Where air quality has been impacted by weather changes in the United States over the last 30 years?

Highlights

• Our findings show significant spatio-temporal variation of climate impact on CO, NO₂, and O_3.

• The site-level analysis allows us to capture this site-to-site heterogeneity of weather trends.

• Our results should be of interest to policy makers to create future strategies related to environmental health.

Abstract

Previous studies have contributed to the understanding on the impacts of weather on air pollution. Most of these investigations focused primarily on future climate scenarios, laboratory experiments, or weather trends-related air pollution at regional scale. In particular, an important limitation of the trends studies is that the estimated weather impact on air pollution may be underestimated or overestimated given that the regional scale does not capture the site-to-site variation of air pollution and weather. The primary objective of this research is to addresses this gap by quantifying weather-associated changes in air pollution at site location (air pollution sites) in the U.S between 1988 and 2018. We quantified the past weather-related increases CO, NO₂, O₃, nitrate, organic carbon, silicon, sodium, sulfate, and SO₂ concentration using Generalized Additive Models (GAMs). We used a framework that derives “penalties” (weather penalty, in μg/m³, ppm or ppb per year) for each season (warm and cold). Three pollutants presented significant results (weather penalties), including CO, NO₂, and O₃. Our findings show significant spatio-temporal variation of climate impact on CO, NO₂, and O₃. For example, in the warm season we estimated a total penalty over the study period for the sites with the highest penalty on CO (site in Boise, Idaho), NO₂ (site in New York City), and O₃ (site in Tucson, Arizona) of 6.18 (95%CI: 0.30; 12.0) ppm, 182.04 (95%CI: 39.33; 324.72) ppb, and 0.09 (95%CI: 0.030; 0.150) ppm, respectively. In the cold season, the estimated total penalty for the sites with the highest penalty on CO (site in Los Angeles, California), NO₂ (site in Washington, Pennsylvania), and O₃ (site in Decatur, Illinois) was 12.01 (95%CI: 1.50; 22.50) ppm, 285.03 (95%CI: 14.37; 555.69) ppb, and −0.066 (95%CI: -0.120; −0.030) ppm, respectively. Our results should be of interest to policy makers to create future strategies related to environmental health and climate change. Climate models typically show large variation in projections of the key variables influencing pollution, and our model based on local scale allowed us to identify statistically robust results.

Introduction

Historical changes in weather conditions have had significant impacts on air quality and health (Jhun et al., 2015; Sun et al., 2015; Tainio et al., 2013). Studies have shown that weather impacts on air pollution varies significantly over space and time given the diversity of air pollutants and the complex mechanisms governing the formation and removal of air pollutants. For example, O₃ is primarily resulted from photochemical reactions between nitrogen oxides (NO_x) and organic compounds hydrocarbons in the presence of sunlight (Fenger, 2009). Low humidity, high temperature, and low wind speed favor O₃ formation (Koo et al., 2012). High temperatures can increase oxidation and production of sulfate particles, but also reduce nitrate particles through volatilization from particle to gas phase (Phalen, 2012; Weichenthal et al., 2016). Temperature is positively correlated with nitrate in California, but negatively correlated in the Southeast – USA (Tai et al., 2010). Relative humidity is negatively correlated with organic and elemental carbon but positively correlated with sulfate and nitrate (Tai et al., 2010). Sulfate dominates in summer days due to more rapid SO₂ oxidation (Hand et al., 2012a; Tai et al., 2010). Warmer summers and less wind (stagnations) favor photochemical reactions so a larger fraction of SO₂ is oxidized to form sulfates. Most sulfate aerosols in the atmosphere come from the photochemical conversion of SO₂ (Roberts and Friedlander, 1976).

These studies have contributed to the understanding on the impacts of weather on air pollution. Most of these focused primarily on future climate scenarios or laboratory experiments. Little research has focused on site-specific spatial and seasonal trends models at site location based on observational data. The primary objective of this research is to addresses this gap by quantifying weather-associated changes in air pollution at site location.

We previously examined the effect of weather changes on PM_2.5 and O₃ trends in U.S. during 1994–2012 and found that temperature increases and wind speed decreases adversely impacted their concentrations (Jhun et al., 2015). We then expanded this analysis to estimate the effect of weather changes on seven major components of ambient PM_2.5, including elemental carbon, organic carbon, nitrate, sulfate, sodium, ammonium, and silicon (Requia et al., 2019). We used similar methods in these studies to understand the influences of weather changes on air quality trends in U.S. The limitation of these studies is that we estimated trends and trend differences at a regional scale (7 regions), rather than site-level scale. This may underestimate or overestimate the regional trends given that the spatial distribution of the air pollution stations over the U.S. it is not homogeneous. In this paper we expand these analyses to quantify the impacts of climate change on air pollution levels at site locations in the U.S, considering a larger period (1988–2018) and a larger list of pollutants (in methods section we describe how the pollutants were selected).