Robert L. de Zafra

On the formation of HNO3 in the Antarctic mid to upper stratosphere in winter

We address the previously unresolved puzzle of nitric acid formation in the polar winter mid to upper stratosphere, first indicated by Limb Infrared Monitor of the Stratosphere observations in the Arctic winter of 1978–1979. Several theoretical studies over the past 2 decades have tried to reproduce these observations with varying success. More recently, the onset, altitude range, and duration of the formation process have been clarified by the Cryogenic Limb Array Etalon Spectrometer onboard UARS and by a series of ground-based observations taken at the South Pole during the Antarctic winters of 1993, 1995, and 1999. Using the Stony Brook-SSt. Petersburg two-dimensional photochemical model, we have reexplored HNO3 formation via both the ion cluster chemistry and heterogeneous chemistry on sulfate aerosols considered by earlier investigators. By including what we believe to be a realistic flux of NOy from the mesosphere, we find that the model can generate observed mixing ratios through a combination of ion-clusterenhanced chemistry in the upper to mid stratosphere, augmented by heterogeneous chemistry on sulfate aerosol below ∼40 km. Results are presented which clarify the relative role of various processes and assumed NOy fluxes. These also point up the need to incorporate more accurate downward NOy fluxes in models being used to simulate the polar stratosphere. Finally, we emphasize the need to consider the influence of repartitioning of NOy or NOx into HNO3 before observed variations in amounts of the former reaching the mid to lower stratosphere in winter and early spring can properly be used as tracers to reflect variations in thermospheric-mesospheric NOy production or transport.

The microwave spectrum of cyanoacetylene in ground and excited vibrational states

Microwave transitions are reported for ten isotopic species of cyanoacetylene in the ground, v 4, v 6, v 6 and v 7 vibrational states in the region 26·5-40·0 GHz. In addition millimetre-wave transitions of HCCCN and DCCCN in the ground v 5, v 6 and v 7 vibrational states in the region 54·5-211·8 GHz have been measured. The combined data have been analysed to yield Bv, Dv, γrs, γltlt′ and qt vibration-rotation parameters, for HCCCN and DCCCN. In addition millimetre-wave measurements pertaining to the v 6 + v 7 and v 5 + v 7 vibrational states have been analysed to give values for rtt′J and approximate values of gtt′ and rtt′ (t = 5, 6; t′ = 7). Rotational constants Bv in the first excited state of the fundamental vibrations v 4, v 5, v 6 and v 7 are combined with infra-red values for v 1, v 2 and v 3 to give Be for both HCCCN and DCCCN.

Millimeter wave spectroscopic measurements over the South Pole. 2. An 11-month cycle of stratospheric ozone observations during 1993-1994

A quasi-continuous record of ozone profiles throughout the stratosphere over the South Pole has been obtained over an 11-month cycle, from February 1993 to January 1994. This record includes the first winter measurements of ozone profiles in the altitude region above ∼30 km. Observations were made approximately every 3 days, using a high-sensitivity millimeter wave spectrometer to quantitatively measure the pressure-broadened ozone rotational emission line at 276.923 GHz. Vertical mixing ratio profiles have been derived from pressure-broadened lineshapes by a deconvolution technique. A number of interesting features are present. We find a persistent double-peaked structure in the mixing ratio profiles, lasting through most of the winter period until the remains of the lower peak are destroyed by spring “ozone hole” chemistry. A new low-altitude peak is reformed in December as the vortex breaks up. With the aid of circumpolar UARS/MLS ozone maps, we interpret the lower peak as due to transport from ozone-rich regions near the edge of the continent, while the profile from ∼30 km upward, composing the “trough” region and upper peak, appears to be the result of normal polar summer photochemistry. This double-peaked structure then becomes “fossilized” within the strong, isolated, fall-winter vortex. The mixing ratio of the upper peak increases after polar sunset, which we interpret as due to poleward mixing causing an erasure of the negative poleward gradient maintained by photochemistry before sunset. Mixing ratio isopleths show a relatively steady downward trend for a 3-month period after the winter vortex pattern is established, preceded by rapid variations in ozone mixing ratios over the 20- to 40-km range. Downward transport rates derived from isopleth slopes in the upper stratosphere are significantly smaller than vertical transport derived from theoretical studies, and we propose an explanation for this discrepancy based on ozone flow from the mesosphere. Descent rates determined from ozone isopleths in the midstratosphere (25 to 35 km) are shown to be in good agreement with recent model estimates of downward transport in the winter vortex, and with the mid to lower stratospheric descent rate inferred from our own South Pole measurements of N2O. Total column measurements are in generally good agreement with those derived from a Dobson photospectrometer at the pole and from local ozonesonde measurements. All three indicate there was no significant increase in total ozone over the pole during the winter of 1993. The onset of the spring ozone hole over the pole was evident by mid- to late-August, well before local stratospheric sunrise on September 11, indicating relatively rapid poleward transport of ozone-depleted air from sunlit regions of the vortex during this period.

Millimeter wave spectroscopic measurements over the South Pole: 1. A study of stratospheric dynamics using N2O observations

Millimeter wave measurements of N2O and O3 [Cheng et al., 1995], along with several other trace gases, have been made nearly continuously from February 1993 through early January 1994 at the Amundsen-Scott Station, South Pole. In order to separate chemical and dynamical effects, this paper uses the observations of the long-lived tracer N2O to study stratospheric dynamics. The main emphasis is on the synoptic evolution of the polar vortex over an entire winter period, and quantitative results are given for various times and altitudes. Diabatic descent rates derived for different altitude levels showed the strongest descent in austral fall at high altitudes, agreeing fairly well with model predictions by Rosenfield et al. (1994). Subsidence was observed to continue until late October, well after polar sunrise. The breakdown of the vortex occurred first in the upper stratosphere, marked in the intrusion of N2O rich air at these altitudes, consistent with trajectory calculations. Our calculated descent rates are not consistent with the idea that the polar vortex is a “flowing processor”, but instead should be viewed as an isolated system.

Millimeter wave spectroscopic measurements over the South Pole: 5. Morphology and evolution of HNO3 vertical distribution, 1993 versus 1995

We compare differences and similarities in the annual stratospheric HNO 3 cycle derived from ground-based measurements at the South Pole during 1993 and 1995, after correcting an error in earlier published profile retrievals for 1993 which led to under estimation of mixing ratios. The data series presented here provide profiling over the range ∼16-48 km, and cover the fall-winter-spring cycle in the behavior of HNO 3 in the extreme Antarctic with a large degree of temporal overlap. With the exception of one gap of 20 days, the combined data sets cover a full annual cycle. The record shows an increase in HNO 3 above 30 km occurring about 20 days before sunset, which appears to be the result of higher altitude heterogeneous conversion of NO x as photolysis diminishes. Both years show a strong increase in HNO 3 beginning about polar sunset, in a layer peaking at about 25 km, as additional NO x is heterogeneously converted to nitric acid. When temperatures drop to the polar stratospheric cloud (PSC) formation range near the end of May, gas phase HNO 3 is rapidly reduced in the lower stratosphere, although at least 2-3 weeks of temperatures ≤192 K appear to be required to complete most of the gas-phase removal at the upper end of the depletion range (22-25 km). Despite a significant difference in residual sulfate loading from the explosion of Mount Pinatubo, there appears to be little gross difference in the timing and effects of PSC formation in removing gas phase HNO 3 in these 2 years, though removal may be more rapid in 1995. Incorporation of gas phase HNO 3 into PSCs appears to be nearly complete up to ∼25 km by midwinter. We also see a repeat of the formation of gas phase HNO 3 in the middle stratosphere in early midwinter of 1995 with about the same timing as in 1993, suggesting that this phenomenon is driven by a repetition of dynamical transport and appropriate temperatures and pressures in the polar night, and not (as has been suggested) by ion-based heterogeneous chemistry that requires triggering by large relativistic electron fluxes. High-altitude HNO 3 production peaks during a period of ∼20 days, but appears to persist for up to ∼40 days in the 40-45 km range, ceasing well before sunrise. This HNO 3 descends rapidly throughout the production period, at a rate in good agreement with theoretically determined midwinter subsidence rates. As noted in earlier studies, later warming of this region above PSC evaporation temperatures does not cause reappearance of large amounts of HNO 3 , indicating that most PSCs gravitationally sink out of the stratosphere before early spring. We present evidence that smaller PSCs do evaporate to ∼1 to 3.5 ppbv of HNO 3 in the lower stratosphere, however, working downward from ∼25 km as temperatures rise during the late winter. There is a delay of ∼15 days after sunrise before photolysis causes significant depletion in the altitude range below ∼30 km, where subsidence has carried virtually all higher-altitude HNO 3 by polar sunrise. Some continued subsidence and photolysis combine to keep mixing ratios less than ∼5 ppbv below 30 km until the final breakdown of the vortex in November brings larger amounts of HNO 3 with air from lower latitudes.

Millimeter wave spectroscopic measurements over the South Pole. 4. O3 and N2O during 1995 and their correlations for two quasi-annual cycles

