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Abstract
Black carbon (BC) aerosol strongly absorbs solar radiation, which warms climate. However, accurate estimation of BC’s climate effect is limited by the uncertainties of its spatiotemporal distribution, especially over remote oceanic areas. The HIAPER Pole-to-Pole Observation (HIPPO) program from 2009 to 2011 intercepted multiple snapshots of BC profiles over Pacific in various seasons, and revealed a 2 to 5 times overestimate of BC by current global models. In this study, we compared the measurements from aircraft campaigns and satellites, and found a robust association between BC concentrations and satellite-retrieved CO, tropospheric NO2, and aerosol optical depth (AOD) (R2 > 0.8). This establishes a basis to construct a satellite-based column BC approximation (sBC*) over remote oceans. The inferred sBC* shows that Asian outflows in spring bring much more BC aerosols to the mid-Pacific than those occurring in other seasons. In addition, inter-annual variability of sBC* is seen over the Northern Pacific, with abundances varying consistently with the springtime Pacific/North American (PNA) index. Our sBC* dataset infers a widespread overestimation of BC loadings and BC Direct Radiative Forcing by current models over North Pacific, which further suggests that large uncertainties exist on aerosol-climate interactions over other remote oceanic areas beyond Pacific.
Highlights
Black carbon is a minute carbonaceous particle, originating from incomplete combustion of fossil fuels and biofuels and biomass burning
The empirical relationship we derived for sBC* follows: sBC⁎ = COα ⋅ NO2β ⋅ exp(γAOD + φ) where sBC* is the predicted satellite-based column Black carbon (BC) approximation; Aerosol Optical Depth (AOD) is the average aerosol optical depth from Moderate Resolution Imaging Spectroradiometer (MODIS) on Aqua and Terra[16]; CO and NO2 are satellite retrievals from AIRS25 and OMI26 respectively
Both quantitative and qualitative analysis strongly supports the assumption that BC aerosols and other air pollutants coexist in remote air, especially in plumes with high BC loadings
Summary
Black carbon is a minute carbonaceous particle, originating from incomplete combustion of fossil fuels and biofuels and biomass burning It strongly absorbs sunlight and imposes considerable positive radiative forcing on global and regional climate through multiple ways[1]. Total climate forcing of BC is estimated to be 1.1 Wm−2 and it is the second largest climate forcer after carbon dioxide[2], while other studies suggest the radiative forcing of BC might be lower[3,4,5,6] This difference is partially caused by the uncertainties in our understanding of BC emissions, atmospheric aging and wet removal processes[2,7,8,9], resulting in a large bias in BC simulation, over remote areas. Such a relationship, if exists, could be employed to infer BC mass from other air pollutants in places and periods of time without available BC measurements
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