Global budget of ethane and constraints on North American sources from INTEX-A aircraft data Yaping Xiao (xyp@io.harvard.edu)1, Jennifer A. Logan1, Daniel J. Jacob1, Rynda Hudman1, Don Blake2, Angela Baker2

1Atmospheric Chemistry Modeling Group, Harvard University; 2University of California, Irvine, CA

Ethane/CO emission ratios (ER)

aff

1. Introduction

Atmospheric C2H6, with fossil-fuel sources similar to those of CH4 (natural gas/oil

leakage, coal mining), could be an excellent tracer for constraining this portion of the CH4 budget through observed correlations of C2H6-CH4. However, application is

limited by uncertainties in the global C2H6 budget, with source estimates in the

literature ranging from 8 to 36 Tg yr-1. In Xiao et al. [2004], we adjusted C2H6 sources

in a top-down fashion to match observations and used the observed correlations of C2H6-CH4 to improve top-down constraints on the sources of CH4. We conduct here a

more in-depth budget analysis of C2H6 with a global 3-D chemical transport model

(GEOS-Chem). Strong correlations of C2H6-CO are observed in aircraft and column

data due to their common sources (biomass burning and biofuel) or co-location of fossil fuel sources, and these are interpreted here in terms of further constraints on C2H6 sources. The INTEX-A aircraft campaign in summer 2004 collected data for

C2H6 and other gases over North America. Here we apply a model simulation of of

the INTEX-A period to investigate the C2H6 source, with the goal of deriving

constraints for the North American CH4 budget.

Our best estimate of global C2H6 emission is 13.5 Tg/yr, as compared to 8-36 Tg/yr from literature.

The model reproduces the general features of atmospheric C2H6 concentrations as compared with a

comprehensive dataset from surface, aircraft and FTIR measurements. The model also captures the regional variation of C2H6-CO correlations observed in the aircraft and column data .

European source could be overestimated as indicated by the bias in simulating surface C2H6

concentrations and C2H6-CO correlations from column data.

Natural gas use is the principal contributor to C2H6 concentrations worldwide, so that long-term trends

in ethane can be used to track that source.

Comparison of model results to observations from the INTEX-A aircraft mission over North America (July-August 2004) supports our U.S. C2H6 emission inventory except for an underestimate in the

South. The U.S. EPA emission inventory (NEI-99) is a factor of 2-3 too low, and this appears to translate into a similar underestimate of national methane emissions from natural gas.

4. Seasonal variation of C2H6

7. Summary

Major sources of ethane (C2H6) are fossil fuel, biofuel and biomass burning [Rudolph, 1995]. Ethane is removed from the atmosphere mainly by OH, with an average lifetime of 2 months.

6. Model simulation of INTEX-A C2H6 observations

5. C2H6-CO correlations GEOS-Chem: GEOS/GMAO assimilated meteorological fields, 30 levels, 2°×2.5° • Simulation for 2001 (full year) uses archived GEOS-Chem 3-D OH fields with

monthly resolution (CH3CCl3 lifetime = 6.3 yr)

• INTEX-A simulation for summer 2004: used for comparison with INTEX-A aircraft data; fully coupled tropospheric simulation of O3-NOx-VOC chemistry

• Biofuel: • East Asia: Streets et al. [2003] inventory • Elsewhere: scaled to CO with ER of 14×10-3 mol mol-1

[Bertschi et al,2003]

• C2H6/CO ER for fossil fuel is about 8-25 ×10-3 mol mol-1, lowest in Asia due to less controlled CO emissions.• C2H6/CO ER for biofuel (14 ×10-3 mol mol-1) is higher than for biomass burning (5 ×10-3 mol mol-1), reflecting more flaming combustion.

•Anthropogenic source (FF) makes a major contribution to surface C2H6 concentrations in northern Hemisphere (NH). •The impact of anthropogenic sources (BF + FF) are also dominant in the Southern Hemisphere (SH).

1.62.4 2.2

0.6

1.6 0 0

0.5

0.8

0 0

1

East Asia Europe North America SH

Biomass burning

Biofuel

Fossil fuel

• fossil fuel and biofuel source contribute 70% and 20% of total C2H6 emissions in northern Hemisphere. • biomass burning composes 50% of total emission in southern Hemisphere

• Observed C2H6/CO slopes are 11-18 ppt/ppb in NH non-biomass burning season, and 5-8 in SH or biomass burning season. (TRACE-P:

9-14 ppt/ppb in Asian outflow in March) • The model captures regional variation of C2H6-CO correlations, and has no distinct bias in estimating the C2H6/CO slopes from aircraft data.

• The C2H6-CO correlations in NH in the model are defined mainly by anthropogenic sources, while in SH they are influenced by both anthropogenic and biomass burning sources.

