P. A. Romashkin

Comparison of MkIV balloon and ER‐2 aircraft measurements of atmospheric trace gases

On May 8, 1997, vertical profiles of over 30 different gases were measured remotely in solar occultation by the Jet Propulsion Laboratory MkIV Interferometer during a balloon flight launched from Fairbanks, Alaska. These gases included H 2 O, N 2 O, CH 4 , CO, NO x , NO y , HCI, ClNO 3 , CCl 2 F 2 , CCl 3 F, CCl 4 , CHClF 2 , CClF 2 CCl 2 F, SF 6 , CH 3 Cl, and C 2 H 6 , all of which were also measured in situ by instruments on board the NASA ER-2 aircraft, which was making flights from Fairbanks during this same early May time period as part of the Photochemistry of Ozone Loss in the Arctic Region in Summer (POLARIS) experiment. A comparison of the gas volume mixing ratios in the upper troposphere and lower stratosphere reveals agreement better than 5% for most gases. The three significant exceptions to this are SF 6 and CCl 4 for which the remote measurements exceed the in situ observations by 15-20% at all altitudes, and H 2 O for which the remote measurements are up to 30% smaller than the in situ observations near the hygropause.

Chemical depletion of Arctic ozone in winter 1999/2000

During Arctic winters with a cold, stable stratospheric circulation, reactions on the surface of polar stratospheric clouds (PSCs) lead to elevated abundances of chlorine monoxide (ClO) that, in the presence of sunlight, destroy ozone. Here we show that PSCs were more widespread during the 1999/2000 Arctic winter than for any other Arctic winter in the past two decades. We have used three fundamentally different approaches to derive the degree of chemical ozone loss from ozonesonde, balloon, aircraft, and satellite instruments. We show that the ozone losses derived from these different instruments and approaches agree very well, resulting in a high level of confidence in the results. Chemical processes led to a 70% reduction of ozone for a region ∼1 km thick of the lower stratosphere, the largest degree of local loss ever reported for the Arctic. The Match analysis of ozonesonde data shows that the accumulated chemical loss of ozone inside the Arctic vortex totaled 117 ± 14 Dobson units (DU) by the end of winter. This loss, combined with dynamical redistribution of air parcels, resulted in a 88 ± 13 DU reduction in total column ozone compared to the amount that would have been present in the absence of any chemical loss. The chemical loss of ozone throughout the winter was nearly balanced by dynamical resupply of ozone to the vortex, resulting in a relatively constant value of total ozone of 340 ± 50 DU between early January and late March. This observation of nearly constant total ozone in the Arctic vortex is in contrast to the increase of total column ozone between January and March that is observed during most years.

Severe and extensive denitrification in the 1999–2000 Arctic winter stratosphere

Observations in the 1999-2000 Arctic winter stratosphere show the most severe and extensive denitrification ever observed in the northern hemisphere. Denitrification was inferred from in situ measurements conducted during high-altitude aircraft flights between January and March 2000. Average removal of more than 60% of the reactive nitrogen reservoir (NO y ) was observed in air masses throughout the core of the Arctic vortex. Denitrification was observed at altitudes between 16 and 21 km, with the most severe denitrification observed at 20 to 21 km. Nitrified air masses were also observed, primarily at lower altitudes. These results show that denitrification in the Arctic lower stratosphere can approach the severity and extent of that previously observed only in the Antarctic.

