Changes in Inorganic Fine Particulate Matter Sensitivities to Precursors Due to Large-Scale US Emissions Reductions * Jareth Holt, Noelle E. Selin and Susan Solomon *Reprinted from Environmental Science & Technology, 49(8): 4834–4841 © 2015 with kind permission from American Chemical Society Reprint 2015-8 The MIT Joint Program on the Science and Policy of Global Change combines cutting-edge scientific research with independent policy analysis to provide a solid foundation for the public and private decisions needed to mitigate and adapt to unavoidable global environmental changes. Being data-driven, the Program uses extensive Earth system and economic data and models to produce quantitative analysis and predictions of the risks of climate change and the challenges of limiting human influence on the environment—essential knowledge for the international dialogue toward a global response to climate change. 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This reprint is one of a series intended to communicate research results and improve public understanding of global environment and energy challenges, thereby contributing to informed debate about climate change and the economic and social implications of policy alternatives. Ronald G. Prinn and John M. Reilly, Program Co-Directors For more information, contact the Program office: MIT Joint Program on the Science and Policy of Global Change Postal Address: Massachusetts Institute of Technology 77 Massachusetts Avenue, E19-411 Cambridge, MA 02139 (USA) Location: Building E19, Room 411 400 Main Street, Cambridge Access: Tel: (617) 253-7492 Fax: (617) 253-9845 Email: [email protected] Website: http://globalchange.mit.edu/ Policy Analysis pubs.acs.org/est Changes in Inorganic Fine Particulate Matter Sensitivities to Precursors Due to Large-Scale US Emissions Reductions Jareth Holt,*,† Noelle E. Selin,†,‡ and Susan Solomon† †Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Building 54-1711, Cambridge, Massachusetts 02139, United States ‡Engineering Systems Division, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States *S Supporting Information ABSTRACT: We examined the impact of large US emissions changes, similar to those estimated to have occurred between 2005 and 2012 (high and low emissions cases, respectively), on inorganic PM2.5 sensitivities to further NOx, SO2, and NH3 emissions reductions using the chemical transport model GEOS-Chem. Sensitivities to SO2 emissions are larger year- round and across the US in the low emissions case than the high emissions case due to more aqueous-phase SO2 oxidation. Sensitivities to winter NOx emissions are larger in the low emissions case, more than 2× those of the high emissions case in parts of the northern Midwest. Sensitivities to NH3 emissions are smaller (∼40%) in the low emissions case, year-round, and ff across the US. Di erences in NOx and NH3 sensitivities indicate an altered atmospheric acidity. Larger sensitivities to SO2 and NOx in the low emissions case imply that reducing these emissions may improve air quality more now than they would have in 2005; conversely, NH3 reductions may not improve air quality as much as previously assumed. 3,9 ■ INTRODUCTION condense. However, multiple reactions oxidize SO2 into 2− ff ff SO4 , and each reaction responds di erently to NOx and Fine particulate matter (PM2.5) adversely a ects cardiovascular 10,11 and respiratory functioning1 and is a key focus of air quality hydrocarbon concentrations. PM2.5 sensitivities have been estimated using a variety of policies such as the National Ambient Air Quality Standards fi ff 12,13 (NAAQS). Designing effective policies requires knowledge of approaches, including nite di erences, direct decomposi- tion,14−17 and adjoint modeling.18,19 Sensitivity estimates how PM2.5 responds to changes in its precursors−its sensitivity to emissions. The precursors of inorganic PM are nitrogen calculate derivatives based around atmospheric conditions 2.5 associated with particular emissions. Extrapolating these oxides (NOx), sulfur dioxide (SO2), and ammonia (NH3). US NO and SO emissions decreased by 42% and 62%, estimates to other levels of emissions is associated with some x 2 degree of error. Zhang et al.17 show that a linear extrapolation respectively, between 