Breakout Summary Report

Bidirectional surface-atmosphere exchange rates are critical pieces needed to quantify aerosolcloud and cloud-aerosol interactions in the coupled Earth-Ocean-Atmosphere climate system. Measurements of these exchange rates via in-situ and remote sensing approaches are widely used within the ASR/ARM program. Example applications include particle (nucleation, accumulation, and coarse mode), trace gas, water vapor, and drizzle flux measurements. Contrary to latent and sensible heat fluxes, which are constrained by the surface energy balance, or ecosystematmosphere CO2 fluxes that are controlled, to the first degree, by photosynthesis and respiration, aerosol and drizzle fluxes are more difficult to validate due to a paucity of known constraints. The main goal of this session is to bring together experimentalists, observationalists, and modelers across all ASR working groups to discuss approaches, progress, and challenges in (1) designing appropriate flux measurements for ARM/ASR campaigns, (2) extracting fluxes from available ARM/ASR datasets, and (3) identifying how flux measurements can best inform process modeling. The expected outcome is a summary of current approaches, progress, and challenges and improved collaboration in addressing these challenges across the various subdisciplines.

The session sought to provide an overview of flux measurement efforts and data usage of fluxes across the ARM program. An overview of activities was presented through 13 invited lightning talks presented by Ashish Singh, Ryan Sullivan, Russell Perkins, Delphine Farmer, Robert Newsom, Virendra Ghate, Nicholas Meskhidze, Jerome Fast, Jian Wang, Ellie Browne, Daniel Feldman, Marcelo Chamecki, and Markus Petters. A copy of the slide deck is attached to this report. The remainder of the session was devoted to discussion and brainstorming. Discussion was performed in small groups followed by reporting to the whole session through oral discussion and entries in a shared Google document. Discussion included the following topics.

Discussion Session 1: Measurements

1. What type of flux measurements do we need to reduce uncertainty in cloud, aerosol, precipitation, and radiation processes that affect the Earth's radiative balance and hydrological cycle?

2. How could we quantify the impact of a flux measurement on reducing uncertainty?

3. If we need to prioritize, what flux measurements should ASR/ARM focus on?

Discussion Session 2: Data Analysis

1. How reliable are current mathematical approaches that calculate fluxes?

2. What flux products are currently available within the ARM/ASR program?

3. What flux products do you wish were available within the ARM/ASR program?

4. Should flux products be handled by the ARM/ASR translators or by individual PIs?

Discussion Session 3: Modelling

1. What flux data is being used to constrain current models?

2. What type of flux data is needed to reduce uncertainty in process models such as large eddy simulation?

Discussion Session 1: Measurements

There was a broad consensus that flux measurements are useful for characterizing aerosol sources and lifetimes, but the flux measurements remain expensive and challenging. Spatial and temporal heterogeneity in fluxes complicate the measurement. Flux measurement validation remains challenging. Various types of closure studies were cited as a means to quantify the ability to quantify transport phenomena through flux measurements, and in turn, reduce uncertainty. Vertical flux profiles e.g., by combining HSRL/Doppler Lidar would be helpful for understanding vertical mixing throughout the PBL. There was consensus that particle fluxes are difficult to interpret without energy or trace gas fluxes. There was agreement that CO2, water, sensible and latent heat flux are generally available and well understood. Hydrometeor fluxes, methane, BVOCs, ozone, NOx, and other trace gases are less frequently available and particle fluxes are rare. Even “simple” NH3 flux measurements are likely useful for understanding new particle formation and constraining emissions in models. Bioaerosol measurements would be useful. Size-resolved particle fluxes with the UHSAS from ARM sites might be feasible, possibly targeting sea spray fluxes and particle number fluxes. Indirect flux measurements could be performed through higher time resolution measurements of above- and below-cloud concentrations of particles and reactive trace gases. The extent to which 10 Hz measurements are needed is unclear. It may be possible to accept some uncertainty in a flux measurement by reducing sampling frequency, thus allowing for more types of measurements. There is a need for space for guest instruments to provide flux measurements during IOPs

Discussion Session 2: Data Analysis

There was consensus that there are two issues that need to be addressed, hardware and software. Hardware involves operation of the instruments, handling data acquisition, and carrying out the measurement. Post-processing software involves complex data analysis. Ideally, ARM/ASR translators would handle this, but involving ARM translators was considered premature due to the complex interplay of hardware and software. Training more students in data analysis, e.g. through summer school might be one option to test various analysis approaches. There is a need to standardize flux data analysis and evaluate what agreement/uncertainty is acceptable for providing flux data. The CO2 community has spent extensive time evaluating this question and found that they can be very accurate and very precise. Expanding to particles and reactive trace gases is more challenging, but it isn’t a math problem - more an instrument, setup, and measurement problem.

Discussion Session 3: Modelling

There was consensus that the fluxes are most useful to constrain processes in models. Surface turbulent heat and water vapor fluxes, and drizzle fluxes are routinely used to constrain models. Models would greatly benefit from flux measurements to constrain/validate emissions, which are generated by the meteorology fields. Examples include soi NOx emission or NH3 emission. Similarly, the deposition flux is directly output by models, which can be compared to measurements. Flux measurement may also be used to validate parameterizations that are implemented in models.

Key Findings

● Flux measurements are valuable for characterizing aerosol sources and lifetimes but remain expensive and challenging.

● Spatial and temporal heterogeneity in fluxes complicates measurements

● Vertical flux profiles (e.g., by combining HSRL/Doppler Lidar) would help understand vertical mixing throughout the PBL.

● Particle fluxes require complementary energy or trace gas flux data for proper interpretation

● CO2, water, sensible and latent heat flux measurements are generally available and better understood, while hydrometeor fluxes, methane, BVOCs, ozone, NOx, and particle fluxes are less common.

● Models would greatly benefit from flux measurements to constrain/validate emissions generated by meteorology fields.

● Flux measurement validation remains challenging.

● The necessity of 10 Hz measurements is unclear; reducing sampling frequency may allow for more types of measurements while accepting some uncertainty.

● Standardization of flux data analysis is needed, with agreement on acceptable uncertainty levels.

● At the ARM sites combine size-resolved in situ particle flux measurements (potentially using UHSAS) with remotely sensed vertical flux profiles from HSRL/Doppler Lidar to better understand sources, sinks, and mixing of particles throughout the PBL.

● Measurements of methane, BVOCs, ozone, NOx, and NH3 fluxes.

● Bioaerosol flux measurements.

● Space for guest instruments to provide flux measurements during IOPs.

● Indirect flux measurements through higher time resolution measurements of above- and below-cloud concentrations.

● Standardized approaches to flux data analysis.

N/A

1. Explore closure studies as a means to quantify transport phenomena through flux measurements. Studies should consider both direct (i.e., in situ and remotely sensed eddy covariance) and indirect (i.e., relaxed eddy accumulation, gradient diffusion) flux measurement approaches.

2. Investigate size-resolved particle fluxes targeting particle number fluxes.

3. Evaluate lessons from the CO2 community's experience in standardizing flux measurements.

● Develop training opportunities (e.g., summer schools) for flux data analysis.

● Establish standards for flux data analysis and acceptable uncertainty levels.

● Explore opportunities for adding flux measurement capabilities at ARM sites.

● Investigate approaches for combining HSRL/Doppler Lidar data for vertical flux profiles.