Ground-based nitric acid measurements at Arrival Heights, Antarctica, using solar and lunar Fourier transform infrared observations

Transcription

1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 109,, doi: /2004jd004665, 2004 Ground-based nitric acid measurements at Arrival Heights, Antarctica, using solar and lunar Fourier transform infrared observations S. W. Wood, 1 R. L. Batchelor, 1,2 A. Goldman, 3 C. P. Rinsland, 4 B. J. Connor, 1 F. J. Murcray, 3 T. M. Stephen, 3,5 and D. N. Heuff 2 Received 19 February 2004; accepted 25 June 2004; published 23 September [1] Nitric acid plays an important role in processes leading to stratospheric ozone loss in polar regions. Spectroscopic absorption measurements of nitric acid have been made during the sunlit part of the Antarctic year at Arrival Heights (78 S, 167 E) since the late 1980s. This paper presents the first extension of these nitric acid measurements through the winter, using the Moon as a light source. Both solar and lunar measurements for the years are presented. For the lunar measurements, additional corrections owing to emission of the atmosphere and the instrument must be made. The measurements show that the column amount of nitric acid within the polar vortex continues to increase after polar sunset reaching values of molecules cm 2 1 month after sunset. When temperatures become low enough for the condensation of water and nitric acid molecules, polar stratospheric clouds form, and rapid depletion of the gaseous nitric acid column is observed. The measurements have captured this event particularly well in At the time of the polar sunrise the column amounts of nitric acid are extremely depleted and are as low as molecules cm 2. During the spring period the site can sample air from both inside and outside the polar vortex. Inside the vortex the observed gradual recovery of HNO 3 in the spring and summer months is consistent with predictions of denitrification occurring during the Antarctic winter. For measurements outside the vortex, higher HNO 3 columns are observed. INDEX TERMS: 0340 Atmospheric Composition and Structure: Middle atmosphere composition and chemistry; 0394 Atmospheric Composition and Structure: Instruments and techniques; 9310 Information Related to Geographic Region: Antarctica; KEYWORDS: stratosphere, polar, winter Citation: Wood, S. W., R. L. Batchelor, A. Goldman, C. P. Rinsland, B. J. Connor, F. J. Murcray, T. M. Stephen, and D. N. Heuff (2004), Ground-based nitric acid measurements at Arrival Heights, Antarctica, using solar and lunar Fourier transform infrared observations, J. Geophys. Res., 109,, doi: /2004jd Introduction [2] Nitric acid (HNO 3 ) is one of the primary reservoir species of stratospheric odd-nitrogen (NO y =NO+NO 2 + HNO 3 +HO 2 NO 2 +2N 2 O 5 + ClONO 2 + minor constituents). Its role in polar ozone depletion is twofold. Firstly, and most importantly, nitric acid has been identified as one of the key constituents of polar stratospheric clouds (PSCs), which are present in the Antarctic winter stratosphere [McCormick and Trepte, 1986; World Meteorological Organization (WMO), 2003, chapter 3, section 3.2.2]. Formation of PSCs is a vital step in polar ozone loss, as 1 National Institute of Water and Atmospheric Research, Omakau, New Zealand. 2 University of Canterbury, Christchurch, New Zealand. 3 Department of Physics, University of Denver, Denver, Colorado, USA. 4 NASA Langley Research Center, Hampton, Virginia, USA. 5 Deceased 15 February Copyright 2004 by the American Geophysical Union /04/2004JD they provide surfaces for heterogeneous chemistry that includes the release of chlorine from its reservoir species into more reactive forms which contribute to catalytic ozone loss [Solomon et al., 1986]. [3] Secondly, nitric acid sequesters reactive odd-nitrogen, NO x (NO + NO 2 ), molecules. This prevents their reaction with the reactive chlorine species ClO and Cl 2 O 2, which are active participants in ozone depletion, and so prevents the conversion of these active species to the chemically inactive reservoir species ClONO 2. If the nitric acid is frozen in PSC particles or is removed permanently by the sedimentation of large PSC particles containing nitric acid (denitrification), then this role is further enhanced, as ozone loss is able to continue for a longer period of time than if photolysis of gaseous nitric acid to NO x were able to occur [WMO, 2003, chapter 3, section 3.2.3]. [4] Measurements of nitric acid have been made using various Fourier transform spectrometers, with the Sun as a light source, at Arrival Heights since the late 1980s [Wood et al., 2002; Connor et al., 1998; Keys et al., 1993; Murcray et al., 1989]. In order to understand the chemical partition- 1of9

