M. Fromm

An assessment of southern hemisphere stratospheric NOx enhancements due to transport from the upper atmosphere

Data from the Halogen Occultation Experiment (HALOE) are used to evaluate the contribution of upper atmospheric NOx to the stratospheric polar vortex. Using CH4 and potential vorticity as tracers, an isolated region of enhanced NOx is shown to occur in the Southern Hemisphere (SH) polar vortex almost every spring from 1991–1996. The magnitude of this enhancement varies according to the Ap auroral activity index. Up to half of the NOx in the mid-stratospheric SH polar vortex may be due to particle precipitation. The peak enhancement occurred in 1991 with a magnitude of 3–5% of the NOy, source due to N2O oxidation.

The Polar Ozone and Aerosol Measurement instrument

The second Polar Ozone and Aerosol Measurement instrument (POAM II) is a spaceborne experiment designed to measure the vertical profiles of ozone, water vapor, nitrogen dioxide, aerosol extinction, and temperature in the polar stratosphere and upper troposphere with a vertical resolution of about 1 km. Measurements are made by the solar occultation technique. The instrument package, which has a mass of less than 25 kg, is carried on the Satellite Pour lObservation de la Terre (SPOT) 3 spacecraft and has a design lifetime of 3–5 years. POAM II has provided data on the south polar ozone hole, north and south polar ozone phenomena, the spatial and temporal variability of stratospheric aerosols and polar stratospheric clouds, and has detected polar mesospheric clouds.

POAM III measurements of dehydration in the Antarctic lower stratosphere

We present measurements of stratospheric water vapor and aerosols in the Antarctic from the POAM III instrument during the period April through December 1998. The measured variations in water vapor enable us to study both descent in the vortex and the effect of dehydration that occurs in the lower stratosphere below ∼23 km when the temperature drops below the frost point in July. There is a temporal correlation between the dehydration that occurs in July and an increase in high aerosol optical depth events in the lower stratosphere, suggesting that these events are due to the presence of ice PSCs. When temperatures warm up there is some rehydration at the highest altitudes of the dehydrated region (∼20–23 km), probably resulting from descent within the vortex. At ∼12 km rehydration is probably the result of mixing in of air from outside the vortex. The temperature increase in October produces little rehydration at 17 km and no clear rehydration at 14 km, suggesting that the water has precipitated out of these layers.

An analysis of POAM II solar occultation observations of polar mesospheric clouds in the southern hemisphere

The second Polar Ozone and Aerosol Measurement (POAM II) instrument is a space-borne visible/near IR photometer which uses the solar occultation technique to measure vertical profiles of ozone, nitrogen dioxide, and water vapor as well as aerosol extinction and atmospheric temperature in the stratosphere and upper troposphere. Here we report on the detection of polar mesospheric clouds (PMCs) in the high-latitude southern hemisphere by POAM II during the 1993 and 1994 summer seasons. These measurements are noteworthy because they are the first measurements of PMCs in atmospheric extinction. The POAM II PMC data set has been analyzed using a simple geometric cloud model. We find that mean cloud altitudes deduced from these data are 82–83 km, consistent with previous ground-based and satellite measurements. In addition, the 0.7 km vertical resolution of POAM II allows for accurate determination of cloud thickness. For the PMCs detected by POAM II we find a mean thickness of 2.4 km. The mean peak slant optical depth was determined to be 1.2×10−3 for the 1993 season and 1.8×10−3 for the 1994 season, corresponding to a cloud extinction coefficient of 3.9×10−6 and 6.1×10−6 km−1, respectively. The multichannel capability of POAM II also makes it possible to study the wavelength dependence of the measured slant optical depth for the clouds with largest extinction. The results of this analysis suggest an upper limit to the modal particle radii for these clouds of approximately 70 nm.

POAM III observations of arctic ozone loss for the 1999/2000 winter

[1]xa0During the Stratospheric Aerosol and Gas Experiment (SAGE) III Ozone Loss and Validation Experiment (SOLVE)/Third European Stratospheric Experiment on Ozone (THESEO) campaign, Polar Ozone and Aerosol Measurement (POAM) III sampled in the vortex core, on the vortex edge, and outside the vortex on a near-daily basis from December 1999 through mid-March 2000. During this period, POAM observed a substantial amount of ozone decline. For example, ozone mixing ratios in the core of the vortex dropped from about 3.5 ppmv in mid-January to about 2 ppmv by mid-March at 500 K. The ozone chemical loss indicated by these measurements is assessed using two methodologies. First, the POAM data is used to construct vortex-averaged ozone profiles, which are advected downward using vortex average descent rates. The maximum ozone loss (1 January to 15 March) is found to be about 1.8 ppmv. In a second approach, the REPROBUS 3-D CTM is used to specify the passive ozone distribution throughout the winter. The chemical loss in the vortex is estimated by performing a point-by-point subtraction of the POAM measurements inside the vortex from the model passive ozone evaluated at the time and location of the POAM measurements. Both ozone loss estimates are in general agreement and they agree well with published loss estimates from ER2 and ozonesonde measurements.

