Elsevier

Atmospheric Environment

Volume 167, October 2017, Pages 116-128
Atmospheric Environment

Source contributions to United States ozone and particulate matter over five decades from 1970 to 2020

https://doi.org/10.1016/j.atmosenv.2017.08.009Get rights and content

Highlights

  • U.S. emissions contributions to ozone and PM2.5 have reduced over five decades.

  • On-road mobile and EGU contributions have declined substantially since 1970.

  • Contributions of non-EGU and area sources are declining less since 2005.

  • Ozone production efficiency rises from 2000 onwards as NOx emissions are reduced.

  • Inter-regional transport is becoming less important in contrast to background ozone.

Abstract

Evaluating long-term air quality trends can demonstrate effectiveness of control strategies and guide future air quality management planning. Observations have shown that ozone (O3) and fine particulate matter (PM2.5) in the US have declined since as early as 1980 in some areas. But observation trends alone cannot separate effects of changes in local and global emissions to US air quality which are important to air quality planners. This study uses a regional model (CAMx) nested within a global model (GEOS-Chem) to characterize regional changes in O3 and PM2.5 due to the intercontinental transport and local/regional emissions representing six modeling years within five decades (1970–2020). We use the CAMx Source Apportionment Technology (OSAT/PSAT) to estimate contributions from 6 source sectors in 7 source regions plus 6 other groups for a total of 48 tagged contributions. On-road mobile sources consistently make the largest U.S. anthropogenic emissions contribution to O3 in all cities examined even though they decline substantially from 1970 to 2005 and also from 2005 to 2020. Off-road mobile source contributions increase from 1970 to 2005 and then decrease after 2005 in all of the cities. The boundary conditions, mostly from intercontinental transport, contribute more than 20 ppb to high maximum daily 8-h average (MDA8) O3 for all six years. We found that lowering NOx emissions raises O3 formation efficiency (OFE) across all emission categories which will limit potential O3 benefits of local NOx strategies in the near future. PM2.5 benefited from adoption of control devices between 1970 and 1980 and has continued to decline through 2005 and expected to decline further by 2020. Area sources such as residential, commercial and fugitive dust emissions stand out as making large contributions to PM2.5 that are not declining. Inter-regional transport is less important in 2020 than 1990 for both pollutants.

Introduction

Since the 1970 Clean Air Act, the U.S. Environmental Protection Agency (EPA) has regulated US criteria air pollutants to improve air quality, visibility and acid deposition. Major regulations include passenger vehicle emission controls for nitrogen oxide (NOx), volatile organic compounds (VOCs) and carbon monoxide (CO), which have tightened progressively from Tier 0 through Tier 3 (EPA, 2016a), New Source Performance Standards for industrial sources affecting VOC, NOx, CO, sulfur dioxide (SO2), and particulate matter (PM) emissions from specific stationary source categories (EPA, 2016b), the Acid Rain Program covering major SOx and NOx sources such as electric utilities (EPA, 2016c), the NOx SIP Call (Federal Register, 1998), to name a few. Previous studies have evaluated long-term trends in air quality and emissions in the US to demonstrate the effectiveness of emission reductions strategies (Simon et al., 2014, Blanchard et al., 2010, Butler et al., 2011, Sather and Cavender, 2012, Hidy and Blanchard, 2015). This information can help in guiding future air quality management plans. The planning in many cases can be complicated by non-controllable factors including intercontinental transport (Jaffe et al., 2003, Lin et al., 2012), meteorology influences (Camalier et al., 2007, Cox and Chu, 1996, Lin et al., 2015), as well as interactions among reacting species.

