Research Program Area: Climate Change
This report provides an assessment of the relative importance of the first indirect aerosol effect of black carbon forcing for California’s climate constrained with measurements by developing a balanced approach between observations, data analyses, and modeling studies. Previous studies in California have indicated that emission control policies have resulted in a 50% decline in BC aerosols. This decline is expected to have a cooling effect due to reduced direct absorption, but the impact on cloud processes is less certain. The report will also address the importance of the internal mixing state and hygroscopicity of the aerosols in addition to total concentration on the secondary aerosol effects.
The study consisted of four primary components: (i) analysis of available measurements and construction of BC case studies constrained by field measurements, (ii) determination of aerosol impact on warm cloud droplet numbers using a sectional parcel model, (iii) downscaling scenarios with reduced BC concentrations compared to observations, and (iv) comparison with global climate models. We identified five field campaigns in California that represent a variety of ambient conditions: clean and polluted marine, polluted urban, and biomass burning dominated. Using co-located ATOFMS measurements of chemical tracers and APS/SMPS number size distributions we described the physical (size and mass) and chemical (both internal and external mixing states) properties of the aerosol. We used our detailed parcel model in conjunction with measured cloud profiles to analyze the evolution of aerosol number size distributions, and determined the total number of activated cloud droplets based on the growth of a mode at 10 micrometers. We repeated the model analysis using scenarios constraining the BC dominated particle types by 50% and 90% to estimate the impact of BC mitigation on cloud droplet population, and compared the local results roughly with those predicted from two GCMs for base (total emissions) and BC mitigated cases. Finally, we also completed a number of sensitivity studies to describe a variety of observed ambient conditions by altering the updraft velocity, allowing for adiabatic cooling, changing the air entrainment rate, and conserving the total available water vapor.
In California, we find that BC-type particles contributed between 16 and 20% of cloud droplets even in the presence of more hygroscopic particles. Reducing BC particle concentration by 50% decreased the cloud droplet concentration by between 6% and 9% resulting in the formation of fewer, larger cloud droplets that correspond to a lower cloud albedo. This trend qualitatively agrees with the GCM calculations of Chen et al. [2010a] when climate feedbacks are excluded and with Jacobson  only when the BC is treated as being hydrophilic. The implications of this comparison are that in addition to total number concentration, the internal mixing state of the BC is also a significant factor in determining the net impact on clouds. For regions like California, where BC mitigation targets primarily fossil fuel sources, the cloud albedo effect of BC particles may partially offset the climate benefits of direct forcing reduction. The study does not consider the cloud burning effects of BC, since it is beyond the scope of this project. As studies have shown, the cloud-burning effects of BC can enhance the climate benefits of direct forcing reduction.
For questions regarding this research project, including available data and progress status, contact: Research Division staff at (916) 445-0753
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