Comparison of POAM III ozone measurements with correlative aircraft and balloon data during SOLVE

[1]xa0The Polar Ozone and Aerosol Measurement (POAM) III instrument operated continuously during the Stratospheric Aerosol and Gas Experiment (SAGE) III Ozone Loss and Validation Experiment (SOLVE) mission, making approximately 1400 ozone profile measurements at high latitudes both inside and outside the Arctic polar vortex. The wealth of ozone measurements obtained from a variety of instruments and platforms during SOLVE provided a unique opportunity to compare correlative measurements with the POAM III data set. In this paper, we validate the POAM III version 3.0 ozone against measurements from seven different instruments that operated as part of the combined SOLVE/THESEO 2000 campaign. These include the airborne UV Differential Absorption Lidar (UV DIAL) and the Airborne Raman Ozone and Temperature Lidar (AROTEL) instruments on the DC-8, the dual-beam UV-Absorption Ozone Photometer on the ER-2, the MkIV Interferometer balloon instrument, the Laboratoire de Physique Moleculaire et Applications and Differential Optical Absorption Spectroscopy (LPMA/DOAS) balloon gondola, the JPL in situ ozone instrument on the Observations of the Middle Stratosphere (OMS) balloon platform, and the Systeme DAnalyze par Observations Zenithales (SAOZ) balloon sonde. The resulting comparisons show a remarkable degree of consistency despite the very different measurement techniques inherent in the data sets and thus provide a strong validation of the POAM III version 3.0 ozone. This is particularly true in the primary 14–30 km region, where there are significant overlaps with all seven instruments. At these altitudes, POAM III agrees with all the data sets to within 7–10% with no detectable bias. The observed differences are within the combined errors of POAM III and the correlative measurements. Above 30 km, only a handful of SOLVE correlative measurements exist and the comparisons are highly variable. Therefore, the results are inconclusive. Below 14 km, the SOLVE comparisons also show a large amount of scatter and it is difficult to evaluate their consistency, although the number of correlative measurements is large. The UV DIAL, DOAS, and JPL/OMS comparisons show differences of up to 15% but no consistent bias. The ER-2, MkIV, and SAOZ comparisons, on the other hand, indicate a high POAM bias of 10–20% at the lower altitudes. In general, the SOLVE validation results presented here are consistent with the validation of the POAM III version 3.0 ozone using SAGE II and Halogen Occultation Experiment (HALOE) satellite data and in situ electrochemical cell (ECC) ozonesonde data.

Correcting the record of volcanic stratospheric aerosol impact: Nabro and Sarychev Peak

Since 2010, several papers have been published that reveal a pattern of discrepancies between stratospheric aerosol data from the Optical Spectrograph and Infrared Imaging System (OSIRIS) instrument and other measurements and model simulations of volcanic plumes from Kasatochi, Sarychev Peak, and Nabro volcanoes. OSIRIS measurements show two discrepancies, a posteruption lag in aerosol onset/increase and a low bias in maximum stratospheric aerosol optical depth. Assumed robustness of the OSIRIS data drove various conclusions, some controversial, such as the contention that the June 2011 Nabro plume was strictly tropospheric, and entered the stratosphere indirectly via the Asian monsoon. Those conclusions were driven by OSIRIS data and a Smithsonian Institution report of strictly tropospheric injection heights. We address the issue of Nabros eruption chronology and injection height, and the reasons for the OSIRIS aerosol discrepancies. We lay out the time line of Nabro injection height with geostationary image data, and stratospheric plume evolution after eruption onset using retrievals of sulfur dioxide and sulfate aerosol. The observations show that Nabro injected sulfur directly to or above the tropopause upon the initial eruption on 12/13 June and again on 16 June 2011. Next, OSIRIS data are examined for nonvolcanic and volcanically perturbed conditions. In nonvolcanic conditions OSIRIS profiles systematically terminate 1–4 km above the tropopause. Additionally, OSIRIS profiles terminate when 750u2009nm aerosol extinction exceeds ∼0.0025u2009km−1, a level that is commonly exceeded after volcanic injections. Our findings largely resolve the discrepancies in published works involving OSIRIS aerosol data and offer a correction to the Nabro injection-height and eruption chronology.