2 ing and processes that precondition the stratosphere for ozone depletion, measurements during the winter are crucial. As such, measurements have been made using the Moon as a light source since 1998, allowing the seasonal behavior of HNO 3 to be observed through all seasons for the first time at the site. Moonlight measurements are much more challenging than their corresponding solar ones. The intensity of the reflected sunlight from the full Moon is 5 orders of magnitude less than that seen from the Sun [Notholt, 1994]. In the 10-micron wavelength region, however, this is increased owing to thermal emission from the Moon itself, allowing us to detect it with a standard mercury-cadmium-telluride detector. The signal is still considerably weaker than for solar measurements, and this means that emissions from the instrument and the atmosphere must also be considered. [5] This paper will outline the procedure used to make nitric acid measurements using the light of the Moon and the procedures used to correct for the emission of the instrument and atmosphere; it will present a time series of nitric acid measurements from Arrival Heights. Additionally, an attempt to quantify the uncertainties in both the solar and lunar data sets will be made, including comparisons between solar and lunar measurements on some days during the spring and autumn periods. 2. Experimental Procedure [6] Measurements were made with a commercial Bruker 120 M Fourier transform infrared spectrometer housed in a heated laboratory at Arrival Heights (77.83 S, E, 200 m above sea level). A mirror tracking system mounted on the roof of the laboratory tracked the Sun or Moon and fed light into the instrument. A standard liquid nitrogen cooled photoconductive mercury-cadmium-telluride detector was used with an uncooled 7 14 mm filter. [7] Each lunar measurement was made up of 20 scans of the interferometer, representing nearly 10 min of integration time, and was at a resolution of 0.02 cm 1. Resolution in this case is defined as 0.9 divided by the scan length. A field of view of 6.4 mrad was used. After each group of lunar measurements a background sky scan was made pointing at the same elevation as the Moon but 10 forward in azimuth. To get adequate signal-to-noise for this measurement in a reasonable timeframe, the resolution was lowered to 0.1 cm 1, but the field of view was kept at 6.4 mrad. This measurement allowed the empirical correction for the broadband emission from the sky tracker and instrument that appear in the lunar spectra as a nonzero baseline. We have also performed similar measurements at different viewing directions and found these to be useful in identifying conditions where the quality of measurements was lowered by the presence of clouds. [8] The lunar measurements required a Moon that was at or very near to full. They could be made for 1 week centered on the full Moon, after which time it was found that the infrared intensity of the Moon was insufficient to allow a signal to be detected. Small amounts of cloud in the direction of the Moon significantly affected the signal and probably account for most of the observed changes in intensity in the measured spectra. Observations were made only when the Moon was visually clear of cloud. At times Figure 1. Averaging kernels showing the sensitivity of the retrieved total columns to changes in HNO 3 as a function of altitude, for typical measurement conditions. The solar measurement (solid line) is shown and is a better measurement. The lunar measurement (dashed line) is also shown. of the year when there was sunlight, measurements were made at a higher spectral resolution of cm 1 and typically consisted of two coadded scans with a smaller field of view of 3.9 mrad. 3. Data Analysis [9] The retrieval of nitric acid column information from the spectral data was performed using the profile retrieval algorithm SFIT2 [Pougatchev et al., 1995; Rinsland et al., 1998]. The SFIT2 forward model uses a line by line calculation of the monochromatic atmospheric spectrum using line parameters from the HITRAN database. Atmospheric ray tracing and the calculation of air masses are based on the FASTCODE algorithm [Meier et al., 2004]. The effect of the instrument line shape is simulated by Fourier transforming the monochromatic atmospheric spectrum, applying appropriate apodisation and phase functions, and then reverse Fourier transforming. The retrieval, or inverse model, uses optimal estimation, based on the formalism of Rodgers [2000], with Newtonian iteration to account for nonlinearities in the spectral calculation. The constraints on the retrieval consist of a priori estimates of the various parameters to be retrieved and of their uncertainty. Averaging kernel calculations, which simulate a measurement and retrieval for typical measurement conditions, show the sensitivity of the retrieved total column to changes in the atmospheric profile of HNO 3 as a function of altitude. These are shown in Figure 1. Both the solar and lunar measurements can be seen to respond to changes in HNO 3 over a range of altitudes. The solar measurement response is better in that its kernel is reasonably close to 1 from 8 35 km in altitude. The averaging kernel for the lunar measurement is more peaked and is responsive to change over a smaller altitude range, km. The lunar 2of9

