models, and Satellites

Citation:

Henneman, L., H. Shen, C. Hogrefe, A. Russell, AND C. Zigler. Four Decades of United States Mobile Source Pollutants: Spatial–Temporal Trends Assessed by Ground-Based Monitors, Air Quality Models, and Satellites. ENVIRONMENTAL SCIENCE & TECHNOLOGY. American Chemical Society, Washington, DC, 55(2):882-892, (2021). https://doi.org/10.1021/acs.est.0c07128

Impact/Purpose:

This manuscript describes the analysis 40+ years of AQS observations, CMAQ simulations for 1990 – 2010 and 2002 – 2014 performed by AESMD, and satellite products to examine trends in traffic-related pollutants (NOx, CO, and EC) and their relationship with various measures of road proximity. The analytical approach is based on a hierarchical linear regression framework. Results suggest that road proximity continues to be a useful substitute for air pollution exposure in epidemiological studies when more direct measures of air pollution exposure are not available, although the influence of traffic over time has reduced at differing rates for different pollutants, and the influence on spatial-temporal trends continues to evolve.

Description:

On-road emissions sources degrade air quality, and these sources have been highly regulated. Epidemiological and environmental justice studies often use road proximity as a proxy for traffic-related air pollution (TRAP) exposure, and other studies employ air quality models or satellite observations. To assess these metrics’ abilities to reproduce observed near-road concentration gradients and changes over time, we apply a hierarchical linear regression to ground-based observations, long-term air quality model simulations using Community Multiscale Air Quality (CMAQ), and satellite products. Across 1980–2019, observed TRAP concentrations decreased, and road proximity was positively correlated with TRAP. For all pollutants, concentrations decreased fastest at locations with higher road proximity, resulting in “flatter” concentration fields in recent years. This flattening unfolded at a relatively constant rate for NOx, whereas the flattening of CO concentration fields has slowed. CMAQ largely captures observed spatial–temporal NO2 trends across 2002–2010 but overstates the relationships between CO and elemental carbon fine particulate matter (EC) road proximity. Satellite NOx measures overstate concentration reductions near roads. We show how this perspective provides evidence that California’s on-road vehicle regulations led to substantial decreases in NO2, NOx, and EC in California, with other states that adopted California’s light-duty automobile standards showing mixed benefits over states that did not adopt these standards.