Analysis of Transpacific Transport of Black Carbon during HIPPO-3 and Implications for Black Carbon Aging

Long-range transport of black carbon (BC) is a growing concern as a result of the great effect of BC on climate change and air quality. We study transpacific transport of BC during HIPPO-3 based on a combination of inverse modeling and sensitivity analysis. We use the GEOS-Chem Model and its adjoint to estimate the origin of BC over the North Pacific and constrain Asian BC emissions. We find that the sources of BC transported to the North Pacific in March and April, 2010 are different. While biomass burning in Southeast Asia contributes about 60% of BC in March, more than 90% BC comes from fossil fuel and biofuel combustion in East Asia in April. GEOS-Chem model generally resolves the spatial and temporal variation of BC concentrations over the North Pacific, but is unable to reproduce the low end and high end of BC observations. We find that using the optimized BC emissions derived from inverse modeling cannot significantly improve model simulations. This indicates that uncertainties remaining in BC transport account for the major biases in BC simulations. The aging process is one of the key factors controlling the wet scavenging and remote concentrations of BC. We conduct several sensitivity tests on BC aging and find that the aging time scale of anthropogenic BC is several hours, smaller than the values used in most global models, while the aging process of biomass burning BC may occur much slower on a time scale of a few days. To evaluate the effect of BC aging and wet deposition on transpacific transport of BC, we develop an idealized BC transport model. We find that the mid-latitude air mass captured by HIPPO-3 aircraft may experience a series of precipitation events, particularly near the East Asia source region. Therefore, transpacific transport of BC is sensitive to BC aging when the aging rate is fast, and the effect of the aging process peaks when the aging time scale is in the range of 1-1.5 d. This further indicates that BC aging close to the sources is essential and need to be simulated at a process level in order to better constrain the global abundance and climate forcing of BC.