R. L. de Zafra

Diurnal variation of stratospheric chlorine monoxide: a critical test of chlorine chemistry in the ozone layer.

This article reports measurements of the column density of stratospheric chlorine monoxide and presents a complete diurnal record of its variation (with 2-hour resolution) obtained from ground-based observations of a millimeter-wave spectral line at 278 gigahertz. Observations were carried out during October and December 1982 from Mauna Kea, Hawaii. The results reported here indicate that the mixing ratio and column density of chlorine monoxide above 30 kilometers during the daytime are ∼ 20 percent lower than model predictions based on 2.1 parts per billion of total stratospheric chlorine. The observed day-to-night variation of chlorine monoxide is, however, in good agreement with recent model predictions, confirms the existence of a nighttime reservoir for chlorine, and verifies the predicted general rate of its storage and retrieval. From this evidence, it appears that the chlorine chemistry above 30 kilometers is close to being understood in current stratospheric models. Models based on this chemistry and measured reaction rates predict a reduction in the total stratospheric ozone content in the range of 3 to 5 percent in the final steady state for an otherwise unperturbed atmosphere, although the percentage decrease in the upper stratosphere is much higher.

Millimeter wave spectroscopic measurements over the South Pole: 3. The behavior of stratospheric nitric acid through polar fall, winter, and spring

We present data from a 9-month series of ground-based measurements of stratospheric nitric acid, made over the South Pole from mid-April 1993 to mid-January 1994. Observations were typically made at 3- to 6-day intervals. Both profiles and column densities have been retrieved from pressure-broadened millimeter-wave emission spectra. These measurements provide the first quasi-continuous series of vertical mixing ratio profiles for this species in the heart of the south polar votex. Conversion of NO x to nitric acid by heterogeneous reactions, and its incorporation into polar stratospheric cloud (PSC) particles, along with subsequent gravitational settling, is considered to be the main denitrifying mechanism in the Antarctic stratosphere, setting up conditions for ozone destruction at the end of winter. In our observations, a small increase in HNO 3 was seen between April and the end of May, after which a rapid loss took place below 25 km. Column density above ∼15 km decreased to <1/4 its maximum within 30 days, and depletion continued until middle to late July, by which time the nitric acid column above 15 km had diminished by more than a factor of 10. The initial depletion was coincident with the onset of a rapid increase in lidar backscatter from polar stratospheric cloud formation at the same altitude range. Gas-phase depletion was tracked as a function of altitude and temperature and found to be consistent with the temperature and partial pressure relationship for formation of ternary mixtures of HNO 3 , H 2 SO 4 , and H 2 O. Depletion occurred ∼3 weeks earlier in 1993 than was seen in 1992 column density measurements by Van Allen et al. [1995]. In late June a new layer of HNO 3 was generated in the vicinity of 40-km altitude and, subsequently, appeared to be carried downward with general vertical transport of air within the vortex. In spring, as temperatures increased, no rapid increase of gas-phase HNO 3 was seen, indicating that gravitational settling had carried PSC-accreted nitric acid to low altitudes. By the end of observations in January 1994, mixing ratios and column densities above ∼15 km had not yet reached more than about half the values seen the previous April, indicating that a rather large increase in stratospheric HNO 3 occurs in the early austral fall over the south polar region.

Chlorine monoxide in the Antarctic spring vortex: 2. A comparison of measured and modeled diurnal cycling over McMurdo Station, 1993

We have derived chlorine monoxide (ClO) mixing ratio profiles within the Antarctic vortex on an hourly basis from ground-based measurements of pressure-broadened emission line spectra. This data set has provided the first opportunity for a detailed comparison between the output of a photochemical model and the measured in situ diurnal behavior of ClO in the Antarctic spring stratosphere. We stress the importance of the diurnal behavior in furnishing a short-term, crucial test of the catalytic chlorine chemistry which determines longer-term ozone depletion. We obtain excellent agreement between our measured and modeled diurnal change using the rate constants recommended in the 1994 Jet Propulsion Laboratory (JPL) evaluation, giving support to current understanding of perturbed chlorine chemistry in the Antarctic spring vortex. We have furthermore found that we can use our data to narrow the listed 1994 JPL uncertainty range for the ClO dimer formation rate constant and the equilibrium constant between ClO dimer formation and thermal dissociation. We show that the new limits we set on the dimer formation rate constant reduce the uncertainty in the daily rate of chlorine catalyzed ozone loss calculated from observed ClO concentrations by ∼40% at 186–196 K. We find that a modeled total ozone loss rate including both chemistry and vertical transport, based on our measurements, agrees rather well with the amount and the linear trend of ozone loss seen throughout September in coincident balloon measurements.

