Zero-emissions vehicle adoption and satellite-measured NO2 air pollution in California, USA, from 2019 to 2023: a longitudinal observational study

Background

Electrifying the transportation sector is a key climate-change mitigation strategy. Reductions in exhaust emissions have anticipated air quality co-benefits; yet, evidence is primarily based on projections. Using observed data in California, USA, we aimed to investigate whether reductions in exhaust emissions from the transition to zero-emissions vehicles (ZEVs: battery electric, plug-in hybrid, and hydrogen fuel cell) were detectable using Tropospheric Monitoring Instrument (TROPOMI) satellite measurements of nitrogen dioxide (NO2) air pollution.

Methods

In this longitudinal observational study, we combined data from 2019 to 2023 on annual light-duty ZEV registrations in 1692 California ZIP code tabulation areas (ZCTAs; cross-walked from ZIP codes) with annual mean TROPOMI-measured NO2. We used longitudinal linear mixed-effects models to assess the association between within-ZCTA ZEV changes and within-ZCTA NO2 changes, adjusting for temporal trends and time-varying potential confounding, or excluding 2020. In positive control analyses, we related internal combustion engine vehicle registrations to NO2. In ground-truth analyses, we related ZEVs to NO2 concentrations using 123 Environmental Protection Agency monitors from 2012 to 2023.

Findings

The median within-ZCTA increase in ZEVs from 2019 to 2023 was 272 (IQR 18 to 839). A within-ZCTA increase of 200 ZEVs was associated with a 1·10% (95% CI −1·19 to −1·00) decrease in annual average NO2. The main findings were supported by sensitivity analyses (–1·32% [–1·43 to –1·21] when excluding the year 2020), ground-truth analysis (–0·87% [–1·76 to 0·03] using NO2 from ground-level monitors), and positive control analysis (0·80% [0·63 to 0·97] increase in annual average NO2 per 800 increase in number of internal combustion engine vehicles).

Interpretation

Using a natural experiment, we found that within-ZCTA increases in ZEV registrations were associated with reductions in NO2 air pollution measured by satellite and replicated with ground-level monitors. This work in California serves as a proof-of-principle for future work using satellite-measured NO2 to quantify effects of climate-change mitigation efforts on combustion-related air pollution within the USA and internationally.