The role of aerosols in climate and weather

Whereas in the past, meteorology and climatology were separate fields, be it only because of disparate time (and length scales as well), it appears today that the two fields are strongly coupled, not only as the climate gives the boundaries for investigating the weather, but also because localized events can influence the larger climatological scales. The specific items on which ESSL scientists focused in this year are related to the role of aerosols in climate and weather, to the coupling of eco-systems, biochemistry and climate, to climate change, climate variability and extreme weather such as hurricanes, to interactions of the water cycle with climate and weather, to the impacts of climate and weather on society and ecosystems and finally to megacities and the effects of urbanization; the latter priority is a highlight for NCAR and concerns the international multi-agency field campaign that took place in Mexico city and combined interactive modeling as well. The laboratory highlights are related to the role of aerosols, to the regional carbon cycle, to a numerical simulation of turbulence, to landfall of hurricanes, to the global and regional water cycles and to polar climate.

Exploring the role of aerosols (formerly Bioemissions and aerosol nucleation)

	Average 3.75 m m cloud reflectance binned as a function of aerosol optical depth, over the Indian Ocean for February – March 2003-2005. In the lower and middle troposphere, cloud reflectance increases as aerosol optical depth increases, which is a quantitative measurement of the aerosol indirect effect. In contrast, small effects are observed in the upper troposphere.   High resolution figure

In order to reduce the uncertainty of the role of aerosols in climate and weather, ESSL scientists are conducting both experimental and modeling studies of aerosol formation, composition, and impacts on climate.

ACD scientists designed and conducted the Niwot Ridge 2006 biogenic particle production and growth study. New particle production and growth has been observed at several remote sites and has been associated with biogenic emissions of volatile organic compounds. Previous studies in the U.S. have only considered a few biogenic VOC and have had little or no information on aerosol chemical composition. The processes controlling particle production, and even the compounds responsible, are not well understood. ACD scientists collaborated with Birgit Wehner (Institute for Tropospheric Research – Liepzig, Germany) to study particle formation and growth at the University of Colorado Mountain Research Station on Niwot Ridge. Particle production and growth events were observed during conditions that brought clean continental air to the site.

ACD scientists are also working with Jana Milford and Detlev Helmig (U. Colorado) to investigate the contribution of biogenic VOC emissions to secondary organic aerosols (SOA) in the U.S. Initial results indicate that sesquiterpenes, monoterpenes, and isoprene all make a significant contribution to regional SOA production.

Another group of ACD scientists are working to understand the impacts of urban emissions on regional air quality through its modification of aerosol physico-chemical properties.   This group specifically focuses its efforts on “ultrafine particles,” those smaller than 100 nm in diameter whose impacts span all scales from local to regional and global.   In March 2006, as part of the MILAGRO campaign, the ACD scientists and collaborators from the University of Minnesota, the Georgia Institute of Technology, the University of Colorado, and the Lawrence Berkeley National Laboratories deployed a suite of instruments to study aerosol formation and growth at the “T1” ground-based site NE northeast of Mexico City during the MILAGRO campaign.   These activities followed the first measurements of ultrafine aerosol size distributions in the Mexico City Metropolitan Area, performed by the group in 2003, where they found that new particle formation events occur intensely and frequently both inside and outside of the metropolitan area.   The preliminary results from MILAGRO show that, once again, new particle formation and subsequent condensational growth occur frequently outside of Mexico City, and are often the dominant processes affecting number concentrations in that area. We also found that these newly formed particles consisted of a complex mixture of both organic and inorganic compounds.

A new facility for investigating biogenic SOA formation and growth was developed by ACD staff in the new NCAR Foothills ACD Laboratory and was successfully used to generate particle formation and growth from vegetation emissions. ACD scientists used this facility to conduct initial studies to demonstrate the utility of the facility. The results show that the mixture of compounds emitted from a plant result in SOA production and growth that cannot be explained by observations of the oxidation of individual compounds. A publication describing the facility and initial results is in review.

In a continuing investigation by ACD scientists, the MOZART and the CAM models have been used to investigate the radiative forcing of ozone and carbon aerosols from wildfires in Alaska and Canada. Some of these results have been incorporated into an exhaustive study of the impact of biomass burning on the boreal forest. This work shows that despite the release of large amounts of carbon dioxide, aerosols and other trace species into the atmosphere by fires, the ability of fire to change surface albedo dominates the net climactic affect of fires, leading to cooling.

Aerosol indirect effects, i.e. the manner in which changes in aerosol loading influence the reflectivity, lifetime, and precipitation characteristics of clouds, is recognized as an important uncertainty in global climate models.   Quantification of these effects as a function of altitude, however, has not been well established. ACD scientists have calculated changes in cloud reflectivity as aerosol loading increases over the Indian Ocean in collaboration with Andy Heymsfield (MMM/NCAR) and Detlef Muller and Patric Seifert of the Leibniz Institute for Tropospheric Research in Leipzig, Germany.   Their study utilized satellite observations of aerosol loading and visible and infrared radiances as measured by the Moderate Resolution Imaging Spectroradiometer (MODIS) experiment. Since the MODIS experiment can determine cloud top pressure and temperature, it was possible to quantify aerosol indirect effects as a function of altitude and cloud phase (i.e. liquid droplets and ice crystals). Figure 1 shows cloud reflectivity as a function of aerosol optical depth derived from the MODIS data, demonstrating an increase in reflectivity for higher aerosol optical depth (i.e. a direct measure of the aerosol indirect effect). The measured reflectivity changes are largest in the lower and middle troposphere (corresponding to the region where liquid droplets are present) and very small in the upper troposphere (corresponding to the region where ice crystals are present).   Ongoing work is aimed at extending these observations over larger regions of the globe.

Plans for FY2007 include analysis of the measurements from the Niwot Ridge and MIRAGE studies and continued work in the field and in the lab on SOA formation, composition, and growth. Also, the work evaluating aerosol indirect effect will be extended to include larger regions of the atmosphere. This work is funded by NSF/NCAR, NSF/Biocompexity, and DOE.

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