3 measurement is thus less accurate, as expected from the lower spectral resolution. Fortunately, the region of sensitivity is where most of the nitric acid is typically located and where we expect the most variability. [10] Averaging kernels for mixing ratios at individual heights in the profile show that there is some vertical profile information in the solar measurements but very little in the lunar measurements. In order to analyze the entire data set in a consistent way, we have restricted this study to retrievals of total columns only. [11] The chosen a priori vertical mixing ratio profile used for this analysis was an average of HNO 3 profiles from high southern latitudes made by the Cryogenic Limb Array Etalon Spectrometer (CLAES) instrument [Roche et al., 1993a] on board the Upper Atmosphere Research Satellite (UARS) between October 1991 and May The annual average was calculated from monthly mean profiles of CLAES measurements available as part of the UARS reference atmosphere project from the Stratospheric Processes and their Role in Climate (SPARC) data center ( Above 40 km we used a standard mixing ratio profile. Below 14 km we smoothly interpolated to an a priori mixing ratio below 8 km of 5 pptv, consistent with that recorded at surface level at the Neumeyer Antarctic station [Jones et al., 1999]. Uncertainties in the a priori profiles were based on the variability of the Figure 3. Sample spectral fit of a typical solar measurement plotted in the same way as Figure 2. The residuals are shown on the same scale as for Figure 2. The greater absorption in this spectrum is due to a higher measurement zenith angle of The retrieved total column from this measurement is molecules cm 2, and it was recorded on 4 April Figure 2. Sample spectral fit of a typical lunar measurement. The recorded spectrum (solid line) and the fit achieved to it (dashed line) are plotted. The residual difference between these two curves is plotted (first panel). The date was 18 April 2003, the zenith angle was 73.17, and the retrieved HNO 3 column, after correction for atmospheric emission, was molecules cm 2. monthly mean CLAES profiles that went into the annual mean, peaking at 0.56 in the lower stratosphere. Outside the altitude range of the CLAES measurements the a priori uncertainties were set to 0.2. National Centers for Environmental Prediction (NCEP) analyses provided to the Network for Detection of Stratospheric Change database for each day of measurement have been used to define the pressure and temperature as a function of altitude. [12] The microwindow used for the retrieval of HNO 3 was cm 1. Previous studies [e.g., Goldman et al., 1999] have used slightly smaller microwindows, but in this study the higher noise in the spectra prompted a wider wavelength range to include more spectral points. Minor interfering gases H 2 O, OCS, and NH 3 were fitted by scaling their reference vertical profiles by a single multiplicative factor. An instrumental line shape for the maximum path difference and field of view of the instrument was assumed. Because the lunar spectra were of variable quality, we used a signal-to-noise ratio determined for each spectrum to define the appropriate value to give to the retrieval. Values ranged from 140 to 390, with most around 250. A single representative signal-to-noise ratio of 300 was assumed for the solar measurements. A sample spectral fit of a lunar spectrum is shown in Figure 2. For comparison, a fit for a solar measurement is shown in Figure 3. Note that the fit allows for the retrieved profile to be different from the a 3of9

