Abstract

AbstractWe use a global chemical transport model (GEOS‐Chem) to interpret aircraft curtain observations of black carbon (BC) aerosol over the Pacific from 85°N to 67°S during the 2009–2011 HIAPER (High‐Performance Instrumented Airborne Platform for Environmental Research) Pole‐to‐Pole Observations (HIPPO) campaigns. Observed concentrations are very low, implying much more efficient scavenging than is usually implemented in models. Our simulation with a global source of 6.5 Tg a−1 and mean tropospheric lifetime of 4.2 days (versus 6.8 ± 1.8 days for the Aerosol Comparisons between Observations and Models (AeroCom) models) successfully simulates BC concentrations in source regions and continental outflow and captures the principal features of the HIPPO data but is still higher by a factor of 2 (1.48 for column loads) over the Pacific. It underestimates BC absorbing aerosol optical depths (AAODs) from the Aerosol Robotic Network by 32% on a global basis. Only 8.7% of global BC loading in GEOS‐Chem is above 5 km, versus 21 ± 11% for the AeroCom models, with important implications for radiative forcing estimates. Our simulation yields a global BC burden of 77 Gg, a global mean BC AAOD of 0.0017, and a top‐of‐atmosphere direct radiative forcing (TOA DRF) of 0.19 W m−2, with a range of 0.17–0.31 W m−2 based on uncertainties in the BC atmospheric distribution. Our TOA DRF is lower than previous estimates (0.27 ± 0.06 W m−2 in AeroCom, 0.65–0.9 W m−2 in more recent studies). We argue that these previous estimates are biased high because of excessive BC concentrations over the oceans and in the free troposphere.

Highlights

  • [2] Black carbon (BC) is of climatic interest as a strong absorber of solar radiation both in the atmosphere [Jacobson, 2001; Koch, 2001; Quinn et al, 2008] and after deposition to snow [Warren and Wiscombe, 1985; Flanner et al, 2007; McConnell et al, 2007]

  • This can be compared to a present-day radiative forcing from CO2 of 1.82 W mÀ2 in recent Intergovernmental Panel on Climate Change (IPCC) report in 2013 (Working Group I Contribution to the IPCC Fifth Assessment Report Climate Change 2013: The physical science basis: Summary for policymakers, http://www.climatechange2013.org/images/uploads/WGIAR5-SPM_Approved27Sep2013.pdf)

  • Observations for individual flights are averaged over the 3-D GEOS-Chem grid, and corresponding model results are sampled along the flight tracks at the same time and location

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Summary

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Our simulation with a global source of 6.5 Tg aÀ1 and mean tropospheric lifetime of 4.2 days (versus 6.8 ± 1.8 days for the Aerosol Comparisons between Observations and Models (AeroCom) models) successfully simulates BC concentrations in source regions and continental outflow and captures the principal features of the HIPPO data but is still higher by a factor of 2 (1.48 for column loads) over the Pacific. It underestimates BC absorbing aerosol optical depths (AAODs) from the Aerosol Robotic Network by 32% on a global basis.

Introduction
Rest of the world
Observed Model
GV flight tracks
Fuel combustion
Load Load above MAC
Conclusions

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