Mary Barth (ACOM/MMM), Annmarie Carlton (University California – Irvine), Sara Lance (Atmospheric Sciences Research Center, University at Albany, SUNY)
The Whiteface Mountain Cloud Chemistry Workshop was held at the Marble Mountain Lodge near Wilmington, New York, 16-17 September 2016 to design a coordinated investigation of the effect of clouds on tropospheric composition. Approximately 40 people attended with expertise ranging from cloud chemistry modeling to measurements of clouds, aerosols, and trace gases.
Clouds cover 60% of the Earth’s surface at a given time and are the primary means by which constituents from the polluted boundary layer are lofted to the free troposphere. Clouds also are aqueous-phase chemical reactors, scavenging soluble gas-phase precursors and supporting oxidation reactions that contribute to increased aerosol mass when the cloud drops evaporate. However, atmospheric chemistry observations (field campaigns and satellite retrievals) typically avoid clouds resulting in a lack of knowledge of cloud chemistry impacts on tropospheric composition.
The workshop reviewed past mountaintop cloud chemistry studies, including historical and current Whiteface Mountain activities, the Great Dun Fell experiments in northern England, and the FEBUKO and HCCT-2010 experiments in Germany, to learn about successful operations and needed improvements with these types of studies. In anticipation of a future field campaign at the Whiteface Mountain Observatory, results from regional and 0-dimensional box model studies were discussed to learn what might be expected in terms of atmospheric composition and cloud chemistry effects on that composition.
Workshop participants agreed on the importance of the following science objectives for future field campaigns.
Quantify the clear-sky bias in chemical characterization of the troposphere.
Identify key oxidants driving aqueous phase chemistry, especially pertaining to organic compounds.
Quantify how aerosol characteristics and gas-phase composition change as a result of cloud processing.
Identify chemical tracers for cloud processing.
Quantify entrainment and transport of chemical constituents into the free troposphere.
Determine the importance of aqueous-phase biological processes on aqueous chemistry.
With these science objectives in mind, workshop participants discussed the capabilities at Whiteface Mountain for conducting a cloud chemistry experiment and how such an experiment would be designed. Whiteface Mountain Summit Observatory routinely collects and analyzes cloud water samples during summertime, while the Marble Mountain Lodge site hosts NADP and CASTNet monitors. Based on hourly data, the summit of Whiteface Mountain experiences cloud 20-60% of the time during summer (June—September), making Whiteface Mountain an excellent location to investigate aqueous organic chemistry in warm, low-level clouds.
The workshop concluded with the decision to begin with small measurement and modeling studies. Workshop participants are currently analyzing cloud water samples from recent cloud events for more in-depth and coordinated analysis of cloud water composition information. Researchers are mining the literature and devising measurement strategies to more fully characterize typical cloud properties and airflow patterns in the region. Model intercomparisons are being pursued for both regional-scale and chemistry box models. A small-scale pilot study is being planned to examine cloud processing of trace gases and aerosols at Whiteface Mountain Observatory, with a focus on water-soluble organic carbon. Based on these efforts, future field intensive operations will be designed.
More information on the workshop can be found at https://www2.acom.ucar.edu/cloud-chemistry. The workshop was sponsored by the U.S. National Science Foundation.