Black Carbon Webinar II

Synopsis

Black carbon is "the second most important human emission in terms of its climate-forcing in the present-day atmosphere; only carbon dioxide is estimated to have a greater forcing." When BC is deposited on snow and ice, it darkens an otherwise bright surface. The darker surface may enhance the absorption of solar radiation resulting in an acceleration of snow and ice melting. In addition, BC particles suspended in the atmosphere absorb solar radiation and heat the surrounding air. Atmospheric BC can also alter cloud properties leading to changes in cloud amount and precipitation. Black carbon has multiple sources including domestic combustion for heating and cooking, diesel combustion related to transportation, fossil fuel and biofuel combustion for power generation, agricultural burning, and wildfires. Identification of the sources and types of black carbon (both the geographical region of the source and the combustion process) is necessary for effectively mitigating its climate impacts. In addition, measurements of black carbon are required to verify whether implemented mitigation strategies that target BC emissions from certain sources are actually leading to reductions in BC concentrations in the Arctic atmosphere and surface. In 2013, NOAA's Arctic Report Card added a black carbon assessment to the Atmosphere Section; the primary conclusions of the assessment are that (1) the average equivalent black carbon concentrations in 2012 at locations Alert (Nunavut, Canada), Barrow (Alaska, USA) and Ny-Alesund (Svalbard, Norway) were similar to average EBC concentrations during the last decade and (2) equivalent black carbon has declined by as much as 55% during the 23 year record at Alert and Barrow (Sharma et al. 2013).

Several issues are currently challenging the Arctic black carbon research community:

In-situ measurements are the most reliable measure of black carbon; however, the most prevalent techniques which involve filter samplers only make proxy black carbon measurements.
Retrievals of aerosols optical depth (AOD) over snow and ice-covered surfaces with passive remote sensing from vis-NIR imagers from space are problematic. Some success over incomplete snow-covered surfaces has been achieved, e.g., with MISR. TOMS, OMI, and probably now OMPS UV passive imaging has some qualitative sensitivity (the Aerosol Index), though with limited sensitivity to near-surface aerosol, and CALIPSO lidar is by far the most sensitive, but with limited coverage.
Standardized ground-based networks such as AeroNET, MPLNet, and BSRN have sparse and sporadic Arctic coverage, and are uncoordinated with the necessary black carbon-in-snow measurements, and some long-standing surface stations have actually been decommissioned in the past few years.
A promising approach to assessing high-latitude aerosol effects is to constrain aerosol transport models with satellite observations taken at lower latitudes, near the aerosol sources (mainly Boreal fires and pollution sites) where and when the surface is not snow-covered.

Speakers

In-situ ground sensing

Patricia Quinn (NOAA)

Satellite remote sensing

Ralph Kahn (NASA)

Transport modeling

Mark Jacobson (Stanford)

Presentations

The Impact of Black Carbon on Arctic Climate

Download NOAA Slides (PDF - 8.8 MB)

Patricia Quinn / NOAA Pacific Marine Environmental Laboratories
Seattle, WA; e-mail: patricia.k.quinn [at] noaa.gov

Black carbon is the second largest contributor to global warming after carbon dioxide. When BC is deposited on snow and ice, it darkens an otherwise bright surface. The darker surface may enhance the absorption of solar radiation resulting in an acceleration of snow and ice melting. In addition, BC particles suspended in the atmosphere absorb solar radiation and heat the surrounding air. Atmospheric BC can also alter cloud properties leading to changes in cloud amount and precipitation. Black carbon has multiple sources including domestic combustion for heating and cooking, diesel combustion related to transportation, fossil fuel and biofuel combustion for power generation, agricultural burning, and wildfires. Identification of the sources of black carbon (both the geographical region of the source and the combustion process) is necessary for effectively mitigating its climate impacts. In addition, measurements of black carbon are required to verify whether implemented mitigation strategies that target BC emissions from certain sources are actually leading to reductions in BC concentrations in the Arctic atmosphere. The Arctic Monitoring and Assessment Programme (AMAP) established an expert group that produced a report "The Impact of Black Carbon on the Arctic Climate" in 2011. The report reviewed the current state of science knowledge about BC properties, measurement, modeling, emissions, distributions, trends, and forcing mechanisms. The report concluded with recommendations for improved characterization of Arctic BC, emissions information and model development to address current science and information needs. The AMAP Expert Group is currently working on a report linking radiative forcing by BC in the Arctic to climate responses. The report will be released in 2015