Impacts from intercontinental transport to U.S. air quality depend on emissions in other regions in the world which have changed significantly in the last five decades. Europe has taken extensive measures to reduce emissions which have resulted in a reduction of pollution recorded between 1980 and 2000 (Lovblad et al., 2004). However, fast growing economies contribute to increased emissions in parts of Asia (Streets and Waldhoff, 2000, Akimoto, 2003, Richter et al., 2005). Such spatial changes in global emissions can potentially change background pollution levels in different regions of US, thus they need to be considered when evaluating U.S. trends. In fact, evidence has suggested recent increase of foreign contributions in the U.S. (Parrish et al., 2009, Parrish et al., 2012, Cooper et al., 2012, Gratz et al., 2015). Therefore, an accurate description of temporal and spatial variations at regional and global scales in long-term emissions inventories is crucial in trend studies.

Air quality models (AQMs) have been increasingly used to demonstrate effectiveness of control scenarios and long-range transport. AQMs can overcome some deficiencies of spatial and temporal data of ambient monitoring networks. For example, Hogrefe et al. (2009) simulated PM2.5 over the northeastern United States for 1988–2005 and integrated with observations to provide greater spatial coverage and speciation information in addition to total mass. AQMs can also help separate the effects of intercontinental transport and domestic contributions (Dolwick et al., 2015, Wild et al., 2012, Hogrefe et al., 2004). To date, most AQM applications in the U.S. are applied for short episodes or single years (Goldberg et al., 2016, Dolwick et al., 2015, Nopmongcol et al., 2014 to name a few). Only a few have extended to multi-years (Bouchet et al., 1999, Pierce et al., 2010, Godowitch et al., 2010, Hogrefe et al., 2011, Xing et al., 2015). Most recent air quality modeling by Xing et al. (2015) applied a regional model with 108 × 108 km resolution across the northern hemisphere over 1990–2010 period. Their trends represent sub-grid variability as well as changes in local emissions. While such an approach allows a direct comparison to observations, it cannot separate effects of changes in local and global emissions to US air quality, which are important to air quality planners.

We focus on how changing global and local emissions influence air quality rather than the effects of inter-annual meteorological variation or long-term climate change. A clear focus on the influence of emission trends can provide useful information to motivate needed inventory improvements. Meteorological changes can mask the trend attributable to changes in precursor emissions (Lin et al., 2015). For this reason the meteorology is held constant (for 2005) and there are no changes to meteorology-dependent emissions (e.g., vegetation, from fires). Air quality planners in the U.S. use the same approach to assess how changes in local emissions influence Ozone (O3) and PM2.5. We simulate intercontinental transport with global emissions and characterize regional changes in O3 and PM2.5 due to the intercontinental transport and local emissions representing six years within five decades (1970–2020). We track contributions from each source sector and each US region throughout the five decades.

Section snippets

Air quality modeling

We use CAMx version 6.1 (ENVIRON, 2014) with the 2005 version of the Carbon Bond chemical mechanism (CB05; Yarwood et al., 2005) to simulate O3 and PM2.5 with anthropogenic emissions for 1970, 1980, 1990, 2000, 2005, and 2020 using the model configurations described in Nopmongcol et al. (2016). The meteorology and all natural emissions, including wild fires, are held constant for 2005 to isolate the effect of changing anthropogenic emissions outside the US. The modeling domain has 36 km

Changes in U.S. emissions of O3 and PM2.5 precursors

U.S. emissions decline from 1970 to 2020 (Table 1; 2020 numbers are based on projections) for all pollutants although at varying rates. Emission totals for the top 5 source categories are provided as Supplemental Information (Figure S2). NOx emissions decline by 59% from 1970 to 2020. Tightening on-road vehicle emission standards reduce NOx emissions from 1980 onward but growing vehicle miles travelled keep on-road vehicles as the largest NOx contributor. EGUs are the second highest NOx

Summary and conclusions

We model O3 and PM2.5 across the U.S. for 2005 meteorological conditions and vary anthropogenic emissions in the U.S. and worldwide from 1970 to 2020. To promote consistent results for this extended time period, during which inventory methods improved, we develop U.S. anthropogenic emission inventories specifically for this study to minimize effects of changing methods. We fix the meteorological year and the natural emissions at 2005 to isolate effects of changing anthropogenic emissions alone.

Acknowledgements

This work was supported by the Electric Power Research Institute.

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