In two separate papers we have previously reported observations of stratospheric O3 and N2O over the South Pole during the 1993 annual cycle. Here we present (1) new O3 and N2O observations at the South Pole in 1995 and (2) correlations between O3 and N2O for two 11-month observations during February 1993 to January 1994 and January-December 1995. Strong similarities exist between the two quasi-annual cycles for both O3 and N2O. A double-peaked profile again dominates O3 vertical distribution in 1995 as in 1993. Features such as a pronounced summer-fall decline in mid-stratospheric O3 followed by an early winter increase, a downward trend in the O3 contour pattern associated with vertical transport, a transient enhancement of middle to upper stratospheric O3 just before local sunrise, the timing of the ozone hole onset, and a dramatic increase of stratospheric O3 during and following vortex breakup all show good consistency between the two annual cycles. N2O observations show a good agreement between the two 11-month cycles in atmospheric descent rate during fall and winter, and in the timing of N2O recovery from diminished values during spring. We use O3-N2O correlations to further investigate the double-peaked vertical distribution of O3. During springtime warmings the O3/N2O ratio shows a tight coupling between O3 and N2O around 20 km, as transport creates the lowaltitude O3 peak. A rapid and systematic decrease of the O3/N2O ratio during summer in the 25 to 30 km region (while N2O is essentially stable) supports the increasingly dominant role of photochemistry in producing the vertical profile for O3 above ∼25 km while leaving a transport-produced layer with a relatively large mixing ratio below ∼25 km. The resulting double-peaked O3 distribution, which persists for many months, can alter the normally negative correlations between O3 and N2O in the lower and middle stratosphere, although in measurements of the N2O/O3 ratios for polar air these perturbations have often been taken to be a hallmark of catalytic ozone depletion by chlorine. The present analysis should help to clarify the influence of the relatively unique O3 vertical distribution of polar ozone when interpreting O3-N2O correlations.

Observation of a strong inverse temperature dependence for the opacity of atmospheric water vapor in the MM continuum near 280 GHz

Using atmospheric opacity measurements made at 278 GHz (9.3 cm−1) at McMurdo Station, Antarctica during the austral springs of 1986 and 1987, combined with measurements of water vapor profile and total column density from near-simultaneous balloon flights, we have determined the attenuation per mm of precipitable water vapor (pwv) at this frequency. Our data were taken at significantly lower temperatures than other measurements in the literature for which accompanying water vapor pressure and temperature data are available. The results show a strong inverse dependence with temperature: measured opacity per mm of pwv is roughly a factor of two times greater at −35°C than at −10°C and three times greater than measurements at the same wavelength at +25°C reported by Zammit and Ade. We briefly review various theories proposed to explain excess absorption in continuum regions. Our lowtemperature measurements demonstrate a significantly greater inverse temperature dependence than embodied in several formulations, theoretical or empirical, proposed to represent mm-wave attenuation as a function of temperature and water vapor. The present results are qualitatively similar to observations of strong inverse temperature dependence in the near IR, but if attributed to water vapor dimer formation, imply a greater binding energy for the dimer than generally proposed by others. There is some independent evidence for a local anomaly in temperature dependence as a function of frequency near 280 GHz. It remains to be established whether our own results are strongly frequency dependent or apply generally to the mm-wave continuum.

Correlated millimeter wave measurements of ClO, N2O, and HNO3 from McMurdo, Antarctica, during polar spring 1994

Ground-based observations of stratospheric ClO, N2O, and HNO3 were made almost continuously at McMurdo Station, Antarctica (77.9°S, 166.6°E), during the austral spring of 1994, using two separate microwave receivers. Vertical profiles of these trace gases have been retrieved from the pressure broadened emission spectra between September 4 and October 8, 1994. In early September, McMurdo was located well inside the polar vortex, and high mixing ratios of chlorine monoxide (up to 1.8 ppbv) were measured in the lower stratosphere. Because of vortex movement, later measurements were taken in edge regions, where ClO was found to be quite variable. This vortex movement also provided an opportunity to study relative changes between all three species. Almost no HNO3 was seen below 20 km during the measurement period, indicating that stratospheric air had been efficiently denitrified by polar stratospheric cloud formation. A significant increase of the nitric acid column was observed only around September 20, when McMurdo was closer to the outer edge of the vortex. At the beginning of the measurements, the vertical profiles of the inert tracer N2O had already descended so far that very little N2O was present above 20 km. During the observation period, the N2O distribution did not show strong changes except for a slight downward trend which increased with altitude. This indicates, as noted in previous years, that subsidence continued in the stratosphere over McMurdo Station until at least early October, when measurements were stopped. The temporal correlations between the behavior of ClO, N2O, HNO3, altitude, and temperature at the 50-hPa level, and of ozone measured by local ozonesondes show that changes in the atmospheric composition were partly due to dynamic effects. A backward trajectory analysis was performed to interpret the ClO data in an attempt to clarify some irregularities.

An Intercomparison of Precipitable Water Vapor Measurements Obtained During the ECOWAR Field Campaign

In this study we present an intercomparison of measurements of very low water vapor column content obtained with a Ground‐Based Millimeter‐wave Spectrometer (GBMS), Vaisala RS92k radiosondes, a Raman Lidar, and an IR Fourier Transform Spectrometer. These sets of measurements were carried out during the primary field campaign of the ECOWAR (Earth COoling by WAter vapor Radiation) project which took place on the Western Italian Alps from 3 to 16 March, 2007.

Procedure for computer-controlled milling of accurate surfaces of revolution for millimeter and far-infrared mirrors

We discuss a simple method for milling accurate off-axis parabolic mirrors with a computer-controlled milling machine. For machines with a built-in circle-cutting routine, an exact paraboloid can be milled with few computer commands and without the use of the spherical or linear approximations that have been discussed in other mirror-cutting procedures in the literature. The method given here can be adapted easily to cut offaxis sections of elliptical or spherical mirrors.