Measurem ents of light alkanes ( C 2 -C 4 ) in firn air at Sum m it, Greenland ( 2 0 0 6 ) and W est Antarctic I ce Sheet Divide, Antarctica ( 2 0 0 5 ) : I s there evidence for a recent decline in polar tropospheric levels? Murat Aydin 1 , Eric Saltzman 1 , Margaret B. Williams 2 , Eric Sofen 3 , and Mark Battle 4 1 UC Irvine, Department of Earth System Science 2 Bay Area Environmental Research Institute/NASA Ames 3 University of Washington, Department of Atmospheric Sciences 4 Bowdoin College, Department of Physics and Astronomy Acknowledgements: S. Montzka and J. Butler (NOAA/ESRL-GMD) – UCI flask data Blake-Rowland Lab. (UCI) – surface flask data E. Atlas (U. of Miami) – firn data Todd Sowers (Penn State) – field NSF/OPP – support ICDS – drilling support
NMHC’s – light alkanes (C 2 H 6 , C 3 H 8 , n-C 4 H 10 ) Precursors of O 3 and CO and a removal mechanism for OH Summer OH lifetimes: τ ethane =1-2 months, τ propane =8-20 days, τ butane =2-10 days Anthropogenic sources Natural sources natural gas and oil leaks biomass burning Automotive, biomass burning oceanic, vegetative, soil emissions (smaller) Firn air records and modeling • Smoothed (low-pass filter) records • Site characteristics impact smoothing • Exact age calculation impossible • All ages (CO 2 , CFC-12, mean) represent an integrated average • Summit and WAIS-D firn modeling with diffusivity tuning using CO 2 and CFC-12.
Summit surface measurements 1997-2007 and seasonality in Summit firn (Blake-Rowland and UCI) n -butane (ppt) 450 300 150 0 1200 propane (ppt) 800 400 0 3000 ethane (ppt) 2000 1000 0 1/1/1997 1/1/1999 1/1/2001 1/1/2003 1/1/2005 1/1/2007 Date � No clear long-term trend � Mean levels • ethane: 1250-1500 ppt • propane: 400-600 ppt • n -butane: 150-250 ppt � Annual mean signal observed in firn below 40 m
Summit UCI firn data (ethane, propane, n -butane) 2200 ~10% damping of peak 2000 propane - butane (ppt) 0 0 100 100 200 200 300 300 400 400 500 500 600 600 1800 ethane (ppt) 0 0 0 1600 1400 20 20 20 Seasonality Summit ethane data 1200 depth in firn (m) Atmospheric history 1000 40 40 40 1900 1920 1940 1960 1980 2000 date Incorporation of 60 60 60 66 m atmospheric signal into firn 76 m 80 80 80 0 1200 1350 1500 1650 1800 1950 2100 20 ethane (ppt) depth in firn (m) 40 Change in alkanes 1970-1990 C 2 H 6 C 3 H 8 n -C 4 H 10 60 Rate 29 ppt/y 9 ppt/y 4 ppt/y 80 Overall 27% 27% 28% 1200 1350 1500 1650 1800 1950 2100 ethane (ppt)
Summit (UCI) and NGRIP (E. Atlas), Greenland data (depths > 40 m) 2100 700 1950 propane (ppt) 1800 600 ethane (ppt) 1650 500 1500 UCI - Summit UCI - Summit 400 E. Atlas - NGRIP E. Atlas - NGRIP 1350 1950 1960 1970 1980 1990 2000 1950 1960 1970 1980 1990 2000 date from mean age date from mean age 280 Similar trends at both sites: 240 1970s peak, decrease during 1980s and on n -butane (ppt) 200 Apparent disagreement C 2 H 6 : bottom of the firn 160 C 3 H 8 : shallow firn UCI - Summit n -C 4 H 10 : shallow firn E. Atlas - NGRIP 120 1950 1960 1970 1980 1990 2000 date from mean age
SH surface measurements 2000-2007 and seasonality in WAIS-D firn (Blake- Rowland and UCI) 400 climatology 20 fit to climatology 350 n -butane (ppt) SPO - Blake&Rowland 15 South Pole 300 10 Cape Grim ethane (ppt) Palmer 250 5 0 200 60 150 propane (ppt) 40 100 20 50 0 2000.0 2000.2 2000.4 2000.6 2000.8 2001.0 400 time of year ethane (ppt) 300 200 100 0 0 1/1/2000 1/1/2002 1/1/2004 1/1/2006 1/1/2008 Date WAIS-D firn depth (m) 20 � No spatial variability, no clear long-term trend 40 � Mean levels 60 • C 2 H 6 : ~200 ppt • C 3 H 8 : ~25 ppt 80 • n -C 4 H 10 : ~8 ppt 140 160 180 200 220 � Annual mean below 40 m at WAIS-D (similar to Summit) ethane (ppt)
WAIS-D UCI firn data (C 2 H 6 , C 3 H 8 , n -C 4 H 10 ) 280 propane and butane (ppt) 240 0 10 20 30 40 50 60 70 ethane (ppt) 200 0 0 0 10 10 10 160 20 20 20 120 WAIS-D firn depth (m) Seasonality 30 30 30 80 1900 1920 1940 1960 1980 2000 40 40 40 date Incorporation of 50 50 50 atmospheric 60 60 60 signal into firn 70 70 70 80 80 80 100 120 140 160 180 200 220 240 260 280 ethane (ppt) WAIS-D ethane • stabilization in 1980s • drop in 1990s (~30%) WAIS-D propane and n -butane • More noise • May be a similar drop in propane
WAIS-D (UCI) and South Pole (E. Atlas) data (depths > 40 m) 320 50 45 280 40 35 240 propane (ppt) ethane (ppt) 30 200 25 20 160 15 E. Atlas - SPO E. Atlas, SPO UCI - WAIS-D UCI, WAIS-D 10 120 Kaspers et al. (2004), DML Kaspers et al. (2004), DML 5 1940 1950 1960 1970 1980 1990 2000 1940 1950 1960 1970 1980 1990 2000 CO 2 - CFC-12 age CO 2 - CFC-12 age • WAIS-D and SPO agree, trends from DML don’t (more/better data from Antarctica • SPO and WAIS-D data together suggest a ~30% decline in ethane � Roughly synchronous with changes at Summit, Greenland • Fidelity of firn record still a question
WHY are NMHC’s decreasing at polar latitudes? (possibly globally!) Sources: Fossil fuel consumption Sink: OH, Cl Sources: Hydrocarbon fuel use efficiency must have improved dramatically? World Dry Natural Gas Consumption • Consumption/production going 90,000 up 80,000 70,000 • Short lifetimes imply source billion cubic feet 60,000 Asia&Oceania loading must be continually Africa dropping 50,000 Mideast Europe 40,000 Cnt&Sth America N. America • Both hemispheres (may be) 30,000 20,000 10,000 0 1980 1982 1984 1986 1988 1990 1992 1994 1996 1998 2000 2002 2004 P2006 Source: Energy Information Administration, Office of Energy Markets and End Use
Can we trust the fidelity of the firn record? Surface air analysis: • Ongoing at UCI with NOAA flasks Firn air data – firn modeling: • back to Summit in 2008 • new SPO firn sampling in 08-09 3000 Rowland-Blake UCI-NOAA Firn 2006 2500 Summit ethane (ppt) 2000 1500 1000 500 1/6/1950 1/6/1960 1/6/1970 1/6/1980 1/6/1990 1/6/2000 1/6/2010 Date
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