Chlorine monoxide in the Antarctic spring vortex: 1. Evolution of midday vertical profiles over McMurdo Station, 1993

We have obtained a prolonged record of emission spectra from chlorine monoxide in the vicinity of McMurdo Station, Antarctica, during formation of the austral spring ozone hole of 1993. These spectra have been processed to obtain vertical mixing ratio profiles by deconvolution of pressure-broadened line shapes. The resulting profiles give a detailed evolution for both altitude distribution and mixing ratio of ClO during development of a major ozone hole event. In early September, very strong emission was observed from pressure-broadened low-altitude ClO. Deconvolutions show that this came from an unusually thick layer, extending well above 20 km in altitude. This layer decreased steadily in thickness through September, accompanied by a shift of the peak mixing ratio from ∼21 km altitude in early September to ∼17–18 km by the end of the month, indicating an apparent descent rate of order 100 meters per day, although we argue that the true descent rate is probably lower than the apparent rate. A brief, significant decrease in ClO content occured in late September when the inner vortex edge (defined by the magnitude of Ertels potential vorticity = 5.2·10−5 at ∼19–20 km) approached McMurdo, signifying that a strong gradient in ClO exists near the inner vortex edge. A rapid and apparently final deactivation of chlorine in the lower stratosphere was observed to start about October 1–2. The findings of initially large values of ClO well above 20 km are consistent with observation of polar stratospheric cloud formation in this range during the austral winter of 1993, and with observations showing increased ozone depletion above 20 km relative to previous years.

Limits on heterogeneous processing in the Antarctic spring vortex from a comparison of measured and modeled chlorine

Forty-day photochemical model runs are compared with ground-based stratospheric ClO observations taken during the austral spring of 1993. Our purpose is to explore the range of required heterogeneous processing within which we can reproduce the duration and degree of chlorine activation within the Antarctic spring vortex. Heterogeneous processing on nitric acid trihydrate (NAT) polar stratospheric clouds (PSCs) or supercooled ternary solution (STS) -type particles is shown to be necessary to maintain chlorine in active forms during ozone hole formation in September, even for small HNO 3 amounts, or chlorine deactivates sooner than observed. The lower limits for the surface areas required are quite small, however. Thus the record ozone losses observed during September of 1993 may be attributed to catalytic loss due to chlorine maintained in active forms by heterogeneous processing despite the sparse particle loading of the Antarctic lower stratosphere at that time. The ozone loss rates predicted by the model during the formation of the springtime Antarctic ozone hole indeed agree quite well with observations. The one-dimensional model is also able to reproduce both the observed timing and rate for subsequent deactivation of chlorine. Renitrification from PSC evaporation is not required for this deactivation, as HCI reformation is very rapid at low ozone values.

Stratospheric ClO profiles from McMurdo Station, Antarctica, spring 1992

The authors describe ground-based measurements of ClO made at McMurdo Station, Antarctica, during September and October 1992. Vertical profiles were retrieved from molecular rotational emission spectra at 278 GHz. Peak mixing ratios of 1.6{+-}0.3 ppbv were seen in mid-September at approximately 18 km altitude, suggestive of somewhat larger quantities than were measured at the same site and season in 1987. As the core of the polar vortex moved away from McMurdo by early October, the ClO mixing ratio at this altitude dropped to less than 0.2 ppbv, coincident with increasing temperature, ozone, and NO{sub 2}. The diurnal variation of ClO was also observed. The lower stratospheric layer from 15 to 27 km was found to reach approximately midday abundance by 2-3 hours after sunrise. The column abundance in this layer began to decrease by the period 4-2 hours before sunset and had declined to approximately one quarter of its midday value by 2-0 hours before sunset. In contrast, the ClO column in the upper stratosphere, from 28 to 50 km, built up slowly until midday and remained large while sunlight persisted. 20 refs., 7 figs., 2 tabs.

