Abstract
The potential impacts of climate change on regional ozone (O3) and fine particulate (PM2.5) air quality in the United States are investigated by linking global climate simulations with regional scale meteorological and chemical transport models. Regional climate at 2000 and at 2030 under three Representative Concentration Pathways (RCPs) is simulated by using the Weather Research and Forecasting (WRF) model to downscale 11-year time slices from the Community Earth System Model (CESM). The downscaled meteorology is then used with the Community Multiscale Air Quality (CMAQ) model to simulate air quality during each of these 11-year periods. The analysis isolates the future air quality differences arising from climate-driven changes in meteorological parameters and specific natural emissions sources that are strongly influenced by meteorology. Other factors that will affect future air quality, such as anthropogenic air pollutant emissions and chemical boundary conditions, are unchanged across the simulations. The regional climate fields represent historical daily maximum and daily minimum temperatures well, with mean biases less than 2 K for most regions of the U.S. and most seasons of the year and good representation of variability. Precipitation in the central and eastern U.S. is well simulated for the historical period, with seasonal and annual biases generally less than 25%, with positive biases exceeding 25% in the western U.S. throughout the year and in part of the eastern U.S. during summer. Maximum daily 8-h ozone (MDA8 O3) is projected to increase during summer and autumn in the central and eastern U.S. The increase in summer mean MDA8 O3 is largest under RCP8.5, exceeding 4 ppb in some locations, with smaller seasonal mean increases of up to 2 ppb simulated during autumn and changes during spring generally less than 1 ppb. Increases are magnified at the upper end of the O3 distribution, particularly where projected increases in temperature are greater. Annual average PM2.5 concentration changes range from -1.0 to 1.0 μg m-3. Organic PM2.5 concentrations increase during summer and autumn due to increased biogenic emissions. Aerosol nitrate decreases during winter, accompanied by lesser decreases in ammonium and sulfate, due to warmer temperatures causing increased partitioning to the gas phase. Among meteorological factors examined to account for modeled changes in pollution, temperature and isoprene emissions are found to have the largest changes and the greatest impact on O3 concentrations.
Highlights
In the United States (US), emissions that lead to the formation of ozone (O3) and atmospheric particulate matter (PM) have declined significantly in recent decades, resulting in substantial improvements in air quality (Parrish et al, 2011; US EPA, 2012) and consequent benefits for human health (Pope III, 2007; Correia et al, 2013)
Because climate models are run without assimilating weather observations, the weather conditions simulated by downscaling a global climate model (GCM) for a particular historical day cannot be expected to correspond to the hourly meteorology that occurred on that day
To account for interannual meteorological variability it is necessary to run the model for periods of several years or even decades, but anthropogenic emissions of pollutants such as NOx, volatile organic compound (VOC), and SO2 can exhibit significant trends that confound the analysis of the impact of using downscaled meteorology
Summary
In the United States (US), emissions that lead to the formation of ozone (O3) and atmospheric particulate matter (PM) have declined significantly in recent decades, resulting in substantial improvements in air quality (Parrish et al, 2011; US EPA, 2012) and consequent benefits for human health (Pope III, 2007; Correia et al, 2013). Modeling studies conducted using mid-21st century climate data project up to 2–8 ppb increases in summer average ozone levels in the US, depending on climate change scenario and time period (e.g., Wu et al, 2008; Nolte et al, 2008; Weaver et al, 2009; Kelly et al, 2012; Trail et al, 2014; Pfister et al, 2014; Gonzalez-Abraham et al, 2015; Fann et al, 2015; He et al, 2016; Dionisio et al, 2017) This deterioration of air quality due to climate change is known as the “climate penalty” (Wu et al, 2008; Rasmussen et al, 2013) and could potentially offset some of the improvement in air quality that would otherwise occur due to reductions in ozone precursor emissions. In addition to presenting changes in seasonal mean quantities, we focus on distributions and examine variability across seasonal and diurnal temporal scales
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