What: A workshop to discuss the current scientific understanding regarding cloud impacts on tropospheric chemistry and to discuss the necessary components for a coordinated study focused on the evaluation of unknown or poorly understood chemical processes occurring within clouds (www2.acom.ucar.edu/cloud-chemistry/whiteface-2016-agenda).
Who: Forty researchers participated, with expertise ranging from chemistry and cloud physics modeling to mountaintop and airborne measurements of trace gases, aerosols, and clouds.
When: 16–17 September 2016
Where: Marble Mountain Lodge, Whiteface Mountain, Wilmington, New York
In situ measurements and remote sensing retrievals of atmospheric chemistry are typically limited to sampling in clear sky. It is reasonable to assume that biased sampling has resulted in a biased understanding of atmospheric chemistry, given the following facts: 1) clouds form frequently within the troposphere, 2) updrafts associated with clouds are the primary means by which emissions from the boundary layer are lofted into the free troposphere (Ching and Alkezweeny 1986; Ching et al. 1988; Vila-Guerau de Arellano et al. 2005), and 3) cloud droplets provide a medium for aqueous-phase chemical reactions that are distinctly different from chemical processes occurring in the gas or aerosol phases (Volkamer et al. 2009; Carlton et al. 2008; Altieri et al. 2008; Perri et al. 2009). Although much research effort has historically focused on cloud-processing impacts on inorganic compounds like sulfate (Hegg and Hobbs 1982; Husain et al. 1991), many questions remain regarding organic compounds (Ervens 2015). An increasing body of experimental evidence indicates that scavenging of soluble gas-phase organic compounds (such as glyoxal and methylglyxal) and subsequent oxidation reactions within fog and cloud droplets can contribute substantially to secondary organic aerosol (SOA) mass following cloud droplet evaporation (Blando and Turpin 2000; Volkamer et al. 2006; Ervens et al. 2011). However, there remains a critical need to identify the key processes impacting organic chemical transformation taking place within clouds and to better understand the conditions under which those processes occur as well as the frequency with which they occur in the troposphere.
During the Whiteface Mountain (WFM) Cloud Chemistry Workshop, participants reviewed past mountaintop cloud chemistry studies, including 1) observations at WFM starting in 1994 and continuing through the present day, 2) the Great Dun Fell experiments from the 1980s and 1990s in northern England, and 3) the 2000s Field Investigations of Budgets and Conversions of Particle Phase Organics in Tropospheric Cloud Processes (FEBUKO) and Hill Cap Cloud Thuringia (HCCT-2010) experiments in Germany. With a focus on “acid rain” deposition, the primary result of these previous studies was a better understanding of cloud-processing impacts on inorganic chemistry, namely, sulfates and nitrates, which have decreased in recent years at WFM as a result of regulatory actions (Schwab et al. 2016a). However, many questions remain regarding cloud processing of organic constituents, which have increased in absolute abundance within aerosol particles measured at the WFM site over the past 15 years, becoming the predominant aerosol chemical constituent (Schwab et al. 2016b). Discussion of previous field studies highlighted general best practices, including choosing a field site with sufficiently frequent cloudy events and a reliably predominant wind direction; locating measurements at in-cloud, upwind, and downwind locations; planning for daytime and nighttime sampling; and releasing tracer gases to characterize wind flow patterns. Airborne and remote sensing vertical profiles of meteorological, chemical, and microphysical observations were also recommended by workshop participants to better constrain the impact of cloud processes such as entrainment, which can alter the cloud droplet size distribution and liquid water content and can also entrain reaction precursors from aloft. As a result of difficulties associated with coordinating intensive observational capabilities with weather phenomena, measurement redundancy was highly recommended, as was planning for multiple field campaigns in the likely event that not all measurements operate perfectly during weather events most conducive to scientific analysis. Workshop participants noted that modeling studies of airflow and cloud dynamics, in addition to modeling studies of trace gas and aerosol chemistry, will be crucial for unraveling the complex interactions and feedback processes associated with such dynamic multiphase systems.
