Beschreibung:
<jats:p>The solar eclipse on 11 August 1999 provided a rare opportunity to observe the remarkably dynamic character of atmospheric photochemistry. OH formation is driven by sunlight, and the rapid changes in light intensity associated with a solar eclipse provide a unique, yet natural perturbation experiment to study the response of OH and the ensuing chemistry. Highly time‐resolved measurements of OH and its rate of primary production were made at ground level during a 97% solar eclipse at Silwood Park, Ascot (51°25′N, 0°41′W) on 11 August 1999. The solar ultraviolet flux fell almost to nighttime levels, and the OH concentration decreased dramatically to below the detection limit of the instrument (2.1×10<jats:sup>5</jats:sup> molecule cm<jats:sup>−3</jats:sup>), before increasing again. The OH concentration is well correlated (r=0.88) to its rate of primary production from ozone photolysis. Shortly after maximum eclipse the concentration of ozone fell to 60% of its value at first contact. The study provides a striking demonstration of the dynamics of photochemical processes in the planetary boundary layer.</jats:p>