Magnitude, trends, and impacts of ambient long-term ozone exposure in the United States from 2000 to 2015

• Main Authors: K. M. Seltzer, D. T. Shindell, P. Kasibhatla, C. S. Malley
• Format: Article
• Language: English
• Published: Copernicus Publications 2020-02-01
• Series: Atmospheric Chemistry and Physics
• Online Access: https://www.atmos-chem-phys.net/20/1757/2020/acp-20-1757-2020.pdf

<p>Long-term exposure to ambient ozone (<span class="inline-formula">O₃</span>) is associated with a variety of impacts, including adverse human-health effects and reduced yields in commercial crops. Ground-level <span class="inline-formula">O₃</span> concentrations for assessments are typically predicted using chemical transport models; however such methods often feature biases that can influence impact estimates. Here, we develop and apply artificial neural networks to empirically model long-term <span class="inline-formula">O₃</span> exposure over the continental United States from 2000 to 2015, and we generate a measurement-based assessment of impacts on human-health and crop yields. Notably, we found that two commonly used human-health averaging metrics, based on separate epidemiological studies, differ in their trends over the study period. The population-weighted, April–September average of the daily 1&thinsp;h maximum concentration peaked in 2002 at 55.9&thinsp;ppb and decreased by <span class="inline-formula">0.43</span> [95&thinsp;% CI: <span class="inline-formula">0.28</span>, <span class="inline-formula">0.57</span>]&thinsp;ppb&thinsp;yr<span class="inline-formula">⁻¹</span> between 2000 and 2015, yielding an <span class="inline-formula">∼18</span>&thinsp;% decrease in normalized human-health impacts. In contrast, there was little change in the population-weighted, annual average of the maximum daily 8&thinsp;h average concentration between 2000 and 2015, which resulted in a <span class="inline-formula">∼5</span>&thinsp;% increase in normalized human-health impacts. In both cases, an aging population structure played a substantial role in modulating these trends. Trends of all agriculture-weighted crop-loss metrics indicated yield improvements, with reductions in the estimated national relative yield loss ranging from 1.7&thinsp;% to 1.9&thinsp;% for maize, 5.1&thinsp;% to 7.1&thinsp;% for soybeans, and 2.7&thinsp;% for wheat. Overall, these results provide a measurement-based estimate of long-term <span class="inline-formula">O₃</span> exposure over the United States, quantify the historical trends of such exposure, and illustrate how different conclusions regarding historical impacts can be made through the use of varying metrics.</p>