Satellite Constraints on Arctic-region Airborne Particles

Download NASA Slides (PDF - 2.2 MB)

Ralph Kahn / NASA Goddard Space Flight Center
Greenbelt MD 20771; e-mail: ralph.kahn [at] nasa.gov

Satellite remote sensing is generally the most practical way to measure aerosol amount and type frequently, over large areas. However, aerosol remote sensing is especially challenging in the polar regions, due to the combination of very bright surface, low sun angle, persistent cloud (including thin cirrus), and generally low aerosol optical depth (AOD). Some success in retrieving AOD over incomplete snow-covered surfaces has been achieved with passive imagers such as MISR. Despite limited coverage, CALIPSO lidar is by far the most sensitive and is the best available space-based source of total-column and height-resolved Arctic aerosol observations, especially at night, when signal/noise is highest. The SAGE passive limb-sounders also provide height-resolved aerosol extinction profiles in the stratosphere and upper troposphere, again with very limited sampling. Passive imagers, such as MODIS, MISR, and TOMS-OMI, provide broader spatial coverage on shorter timescales, making event-resolved studies possible. Such observations can be acquired reliably at lower latitudes, near the aerosol sources (mainly Boreal fires and pollution sites) where and when the surface is not snow-covered, and the AOD and sun elevation angle are higher. A promising approach to assessing high-latitude aerosol effects from passive imagers is to constrain chemical transport models with satellite observations at lower latitudes, and use the models to simulate conditions in the Arctic. Similarly, gas molecules such as CO and SO2, mapped globally from space by AIRS, OMI, and other instruments, can serve as smoke and particle pollution tracers for constraining transport model simulations. Given the limitations of each approach, the combination of active and passive satellite measurements, suborbital observations for validation and additional detail, and transport modeling constrained by observations, is required to complete the Arctic aerosol picture.

Contributions of Cross-Polar flights to Arctic Black Carbon

Download Stanford Slides (PDF - 960 KB)

Mark Jacobson / Stanford University
Stanford, CA 94305 e-mail: Jacobson [at] stanford.edu

Climate data suggest greater warming over the Arctic than lower latitudes, and the most abundant direct source of black carbon and other climate-relevant pollutants over the Arctic is cross-polar flights by international aviation. A relevant question is whether rerouting cross-polar flights to circumnavigate the Arctic Circle reduces or enhances such warming. To study this issue, a model accounting for sub-grid exhaust plumes from each individual commercial flight worldwide was used with 2006 global aircraft emission inventories that treated cross-polar flights and flights rerouted around the Arctic Circle (66.56083 oN), respectively. Rerouting increased fuel use by 0.056% in the global average, mostly right outside the Arctic Circle, but most of the associated black carbon and other emissions were removed faster because they were now over latitudes of greater precipitation and lesser stability. Rerouting also reduced fuel use and emissions within the Arctic Circle by 83% and delayed pollutant transport to the Arctic. The Arctic reduction in pollutants, particularly of black carbon, decreased Arctic and global temperature and increased Arctic sea ice over 22 years. Although the slight increase in total CO2 emissions due to rerouting may dampen the benefit of rerouting over more decades, rerouting or even partial rerouting (allowing cross-polar flights during polar night only) may delay the elimination of Arctic sea ice, which will otherwise likely occur within the next 2-3 decades due to global warming in general. Rerouting may increase worldwide fuel plus operational costs by only ~$99 million/yr, 47-55 times less than an estimated 2025 U.S.-alone cost savings due to the global warming reduction from rerouting.