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.

中文翻译：

---

洛杉矶县减少颗粒物 2.5：反事实评估

自 2005 年洛杉矶县被指定为未达标地区以来，洛杉矶（LA）县的大气 PM2.5 浓度一直呈下降趋势。但是，未达标是否是该县 PM2.5 下降的根本原因需要进一步的实证调查。用于研究未达标对空气质量的影响的传统统计方法存在的问题包括不确定性协变量、混杂、模型指定错误和未检测到的总体影响。我们的研究成功地使用了项目评估的面板数据方法 (PAMPE) 来比较实际结果和反事实结果之间的差异，以揭示与未完成分配相关的治疗效果，而没有与先前研究相关的负担。我们的结果表明，在监测级别，在洛杉矶县被指定为未达标地区后，污染较重的地区获得的空气质量改善更大。平均而言，我们研究中得出的反事实减少率范围从 - 0.01 到 - 0.15%。PM2.5 水平升高的案例发生在研究期间完全或部分合规的两个监测器上，表明监管监督确实存在空间异质性。