Export efficiency of black carbon aerosol in continental outflow: Global implications

Rokjin J Park,Tami C Bond,Fred L Eisele,Daniel J Jacob,Alan R Bandy,Antony D Clarke,Glen W Sachse,Rodney J Weber,Donald C Thornton,Paul I Palmer,Mark A Zondlo

doi:10.1029/2004jd005432

Abstract

We use aircraft observations of Asian outflow from the NASA Transport and Chemical Evolution over the Pacific (TRACE‐P) mission over the NW Pacific in March–April 2001 to estimate the export efficiency of black carbon (BC) aerosol during lifting to the free troposphere, as limited by scavenging from the wet processes (warm conveyor belts and convection) associated with this lifting. Our estimate is based on the enhancement ratio of BC relative to CO in Asian outflow observed at different altitudes and is normalized to the enhancement ratio observed in boundary layer outflow (0–1 km). We similarly estimate export efficiencies of sulfur oxides (SOx = SO2(g) + fine SO42−) and total inorganic nitrate (HNO3T = HNO3(g) + fine NO3−) for comparison to BC. Normalized export efficiencies for BC are 0.63–0.74 at 2–4 km altitude and 0.27–0.38 at 4–6 km. Values at 2–4 km altitude are higher than for SOx (0.48–0.66) and HNO3T (0.29–0.62), implying that BC is scavenged in wet updrafts but not as efficiently as sulfate or nitrate. Simulation of the TRACE‐P period with a global three‐dimensional model (GEOS‐CHEM) indicates that a model timescale of 1 ± 1 days for conversion of fresh hydrophobic to hydrophilic BC provides a successful fit to the export efficiencies observed in TRACE‐P. The resulting mean atmospheric lifetime of BC is 5.8 ± 1.8 days, the global burden is 0.11 ± 0.03 Tg C, and the decrease in Arctic snow albedo due to BC deposition is 3.1 ± 2.5%.

Highlights

[2] Black carbon (BC), operationally defined as the lightabsorbing fraction of carbonaceous aerosols, has complex climatic implications involving atmospheric heating and surface cooling [National Research Council, 2005]
[3] We present here an assessment of the export efficiency of BC in Asian outflow using aircraft observations over the NW Pacific from the NASA Transport and Chemical Evolution over the Pacific (TRACE-P) mission in PARK ET AL.: EXPORT EFFICIENCY OF BLACK CARBON
A mass absorption efficiency of 7 m2 gÀ1 [Clarke et al, 2004] was assumed to convert to mass concentration. This conversion factor was derived from collocated measurements of particle/soot absorption photometer (PSAP) absorption and black carbon mass concentrations from the Asian Pacific Regional Aerosol Characterization Experiment (ACE-Asia) aircraft mission conducted over the NW Pacific concurrently with TRACE-P [Huebert et al, 2003]

Summary

Introduction

[2] Black carbon (BC), operationally defined as the lightabsorbing fraction of carbonaceous aerosols, has complex climatic implications involving atmospheric heating and surface cooling [National Research Council, 2005]. By reference to CO as an inert combustion tracer, Koike et al [2003] computed the altitude-dependent export efficiencies in east Asian outflow as the ratios of the species enhancements above their background to the corresponding CO enhancements, normalized by the emission ratio for the source region.

Results

Conclusion