An Investigation Of Polar Stratospheric Clouds Using Remote Sensors, In Situ Instruments, And The Perseus Remotely Piloted Aircraft

Abstract

Motivation Polar stratospheric clouds (PSCs) are believed to catalyze a series of chemical reactions that simultaneously remove oxides of nitrogen (NO,) from the air and activate a type of chlorine chemistry that destroys ozone rapidly (1). The total amount of ozone that is subsequently lost depends critically on the timing of, and on the rate of release of, NO, back into the airmass to bind with reactive chlorine. PSCs composed of nitric acid and water (called “Type I” PSCs) form when temperatures in the lower stratosphere drop below about 196-200 K (depending on altitude), and water ice clouds (called “Type 11” PSCs) form at temperatures another 5 to 10 degrees lower. Temperatures for Type I PSCs occur annually over both polar regions during the winter season (with few exceptions over the Arctic), while colder temperatures for Type I1 PSCs occur regularly over Antarctica, but less frequently over the Arctic. it is thought that nitric acid can be irreversibly removed from the air when larger PSC particles, especially those composed mainly of water ice, sediment (or snow) to lower altitudes. If this denitrification is extensive enough, as it is over Antarctica each austral winter, there is insufficient nitric acid remaining as a source of NO, to bind the reactive chlorine in springtime, and a large “ozone hole” forms [l]. Over the Arctic, denitrification has also been observed, but on a much smaller scale than over Antarctica, so ozone depletion is more limited in both altitudinal and geographic extent.