4 Figure 4. The contribution of atmospheric emission to the lunar measurements of HNO 3, plotted as a function of atmospheric temperature. Typical average weighted stratospheric temperatures (as explained in text) are 235 K in January, 215 K in April, 205 K in May, and 193 K in June August. Also shown is the identical calculation for a solar measurement, demonstrating that the effect can be neglected. priori profile and that the retrieval will be varying the profiles more in the stratospheric region where we have stated that the uncertainty or variability is higher. An examination of the retrieved profiles shows that this is indeed the case. Since the uncertainties are based on independent measurements of the HNO 3 vertical profile, these retrievals are constrained to behave more like the real atmosphere than a simpler analysis such as scaling the entire vertical profile by a single scaling factor. [13] Because of the low infrared intensity of the moonlight, an offset in the spectra due to emission of the atmosphere, tracker and the instrument is seen. The sections below give descriptions of how this emission was corrected for Zero-Level Correction [14] Emission from the tracker and instrument is seen as a broadband emission causing the baseline of the spectra to shift above zero. Correction for this was made for each measurement using the background sky scans described in section 2. In the wave number region that the nitric acid retrieval uses, the zero-level shift was calculated from the difference between the average height of the background sky spectra and the average height of the unabsorbed parts of the lunar spectra. The effect of this correction is to increase the depth of the absorption lines by shifting the baseline back to zero. The zero-level shift was then input directly as a parameter to be used by SFIT2 for the retrieval Atmospheric Emission [15] As well as broadband emission, the nitric acid in the atmosphere is emitting some radiation specifically in the regions where we see absorption lines in the lunar spectrum. This emission is a function of the temperature and the amount of nitric acid in the path. An ideal correction for this would be the subtraction of a background sky spectrum taken at the same zenith angle but different azimuthal angle from the Moon. In order to do this, however, we would need a spectrum of similar characteristics and resolution to the lunar spectra in order to not introduce phase effects, and this is not practical [Meier, 1997]. Instead, we have calculated the emission contribution as described by Notholt et al. [1997], and further in the work of Notholt and Lehmann [2003], for the wave number at the center of our retrieval microwindow ( cm 1 ). An effective temperature calculated for each measurement using the appropriate temperature profile weighted by the retrieved number density profile of HNO 3 has been used. A correction factor for each lunar measurement has been applied directly to the retrieved columns. The emission contribution depends strongly on the time of year, between 8% early in the winter season and 4% in the colder months (see Figure 4). A calculation of the effect of this emission in a solar measurement shows that the effect is negligible (dashed line, Figure 4). 4. Error Analysis [16] We have attempted to quantify the various uncertainties in these measurements. It is usual to classify errors as either random or systematic, but this isn t always a clear distinction. Random uncertainties from independent sources will give different effects from one day to the next and must be taken into consideration for comparisons between individual measurements. Systematic uncertainties tend to bias the measurements in a more consistent way and are discussed separately. They include both fixed and variable errors. The various error sources are summarized in Table 1, with some discussion below Random Uncertainties Measurement Error [17] The simplest uncertainty to quantify is that arising from the random noise in the raw measurement, i.e., the recorded spectrum. We have assumed a typical signal-tonoise ratio of 250 in the spectra and have assumed that the errors in individual spectral points are uncorrelated. From this we can calculate the resulting uncertainty in the total Table 1. Summary of Error Sources in Solar and Lunar Measurements Lunar Error, % Solar Error, % Random uncertainties Measurement error (from spectral noise) 8 2 Error from changing zenith angles Error from zero-level determination Temperature uncertainty Total random uncertainty Other uncertainties Line parameter uncertainty Smoothing error from an assumed a priori 4 2 uncertainty (see text) Total of other uncertainties Estimated total accuracy Additional error at low columns (see text) of9

