Long-range transport of black carbon to the Pacific Ocean and its dependence on aging timescale

Abstract

Abstract. Improving the ability of global models to predict concentrations of black carbon (BC) over the Pacific Ocean is essential to evaluate the impact of BC on marine climate. In this study, we tag BC tracers from 13 source regions around the globe in a global chemical transport model, Model for Ozone and Related Chemical Tracers, version 4 (MOZART-4). Numerous sensitivity simulations are carried out varying the aging timescale of BC emitted from each source region. The aging timescale for each source region is optimized by minimizing errors in vertical profiles of BC mass mixing ratios between simulations and HIAPER Pole-to-Pole Observations (HIPPO). For most HIPPO deployments, in the Northern Hemisphere, optimized aging timescales are less than half a day for BC emitted from tropical and midlatitude source regions and about 1 week for BC emitted from high-latitude regions in all seasons except summer. We find that East Asian emissions contribute most to the BC loading over the North Pacific, while South American, African and Australian emissions dominate BC loadings over the South Pacific. Dominant source regions contributing to BC loadings in other parts of the globe are also assessed. The lifetime of BC originating from East Asia (i.e., the world's largest BC emitter) is found to be only 2.2 days, much shorter than the global average lifetime of 4.9 days, making the contribution from East Asia to the global BC burden only 36 % of that from the second largest emitter, Africa. Thus, evaluating only relative emission rates without accounting for differences in aging timescales and deposition rates is not predictive of the contribution of a given source region to climate impacts. Our simulations indicate that the lifetime of BC increases nearly linearly with aging timescale for all source regions. When the aging rate is fast, the lifetime of BC is largely determined by factors that control local deposition rates (e.g., precipitation). The sensitivity of lifetime to aging timescale depends strongly on the initial hygroscopicity of freshly emitted BC. Our findings suggest that the aging timescale of BC varies significantly by region and season and can strongly influence the contribution of source regions to BC burdens around the globe. Therefore, improving parameterizations of the aging process for BC is important for enhancing the predictive skill of global models. Future observations that investigate the evolution of the hygroscopicity of BC as it ages from different source regions to the remote atmosphere are urgently needed.

In situ measurements of water uptake by black carbon‐containing aerosol in wildfire plumes

“…In less efficient open combustion sources, on the other hand, BC is coemitted with a wide suite of condensable species generating relatively large BC that is thickly coated shortly after emission [Schwarz et al, 2008]. A few recent studies have investigated factors affecting BC removal rates; Moteki et al [2012] and Taylor et al [2014] report size-dependent removal of BC in air parcels that have undergone wet scavenging, and Zhang et al [2015] infer, based on remote atmosphere comparisons between modeled and measured BC loadings, that the average BC lifetime varies from less than 1 day to more than a week depending on the source region, season, and latitude. Nonetheless, inferred or prescribed rates of BC conversion from hydrophobic to hydrophilic modes in global models vary widely [Cape et al, 2012;Chung and Seinfeld, 2002;Cooke and Wilson, 1996;Samset et al, 2014] and are rarely constrained by direct observation of BC aerosol hygroscopicity.…”

Climate Feedback on Aerosol Emission and Atmospheric Concentrations

“…This would lead to less wet deposition, increasing aerosol lifetimes and burdens [79]. The importance of initial hygroscopicity and aerosol aging, e.g., by nitric acid, on particle for wet removal efficiency and thus the remote distribution of soot aerosol has been shown by [80,81].…”

Influence of cloud microphysical processes on black carbon wet removal, global distributions, and radiative forcing

“…BC aerosols are emitted in a combination of 80 % hydrophobic BC phobic and 20 % hydrophilic BC philic . Although the aging time has been estimated in the range of hours to 2 weeks (Fierce et al, 2015;Zhang et al, 2015;Matsui, 2016), a fixed e-folding aging time (36 h) is assumed to convert BC phobic to BC philic . In our study the activation rate is diagnosed from the cloud droplet number concentration (CDNC) tendency (no.…”

“…Dry and wet deposition are the sinks of BC. Previous literature suggests that global total wet deposition is 3-6 times larger than dry deposition (Jurado et al, 2008;Huang et al, 2010;Zhang et al, 2015). In the remote troposphere, wet scavenging is considered to be the most important source of BC simulation uncertainties (Koch et al, 2009;Schwarz et al, 2010;Croft et al, 2010;Liu et al, 2011;Wang et al, 2014).…”