The abatement of particulate matter 2.5 in Los Angeles County: a counterfactual evaluation

Abstract

The ambient PM2.5 concentration in Los Angeles (LA) County has been on a decreasing trend since LA County was designated as a nonattainment area in 2005. However, whether the nonattainment assignment is the underlying cause of the county’s reductions in PM2.5 requires further empirical investigation. Traditional statistical approaches used to study the impact of nonattainment designation on air quality present problems involving indeterministic covariates, confoundedness, model misspecification, and undetected effects at the aggregate level. Our study successfully uses the Panel Data Approach for Program Evaluation (PAMPE) to compare the differences between the actual outcomes and counterfactual outcomes to reveal the treatment effects associated with nonattainment assignment without the burden associated with previous studies. Our results show that, at the monitor level, the air quality improvements obtained by the more-polluted areas were greater after LA County was designated as a nonattainment area. On average, the counterfactual reduction rates derived in our study range from − 0.01 to − 0.15%. Cases in which PM2.5 levels increased occurred at two monitors that were fully or partially compliant during the study period, suggesting the regulatory oversight is indeed spatially heterogeneous.