This session examines the variability of trace gases and aerosols in the troposphere,
and how the chemical heterogeneity challenges our ability to compare models and measurements. Papers will examine how transport, chemicophysical processes, and emissions can create the chemical patterns we observe, and how different modeling approaches (Lagrangian and Eulerian) are able to establish and sustain them. Resolved advective transport (e.g., jet streams, frontal systems) as well as convection, turbulence, precipitation and scavenging, create a mixture of chemically distinct air masses. We will examine how this heterogeneity can be observed and assessed in models. Even over the remote ocean basins we find a mixture of anthropogenic pollution mixed with the chemistry of the marine boundary layer. Pollution or land-based signatures are useful in tracing the history of air parcels, but it remains uncertain whether this diluted pollution can influence the chemical reactivity.
In addition, we look for observations and models that can evaluate the chemical patterns in terms of which combinations, or covariation of key species, makes some air parcels more reactive than others, and thus controls the evolution of tropospheric ozone and methane. Likewise, we look for observations and modeling techniques that help us understand how aerosols are modified by atmospheric processes, from generation of new particles to removal by precipitation. A fundamental evaluation of our current chemistry-climate models can be based on matching this heterogeneity.