Cleaner air has contributed one-fifth of US maize and soybean yield gains since 1999

Crop productivity is potentially affected by several air pollutants, although these are usually studied in isolation. A significant challenge to understanding the effects of multiple pollutants in many regions is the dearth of air quality data near agricultural fields. Here we empirically estimate the effect of four key pollutants (ozone (O₃), particulate matter (PM), sulfur dioxide (SO₂), and nitrogen dioxide (NO₂)) on maize and soybean yields in the United States using a combination of administrative data and satellite-derived yield estimates. We identify clear negative effects of exposure to O₃, PM, and SO₂ in both crops, using yields measured in the vicinity of monitoring stations. We also show that while stations measuring NO₂ are too sparse to reliably estimate a yield effect, the strong gradient of NO₂ concentrations near power plants allows us to more precisely estimate NO₂ effects using satellite measured yield gradients. The presence of some powerplants that turn on and others that shut down during the study period are particularly useful for attributing yield gradients to pollution. We estimate that total yield losses from these pollutants averaged roughly 5% for both maize and soybean over the past two decades. While all four pollutants have statistically significant effects, PM and NO₂ appear more damaging to crops at current levels than O₃ and SO₂. Finally, we find that the significant improvement in air quality since 1999 has halved the impact of poor air quality on major crops and contributed to yield increases that represent roughly 20% of overall yield gains over that period. (letter)