Ethane-CO correlation in column data

Ethane-CO correlations in aircraft data

North American emissions of ethane (monthly mean in July-August)

8. References• Duncan, B.N., R.V. Martin, A.C. Staudt, R. Yevich, J.A. Logan, Interannual and Seasonal Variability of Biomass Burning Emissions Constrained by Satellite Observations, J. Geophys. Res., 108(D2), 4040, doi:10.1029/2002JD002378, 2003. • Hudman, R.C., et al. (2005), A multi-platform analysis of the North American reactive nitrogen budget during the ICARTT summer intensive, J. Geophys. Res., in preparation.• Rudolph, J., The tropospheric distribution and budget of ethane, J. Geophys. Res., 100, 11,369– 11,381, 1995.• Streets, D.G., T. C. Bond, et al., An inventory of gaseous and primary aerosol emissions in Asia in the year 2000, J. Geophys. Res., 108 (D21), 8809, doi:10.1029/2002JD003093, 2003. • Turquety S., D.J. Jacob, et al. (2005), Improved representation of boreal fire emissions for the ICARTT period, J. Geophys. Res., in preparation • Wang, J.S., J.A. Logan, M.B. McElroy, B.N. Duncan, I.A. Megretskaia, and R.M. Yantosca, A 3-D model analysis of the slowdown and interannual variability in the methane growth rate from 1988 to 1997, Global Biogeochem. Cycles, 18, GB3011, doi:10.1029/2003GB002180, 2004 • Xiao Y.P.,  D. J. Jacob, J. S. Wang, J. A. Logan, P. I. Palmer, P. Suntharalingam, R. M. Yantosca, G. W. Sachse, D. R. Blake, D. G. Streets, Constraints on Asian and European sources of methane from CH4-C2H6-CO correlations in Asian outflow, J. Geophys. Res., 109, D15S16, doi:10.1029/2003JD004475, 2004.

Acknowledgment. This work was funded by NSF Atmospheric Chemistry Program.

<2km

2-6km

>6km

*Southwestern US [25N-40N, 105W-88W]: C2H6 emission annual total of 0.48 in GEOS-Chem is consistent with 0.3-0.5 Tg estimated by Katzenstein et al. [2003] based on surface C2H6 observations

East Asia EU NA SH

FF BF BB FF FF FF BF BB

C2H6, Tg 1.6 1.6 0.8 2.4 2.2 0.6 0.5 1.0

CO, Tg 198 109 145 89 119 35 34 203

C2H6/CO,10-3 mol mol-1 7.7 14 5 25 17 16 14 5

2. Atmospheric ethane measurements

3. GEOS-Chem and emissions

* surface sites+ column stations□ aircraft regions.

3. Continued…

NEI-99 C2H6 emission inventory from U.S. EPA is low by a factor of 3 compared to GEOS-Chem and is inconsistent with INTEX-A observations.

The primary objective of INTEX-A was to characterize chemical outflow from North America and better understand the sources

INTEX-A (July 1 – Aug. 15, 2004)

Midwest OffshoreSouthNortheast

Low-altitude measurements over the Atlantic Ocean [Penkett et al., 1993].

Biomass burning source of C2H6 is minor compared to fossil fuel source.

Significant model bias in Europe: fossil fuel source is too high?

• Overestimated slope in winter at Spitsbergen and Mauna Loa where European fossil fuel source contributes 40%-50%. C2H6 European source might be too high. • Underestimated slope at Lauder: the simulated slope is more parallel to ER from biomass burning, but this source contributes only 30% of the C2H6 column at Lauder in September.

black: datared: model

Biofuel

atmosphere

Natural gas/oil leakage

Biomass burning

black: datared: model

CO

C2 H

6

C2H6

Emissions• Fossil fuel:

• East Asia: Streets et al. [2003] inventory

• Elsewhere: scaled to those of CH4 [Wang et al.,2004],

with C2H6 emissions adjusted to observations in a top-

down fashion. The CH4/C2H6 emission ratios (ER) vary

over the range 8-25 mol mol-1 *U.S. emissions used for INTEX-A simulation: see below.

• Biomass burning: • For 2001 simulation: scaled to climatological inventory of CO [Duncan et al., 2003] with ER

of 5 ×10-3 mol mol-1

• For INTEX-A simulation: daily inventory with major fires in Alaska and Canada in summer 2004, and with injection of emissions into free troposphere [Turquety et al., in preparation]

as compared to 9.6 Tg estimated by EIA (Emissions of Greenhouse Gases in the United States, 2003).

as shown by the ground samples near natural gas/oil source region in southwest US [data from Angela Baker]

Estimated CH4 emission from natural gas/oil source in U.S.

C2H6 emission in U.S.

1.8 TgCH4/C2H6 ER from fossil fuel

19 mol mol-1CH4 fossil fuel emission in U.S.

18 TgX =

Observations GEOS-Chem

black: datared: model

The model shows systematic underestimate of C2H6 concentrations in upper troposphere

GEOS-Chem fossil fuel: 0.14 Tg NEI-99 fossil fuel: 0.05 Tg

Climatological biomass burning: 0.016 Tg 2004 biomass burning: 0.023 Tg

Tagged simulation in year 2001 shows 50% of C2H6 concentrations in upper troposphere is contributed by North American source. convection is too weak in the model?

The model underestimates ethane concentrations by about 500 ppt in the boundary layer of south region, indicating an underestimate of C2H6 source in this region