The parial lifeime of carbon tetrachloride with respect to

The par(al life(me of carbon tetrachloride with respect to the soil sink Robert C. Rhew ([email protected]) Department of Geography/ Berkeley Atmospheric Sciences Center, University of California, Berkeley, Berkeley, CA 94720-­‐4740 Abstract In the atmospheric budget of carbon tetrachloride, the magnitude of the soil sink remains highly uncertain, with CCl4 par(al life(mes with respect to soil ranging from ~100 to 907 years. This uncertainty arises from the limited coverage of field observa(ons combined with the methods of extrapola(ng results to the global scale. Here, we add to the published CCl4 fluxes with addi(onal field measurements, and we develop a land cover classifica(on scheme to align more closely with the measurement sites to re-­‐evaluate the global carbon tetrachloride soil sink. Upda(ng fluxes from previously unmeasured biomes combined with a biome surface area based on AVHRR measurements, we suggest an updated par(al life(me of CCl4 with respect to the soil sink to be 378 (328-­‐445) years. Including longer ocean and stratospheric par(al life(mes, as presented in this workshop, the CCl4 tropospheric life(me may be increased from the 26 years reported in the 2014 WMO report to 31 (26-­‐35) years. I. Introduc(on The tropospheric life(me (τatm) of CCl4 may be calculated from its par(al life(mes with respect to individual loss rates: photolysis in the stratosphere (τstrat), degrada(on in surface ocean waters (τoce), and degrada(on in terrestrial soils (τsoil) [WMO, 2014]. (τatm) -­‐1 = (τstrat) -­‐1 + (τoce) -­‐1 + (τsoil)-­‐1 In the WMO 2014 report, the es(mated par(al life(mes were: τstrat = 44 years, τoce= 94 (82-­‐191) years, and τsoil = 195 (108-­‐907) years; together these yield an overall tropospheric life(me of 26 years. Recently, Happell et al. (2014) provided a revised soil par(al life(me es(mate of 245 years, based on a much larger set of field measurements. This revision would increase the total atmospheric life(me of CCl4 from 26 to 27 years, assuming the par(al life(mes of stratospheric and oceanic loss remained the same. A large soil sink exacerbates the imbalance in the present CCl4 budget, which suggests a large missing CCl4 source to the atmosphere, an underes(mated life(me, and/or overes(mated sinks. This study seeks to refine the soil sink es(mate by a) providing addi(onal field flux measurements; and b) by examining the biome surface areas used for the global extrapola(ons. III. Additonal flux measurements Tundra
R_2008b: CHCl3 fluxes were reported from Barrow
and Toolik Lake, Alaska, from 2005-2006 [Rhew et
al. (2008b)] . An average CCl4 tundra flux of -1.2 ±
2.2 nmol m-2 d-1 was also reported (Barrow = -0.9 ±
2, and Toolik Lake -2.5 ± 2.5). The mean with
averaged flux errors is -1.2 ± 0.2. Here negative
fluxes indicate uptake. These flux values are
roughly in agreement with those presented by
HMG_2014 but are lower than HR_2003 (both of
which used boreal forest measurements).
Boreal forest
RAS_2003: Rhew et al. (2003) reported 2 boreal
forest flux measurements for CH3Br and CH3Cl at
Bonanza Creek Experimental Forest, Fairbanks AK,
using an aluminum chamber. CCl4 flux
measurements were measured but not previously
reported: 2.8 ± 1.1 nmol m-2 d-1 (positive indicating
emissions).
Oak-Savanna Woodland
R_2010: Rhew et al. (2010) reported CH3Br and
CH3Cl gross and net fluxes using a stable isotope
tracer method, involving the addition of 13C labeled
tracer gases. Three chambers did not have tracers
injected, and these chambers showed an average
flux of -1.6 ± 1.2 nmol m-2 d-1 (ave ± sd) or -1.6 ± 0.9
(ave ± error).
Table 1. Summary of published and new flux
measurements
Best es(mate fluxes are highlighted in bold. V. Soil sink par(al life(me IV. Biome classifica(ons Deriving a biome surface area dataset to
coincide with field measurement data
We utilize data from the Advanced Very High Resolution Radiometer
(AVHRR) Interrupted Goode Homolosine Projection, which provides an
equal-area map projection from which it is straightforward to derive
surface areas. We divide and quantify pixels for each of the 24
prescribed land cover types into 4 latitudinally defined climatic zones:
Tropical (25 °S to 25 °N), Subtropical (35°S to 25 °S and to 25 °N to
35°N), Temperate (55°S to 35 °S and 35°N to 55°N), and Boreal/Arctic
(90°S to 55°S and 55°N to 90°N). We then reclassify each latitudinally
defined ecosystems into the following 13 biomes: (1) Urban; (2)
Agriculture; (3) Grassland; (4) Shrubland; (5) Savanna; (6) Tropical/
Subtropical Forest; (7) Temperate Forest; (8) Boreal Forest; (9) Wetland;
(10) Desert; (11) Tundra; (12) Snow/Ice; and (13) Water.
