Chemical Processing of Organics within Clouds (CPOC)
Pilot Study at Whiteface Mountain Research Observatory, Wilmington, NY
Motivations for the 2017 CPOC pilot study and ongoing studies:
Aqueous chemical processing within cloud and fog water has been identified as a key process in the formation of secondary organic aerosol (SOA) mass. Organic compounds are found abundantly within aerosol particles throughout the troposphere, but there remain major uncertainties regarding how organics chemically transform and end up in the aerosol phase. While clouds clean the atmosphere by removing aerosol particles via rain and snow, clouds can also impart mass to pre-existing aerosol particles by acting as aqueous phase micro-reactors. Aerosols, in turn, control cloud microphysical properties, which can affect whether, when and where rain falls. Thus, aerosols and clouds are inextricably linked, and one cannot be understood without the other. Decades of research suggests that the impact of human emissions on clouds exerts a net cooling effect that may be nearly as large as the warming effect from CO2 (although over a much shorter timescale). However, the aerosol-cloud relationship continues to be a highly uncertain aspect of Earth’s climate.
Whiteface Mountain (WFM) in northern New York State is one of the highest peaks in the Adirondacks and is an important sampling site for cloud chemistry because:
1) the mountain top is within cloud 25-60% of the time during summer months,
2) automated cloud water sampling often produces ample cloud liquid water (~1 liter) for each cloud event,
3) decades of chemically-speciated measurements of cloud and rain water have been collected at the site, and
4) due to its strategic location, the summit frequently intercepts pollution plumes that have undergone varying degrees of ‘aging’ during transport from various sources.
The large volumes of cloud water that are routinely collected at WFM allow for the ability to characterize a wide array of chemical compounds. Long-term measurements from WFM have provided a unique and broad view of regional air quality and cloud-aerosol interactions, particularly relevant for acid deposition to the Adirondack ecosystem. The CPOC pilot study and future planned intensive measurement campaigns at WFM will help researchers better understand the impact clouds have on the chemical processing of organics within our ever-changing atmosphere, at the intersection of air quality and climate research.
The pilot study that took place at WFM in August 2017 included upwind below-cloud measurements of aerosol chemical and physical properties, similar to measurements obtained at the summit. Under certain conditions, aerosols measured at the summit between cloud events are representative of cloud droplet residuals detrained from the frequent shallow cumulus intercepting the summit. These pre- and post- ‘cloud processed’ aerosols can then be compared with the chemical composition of cloud water samples to evaluate changes to the organic constituents in the aqueous and aerosol phases. Measured CCN concentrations can also be linked to the observed cloud droplet concentrations using meteorological observations, to better constrain mixing processes occurring at the mountaintop. Wind LIDAR and radiosonde observations are used to help link the below-cloud and summit observations with the aerosol source regions.
An NSF-funded Cloud Chemistry Workshop in Sept 2016 brought together key researchers at WFM to lay out the most pertinent scientific questions relevant to heterogeneous chemistry occurring within fogs and clouds and to discuss preliminary model intercomparisons. The workshop culminated in a plan to coordinate chemical analyses of cloud water samples focused on chemical constituents thought to be most relevant for SOA formation. Workshop participants also recommended that a pilot study be conducted at WFM to better characterize the meteorological conditions, airflow patterns and clouds intercepting the site, in preparation for future intensive field operations focused on the chemical processing of organics within clouds.
Scientific objectives highlighted by the 2016 Cloud Chemistry Workshop participants:
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.
For more information about the 2016 Workshop, see: