Monday, 28 May 2012

Science at Sunrise

Guest Writer: Glenn M. Wolfe, University of Wisconsin, Madison

Like a delicious cake, that atmosphere is comprised of many layers. The thickness of – and mixing between – layers is determined by a number of meteorological phenomena that vary across time and space. Most terrestrial life on the Earth dwells in the lowermost region, the planetary boundary layer (PBL). Because humans are creatures of the surface, the immediate impact of our emissions is strongly felt here. This brings to mind the intriguing question of how the atmosphere would respond to a future world with flying cars, but I diverge…
The height of the PBL is mainly controlled by the sun. Solar heating at the ground warms the air, which rises while mixing turbulently with surrounding air (for an example of turbulent mixing, watch the steam rise from your coffee). The vertical extent of this mixing can be as much as 1 or 2 km on a hot summer’s day. At night, the PBL collapses and the region of mixing at the surface is shallower (less than 100 m).
These boundary layer dynamics can impact air quality. For example, pretend that the PBL is a box. We emit gases into this box at a constant rate.  If the height of our PBL box decreases by a factor of 10, then the concentration of gases (the amount per unit volume) will increase by a similar factor. In other words, we’re putting the same amount of pollutants into a smaller box. On the other hand, many secondary pollutants – such as ozone and particulate matter, the key components of urban smog – are only produced through sunlight-driven processing of surface emissions.
As a scientific platform, the Zeppelin affords the unique opportunity to characterize the transition from nighttime to daytime boundary layers – to unravel the complex confluence of chemistry and meteorology at sunrise. That is, if you don’t mind working by moonlight.

At 2:15 AM, a dozen drowsy scientists rallied in the hangar to begin flight preparations. An early start was needed to build two instruments into the cabin for the gas-phase photochemistry package (strong winds precluded doing this on the previous day). In defiance of sleep deprivation, spirits were high and we were all excited for this ambitious flight plan. The Zeppelin ground crew and pilots also worked efficiently to ensure an on-time take-off, and at 4:30 AM the airship departed for the Cabauw tower.

After conducting several height profiles around sunrise (6:00 AM), the airship returned to Rotterdam to refuel and then sped back to Cabauw. Based on computer models and previous tower measurements, our Dutch collaborators predicted that the daytime boundary layer would begin to develop around 8:00 AM. To capture this growth and its influence on atmospheric composition, the Zeppelin flew continuous height profiles near the tower.

After five such profiles, the airship returned to Rotterdam and docked at 11:00 AM. The scientists followed their normal routine of post-flight calibrations and celebrated a successful conclusion to our journey in The Netherlands.
We slept well that night.
Preliminary data shows that the Zeppelin did indeed dip in and out of the nascent PBL. Equally remarkable is the fact that all instruments worked properly throughout the flight – a rare occurrence on any field campaign. This truly unique dataset will provide new insights on the coupling between human activities and natural processes.

Google Earth overlay of the Zeppelin flight path (in yellow). The inset shows the altitude in meters as a function of time in blue.

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