Participants included Carmen Benkovitz (BNL), Chris Bischoff (ANL), John DeLuisi (NOAA), Huiting Mao (SUNY Albany), Petropavloskikh (NOAA), Joyce Penner (Univ. of Mich.), Don Heath (RSI, Inc.), and Betsy Weatherhead (Univ. of Colorado).

1. Focused Discussion

Two topical areas, climate-chemical processes and climate-chemistry interaction, were used for focused discussion. The first topic concerns the processes that are important to both atmospheric chemistry and climate, while the second topic deals with the subsequent effects of these processes on climate, atmospheric chemistry, or both.

1.1 Climate-Chemical Processes

• Increased cirrus cloud due to global warming and frequency of contrails associated with aircraft emissions of particles in the upper troposphere and lower stratosphere may affect the photodissociation rates (J-values).
• Changes in the frequency of lightning events due to climate changes may potentially change the NO_x production rate.
• Accuracy in representing cloud and precipitation processes in large-scale chemistry-transport models (CTMs) needs to be addressed. In particular, aerosol scavenging is sensitive to any change in the frequency of precipitation or its nature, e.g., strength of convection.
• The effects of relative humidity in influencing aerosol growth need to be improved in models with interactive climate and chemistry.

Because of the dependence of atmospheric chemistry on the climate state (clouds moisture, winds etc.), CTMs need to examine this aspect by either using various simulated climate states or objective analysis datasets.

• Emissions of volatile organic compounds (VOCs) from vegetation are sensitive to the nature of the vegetation as well as to temperature and humidity. Any change in climate state might result in changes in the magnitude and species mix of such emissions.
• Because of the larger sensitivity of radiative forcing to changes in upper tropospheric ozone, the aspects of transport in 3-D CTM need to be addressed.
• The influence of anthropogenic aerosols on cloud droplet concentrations is established leading to the potential for influence on precipitation removal, the radiation field, and aqueous chemistry. However, this influence has been virtually unexamined to date. Absorbing aerosol may influence cloud heating with potential further consequences on cloud frequency and precipitation and associated influences on atmospheric chemistry.

• There exists a misconnection and disconnection between regional and global models as well as between measurements and modeling efforts.
• Although there is increasing reliance on CTMs, there are relatively few head-to-head comparisons of such models, and even fewer where there are measurement data that can serve as arbiter. There exists a need to conduct model intercomparisons, not just in comparison of concentrations of model species but also, for example, through examination of ratios concentrations of key species. The latter would allow examination of the accuracy of the treatment of fast chemistry as distinct from the accuracy of the transport schemes.
• There exists a lack of good measurement of concurrent fields that influence tropospheric ozone and a lack of systematic programs of sondes or satellite data sets by which such data might be obtained on a regular basis. Don Heath called attention to an approach based on the Umkehr method that can get column ozone NO₂, SO₂, and aerosol optical depth from zenith radiance measurements.

While individual research efforts will continue, there is a need to develop an ACP community model as platform for testing modules and examining sensitivity to different representations of physical and chemical processes. It would lead to a reduction in duplication of efforts and in the mean time to accommodate a diversity of approaches. However, issues of infrastructure to document, to maintain, and to make it available to users need to be addressed.