N2O as an indicator of Arctic vortex dynamics: Correlations with O3 over Thule, Greenland in February and March, 1992

We have recovered vertical profiles of stratospheric N2O from spectra observed using a ground-based mm-wave spectrometer during the Arctic spring. The measurements were made from Thule, Greenland (76.3°N, 68.4°W) on nine occasions from late February to late March, 1992 as part of the Upper Atmosphere Research Satellite (UARS) Correlative Measurements Program and the European Arctic Stratospheric Ozone Experiment (EASOE). During late February Thule was under the inside edge of the Arctic vortex and mixing ratio profiles measured in that period are substantially reduced from typical high-latitude summer values. By late March the polar vortex had moved well away from Thule and N2O mixing ratios were greatly increased, coinciding with a basic change in circulation that brought in air from the Aleutian high. The motion of the vortex is also illustrated in the change in potential vorticity above Thule. A correlation with ozone balloonsonde data from Thule is made and compared to similar analyses of the Airborne Arctic Stratospheric Expedition (AASE) measurements. Within the sensitivity of our analysis, the correlation of N2O and O3 shows no evidence of ozone depletion within the vortex during this period; however, there is a distinct difference in the correlation inside and outside the vortex.

A quasi-continuous record of atmospheric opacity at λ = 1.1 mm over 34 days at Mauna Kea observatory

A quasi-continuous record of atmospheric attenuation is obtained. The data were gathered during a 24-day period in September and October 1982 and a 10-day period in December of that year. The opacity is arrived at by measuring the thermal emission of the atmosphere over a bandwidth of approximately 300 MHz. Using an experimental relationship established by Zammit and Ade (1981), opacity measurements at 1.1 mm are converted to the precipitable water vapor column overhead. With the precipitable water vapor, estimates of opacity due to water vapor can be made for other mm and FIR wavelengths. These estimates require model absorption curves for the atmosphere.

The chlorine budget of the lower polar stratosphere: Upper limits on ClO, and Implications of new Cl2O2 photolysis cross sections

Chlorine catalytic chemistry, which destroys ozone while cycling chlorine between Cl, ClO, and Cl2O2, is the primary cause of the springtime Antarctic ozone hole. We have calculated the concentrations of Cl2O2 which are in equilibrium with midday ground-based, aircraft, and satellite observations of ClO in the Antarctic spring lower stratosphere. Two significant conclusions are presented here: (1) Using the JPL 94 recommended rates and photolysis cross sections, more than ∼2.0 ppbv ClO in the polar lower stratosphere causes inferred total active chlorine to exceed the total chlorine budget. This limit is smaller than some reported ClO measurements. (2) Using smaller cross sections recently measured by Huder and DeMore [1995], the amount of Cl2O2 in midday equilibrium with measured ClO is approximately doubled. Activated chlorine inferred from many measurements then exceeds total chlorine in the lower stratosphere, suggesting these cross sections may be too small.

Accuracy of profile retrievals from mm-wave spectra of ClO and N/sub 2/O

The retrieval accuracy of a Chahine-Twomey retrieval algorithm, acting on pressure-broadened mm-wave rotational spectra measured with ground-based equipment, is explored for two radically different profiles, the first representing a two-layered Antarctic ClO distribution having a maximum mixing ratio of /spl sim/1.5 ppbv and no tropospheric component, and the second representing a typical polar N/sub 2/O profile, with a tropospheric mixing ratio of 300 ppbv dropping rapidly in the low to mid-stratosphere. The uncertainties or errors due to intrinsic retrieval limits, practical levels of random noise, uncertainties in input parameters (atmospheric temperature and pressure profiles, pressure broadening), and in calibration of instrument sensitivity are all explored, and an overall error budget established a function of altitude.<<ETX>>