5 column following the formula given in Rodgers [1990]. Values shown in Table 1 are two sigma errors in order to make them comparable to errors calculated from perturbation tests Zenith Angle Effects [18] During a 10-min lunar measurement the Moon can move up to 0.4 degrees in zenith angle, depending on its location in its orbit. Uncertainty in the total HNO 3 column amount due to assuming an average zenith angle has been quantified by looking at the extreme cases of lunar zenith angles at the start and finish times. This error varies considerably depending on whether the measurement was made when the Moon was close to its north transit, with zenith angle changing very slowly, or at a time with more rapid change in zenith angle. Solar scans were typically 3 min long, so this effect was smaller by a factor of Temperature Effects [19] From figures provided with the data, the uncertainty in the NCEP temperatures is typically ±2.5 K for this site at the heights where the measurement is sensitive and is larger at higher altitudes. The temperature uncertainty affects the retrieved columns in two ways: first in the retrieval itself, because of temperature dependencies in the absorption, and second, for the lunar measurements, in the emission correction procedure. The tabulated lunar value includes both processes Zero-Level Shift [20] For lunar measurements the background radiation can be measured with an estimated uncertainty of ±5%. This background can represent anything from 14 to 60% of the total radiation measured, depending on the varying amount of lunar radiation. Measurements where the background was >60% of the total were discarded. For a good measurement with no apparent attenuation this uncertainty is at its smallest and becomes more significant when the measurement has been degraded by subvisible cloud. While an explicit correction for zero-level shift is not made for solar measurements, there is a small uncertainty in the zero level due to possible nonlinearities in the detector. There are no saturated lines close to the microwindow to check this precisely, but saturated lines at other points in the recorded spectrum indicate there may be an uncertainty of 1 2% in the retrieved column due to uncertainty in the true zero level. In a recent intercomparison of measurements of HNO 3 this was one of the larger errors affecting the repeatability of measurements made simultaneously with different instruments [Goldman et al., 1999] Smoothing Error [21] With an optimal estimation technique, there is some uncertainty in the retrieved result because of the uncertainty in the a priori information that is provided to the retrieval. This is called the smoothing error. The smoothing error can have both random and systematic components, which are influenced by the a priori used in the retrievals. The smoothing error is difficult to objectively estimate, because to do so requires knowledge of the variability of the quantity being measured, preferably from some independent measurements of the same quantity [Rodgers, 1990, 2000]. [22] The smoothing error can be avoided or bypassed in some situations. We can consider the measurement as a smoothed version of the real atmospheric state, smoothed with a known function defined by the averaging kernel and the a priori information. If the measurements were then to be compared with independent measurements or model predictions, the same smoothing function could be applied to those, eliminating the smoothing error. For the purposes of this study we have made an estimate of the smoothing error in the total columns reported, but note the limitations of that estimate. [23] For calculating the smoothing error, we have used the formalism of Rodgers [1990]. We have used the same a priori uncertainty values that were used in the retrieval except for allowing for a greater variability in the troposphere to reflect our real lack of knowledge in this region. This estimate is only as good as the assumption of the a priori uncertainties. However, since they are based largely on measured data (CLAES), we feel they are realistic. The resulting uncertainty in total column is relatively small compared to other sources of error and thus not critical to our results Systematic Uncertainties Line Parameter Errors [24] Spectroscopic line parameters have been taken from the HITRAN2000 line list, with updates included as released up until 2001 [Rothman et al., 2003]. The HNO 3 line half widths are the same as used in the previous HITRAN edition. The errors in these are at least 15% [Goldman et al., 1998]. Uncertainties in the line intensities have been recently investigated [Toth et al., 2004; Flaud et al., 2003; Chackerian et al., 2003], and it can be conservatively estimated that these are 10%. These uncertainties, while large, are expected to be consistent across the entire data set Bias at Low Columns [25] Given that this data set has a high degree of variability, it was considered prudent to check the estimate of smoothing error for periods when the amount of HNO 3 is a long way from the a priori value. In general, we believe this is simulated well by our calculation of smoothing error, but for times of very low total column the bias may be larger than expected from theoretical calculations. Perturbation tests, using a priori profiles that are more likely to reflect the real atmospheric profile for a given time