Influence of Radical Recyling on Spatial Distributions of HOx in the Planetary Boundary Layer - Zeppelin-Based Observations

Abstract:

Tropospheric OH is the most important oxidant in the chemical degradation of atmospheric pollutants leading to a large variety of oxidised products. Often, OH reactions produce hydroperoxy radicals (HO₂) which can recycle OH by reaction with NO or O₃. The chemical interconversion of OH and HO₂(collectively HOx) occurs on a fast time scale of seconds to minutes. Owing to their high reactivity and short chemical lifetime, substantial spatial variability of HOx is expected in ambient air with inhomogeneous trace-gas distribution. This is particularly the case in the planetary boundary layer (PBL) where most tropospheric pollutants are emitted near Earth's surface and are then distributed by transport.

In summer 2012, a Zeppelin NT was used as an airborne platform to investigate the spatial variation of HOx and other trace gases in the PBL in the Po Valley (Italy) as part of the Pan-European Gas-AeroSOls-climate interaction Study (PEGASOS). HOx and OH reactivity were measured by laser-induced fluorescence. Other on-board measurements included O₃, CO, NO, NO₂, HCHO, HONO, VOCs, photolysis frequencies, particle number concentration, and meteorological parameters. Due to the slow flight speed and precise navigation of the Zeppelin, the concentrations of HOx and trace gases could be measured with high spatial resolution. Vertical profiles were recorded repeatedly at altitudes between 75 m and 900m above ground. In the morning, measured vertical distributions of trace gases such as CO, NOx or VOCs visualise the dynamically evolving structure of the PBL. They show a pronounced effect on the radical cycling of HOx and therefore on the concentration profiles of OH and HO₂. This presentation will show examples of the Zeppelin-based observations and discuss the role of HOx radical recycling in the evolving PBL.