A. Waibel

Aircraft measurements of O3, HNO3 and N2O in the winter Arctic lower stratosphere during the Stratosphere‐Troposphere Experiment by Aircraft Measurements (STREAM) 1

Simultaneous in situ measurements of O3, HNO3, and N2O were performed in the Arctic (68°–74°N) lower stratosphere during February 1993 on board a Cessna Citation aircraft up to 12.5 km altitude, during the first Stratosphere-Troposphere Experiment by Aircraft Measurements (STREAM) campaign. Strong variations in the concentrations, distributions, and ratios of these trace gases were found from the maximum altitude down to the tropopause. Close to the tropopause, vortex air was present with relatively low N2O concentrations. The observed N2O-HNO3 relation agrees with earlier measurements of total nitrogen and N2O inside the vortex, suggesting subsidence of vortex air across the bottom of the vortex. This air also contained low O3 concentrations relative to N2O, indicating enhanced O3 loss by chemical reactions involving stratospheric particles. Based on trajectory calculations and assuming a potential temperature cooling rate of 0.6 K d−1, we estimate an O3 loss of 4–7 ppbv d−1 (0.9–1.2% d−1), in the Arctic lower stratosphere for the period January–February 1993. Air parcels originating from middle latitudes, containing relatively low O3 and N2O concentrations, may have originated from the vortex earlier in the winter. In addition, the results also show high HNO3 concentrations relative to O3 and N2O. Air parcels originating from high latitudes may have been enriched in HNO3 by sedimentation and evaporation of nitric acid containing particles, which would explain the relatively high HNO3 concentrations and HNO3/O3 ratios measured. Heterogeneous chemistry on sulfuric acid particles, probably enhanced in concentration by gravitational settling of the Pinatubo aerosol, is the most plausible explanation for the observed high HNO3 concentrations relative to N2O in air parcels originating from midlatitudes.

Highly elevated carbon monoxide concentrations in the upper troposphere and lowermost stratosphere at northern midlatitudes during the STREAM II summer campaign in 1994

Abstract During the second Stratosphere Troposphere Experiment by Aircraft Measurements (STREAM II) five flights up to 12 km altitude were performed over the North Sea, Great Britain, The Netherlands, and Germany, with a Cessna Citation II twin jet aircraft during the period 20–29 July 1994. Carbon monoxide (CO) concentrations in the lowermost stratosphere, which were measured by tunable diode laser absorption spectroscopy, varied between typical background values around 40 parts per billion by volume (ppbv) and highly elevated levels of up to 300 ppbv. The high CO concentrations were observed before a cold front passed over western Europe. The enhanced CO abundance was most probably caused by boreal fires in Canada in connection with convective mixing and subsequent horizontal advection over the North Atlantic Ocean.

Observations of high concentrations of total reactive nitrogen (NO y ) and nitric acid (HNO3) in the lower Arctic stratosphere during the Stratosphere‐Troposphere Experiment by Aircraft Measurements (STREAM) II campaign in February 1995

Simultaneous in situ measurements of NO y , HNO 3 , O 3 , N 2 O, and CO have been performed in the lower stratosphere during the Stratosphere-Troposphere Experiment by Aircraft Measurements (STREAM) II intensive winter campaign in February 1995 from Kiruna airport (northern Sweden) with a Cessna Citation II twinjet aircraft up to a maximum altitude of 12.8 km. The flights were coordinated with the Arctic Second European Stratospheric Arctic and Midlatitude Experiment (SESAME) winter campaign. Strongly elevated levels of total reactive nitrogen (NO Y ) and its most abundant contributing species, nitric acid (HNO 3 ), with mixing ratios up to 9 parts per billion by volume (ppbv), were observed during all flights at altitudes near 12 km. On average, the measured NO concentrations exceed the expected levels by a factor of 2-3. Normal background O Y has been calculated from observed N 2 O mixing ratios using the NO Y -N 2 O correlation reported for the undisturbed northern hemisphere. This indicates that subsidence of air in the vortex alone cannot explain these findings. We propose that the elevated NO Y concentrations were caused by nitrification of the lower stratosphere associated with sedimentation and evaporation of polar stratospheric cloud particles that carry down HNO 3 from higher altitudes, that is, from altitudes up to about 25 km.

In situ trace gas and particle measurements in the summer lower stratosphere during STREAM II : Implications for O-3 production

In situ aircraft measurements of O3, CO,HNO3, and aerosol particles are presented,performed over the North Sea region in the summerlower stratosphere during the STREAM II campaign(Stratosphere Troposphere Experiments by AircraftMeasurements) in July 1994. Occasionally, high COconcentrations of 200-300 pbbv were measured in thelowermost stratosphere, together with relatively highHNO3 concentrations up to 1.6 ppbv. The particlenumber concentration (at standard pressure andtemperature) between 0.018-1 μm decreased acrossthe tropopause, from >1000 cm-3 in the uppertroposphere to <500 cm-3 in the lowermoststratosphere. Since the CO sources are found in thetroposphere, the elevated CO mixing ratios areattributed to mixing of polluted tropospheric air intothe lowermost extratropical stratosphere. Further wehave used a chemical model to illustrate that nitrogenoxide reservoir species (mainly HNO3) determinethe availability of NOx (=NO + NO2) andtherefore largely control the total net O3production in the lower kilometers of thestratosphere. Model simulations, applying additionalNOx perturbations from aircraft, show that theO3 production efficiency of NOx is smallerthan previously assumed, under conditions withrelatively high HNO3 mixing ratios, as observedduring STREAM II. The model simulations furthersuggest a relatively high O3 productionefficiency from CO oxidation, as a result of therelatively high ambient HNO3 and NOxconcentrations, implying that upward transport of COrich air enhances O3 production in the lowermoststratosphere. Analysis of the measurements and themodel calculations suggest that the lowermoststratosphere is a transition region in which thechemistry deviates from both the upper troposphere andlower stratosphere.

Analyzing atmospheric trace gases and aerosols using passenger aircraft

CARIBIC (Civil Aircraft for the Regular Investigation of the Atmosphere Based on an Instrument Container) resumed regular measurement flights with an extended scientific payload in December 2004. After an automated measurement container was successfully deployed on intercontinental flights using a Boeing 767 from 1997 to 2002, a far more powerful package is deployed using a new Airbus A340-600 made available by Lufthansa German Airlines (Star Alliance). The new CARIBIC system will help address a range of current atmospheric science questions during its projected lifetime of 10 years. European and Japanese scientists are developing a variety of atmospheric chemistry research and monitoring projects based on the use of passenger aircraft. This is a logical approach with a main advantage being that near-global coverage is obtained, in contrast to limited coverage through research aircraft-based expeditions. Moreover, highly detailed and consistent data sets can be acquired, as compared to satellite observations in general. In addition, even compared to land-based observatories, operational costs are moderate.