Projecting policy‐relevant metrics for high summertime ozone pollution events over the eastern United States due to climate and emission changes during the 21st century

Abstract

Over the eastern United States (EUS), nitrogen oxides (NO x ) emission controls have led to improved air quality over the past two decades, but concerns have been raised that climate warming may offset some of these gains. Here we analyze the effect of changing emissions and climate, in isolation and combination, on EUS summertime surface ozone (O 3 ) over the recent past and the 21st century in an ensemble of simulations performed with the Geophysical Fluid Dynamics Laboratory CM3 chemistry-climate model. The simulated summertime EUS O 3 is biased high but captures the structure of observed changes in regional O 3 distributions following NO x emission reductions. We introduce a statistical bias correction, which allows derivation of policy-relevant statistics by assuming a stationary mean state bias in the model, but accurate simulation of changes at each quantile of the distribution. We contrast two different 21st century scenarios: (i) representative concentration pathway (RCP) 4.5 and (ii) simulations with well-mixed greenhouse gases (WMGG) following RCP4.5 but with emissions of air pollutants and precursors held fixed at 2005 levels (RCP4.5_WMGG). We find under RCP4.5 no exceedance of maximum daily 8 hour average ozone above 75 ppb by mid-21st century, reflecting the U.S. NO x emissions reductions projected in RCP4.5, while more than half of the EUS exceeds this level by the end of the 21st century under RCP4.5_WMGG. Further, we find a simple relationship between the changes in estimated 1 year return levels and regional NO x emission changes, implying that our results can be generalized to estimate changes in the frequency of EUS pollution events under different regional NO x emission scenarios.

Tropospheric Ozone Assessment Report: Assessment of global-scale model performance for global and regional ozone distributions, variability, and trends

“…Feedbacks from local anthropogenic and biogenic emissions and atmospheric chemistry during these meteorologically driven events will influence the severity of the event, and add another layer of uncertainty to projecting future changes. Whether or not our projections are hindered, several studies argue that global chemistry models are able to capture the impact of such large-scale synoptic processes on ozone levels (Fiore et al, 2003(Fiore et al, , 2015Jacob and Winner, 2009). Figure 7 presents a further example, showing a favorable comparison of extreme ozone levels from a global CTM simulation (Murray, 2016) against the same ozone metric from the TOAR-Surface Ozone Database during a heatwave over the U.S.…”

“…Embedding tropospheric chemistry within a GCM (CCMs and chemistry GCMs) opens up the possibility of studying a large range of Earth system feedbacks, such as climatedependent biogenic emissions (Sanderson et al, 2003;Hauglustaine et al, 2005;Hedegaard et al, 2008Hedegaard et al, , 2013Heald et al, 2009;Young et al, 2009;Ganzeveld et al, 2010), vegetation-ozone interactions (Sitch et al, 2007), as well as the impacts of climate change on tropospheric chemistry (e.g., Johnson et al, 1999;Zeng andPyle, 2003, 2005;John et al, 2012;Doherty et al, 2013;Val Martin et al, 2015) and air quality (see Table S3 of Fiore et al (2015) for examples).…”

“…2. The number and/or frequency of events above a fixed value that is considered extreme at present, including values that are relevant to attaining ambient air quality standards (AAQS) (e.g., Murazaki and Hess, 2006;Wu et al, 2008;Gao et al, 2013;Pfister et al, 2014;Rieder et al, 2015). 3.…”

“…3. Statistical methods from extreme value theory (EVT) to analyze ozone extremes in observations and CCM simulations (Rieder et al, 2013(Rieder et al, , 2015. The EVT approach is useful as it combines the frequency and intensity aspect of ozone extremes by focusing on so-called 'T-year ozone return values', which describe the probability of exceeding a value of intensity × within a time window T. 4.…”

Co-occurrence of extremes in surface ozone, particulate matter, and temperature over eastern North America

“…In any case, the extremes often co-occur as a result of their shared underlying drivers, greatly increasing the risks to human health (14). The imperative to understand the co-occurrence of health extremes is driven in part by the recognition that episodes of extreme temperatures (15)(16)(17)(18) and poor air quality (19)(20)(21)(22)(23)(24) may become more frequent, longer lasting, and more intense in a warming climate, in which many climate-driven feedbacks can alter air quality independent of emissions (e.g., 8,10,25,26).…”