of the year than an annual mean profile, have confirmed this. The variations in retrieved column of HNO 3 from carrying out these tests are consistent with the quoted smoothing errors for all measurements, except for times of extreme depletion, which can be double the calculated smoothing error for solar measurements and as much as 5 times that calculated for the lower quality lunar measurements. Consequently, we believe that the lowest column amounts may be even lower than those presented here Solar and Lunar Comparisons [26] As a further check of our error estimates, we have examined the few places in the data record where both solar and lunar measurements were made within the same 24-hour period. All of these comparisons are near the equinoxes, so the Sun is very low in the sky, and hence the solar measurement may have a little more uncertainty owing to the difficulty in accurately modeling refraction at high air masses. Those comparisons that occur in the autumn when the nitric acid column is high show good agreement, 5of9

6 Figure 5. Time series of nitric acid total column measurements made from Arrival Heights, Antarctica, between 1998 and Solar measurements begin at polar sunrise in late August and continue through until the Sun sets in April. Lunar measurements are only possible for 1 week around the full Moon. Typical uncertainty, excluding the constant bias from line parameter uncertainties, is 9% for the lunar measurements (diamonds) and 4% for the solar measurements (pluses); see Table 1 and text for details. Error bars have been omitted for clarity. typically within 10% on individual measurements, which is within our calculated random and smoothing uncertainties. Those during spring, when columns are much lower, do not always agree as well, with discrepancies of sometimes 20 30%, although the largest absolute differences encountered between individual measurements for both spring and autumn periods were actually quite similar. The poorer agreement in spring is not too surprising, as the spring period is really more variable owing to changes in vortex location, and we know there is a larger relative error in the measurements when column amounts are extremely low, as discussed in section There is no evidence in these comparisons of a clear bias between solar and lunar measurements, as reported by Becker and Notholt [2000] in a similar comparison of measurements in the Arctic, nor is there evidence that errors in the measurements exceed our estimates. 5. Results and Discussion [27] Figure 5 shows a time series of both solar and lunar HNO 3 measurements made from Arrival Heights between 1998 and In midlatitude regions, HNO 3 tends to have a winter maximum. One reason for this is that the photolysis of nitric acid by sunlight and the conversion of NO and NO 2 to N 2 O 5 and HNO 3 during periods of darkness will tend to produce more HNO 3 in winter than in summer. There is also a strong latitudinal gradient with HNO 3 increasing toward the poles owing to transport. Both processes will contribute to the observed seasonal behavior at high latitudes. However, these measurements show additional features that are specific to conditions inside the Antarctic polar vortex. [28] From the beginning of each year until sunset in April the solar measurements show an increase in nitric acid total column amount to molecules cm 2. From sunset the lunar measurements show that these column amounts continue to increase, reaching values of around molecules cm 2, until temperatures inside the vortex are low enough for nitric acid to condense into PSC particles. On the basis of NCEP temperature measurements the timing of this first potential PSC formation is variable from year to year [see also WMO, 2003, chapter 3, section 3.2.1]. However, in all years we do observe a dramatic drop in the nitric acid column amount at some point during the winter. [29] Figure 6 shows the clearest of these events in the time series so far, which occurred during a full Moon period in The vertical line shown is the day of the first PSC, a thin mixed-phase (liquid and nitric acid trihydrate) cloud, observed by a lidar at nearby McMurdo Station. The onset day may be a day or two prior to that shown owing to a weather-related lack of measurements (P. Massoli, personal communication, 2003). Following this drop, total column measurements are seen to stay low, at values of molecules cm 2, for several months. This is consistent with ongoing observations of polar stratospheric clouds. [30] As the sunlight returns in the spring, because transport is still limited by the vortex, in situ processes, such as the reevaporation of PSCs, drive the slow recovery of HNO 3 within the vortex. The recovery may be further slowed by denitrification due to sedimentation