Methane emissions from Alaska in 2012 from CARVE airborne observations

Abstract

We determined methane (CH4) emissions from Alaska using airborne measurements from the Carbon Arctic Reservoirs Vulnerability Experiment (CARVE). Atmospheric sampling was conducted between May and September 2012 and analyzed using a customized version of the polar weather research and forecast model linked to a Lagrangian particle dispersion model (stochastic time-inverted Lagrangian transport model). We estimated growing season CH4 fluxes of 8 ± 2 mg CH4⋅m(-2)⋅d(-1) averaged over all of Alaska, corresponding to fluxes from wetlands of 56(-13)(+22) mg CH4⋅m(-2)⋅d(-1) if we assumed that wetlands are the only source from the land surface (all uncertainties are 95% confidence intervals from a bootstrapping analysis). Fluxes roughly doubled from May to July, then decreased gradually in August and September. Integrated emissions totaled 2.1 ± 0.5 Tg CH4 for Alaska from May to September 2012, close to the average (2.3; a range of 0.7 to 6 Tg CH4) predicted by various land surface models and inversion analyses for the growing season. Methane emissions from boreal Alaska were larger than from the North Slope; the monthly regional flux estimates showed no evidence of enhanced emissions during early spring or late fall, although these bursts may be more localized in time and space than can be detected by our analysis. These results provide an important baseline to which future studies can be compared.

Keywords: Alaska; Arctic; boreal; methane; tundra.

Fig. 1.

Location of flight tracks (gray) and vertical profiles (black) during CARVE 2012. Background colors are elevation categories based on US EPA level III ecoregions (31).

Fig. 2.

Sample CH₄ vertical profile (A) used for column analysis and corresponding O₃ profile (B) from September 22, 2012. The dashed purple line represents the identified top of the residual layer, and the hatched areas are used to determine the column enhancement.

Fig. 3.

Estimated mean CH₄ fluxes from the column analysis for (A) the entire study period (May to September 2012) and (B) by month. Error bars are the 95th percentile determined from the bootstrapping analysis described in the text.

Fig. 4.

Covariance of O₃ and CH₄ below 1,500 m agl for June. See Fig. S3 for other months.

Fig. 5.

Estimated CH₄ fluxes from the O₃:CH₄ analysis assuming a constant and seasonally varying O₃ $v_{\text{D}}$, red and black points, respectively. Error bars reflect the uncertainty in the O₃ $v_{\text{D}}$ (∼33%).