Also presented at the workshop were preliminary modeling intercomparisons focused on the summer months (June–September) within New York State and surrounding regions, including water-soluble gases and ozone concentrations simulated by the Weather Research and Forecasting Model coupled to Chemistry (WRF-Chem) and the Community Multiscale Air Quality (CMAQ) model. Simulations of gas-phase glyoxal mixing ratios showed a factor of ∼2 discrepancy between the WRF-Chem and CMAQ models (Fig. 1). Discrepancies between modeled methyl glyoxal mixing ratios were a factor of 2–20. Workshop participants suggested that these discrepancies are most likely caused by different methods of producing isoprene emissions and the fact that the simulations represented different years. However, the discrepancies illustrate that assessing the role of cloud chemistry on isoprene products is highly sensitive to model conditions and assumptions.
Workshop participants agreed on the importance of the following science objectives for future field campaigns and modeling studies:
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 the entrainment and transport of chemical constituents into the free troposphere; and
determine the importance of aqueous-phase biological processes on aqueous chemistry.
With these science objectives and recommendations in mind, workshop participants discussed the advantages and disadvantages of the WFM site for conducting a cloud chemistry experiment. Cloud water samples have been routinely collected and analyzed during the months of June–September at WFM since 1995, providing a baseline regional climatology of summertime cloud water chemistry, including pH and inorganic ion content. Based on this climatology, the summit of WFM experiences clouds 20%–60% of the time during the summer (Schwab et al. 2016b), suggesting a high likelihood that a field deployment to WFM would encounter ideal conditions to investigate warm, low-level cloud processes, in contrast to other locations where surface relative humidity in the summertime is relatively low, resulting in higher cloud base than is often accessible for the laboratory measurements (e.g., at Storm Peak Laboratory in Colorado). The historical trace gas, aerosol, and meteorological observations collected year round by the Adirondack Lake Survey Corporation (ALSC), State University of New York (SUNY) Atmospheric Sciences Research Center (ASRC), and New York State Department of Environmental Conservation (NYSDEC) at the WFM site over past decades also provide a broader context to any future intensive field operation that will take place in the region (Schwab et al. 2016a; Brandt et al. 2016). The lack of local pollution sources, the persistent wind direction, and the expectation of no land-use change within the Adirondacks were also pointed out as positive attributes of the WFM site. The recent rollout of the New York State Mesonet with a monitoring station at the WFM site will be helpful for characterizing and anticipating weather patterns. A hairpin turn on the road up to the summit of WFM (known locally as the “Lake Placid turn”) was highlighted as a potential site for below-cloud and up-wind trace gas and aerosol characterization using mobile research facilities. Concerns were raised about the possibility of complex airflow patterns due to the mountainous terrain thereby complicating the analysis, and ended in the recommendation to enlist experts in mountain meteorology and to deploy meteorological instrumentation with sufficient spatiotemporal resolution to constrain modeling efforts of airflow dynamics. This concern also renewed calls for measurements from airborne platforms. Modifications to the cloud water collection procedure, including sterilization protocols and below-freezing storage temperature, were discussed, with an appreciation for the possibility of organic chemical reactions occurring prior to chemical analysis because of biological activity subsequent to cloud water collection.
The workshop concluded with the decision to start with near-term, focused measurement and modeling intercomparison studies. Researchers from the workshop have begun a coordinated analysis of cloud water samples from recent cloud events, with the expectation of obtaining more in-depth analysis of cloud water composition than has ever been done before and with the goal of further refining research plans and needs for future coordinated intensive field operations. Further modeling efforts are also being pursued, using both regional-scale and chemistry box models. A small-scale pilot study at WFM is being planned for summer 2017. Towards the goals of the pilot study, infrastructure developments at the summit of WFM are being pursued, including addition of instrumentation to measure cloud droplet and drizzle size distributions, as well as addition of an inlet to sample interstitial aerosol for online microphysical analysis. These efforts will aid in future coordinate intensive field operations.
Funding for the workshop was provided by a National Science Foundation (NSF) Funding Opportunity for Conferences, Symposia and Workshops (Grant AGS1638579). The authors would like to thank local organizers Paul Casson, Shari Kent, Richard Brandt, and Michelle Casson for assisting with meeting logistics and catering at the workshop. Special thanks to Jed Dukett and the ALSC for collecting, cataloging, and distributing cloud water samples to participants for coordinated analysis post workshop.