John S. Hornstein

Observations of boreal forest fire smoke in the stratosphere by POAM III, SAGE II, and lidar in 1998

A substantial increase in stratospheric aerosol was recorded between May and October 1998 between 55° and 70°N. This phenomenon was recorded in the absence of reported volcanic eruptions with stratospheric impact potential. The POAM III and SAGE II instruments made numerous measurements of layers of enhanced aerosol extinction substantially higher than typical values 3 to 5 km above the tropopause. A comparison of these observations with lidar profiles, TOMS aerosol index data, and forest fire statistics reveals a strong link between stratospheric aerosol and forest fire smoke. Our analysis strongly suggests that smoke from boreal forest fires was lofted across the tropopause in substantial amounts in several episodes occurring in Canada and eastern Russia. Observations reveal a broad zonal increase in stratospheric aerosol that persisted for at least three months.

Observations of Antarctic polar stratospheric clouds by POAM II: 1994-1996

The Polar Ozone and Aerosol Measurement (POAM) II solar occultation instrument has made extensive measurements of polar stratospheric clouds (PSCs) since launch in September 1993. In a polar orbit similar to that of the Stratospheric Aerosol Measurement (SAM) II experiment but measuring to within 2 latitude of the south pole, POAM II observations of PSCs provide a valuable geographic and temporal extension of the SAM II PSC climatology. The cloud detection algorithm used to identify PSCs from POAM II measurements is described. POAM II PSC data are also examined in comparison with coincident lidar PSC observations. Results from the 1994 to 1996 Antarctic fall/winter/spring seasons are presented and related qualitatively to the SAM II PSC climatology. The frequency of PSC occurrence increases during the Antarctic winter, reaching a maximum of about 71% of all POAM II measurements in August. There is a strong longitudinal variation in the cloud frequency, which is closely related to longitudinal temperature patterns. A broad minimum in PSC frequency is centered near the international dateline and a broad maximum is centered about 315°E, in the lee of the Antarctic Peninsula, where the PSC frequency is about twice that near the minimum. In May, PSCs are observed at an average altitude of 24 km, with the altitudes moving downward as the altitude of the coldest air descends within the polar vortex during the winter. By October the average PSC altitude is 17 km.

An analysis of Polar Ozone and Aerosol Measurement (POAM) II Arctic polar stratospheric cloud observations, 1993–1996

The Polar Ozone and Aerosol Measurement (POAM) II instrument made numerous observations of polar stratospheric clouds (PSCs) in the northern hemisphere winters of 1993/1994 through 1995/1996. An updated POAM II PSC detection algorithm, described herein, is applied to POAM II 1060 nm aerosol extinction profiles to distinguish PSCs from noncloud measurements. The impact of the updated algorithm on previously published Antarctic PSC statistics is discussed. Operating sporadically in the 1993/1994 PSC season (defined as November through April), but continuously in the following two winters, POAM II made approximately 340 PSC profile measurements. We analyze the POAM II PSC observations with respect to polar vortex location, temperature, longitude, time, and altitude. Daily PSC probability (defined as the number of PSC profiles inside the polar vortex relative to the number of all profiles located inside the vortex) exceeds 60% during the most intense PSC episodes. There is considerable year-to-year variability in PSC probability, preferred location, and timing of onset and final appearance. The POAM II data also reveal interannual differences in the seasonal change of PSC altitude. Certain observations of opaque clouds are used to infer Type II PSCs. The pattern of observed PSCs is discussed with respect to recent studies of Arctic ozone loss.

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 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 and Simulation of Stratospheric Ozone Distributions during the 2002 Austral Winter

Abstract Satellite-based solar occultation measurements during the 2002 austral winter have been used to reconstruct global, three-dimensional ozone distributions. The reconstruction method uses correlations between potential vorticity and ozone to derive “proxy” distributions from the geographically limited occultation observations. Ozone profiles from the Halogen Occultation Experiment (HALOE), the Polar Ozone and Aerosol Measurement III (POAM III), and the Stratospheric Aerosol and Gas Experiment II and III (SAGE II and III) are incorporated into the analysis. Because this is one of the first uses of SAGE III data in a scientific analysis, preliminary validation results are shown. The reconstruction method is described, with particular emphasis on uncertainties caused by noisy and/or multivalued correlations. The evolution of the solar occultation data and proxy ozone fields throughout the winter is described, and differences with respect to previous winters are characterized. The results support the i...