Closure of the total hydrogen budget of the northern extratropical lower stratosphere

Methane (CH4), molecular hydrogen (H2), and water vapor (H2O) were measured concurrently on board the NASA ER-2 aircraft during the 1995–1996 Stratospheric Tracers of Atmospheric Transport (STRAT) and 1997 Photochemistry of Ozone Loss in the Arctic Region in Summer (POLARIS) campaigns. Correlations between these three main hydrogen reservoirs in the northern extratropical lower stratosphere are examined to evaluate H2O production from CH4 and H2 oxidation. The expected ratio of stratospheric H2O production (PH2O)to CH4 destruction (LCH4) = −1.973±0.003 is calculated from an evaluation of CH4 and H2 oxidation reactions and the relationship between H2 and CH4 mixing ratios measured during STRAT. Correlations between H2O and CH4 were tight and linear only for air masses with mean ages ≥3.8 years, restricting this analysis predominantly to latitudes between 40° and 90°N and potential temperatures between 470 and 540 K. The mean observed ΔH2O/CH4 (−2.15±0.18) is in statistical agreement with the expected PH2O/LCH4. The annual mean stratospheric entry mixing ratio for H2O calculated from this slope is 4.0 ± 0.3 ppm. The quantity H2O + 2·CH4 is quasi-conserved at 7.4 ± 0.5 ppm in older air masses in the northern extratropical lower stratosphere. Significant departure of H2O + 2·CH4 from the mean value is a sensitive indicator of processes which influence H2O without affecting CH4, such as dehydration in a polar vortex or near the tropical tropopause. No significant trend is observed in ER-2 aircraft data for H2O + 2·CH4 in the lower stratosphere from 1993 through 1997.

In Situ Measurements of Long-Lived Trace Gases in the Lower Stratosphere by Gas Chromatography

Abstract Detailed information on the four-channel Airborne Chromatograph for Atmospheric Trace Species (ACATS-IV), used to measure long-lived atmospheric trace gases, is presented. Since ACATS-IV was last described in the literature, the temporal resolution of some measurements was tripled during 1997–99, chromatography was significantly changed, and data processing improved. ACATS-IV presently measures CCl3F [chlorofluorocarbon (CFC)-11], CCl2FCClF2 (CFC-113), CH3CCl3 (methyl chloroform), CCl4 (carbon tetrachloride), CH4 (methane), H2 (hydrogen), and CHCl3 (chloroform) every 140 s, and N2O (nitrous oxide), CCl2F2 (CFC-12), CBrClF2 (halon-1211), and SF6 (sulfur hexafluoride) every 70 s. An in-depth description of the instrument operation, standardization, calibration, and data processing is provided, along with a discussion of precision and uncertainties of ambient air measurements for several airborne missions.

The budget and partitioning of stratospheric chlorine during the 1997 Arctic summer

Volume mixing ratio profiles of HCl, HOCl, ClNO 3 , CH 3 Cl, CFC-12, CFC-11, CCl 4 , HCFC-22, and CFC-113 were measured simultaneously from 9 to 38 km by the Jet Propulsion Laboratory MkIV Fourier Transform Infrared solar absorption spectrometer during two balloon flights from Fairbanks, Alaska (64.8°N), on May 8 and July 8, 1997. The altitude variation of total organic chlorine (CCl y ), total inorganic chlorine (Cl y ), and the nearly constant value (3.7±0.2 ppbv) of their sum (Cl TOT ) demonstrates that the stratospheric chlorine species available to react with O 3 are supplied by the decomposition of organic chlorinated compounds whose abundances are well quantified. Measured profiles of HCl and ClNO 3 agree well with profiles found by photochemical model (differences < 10% for altitudes below 35 km) constrained by various other constituents measured by MkIV. The production of HCl by ClO + OH plays a relatively small role in the partitioning of HCl and ClNO 3 for the sampled air masses. However, better agreement with the measured profiles of HCl and ClNO 3 is obtained when this source of HCl is included in the model. Both the measured and calculated [ClNO 3 ]/[HCl] ratios exhibit the expected near linear variation with [O 3 ] 2 /[CH 4 ] over a broad range of altitudes. MkIV measurements of HCl, ClNO 3 , and CCl y agree well with ER-2 in situ observations of these quantities for directly comparable air masses. These results demonstrate good understanding of the budget of stratospheric chlorine and that the partitioning of inorganic chlorine is accurately described by photochemical models that employ JPL97 reaction rates and production of HCl from ClO + OH for the environmental conditions encountered: relatively warm temperatures, long periods of solar illumination, and relatively low aerosol surface areas.

