| Summary: | We present C 2 –C 6 nonmethane hydrocarbon (NMHC) measurements from canister samples obtained in the extratropical lower stratosphere during the fall (November/December 1995), winter (March 1997), and summer seasons (July 1998) as part of the stratosphere-troposphere experiments by aircraft measurements campaign. The flights were carried out from Amsterdam (Netherlands, 52°N, 4.5°E) during fall, from Kiruna (Sweden, 68°N, 20°E) during winter, and from Timmins (Canada, 48.2°N, 70.3°W) during summer. The NMHC measurements have been evaluated along with concurrent in situ measurements of acetone (CH 3 COCH 3 ), CO, O 3 , N 2 O, and CFC-12 (CCl 2 F 2 ). The vertical distributions of NMHC and acetone as a function of O 3 and potential temperature in the lowermost stratosphere show a strong seasonality. Enhanced concentrations of NMHC + CH 3 COCH 3 were found during July up to potential temperatures of Θ = 370 K, whereas during March this was limited to Θ = 340 K, in agreement with stronger isentropic cross-tropopause transport during summer. Increasing methyl chloride (CH 3 Cl) concentrations with altitude were measured during July, pointing to mixing at the subtropical tropopause. During summer and fall, mean NMHC + acetone concentrations were more than a factor of 2 higher than that during winter. Box model calculations indicate that the observed acetone levels of 0.5–1 ppbv can explain 30–50% of the enhanced OH radical concentrations in the summertime lowermost stratosphere. Using mass balance calculations, we show that a significant tropospheric fraction (≤30%) was present up to Θ = 370 K in the summertime lowermost stratosphere. During winter, the tropospheric fraction approached zero at about Θ = 350 K. The time between selected troposphere-to-stratosphere mixing events and the aircraft measurements has been estimated at 3–14 days. Our results emphasize that isentropic cross-tropopause transport can be a fast process occurring on timescales of days to weeks.
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