of PSC particles, which would be demonstrated by low HNO 3 columns persisting into periods when temperatures are well over PSC temperatures. In the spring period the movement of the vortex means that the air sampled may sometimes be outside of the vortex. Hence the measurements show a much larger variability owing to the strong contrast in HNO 3 contained in air inside and outside the polar vortex. Where periods outside of the vortex are brief, they produce apparent outliers in the HNO 3 record. Where the periods are longer, the measurements appear to show a more typical seasonal behavior, with HNO 3 being high and then slowly decreasing with increasing day length and photolysis. [31] Figure 7 shows HNO 3 measurements for two selected spring periods, 1999 and 2002, with the associated time series of the isentropic potential vorticity (PV) at a potential temperature of 550 K, calculated from NCEP/National Center for Atmospheric Research (NCAR) analyses, and temperature at 50 hpa, also from NCEP/NCAR, above the 6of9

7 Figure 6. Time series of nitric acid total column measurements made at Arrival Heights through the winter months of The line at day 149 indicates the date of the first PSC observed by the lidar at nearby McMurdo Station, as explained in the text. Error bars show the estimated uncertainty in the columns, excluding the line parameter uncertainty. site. High absolute values of PV are associated with vortex air, and low values (at the top of the PV plots) indicate air that is from outside the vortex. The calculated position of the vortex boundary using the Nash PV boundary criteria is plotted as a dashed line to show which days are inside and outside the vortex (G. Bodeker, personal communication, 2004). In 1999, Arrival Heights was largely inside of the vortex during the spring period, and we observe the slow Figure 7. (a and b) Time series of nitric acid measurements at Arrival Heights for two selected spring periods, 1999 and 2002, respectively. (c and d) Time series of isentropic potential vorticity (PV) at a potential temperature of 550 K above the site (solid line) from National Centers for Environmental Prediction (NCEP)/National Center for Atmospheric Research (NCAR) analyses for the same two periods. High absolute values of PV (plotted at the bottom of the graph) are associated with air from inside the polar vortex. The vortex boundary is plotted (dashed line). Low absolute values of PV show that Arrival Heights is outside of the polar vortex. (e and f ) NCEP temperatures at 50 hpa above the site for the same periods in 1999 and Temperatures lower than 197 K are required for any PSCs containing nitric acid to exist. 7of9

8 increase of HNO 3 in that region. Brief times outside of the vortex show higher HNO 3 on days 290 and 324 and a longer period from days 337 to 347. In 2002 the site was outside of the vortex for much of the time, owing to unusual perturbations of the vortex [Allen et al., 2003], and we observe high values of HNO 3 during these periods. The highest columns around day 270 are indications of the collar region of high HNO 3 surrounding the vortex, as first observed by the CLAES instrument [Roche et al., 1993b], being over the measurement site. These then gradually decrease to midsummer values at the end of the year. The 1999 data also show low HNO 3 values persisting inside the vortex when stratospheric temperatures have increased well above those at which PSCs can exist, as shown in the second panel. This is a clear indication of denitrification having occurred during the winter. Regardless of the exact date of vortex breakdown and regardless of whether Arrival Heights has been inside or outside of it, by the end of the year (midsummer) the HNO 3 measurements have returned to a value of around molecules cm 2 and have resumed the more normal increase with decreasing day length. [32] In contrast to the sharp winter changes seen in these Antarctic measurements, measurements of nitric acid using a similar technique in the Arctic have shown a gradual change between a maximum in the winter and a minimum in the summer. Midsummer and autumn measurements made at Ny Ålesund, Spitsbergen (78.9 N, 11.9 E) [Notholt et al., 1997], were in agreement with comparable measurements made at Arrival Heights. Winter measurements in the Arctic were not seen to drop dramatically, instead continuing to increase to maximum values of around molecules cm 2 in the middle of winter [Notholt et al., 1997]. Without the large perturbation in HNO 3 from PSCs the return to the lower summer values was more gradual. These findings are consistent with HNO 3 measurements made by the Microwave Limb Sounder in both hemispheres, which observed much larger depletions of HNO 3 in the Antarctic than in the Arctic [Santee et al., 1999, 2000], which can be attributed to the differences in the extent and type of PSCs in the two polar regions [Tabazadeh et al., 2000]. 