Factors controlling the oxidative capacity of the troposphere since the Last Glacial Maximum

Transcription

1 Factors controlling the oxidative capacity of the troposphere since the Last Glacial Maximum Lee T. Murray 1, Loretta J. Mickley 1, Jed O. Kaplan 2, Eric D. Sofen 3, Mirjam Pfeiffer 2, and Becky Alexander 3 1. School of Engineering and Applied Sciences, Harvard University, Cambridge, MA, USA 2. Environmental Engineering Institute, École Polytechnique Fédérale de Lausanne, Lausanne, Switzerland 3. Department of Atmospheric Sciences, University of Washington, WA, USA

2 GEOS-Chem at and since the Last Glacial Maximum GOAL: Test sensitivity of OH to range of uncertainty in climate and emissions over glacial-interglacial periods ICE age Chemistry And Proxies (ICECAP) project Climate GISS ModelE G (Schmidt et al., 26) Stratospheric chemistry Linoz (McLinden et al., 2) Tropospheric chemistry GEOS-Chem CTM (v9-1-3) ( Vegetation BIOME4-TG (Kaplan et al., 26) Fire LMfire (Pfeiffer and Kaplan, 212) Isotope chemistry GEOS-Chem 17 O offline aerosol CTM (Sofen et al., 211) Ice Core Records E. Sofen poster L. Geng poster 4 climate scenarios Present Day (ca. 199s) Preindustrial (ca. 177s) Warm (~21ka) Cold (~21ka) 3 past emission scenarios Low Fire High Fire Fixed Lightning Uncertainty in Uncertainty in past fire emissions Uncertainty in past lightning Tg CO a Fire CO Emissions LMfire [Pfeiffer et al., in prep.] Scaled to Charcoal Data [Power et al., 28] Fire Scenario High Low Present Tg N a Lightning NO x Lightning Scenario Fixed Variable Cold Warm Preind. Present Cold Warm Preind. Present

3 Net reduced OH burdens in all past atmospheres Murray et al. [213] Naik et al. [212] % in OH from (-Preind.)/Preind. Preind. to Present Percent change [%] OH relatively buffered; our present day consistent with large spread in models We find decreased OH at Murray et al. [213] Kaplan et al. [26] Valdes et al. [25] Karol et al. [1995] Martinerie et al. [1995] type Thompson et al. [1993] 1D model Pinto and Khalil [1991]; Lu and Khalil [1991] 2D model Valentin [199] 3D model McElroy [1989] % OH in OH (-Preind.)/Preind. from to [Sowers and Bender, 1995] Temp. CO2 CH Percent change [%] Ice Age Ice Age Ice Age time Most earlier studies found increases necessary to match ice-core methane decreases What controls OH in our simulations? Why are we different? Methane budget implications?

4 OH sensitive to photolysis, water vapor, emissions We find a simple linear relationship explains 67% of variability in simulated mean OH across glacial-interglacial periods Ozone photolysis rate (s -1 ) Mass-weighted tropospheric mean / Specific humidity (g/kg) Total NOx source (Tmol a -1 ) Total reactive carbon source (Tmol a -1 ) OH [1 5 molec cm -3 ] y = x R 2 =.69 Present GEOS4 Present GISS Preindustrial Warm Cold High fire Low fire Fixed lightning Derived from steady-state equations of the O3-CO-HOx-NOx system following Wang and Jacob [1998], who were able to consider JO3 and q as constant 1 8 J O3 q S N /(S C 3/2 ) [s -1 g/kg {mol a -1 } -1/2 ] OH most correlated with JO3, then q, then SN/(SC 3/2 )

5 Chemistry/climate-driven changes in overhead ozone J / In colder climates Stratospheric circulation decelerates Less stratospheric NOx & ClOx Lower tropopause More tropical overhead ozone (converse in warmer climates) Zonal mean ozone number densities [molec cm -3 ] Present - Preind. O3 Production 4-6% decrease (1-12 DU) Pressure [mb] Warm - Preind. 3-6% increase (1-12 DU) Cold - Preind. 1-13% increase (26-3 DU) Earlier studies used present-day stratospheric boundary conditions

6 Tropospheric H2O changes with surface temperature q / Surface and atmospheric temperature decreases drive large decreases in water vapor at relative to preindustrial; small increases in present day Zonal mean specific humidity [g/kg] Pressure [mb] 23% decrease 5% decrease

7 OH sensitivity to changing emissions S N S C / Simulated mean OH not sensitive to Fires Fuel combustion OH [1 5 molec cm 3 ] fix lightning high fire low fire present Present GEOS4 Present GISS Preindustrial Warm Cold.41 but sensitive to Lightning Tg N a Fire NO x L L S S Tg N a Fuel NO x L L S S Tg N a Soil NO x L L S S Tg N a Lightning NO x L L S S Lightning makes a large percentage of SN, and SN SC; ratio easily perturbed by lightning Tg CO a R 2 = 1. R 2 =.98 R 2 =.92 Fire CO L L S S Tg CO a Fuel CO L L S S Tg C a Fuel, fire, and biogenics all have correlated changes in SN and SC Biogenic VOC L L S S R 2 =.4 Murray et al. [213] Kaplan et al. [26] Valdes et al. [25] Earlier studies did not vary SN or overhead ozone, favoring SC OH at Percent change [%]

8 Implications for methane budget Methane varies by ± 5% What drives glacial-interglacial variability in methane? Temp. CO2 CH4 Wetland emissions? Khalil and Rassmussen [1987] Crutzen and Brühl [1993] Chappellez et al. [1993, 1997] Martinerie et al. [1995] Brook et al. [2] Fischer et al. [28] Singarayer et al. [211] Levine et al. [211a,211b] OH sink? Kaplan [22] Valdes et al. [25] Kaplan et al. [26] Our static/increased methane lifetimes at consistent with a higher role for wetland emissions [Sowers and Bender, 1995] Implied source changes from LCH4 consistent with previous bottom-up emission estimates Murray et al. [213] Weber et al. [21] Kaplan et al. [26] Valdes et al. [25] Decrease in methane emissions (Implied from LCH4) Global methane lifetime against oxidation by tropospheric OH, = ] + ( ) ] Can t treat CH4 distribution constant on glacial timescales; kch4+oh is not enough alone Percent [%] Decreases in integrated methane loss rate from [OH] and T attenuated by increases in tropical fraction of methane burden at We prescribe methane from ice cores; 33% higher fraction in tropics at. Supported by methane emission models. Methane Kaplan et al. [26] Latitude Preind.

9 Chemical feedbacks on glacial cycles? Next step: Put aerosols back into G and re-run to calculate radiative forcing (RF) RF relative to 177s 199s RF relative to 177s Sulfate POA/SOA W m-2 Large decrease in organic aerosol at leads to regional warming, providing a negative feedback; comparable in magnitude to present-day inorganic RF.