POAM II ozone observations in the Antarctic ozone hole in 1994, 1995, and 1996

We present an overview of Polar Ozone and Aerosol Measurement (POAM) II satellite-based observations of ozone in the Antarctic ozone hole in 1994, 1995, and 1996. The POAM II observations are consistent with previous observations suggesting that ozone loss in the ozone hole is confined to the polar vortex. Ozone concentrations are observed to decrease by nearly a factor of 10 near 20 km during the ozone hole formation period, and a reduction in ozone was observed up to 24 km. The timing of ozone loss and recovery was similar in each year. Ozone concentrations begin to decrease in July, and the period of largest depletion observed by POAM II occurs between early September and early October, when the observations are obtained at high southern latitudes (82°–88°S) near the vortex center. However, ozone concentrations were consistently lower (by about 10%) in 1996, throughout the ozone hole altitude region and time period, than in the other two years. We have also used the POAM II observations to compute vertical profiles of monthly averaged ozone photochemical loss rates as a function of potential temperature in August (450–800 K) and September (450–700 K) of each year, incorporating a correction for diabatic descent. We find that the ozone loss rates are not significantly different from zero in August 1994 at any potential temperature level. However, we do find significant chemical loss in August 1995 below 600 K, and in August 1996 at all levels up to 700 K. Maximum monthly averaged ozone chemical loss rates occurred in September near 500 K in each year (1994: 0.1±0.004 parts per million by volume per day (ppmv/d); 1995 and 1996: 0.08±0.004 ppmv/d). Generally, in September, loss rates were larger in 1994 than in 1995 and 1996 below 550 K, and above 550 K the largest loss rates occurred in 1996. We find significant chemical loss up to at least 700 K in September in all three years. Finally, the POAM II observations show that in late spring, after the ozone hole chemical processing has been completed, ozone mixing ratios are lower inside the Antarctic vortex (relative to outside the vortex) at all levels between at least 450 K and 1500 K, presumably resulting from a combination of dynamical and chemical effects.

Reconstruction of three‐dimensional ozone fields using POAM III during SOLVE

[1]xa0In this paper we demonstrate the utility of the Polar Ozone and Aerosol Measurement (POAM) III data for providing semiglobal three-dimensional ozone fields during the Stratospheric Aerosol and Gas Experiment (SAGE) III Ozone Loss and Validation Experiment (SOLVE) winter. As a solar occultation instrument, POAM III measurements were limited to latitudes of 63°N to 68°N during the SOLVE campaign but covered a wide range of potential vorticity. Using established mapping techniques, we have used the relation between potential vorticity and ozone measured by POAM III to calculate three-dimensional ozone mixing ratio fields throughout the Northern Hemisphere on a daily basis during the 1999/2000 winter. To validate the results, we have extensively compared profiles obtained from ozonesondes and the Halogen Occultation Experiment to the proxy O3 interpolated horizontally and vertically to the correlative measurement locations. On average, the proxy O3 agrees with the correlative observations to better than ∼5%, at potential temperatures below about 900 K and latitudes above about 30°N, demonstrating the reliability of the reconstructed O3 fields in these regions. We discuss the application of the POAM proxy ozone profiles for calculating photolysis rates along the ER-2 and DC-8 flight tracks during the SOLVE campaign, and we present a qualitative picture of the evolution of polar stratospheric ozone throughout the winter.

Observations and analysis of polar stratospheric clouds detected by POAM III during the 1999/2000 Northern Hemisphere winter

[1]xa0We present an overview of polar stratospheric cloud (PSC) measurements obtained by POAM III in the 1999/2000 Northern Hemisphere winter. PSCs were observed at POAM latitudes from mid-November to 15 March. PSCs in the early season generally occurred between 17 and 25 km. The central altitude of the PSC observations, roughly 21 km, is unchanged between November and late January. PSCs were not observed between 7 and 27 February. When they reappeared, they formed at distinctly lower altitudes, centered roughly at 16 km. We also present both qualitative and quantitative comparisons with airborne lidar and in situ balloon measurements of PSCs obtained over the Norwegian Sea and Scandinavia over the 25–27 January time period. We find that the large-scale PSC altitude features and morphology are well reproduced in the POAM measurements. Finally, we use PSC occurrence probabilities, analyzed as a function of ambient temperature relative to the NAT saturation point, to infer irreversible denitrification. This denitrification is observed to maximize in late February at levels of at least 75% in the 19–21 km region, with similar values in the 16–18 km region. No denitrification was inferred above 21 km or below 16 km.