An overview of POAM II aerosol measurments at 1.06 µm

We present aerosol measurements by the Polar Ozone and Aerosol Measurement (POAM) II instrument, from November 1993 through February 1996. Aerosol extinction profiles are in good agreement with coincident SAGE II measurements from 15 to 24 km. Extinction is continuing to decline since the eruption of Mt. Pinatubo. Seasonal variation in extinction in both hemispheres is examined on constant ground-referenced and tropopause-referenced altitude surfaces. Aerosol extinction peaks in the winter in the north outside the vortex. This also appears to occur in the south, but cannot be followed throughout the winter since POAM II measurements occur entirely inside the vortex at this time. A summer maximum at 19 km and above in the south is observed when the extinction ratio outside the vortex is plotted at a constant tropopause-referenced altitude. This is not true for the north. We conclude that in late spring in the south POAM II is sampling low extinction, vortex-processed air, or that an unidentified mechanism operates in the south, but not in the north. Signatures of polar stratospheric clouds (PSCs) are prevalent in the southern hemisphere data. PSCs are also observed in the northern hemisphere, but much less frequently. Evidence of enhanced diabatic descent in the polar vortex in both hemispheres is observed at 500 and 600 K. At 400 K the distinction between extinction ratios inside and outside the vortex in early winter is not as clear as at higher altitudes.

Preliminary results from POAM II: Stratospheric ozone at high northern latitudes

We present results from the first 18 months of measurements by the Polar Ozone and Aerosol Measurement experiment (POAM II;) in the northern hemisphere. POAM II was launched on 25 September 1993, onboard the SPOT-3 satellite into a sun-synchronous polar orbit. The results presented here span a latitude range from about 55° to 71°, and an altitude range from about 18 to 35 km. Our results compare well with SAGE II data from 1986 to 1991. The observed variability in the zonally-averaged ozone retrievals is due to a convolution of the true seasonal variation in ozone with the seasonal variation in latitude of the POAM II measurements. The seasonal variations in ozone at different altitudes reflect a transition between summer-time photochemical and dynamical control around 25 km. The longitudinal variation in ozone densities correlates well with the position of the measurement with respect to the polar vortex. Mixing ratios inside the vortex during the winters of 1993–1994 and 1994–1995 are indicative of enhanced descent within the vortex, as well as other dynamical processes and possibly chemical depletion.

First results from POAM II: The dissipation of the 1993 Antarctic Ozone Hole

POAM II is a space-borne instrument which uses the solar occultation technique to measure the vertical distribution of ozone, aerosols and polar stratospheric clouds, and other properties of the stratosphere and mesosphere. POAM II was launched aboard the SPOT 3 satellite in time to observe the dissipation of the 1993 Antarctic ozone hole. POAM data indicates that the Antarctic ozone hole dissipates from the top downward. It also supports the hypothesis that the Antarctic vortex is an effective containment vessel. However the strength of the containment appears to decrease markedly at altitudes below 18 km

A comparative study of POAMII and electrochemical concentration cell ozonesonde measurements obtained over northern Europe

Ozone profiles measured by the Polar Ozone and Aerosol Measurement (POAM II) instrument (version 5 retrievals) flown on the SPOT 3 spacecraft are compared with coincident electrochemical concentration cell (ECC) ozonesonde profiles at 11 stations over polar northern regions. The comparison study extends from December 1993 to March 1996 and contains about 500 coincidences. The satellite and ozonesonde profile coincidences are averaged separately for winter and nonwinter periods and for winter coincidences occurring inside and outside of the polar vortex. The mean differences versus altitude show a dependence on the latitude separation between the sondes and the POAM II measurements. The best estimate of the biases between the POAM II and the ECC ozone measurements is obtained after removal of the latitudinal gradient effect on the relative mean differences for the dynamically quiet nonwinter periods. The biases are negative at all altitudes indicating that the POAM version 5 retrievals are low relative to the ECC sondes. The magnitude of this bias ranges from 2–3% near the lowest and highest altitudes of profile overlap (17 and 30 km) to 7.6% from 21 to 24 km, which is comparable to the evaluated errors of both instruments.