Atmospheric Chemistry in the Earth System

Chemistry in the atmospheric boundary layer

Chemical and physical processes in the planetary boundary layer critically depend on fine scale properties of the Earth’s surface. The exchange of gases between soil, vegetation, and surface water on the one hand side and the atmosphere on the other hand side is accomplished by micro-scale transport phenomena. This leads to high spatial heterogeneity of co-reactant gases (segregation) at scales much lower than resolved even by air quality models. Moreover, pollution levels depend on photochemical kinetics which is inherently non-linear with respect to the concentration of precursors. Therefore, we study the influence of more elaborate representations of the surface and boundary layer processes on lower tropospheric chemistry. In particular we implement and investigate the impact of:

• Urban schemes (esp. heat island effects)
• Canopy schemes for forests (focus on shading effects and change in turbulent mixing)
• High-resolution emission / external data sets
• Different boundary layer parametrisations for turbulent exchange

Pollution and weather

Weather determines the physical conditions under which pollution and pollutant emissions develop. Vice versa, gaseous and particulate pollutants affect local radiative forcing and cloud formation. Moreover, the short-term responses of terrestrial vegetation to weather extremes involve significant changes of trace gases emission and removal. Specifically, we investigate:

• The impact of evapo-transpiration on pollutants during droughts and heat waves
• The role of temperature and humidity in the photochemical production of ozone during weather extremes
• The links between humidity and atmospheric brown carbon and the related regional warming

Atmospheric composition and climate

The chemistry of the atmosphere mediates diverse couplings between the biosphere and climate. We investigate the short-term feedbacks which may counteract or dampen perturbations of the physical state of the atmosphere. We are interested in building mechanistic representations of how volatile organic compounds may affect aerosol and cloud formation and thus the radiative balance of the atmosphere. In particular we focus on the significance of:

• Non-methane hydrocarbons from terrestrial vegetation
• Organo-sulfur compounds from ocean phytoplankton

Ongoing model development

The assessment and the exploration of the role of atmospheric chemistry in the Earth system rely on efficient use of supercomputers for ever increasing detail and resolution of simulations. In this context we push the frontiers of knowledge along with the technical frontiers for atmospheric chemistry models within the HGF-ESM project. Our developments are targeted to efficiently use upcoming exascale supercomputing technologies for our simulations. Specifically, we

• are engaged in the porting of the MESSy submodel MECCA to GPUs with CUDA C, CUDA libraries and OpenACC
• implement and test matrix-free integrators for large and stiff ODE systems with higher order time-stepping schemes.
• develop a prototype “MESSy dwarf”, i.e., a framework to run and tests each MESSy submodel or a combination of MESSy submodels outside of the MESSy legacy models in a carefully refined (test) setup.
• take actively part in the development of ICON/MESSy which probably will replace ECHAM5/MESSy in the future.

These developments can be considered as IEK-8’s contribution to the Helmholtz Association project PL-ExaESM.