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Tropical Rainforest-Atmosphere iNteractionS:<br/>Achieving a Coupled CLoud-Aerosol Interactions Model

Final Report Summary - TRANS-ACCLAIM (Tropical Rainforest-Atmosphere iNteractionS:<br/>Achieving a Coupled CLoud-Aerosol Interactions Model)

The interactions between atmospheric aerosols and clouds account for a substantial uncertainty in our understanding of the climate system. These interactions exemplify one of the ways in which humans impact our environment and vice-versa. Detailed fundamental knowledge of the processes involved and their associated effects is critical for understanding the climate as a whole and its sensitivity to anthropogenic forcing. The Tropical Rainforest-Atmosphere iNteractionS: Achieving a Coupled Cloud-Aerosol Interactions Model (TRANS-ACCLAIM) project has focused on the role that tropical deep convective clouds play in the redistribution and budget of atmospheric aerosols, and in particular organic aerosol chemistry. The objectives of this project are to quantify the sources and sinks of particles in deep convective cloud events, and to assess the importance of numerical model representation to relevant chemical properties. These objectives are achieved through the development and application of new atmospheric model tools combined with analysis of appropriate in situ measurements.

The majority of the project focused on development, application and evaluation of the Cloud Resolving Model with Organics (CRM-ORG), a unique, three-dimensional, high-resolution, limited-area model suitable for representing the formation and evolution of a deep convective cloud event. The model explicitly takes into account the production of aerosols from oxidation of trace gases, condensation of the oxidation products to the particulate phase, activation of particles to form cloud droplets, scavenging of particles by droplets, rain drops, ice crystals and graupel, aqueous-phase chemistry of scavenged species and release of aerosols from detrained, evaporated hydrometeors.

TRANS-ACCLAIM has achieved the following specific results:
• With CRM-ORG, we have shown that the enhancement of ultrafine particles in the upper troposphere downwind of deep convective clouds could come from in-cloud nucleation of semivolatile organic vapors transported directly from the atmospheric boundary layer within the cloud convective core. This points urgently to the need for temperature-dependent new particle formation mechanisms; new, more detailed parameterizations are necessary for informing an accurate view of new particle formation rates throughout the remote atmosphere.
• Accurate estimates for organic compound chemical properties (e.g. volatility, surface tension, affinity for ice hydrometeor surfaces) are all critical for predicting new particle formation in the outflow of deep convective clouds. It will be important for the atmospheric chemistry experimental community to address these gaps in the future.
• During the Amazon clean season, sulfuric acid is not predicted by CRM-ORG to lead to substantial new particle formation in connection with deep convective events. This result is relevant for the community engaged in measuring new particle formation rates in laboratory experiments; more work on organic vapor driven new particle formation mechanisms is needed.
• Sensitivity studies exploring the effect of organic aerosol loading and vertical profile on cloud properties have shown modest effects for these relatively clean conditions (less than 2000 particles cm-3 in the boundary layer). This finding is important for the climate modeling community as it relieves large-scale models from including aerosol effects on clouds at low concentrations.
• At larger scales relevant for regional and global models, the effect of cloud cover on new particle formation is actually quite complicated as rates below/above the clouds may be reduced/enhanced due to reduced/enhanced photochemical activity.
• Bulk viscosity measurements and mass transfer analysis have suggested that moderately cold temperature (less than 290 K) and low water relative humidity are associated with highly viscous behavior of organics. These compounds may therefore behave like semisolids at high altitudes. However, it remains unclear what this finding implies for high relative humidity conditions in clouds.
• A unified framework for classifying organic compounds by volatility and phase provides a consistent connection between atmospheric models (e.g. CRM-ORG), smog chamber experiments and emissions inventories used for research and regulatory efforts. This result, targeted to the public policy community and scientific community, has enormous importance for disseminating information about and regulating organic aerosol in effective and comprehensible ways.

The development of CRM-ORG is an important step forward in the effort to understand the feedbacks between meteorology and chemistry in the atmosphere. The application of the model has shown that it is crucial to understand how assumptions made with respect to chemical behavior at the Earth’s surface will impact our picture of what happens in the upper troposphere, where interactions with high-level clouds could be important for the Earth’s radiation budget. This model will continue to be used to explore the role of convective clouds in the removal and processing of particles in the tropics and throughout the world. This work also motivates atmospheric researchers in general to consider the consequences of highly dynamic and complex, yet frequent, events in the atmosphere. Events like these may have profound impact on the climate system (e.g. through their modification of aerosol population) and we are only beginning to understand the ways in which these impacts manifest.