II. Background literature Table 2: Global uptake rates of CCl4 using
different biome classification schemes
Biomes based on AVHRR classifications (this
study)
a. Biomes combined the following sub-­‐biome categories from Maihews (1983): Agriculture (W); Grassland (N, O, P, Q, R, S, T); Shrubland (H,I,J,K,L); Savanna (C,D,E,F,G); Tropical/
Subtropical Forest (1,2,3,7,9); Temperate Forest (4,5,6,8); Boreal Forest (A,B,G); Desert (U); Tundra (M); and Snow-­‐Ice (V). b. Biomes combined the following sub-­‐biome categories from Poier et al (1996): Agriculture (7); Grassland (5); Shrubland (9,11); Savanna (12); Tropical/Subtropical Forest (13,14); Temperate Forest (6,10); Boreal Forest (3); Desert (4,8); and Tundra (1,2). c. For consistency, representa(ve biome fluxes are assumed to be first order fluxes that scale linearly with ambient concentra(ons, which is set to 90 ppt. Because the shrubland and desert ecosystems are es(mated to have below measurement capacity fluxes (<0.5 nmol m-­‐2 d-­‐1), the fluxes are es(mated at 0.25 ± 0.25 nmol m-­‐2 d-­‐1. d. Ac(ve period modified slightly from Happell et al., 2014, to incorporate an es(mated 10% error Table 3. Revised lifetimes
DATA SOURCE: Advanced Very High Resolution Radiometer (AVHRR) Global Land Cover Characteristics (GLCC) data set with a nominal spatial resolution of 1 km that is
stored in the Interrupted Goode Homolosine projection (gusgs2_0g.img database accessed from http://edc2.usgs.gov/glcc/globdoc2_0.php in September 2015). The data set is
derived from a 12-month period (April 1992-March 1993) of observations. The USGS Land Use/Land Cover (Modified Level 2) classification scheme has been modified as
described in the text to align the 24 classifications to the 13 land cover (and water body) classifications presented here. Surface area calculations derived from reading every
2nd point. Above plot generated by reading every 10th point.
Biomes based on Matthews (1983) classifications
Vegetation land cover based on Matthews (1983)
cultivation
75N
rainforest
60N
45N
forests
30N
boreal
15N
woodland
0
shrubland
15S
tundra
30S
grassland
45S
desert
60S
ice
75S
HR_2003: Happell and Roche (2003) derived fluxes from several ecosystems using a soil gas concentra(on gradient method: F = De (dC/dz). L_2007: Liu (2007) used a similar diffusion model with dC/
dz down to 150 cm, finding ~half the forest flux of HR_2003. RMW_2008: Rhew et al. (2008) used sta(c flux chambers in southern California shrublands over several seasons, with fluxes smaller than HR_2003. HMG_2014: Happell et al. (2014) used both sta(c chambers and soil gradients in mul(ple ecosystems to adjust the soil sink to ~1/3 of the HR_2003 fluxes. Biome uptake rates use the ‘best estimate’ CCl4 uptake rates from Table
1 combined with the surface areas from three different studies: this
study, Matthews (1983) and Potter et al. (1996). Resulting global uptake
rates are similar, ranging from 6.1 (this study) to 6.8 Gg yr-1 (Potter et al.
study). (See Table 2)
water
120W
60W
0
60E
120E
DATA SOURCE: Matthews (1983) Global vegetation and land use: new high-resolution data bases for climate studies, J. Climate and Applied Meteorology. The data set
provides a 1 ° latitude by 1° longitude resolution, which translates to pixel sizes of 2100 km2 near the poles and 12000 km2 at the equator. The 32 land cover classifications in
Matthews have been modified to align with the 13 land cover (and water body) classifications presented here. Surface areas are calculated using the areas provided by
Matthews (1983).
Comparison with Potter (1996) and FAO (2011)
The Potter et al. (1996) (P_1996) database utilized by HMG_2014 had a
40-50% larger surface area for tropical/subtropical forest and boreal
forest, and twice the size of tundra, as Matthews (1983) and this study.
P_1996 may overestimate the surface area of forests (56.5 x 1012 m2); a
FAO 2011 study indicates a total forest area of 40.3 x 1012 m2,
comparable to this study (39.6 x 1012 m2) and Matthews (41.8 x 1012 m2).
The partial atmospheric lifetime of CCl4 with respect to soil is calculated using a tropospheric mass of 1.46 x 1020 moles, tropospheric fraction of 0.886, and tropospheric
concentration of 90 ppt (Yvon Lewis and Butler, 2002; Happell et al., 2014).
Using updated fluxes and biome surface areas, the partial lifetime of
CCl4 with respect to the soil sink is increased to 380 (330-450) years
from 245 years. The smaller global soil sink is due to the addition of the
savanna and tundra biome flux measurements, the further subdivision of
representative biomes, and the updated biome surface areas.
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Acknowledgments: Thanks to the many field and lab assistants who helped conduct the flux measurements; W. Reeburgh for the early flux chamber work; R.F.
Weiss, B. Miller and C. Harth for calibration standards and southern California measurements; and J. Chiang for assistance with AVHRR data and plots.