Global budget and radiative forcing of black carbon aerosol: Constraints from pole‐to‐pole (HIPPO) observations across the Pacific

Abstract

[1] We 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 À1 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 À2 , with a range of 0.17-0.31 W m À2 based on uncertainties in the BC atmospheric distribution. Our TOA DRF is lower than previous estimates (0.27 ± 0.06 W m À2 in AeroCom, 0.65-0.9 W m À2 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.

Modelled black carbon radiative forcing and atmospheric lifetime in AeroCom Phase II constrained by aircraft observations

“…Figure 2 shows that some models more closely reproduce the measurements than others, both in magnitude and shape. Several studies have suggested that to reproduce HIPPO data, a low modelled atmospheric lifetime, or a short ageing timescale, of BC is required Wang et al, 2014a). Here we can test this supposition for a larger set of models.…”

“…Meteorology and treatment of BC aging and wet scavenging also vary, and will impact the vertical profiles (Kipling et al, 2013;Bauer et al, 2013). Further model experiments, in line with the single-model study in Wang et al (2014a), are required to address the reasons behind the relationship found in Fig. 3. …”

“…For example, Wang et al (2014a) compared results from the GEOS-Chem model (http: //geos-chem.org) with results from the HIPPO flight campaign, and concluded that to reproduce HIPPO, more wet removal was required than is represented in most present models. Based on this, and on revised estimates on direct RF of BC when taking HIPPO constraints into account, they argue that previous model estimates may be biased high due to elevated BC concentrations in the free troposphere.…”

Standard climate models radiation codes underestimate black carbon radiative forcing

“…However, the diversity in existing estimates of the climate effect of BC is large (Bond et al, 2013;Boucher et al, 2013;Myhre et al, 2013). The causes for the diversity in estimates are many, from emissions (Amann et al, 2013;Cohen and Wang, 2014;Lam et al, 2012;Stohl et al, 2013;Wang et al, 2014b), lifetime and abundance (Hodnebrog et al, 2014;Samset et al, 2014;Wang et al, 2014a) to radiative efficiency Zarzycki and Bond, 2010).…”

Interpreting aerosol lifetimes using the GEOS-Chem model and constraints from radionuclide measurements

“…The global, annual mean lifetimes of the carbonaceous aerosols and dust are about 6 days, which is close to the mean lifetime of 137 Cs when injected uniformly into the surface layer (simulation USFC). A shorter black carbon (BC) lifetime (4.2 days) in the recent GEOS-Chem simulation of Wang et al (2014) is attributed to their modified impaction scavenging of hydrophobic BC in convective updrafts and scavenging of hydrophilic BC in ice clouds. The sulfate-nitrate-ammonium system has a shorter lifetime, 2.6 days.…”