Effect of the tropospheric trend on the stratospheric tracer-tracer correlations: Methyl chloroform

In situ measurements in the lower stratosphere in 1997 produced distinct intersecting correlations between methyl chloroform (CH 3 CCl 3 ) and chlorofluorocarbon- 11 (CFC-11) for air parcels sampled during spring and summer in the same physical space. The disagreement between spring and summer correlations in the stratosphere below 225 ppt of CFC-I 1 is in the opposite direction than is expected from the documented tropospheric trend of-15 ppt yr -1 . Summertime air parcels were enriched in CH 3 CCl 3 compared to spring parcels. Mean ages based on sulfur hexafluoride (SF 6 ) for CH 3 CCl 3 -rich air masses observed during summer were older by about a year than mean ages of CH 3 CCl 3 -depleted air parcels sampled during spring. The ages of stratospheric air masses and the documented history of methyl chloroform mixing ratios in the troposphere were used to normalize the stratospheric input of methyl chloroform, as if it had no tropospheric trend. The normalized correlations with and without parameterization for phyotoylic loss on the age distribution are more compact and revealed hidden anomalous mixing lines between polar vortex and midlatitude air. Higher mixing ratios of CH 3 CCl 3 , which the older air had at the time of the stratospheric entry, caused the summer correlation to be elevated on the tracer-tracer plot below 225 ppt of CFC- 11. The ongoing tropospheric decrease of CH 3 CCl 3 caused an inflection point and crossing of spring and summer correlations at 225 ppt of CFC-11. These results suggest that the tropospheric history is especially important for correlations of species that were increasing in the 1980s and are now decreasing (some CFCs, CCl 4 , and CH 3 CCl 3 ). Distinct correlations between CH 3 CCl 3 and CFC-11 observed for spring and summer 1997 measurements are the result of the 1991 tropospheric maximum of CH 3 CCl 3 [Prinn et al., 1995] that has propagated into the stratosphere, where air parcels with high CH 3 CCl 3 were sampled at 20 km altitude and 60°-90°N.

Comparison of in situ N2O and CH4 measurements in the upper troposphere and lower stratosphere during STRAT and POLARIS

Nitrous oxide (N 2 O) and methane (CH 4 ) were measured in the upper troposphere and lower stratosphere by multiple instruments aboard the NASA ER-2 aircraft during the 1995-1996 Stratospheric Tracers of Atmospheric Transport (STRAT) and 1997 Photochemistry of Ozone Loss in the Arctic Region in Summer (POLARIS) campaigns. Differences between coincidental, in situ measurements are examined to evaluate the agreement and variability in the agreement between these instruments during each flight. Mean N 2 O measurement differences for each flight were much smaller than limits calculated from quoted values of N 2 O measurement accuracy and for all but two flights were ≤8.7 ppb (3.5%). Mean CH 4 measurement differences for flights were similarly much smaller than calculated limits and for all but three flights were ≤65 ppb (4.4%). Typical agreement between instruments during flights averaged 6.2 ppb (2.5%) for N 2 O and 43 ppb (2.9%) for CH 4 . In contrast, for about half of the flights, the variability of N 2 O and CH 4 measurement differences exceeded limits calculated from quoted values of measurement precision. The typical measurement difference variability (1σ) during a flight averaged ±8.0 ppb (3.2%) for N 2 O and ±43 ppb (2.9%) for CH 4 . For some flights, large differences or variations in differences are attributable to the poor measurement accuracy or precision of one instrument. It is demonstrated that small offsets between the computer clocks of these instruments can result in significant differences between their coincidental N 2 O and CH 4 data, especially when there is high spatial variability in tracer abundance along a flight track.