Characterizing the Tropospheric Ozone Response to Methane Emission Controls and the Benefits to Climate and Air Quality Arlene M
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JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 113, D08307, doi:10.1029/2007JD009162, 2008 Click Here for Full Article Characterizing the tropospheric ozone response to methane emission controls and the benefits to climate and air quality Arlene M. Fiore,1 J. Jason West,2,3,4 Larry W. Horowitz,1 Vaishali Naik,3 and M. Daniel Schwarzkopf1 Received 12 July 2007; revised 2 November 2007; accepted 12 December 2007; published 30 April 2008. [1] Reducing methane (CH4) emissions is an attractive option for jointly addressing climate and ozone (O3) air quality goals. With multidecadal full-chemistry transient simulations in the MOZART-2 tropospheric chemistry model, we show that tropospheric O3 responds approximately linearly to changes in CH4 emissions over a range of À1 anthropogenic emissions from 0–430 Tg CH4 a (0.11–0.16 Tg tropospheric O3 or À1 11–15 ppt global mean surface O3 decrease per Tg a CH4 reduced). We find that neither the air quality nor climate benefits depend strongly on the location of the CH4 emission reductions, implying that the lowest cost emission controls can be targeted. With a series of future (2005–2030) transient simulations, we demonstrate that cost-effective CH4 controls would offset the positive climate forcing from CH4 and O3 that would otherwise occur (from increases in NOx and CH4 emissions in the baseline scenario) and À2 improve O3 air quality. We estimate that anthropogenic CH4 contributes 0.7 Wm to climate forcing and 4 ppb to surface O3 in 2030 under the baseline scenario. Although the response of surface O3 to CH4 is relatively uniform spatially compared to that from other O3 precursors, it is strongest in regions where surface air mixes frequently with the free troposphere and where the local O3 formation regime is NOx-saturated. In the model, CH4 oxidation within the boundary layer (below 2.5 km) contributes more to surface O3 than CH4 oxidation in the free troposphere. In NOx-saturated regions, the surface O3 sensitivity to CH4 can be twice that of the global mean, with >70% of this sensitivity resulting from boundary layer oxidation of CH4. Accurately representing the NOx distribution is thus crucial for quantifying the O3 sensitivity to CH4. Citation: Fiore, A. M., J. J. West, L. W. Horowitz, V. Naik, and M. D. Schwarzkopf (2008), Characterizing the tropospheric ozone response to methane emission controls and the benefits to climate and air quality, J. Geophys. Res., 113, D08307, doi:10.1029/2007JD009162. 1. Introduction [e.g., Marenco et al., 1994; Wang and Jacob, 1998]. Here, we characterize the response of tropospheric O3 to controls [2] Methane (CH ) emission controls are currently re- 4 on CH emissions, analyze the dominant processes deter- ceiving attention as a viable low-cost strategy for abating 4 mining the distribution of this response, and quantify the surface ozone (O ) pollution while simultaneously slowing 3 resulting benefits to air quality and climate. greenhouse warming [Hansen et al., 2000; Fiore et al., [3] With a lifetime of approximately a decade, CH is 2002a; Dentener et al., 2005; EMEP, 2005; West and Fiore, 4 fairly well-mixed in the atmosphere. Sources of atmospheric 2005; West et al., 2006]. In the presence of nitrogen oxides CH include wetlands, ruminants, energy, rice agriculture, (NO ), tropospheric CH oxidation leads to the formation of 4 x 4 landfills, wastewater, biomass burning, oceans, and ter- O [Crutzen, 1973]. Over the last century, global back- 3 mites. Anthropogenic emissions are estimated to contribute ground O3 concentrations have risen by at least a factor of at least 60% to total CH4 emissions, with individual studies two, due mainly to increases in CH4 and NOx emissions À1 reporting a range of 500 to 610 Tg a for total CH4 emissions [Denman et al., 2007]. The dominant CH4 sink 1NOAA Geophysical Fluid Dynamics Laboratory, Princeton, New is reaction with the hydroxyl radical (OH) in the tropo- Jersey, USA. sphere. If sufficient quantities of NOx are available, CH4 2 Atmospheric and Oceanic Sciences Program, Princeton University, oxidation produces O3 via reactions of peroxy radicals with Princeton, New Jersey, USA. 3 NOx. In a low-NOx environment, formation of methyl hydro- Woodrow Wilson School of Public and International Affairs, Princeton peroxide (CH OOH) suppresses O production and may University, Princeton, New Jersey, USA. 3 3 4Now at University of North Carolina at Chapel Hill, Chapel Hill, North provide a net O3 sink. In an extremely low-NOx environment, Carolina, USA. CH4 oxidation may also decrease O3 levels by HO2 reacting preferentially with O3 rather than with NO. Under present- Copyright 2008 by the American Geophysical Union. day tropospheric conditions, however, Spivakovsky et al. 0148-0227/08/2007JD009162$09.00 D08307 1of16 D08307 FIORE ET AL.: OZONE RESPONSE TO METHANE D08307 [2000] show that HO2 + NO is more important than HO2 +O3 globally as a source of OH (their Figure 10), implying that increases in CH4 abundances should yield a net global increase in the tropospheric O3 burden, as has been reported in prior modeling studies [e.g., Prather et al., 2001]. Previ- ously, CH4 and NOx emission reductions have been shown to be the most effective means of lowering tropospheric O3: reductions in anthropogenic NOx emissions decrease surface O3 in polluted source regions by up to four times more than equivalent percentage reductions in anthropogenic CH4 emissions, while CH4 reductions have a stronger impact on the tropospheric O3 burden, and a similar influence to NOx on global average surface O3 concentrations [Fiore et al., 2002a; West et al., 2008]. [4] To date, most chemical transport model (CTM) stud- Figure 1. The change in tropospheric O3 burden (Tg) as a ies have applied a uniform CH mixing ratio to avoid the 4 function of the change in anthropogenic CH4 emissions computational expense of multidecadal simulations required (Tg aÀ1), compiled from modeling studies in the literature for CH4 to reach a steady state [e.g., Prather et al., 2001; encompassing a range of modeling approaches: (1) full- Stevenson et al., 2006]. We have previously adopted this chemistry, transient simulations (pink filled diamonds: TM3 approach to evaluate the benefits to human health, agricul- [Dentener et al., 2005]; blue filled circle: RGLOB-BASE in ture, and commercial forests resulting from lower O3 due to this work; blue filled triangles: 2030 results from future CH4 emission reductions [West and Fiore, 2005; West et al., scenarios A, B, and C in this work), (2) steady state 2006]. In such simulations, termed ‘‘steady state’’ in our simulations in which a uniform, fixed CH4 concentration is analysis below, the uniform CH mixing ratio is adjusted to 4 adjusted to reflect a change in CH4 emissions (red x: GEOS- reflect a desired CH4 emission change, accounting for the Chem [Fiore et al., 2002a]; black upside down triangle: non-linear feedback of CH on its own lifetime through OH 4 Recommended based on IPCC TAR models, with the O3 [Prather, 1996; Prather et al., 2001]. Since the relationship burden change reduced by 25% to correct for the inclusion between CH emissions and CH concentrations is non- 4 4 of stratospheric O3 in the reported results [Prather et al., linear, it is important to assess the degree to which this non- 2001]; black +: MOZART-2 [West et al., 2006, 2008]; open linearity affects the accuracy of estimates obtained by black circle: addition of CH4 to the pre-industrial atmo- scaling results (e.g., changes in O3 concentrations) from sphere in a global CTM [Wang and Jacob, 1998]; open blue one CH perturbation to another. 4 triangle: CH4-700 simulation in this work), and (3) a hybrid [5] In Figure 1, we compile estimates of the response of approach where the initial CH4 trends from a transient tropospheric O3 to changes in CH4 emissions from several simulation were extrapolated exponentially using the global CTMs in the literature, to investigate whether model’s CH4 perturbation time of 12.6 years, followed by changes in O3 scale linearly with changes in CH4 emissions. 5 additional years of simulation (green squares: GISS We include results from transient, full-chemistry simula- [Shindell et al., 2005; personal communication, 2006]). All tions [Dentener et al., 2005], from ‘‘steady state’’ simula- blue symbols (from RGLOB-BASE and the differences of tions with a uniform, fixed CH concentration [Wang and 4 the 2030 A, B, C, CH4-700 and 2030 CLE simulations) Jacob, 1998; Prather et al., 2001; Fiore et al., 2002a; West denote the O3 burden change adjusted to steady state as et al., 2006, 2008], and from a hybrid modeling approach described in section 3. The two left-most points depict [Shindell et al., 2005]. Despite variations in the simulation simulations in which anthropogenic CH4 emissions are type, the total CH4 emissions, the anthropogenic fraction of eliminated. CH4 emissions, and the emissions of other species that affect OH, Figure 1 shows that the tropospheric O3 burden responds roughly linearly to changes in anthropogenic CH 4 [6] Designing effective CH4 controls to combat O3 air emissions across the models. Estimates from the individual pollution requires knowledge of the magnitude and spatial studies range from 0.12–0.16 Tg tropospheric O per Tg aÀ1 3 pattern of the surface O3 response to changes in CH4 change in CH emissions. Although the feedback between 4 emissions. The sensitivity of O3 to changes in CH4 should CH and OH will cause the CH concentration to respond in 4 4 depend on the emission ratio of NOx to non-methane a strongly non-linear manner for sufficiently large increases volatile organic compounds (NMVOC) and carbon monox- in CH4 emissions, the relationship is approximately linear ide (CO), which affects the abundance of OH [Wang and for the range of emission perturbations considered in Jacob, 1998; West et al., 2006].