Abstract
The representations of cloud processing of gases and aerosols in some of the current state-of-the-art regional air quality models in North America and Europe are reviewed. Key processes reviewed include aerosol activation (or nucleation scavenging of aerosols), aqueous-phase chemistry, and wet deposition/removal of atmospheric tracers. It was found that models vary considerably in the parameterizations or algorithms used in representing these processes. As an emerging area of research, the current understanding of the uptake of water soluble organics by cloud droplets and the potential aqueous-phase reaction pathways leading to the atmospheric secondary organic aerosol (SOA) formation is also reviewed. Sensitivity tests using the AURAMS model have been conducted in order to assess the impact on modeled regional particulate matter (PM) from: (1) the different aerosol activation schemes, (2) the different below-cloud particle scavenging algorithms, and (3) the inclusion of cloud processing of water soluble organics as a potential pathway for the formation of atmospheric SOA. It was found that the modeled droplet number concentrations and ambient PM size distributions were strongly affected by the use of different aerosol activation schemes. The impact on the modeled average ambient PM mass concentration was found to be limited in terms of averaged PM2.5 concentration (~a few percents) but more significant in terms of PM1.0 (up to 10 percents). The modeled ambient PM was found to be moderately sensitive to the below-cloud particle scavenging algorithms, with relative differences up to 10% and 20% in terms of PM2.5 and PM10, respectively, when using the two different algorithms for the scavenging coefficient (Λ) corresponding to the lower and upper bounds in the parameterization for Λ. The model simulation with the additional cloud uptake and processing of water-soluble organic gases was shown to improve the evaluation statistics for modeled PM2.5 OA compared to the IMPROVE network data, and it was demonstrated that the cloud processing of water-soluble organics can indeed be an important mechanism in addition to the traditional secondary organic gas uptake to the particle organic phase.
Highlights
Clouds play an active role in the processing and cycling of chemical tracers in the atmosphere
In this paper we reviewed the representations of cloud processing of gases and aerosols in some of the current regional air quality models in North America and Europe
We have focused on a number of key processes: aerosol activation, aqueous-phase chemistry, and wet deposition by precipitation, and we have mostly focused on warm cloud processes
Summary
Clouds play an active role in the processing and cycling of chemical tracers in the atmosphere. This is of particular importance for looking at the feedback to radiation through cloud and aerosol optical properties and to cloud microphysics, e.g., precipitation production and subsequent aerosol activation, in fully coupled models (see Zhang [120] for a detailed review of online coupled meteorology and chemistry models; note that while AURAMS is an off-line model, i.e., it does not include the feedback from aerosol to cloud microphysics and dynamics, the change in aerosol size distribution as a result of change in droplet number may affect the diagnosed supersaturation in subsequent aerosol activation) It demonstrates the strong influence on modeled aerosol size spectrum from the activation parameterization, the impact on overall modeled bulk aerosol mass is less significant. The variability tested here (the two different activation schemes) does not cover the full range of variability in the parameterization of the various factors affecting aerosol activation, such as influence of organic surfactants (e.g., [122,123]), updraft velocity (e.g., [113,123]), and entrainment (e.g., [124])
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