2005 and 2012, while NH3 emissions remained approximately constant.2 These emissions changes underestimates the PM2.5 response to a 50% decrease in NOx emissions by 15%, averaged over the US, with local are potentially large enough to change the sensitivity of PM2.5 to future emissions reductions and hence change the expected underestimates up to 50%. Higher-order sensitivity analysis fi can more accurately predict responses beyond the linear bene ts of air quality policies. We analyze where and to what 14 extent large changes in precursor emissions, similar to those range, but these approaches are computationally demanding. Studies of health and economic impacts of emissions often that occurred between 2005 and 2012, alter US PM2.5 sensitivities to further emissions reductions. apply estimates of PM2.5 sensitivities to parametrize how air ff quality responds to emissions. Muller and co-workers12,20,21 use Several chemical mechanisms a ect PM2.5 concentrations. the integrated assessment model APEEP to calculate marginal Nitric acid (HNO3) is formed from NOx, and the fraction of damages (in US dollars per ton) by increasing emissions from NH3 and HNO3 in particulates (the partitioning of these species) depends on ambient temperature, humidity, and one source by one ton and tracing impacts on human health, agriculture, and other vulnerable structures. The base case in acidity. NH3 is the primary basic species, forming ammonium + − APEEP uses the EPA’s 2002 National Emissions Inventory (NH4 ) in particles to neutralize acidic nitrate (NO3 ) and 21 2− (NEI02), but Muller implements the 2005 inventory sulfate (SO4 , formed from SO2). PM2.5 sensitivities to NH3 emissions are large, and it has been argued that reducing NH3 is ff 3−8 a cost-e ective strategy to reduce PM2.5. Sulfate is not Received: January 1, 2015 fl volatile like NH3 and HNO3, but it in uences the ambient Revised: March 25, 2015 2− acidity. PM2.5 concentrations can even increase as SO4 Accepted: March 27, 2015 concentrations decrease, by allowing more HNO3 to Published: March 27, 2015 © 2015 American Chemical Society 4834 DOI: 10.1021/acs.est.5b00008 Environ. Sci. Technol. 2015, 49, 4834−4841 Environmental Science & Technology Policy Analysis (NEI05). Fann, Baker, and Fulcher22,23 use the CAMx Particle seen in GEOS-Chem.28,38,39 These features improve simulated ff Source Apportionment Technology to attribute PM2.5 concen- NOx concentrations (especially the daytime-nighttime di er- − 28,29,38 trations to emissions from each economic sector. Their ence) and reduce the high NO3 bias noted previously. simulations use NEI05 and projections for 2016 based on Emissions. We use anthropogenic emissions from the proposed air quality rules. Similarly, Fann, Fulcher, and EDGAR and RETRO global inventories plus several regional Hubbell24 use NEI02 with projections for 2015 as the inventories (over e.g., China, Europe).40 The US inventory is emissions inventory for EPA’s Response Surface Model of air EPA’s National Emissions Inventory for 2005 (NEI05). NEI05 25 quality. In these studies, the projections based on then- provides emissions of NOx, SO2, NH3, several hydrocarbon proposed rules exhibit 30% decreases in national NOx and SO2 species, and primary PM. We seasonally adjust NH3 emissions emissions between 2001 and 2010.25 These emissions actually from NEI05 following Zhang et al.,38 decreasing winter decreased by 34% and 53%, respectively, and NH3 emissions emissions to better match observations, consistent with increased by 17%. Whether sensitivities calculated using older process-based inventories.41 (2002 and 2005) emissions, or even past estimates of current We created two groups of simulations, based around a high emissions, are sufficiently accurate for health and economic and low emissions case, to test the influence of large-scale impact assessment depends on the nonlinear response of PM2.5. emissions reductions on PM2.5 sensitivity. The national, annual fl We evaluate the in uence of large NOx and SO2 emissions total emissions of NOx, SO2, and NH3 as reported to the US 2 changes (comparable to those that occurred in the US between EPA for 2005 and 2012 exhibit a 42% decrease in NOx, a 62% 2005 and 2012) on PM2.5 sensitivities and identify the most decrease