6. Conclusions [33] Nitric acid measurements using the Moon as a light source have been made, allowing us to present a time series of total column measurements throughout the years Measurements show the total column increasing after the polar sunset to amounts of molecules cm 2 before depleting rapidly with the formation of polar stratospheric clouds. Measurements made during the 2002 winter show this process particularly clearly. Column amounts of around molecules cm 2 are reached by the end of June and maintained throughout the polar night. After sunrise the gradual increase of nitric acid from these extremely low values is indicative of permanent denitrification having occurred within the vortex during the winter period. In some years, dynamical processes that bring air containing large amounts of nitric acid over the measurement site modify this gradual increase. Further work correlating these nitric acid measurements with PSC formation, including the quantification of the rate of loss of gaseous nitric acid and the implications for denitrification, is being undertaken. [34] Acknowledgments. Work in New Zealand and Antarctica was funded by the New Zealand Foundation for Research Science and Technology (contract CO1X0204) and also supported by the New Zealand Antarctic Institute and the University of Denver and University of Canterbury. Work at the NASA Langley Research Center was supported by NASA s Upper Atmosphere Research Program and the Atmospheric Chemistry Modeling and Analysis Program (ACMAP). Work at University of Denver was supported in part by NASA and NSF. G. E. Bodeker provided the software and data for the PV calculations. Thanks should go to all the work done by the Scott Base science technicians who made the measurements, to Justus Notholt of University of Bremen, and to Paola Massoli of Istituto di Fisica dell Atmosfera. References Allen, D. R., R. M. Bevilacqua, G. E. Nedoluha, C. E. Randall, and G. L. Manney (2003), Unusual stratospheric transport and mixing during the 2002 Antarctic winter, Geophys. Res. Lett., 30(12), 1599, doi: / 2003GL Becker, E., and J. Notholt (2000), Intercomparison and validation of FTIR measurements with the Sun, the Moon and emission in the Arctic, J. Quant. Spectros. Radiat. Transfer, 65, Chackerian, C., S. W. Sharpe, and T. A. Blake (2003), Anhydrous nitric acid integrated absorption cross sections: cm 1, J. Quant. Spectros. Radiat. Transfer, 82, Connor, B. J., N. B. Jones, S. W. Wood, J. G. Keys, C. P. Rinsland, and F. J. Murcray (1998), Retrieval of HCl and HNO 3 profiles from ground-based FTIR data using SFIT2, in Proceedings of the XVIII Quadrennial Ozone Symposium, edited by R. D. Bojkov and G. Visconti, pp , Parco Sci. e Technol. d Abruzzio, L Aquila, Italy. Flaud, J.-M., C. Picolo, B. Carli, A. Perrin, L. H. Coudert, J.-L. Teffo, and L. R. Brown (2003), Molecular line parameters for the MIPAS (Michelson Interferometer for Passive Atmospheric Sounding) Experiment, J. Atmos. Oceanic Opt., 16, Goldman, A., C. P. Rinsland, A. Perrin, and J.-M. Flaud (1998), HNO 3 line parameters: 1996 HITRAN update and new results, J. Quant. Spectros. Radiat. Transfer, 60, Goldman, A., C. Paton-Walsh, W. Bell, G. C. Toon, J.-F. Blasvier, B. Sen, M. T. Coffey, J. W. Hannigan, and W. G. Mankin (1999), Network for the Detection of Stratospheric Change Fourier transform infrared intercomparison at Table Mountain Facility, November 1996, J. Geophys. Res., 104(D23), 30,481 30,503. Jones, A. E., R. Weller, A. Minikin, E. W. Wolff, W. T. Sturges, H. P. McIntyre, S. R. Leonard, O. Schrems, and S. Bauguitte (1999), Oxidized nitrogen chemistry and speciation in the Antarctic troposphere, J. Geophys. Res., 104(D17), 21,355 21,366. Keys, J. G., P. V. Johnston, R. D. Blatherwick, and F. J. Murcray (1993), Evidence of heterogeneous reactions involving nitrogen compounds in the Antarctic stratosphere, Nature, 361, McCormick, M. P., and C. R. Trepte (1986), SAM II measurements of Antarctic PSCs and aerosols, Geophys. Res. Lett., 13(12), Meier, A. (1997), Determination of atmospheric trace gas amounts and corresponding natural isotopic ratios by means of ground-based FTIR spectroscopy in the high Arctic, in Reports on Polar Research, pp. 309, Alfred Wegener Inst. for Polar and Mar. Res., Bremerhaven, Germany. Meier, A., A. Goldman, P. Manning, T. Stephen, C. Rinsland, N. B. Jones, and S. W. Wood (2004), Improvements to air mass calculations for ground-based infrared measurements, J. Quant. Spectros. Radiat. Transfer, 83, Murcray, F. J., A. Goldman, R. Blatherwick, W. A. Matthews, and N. B. Jones (1989), HNO 3 and HCl amounts over McMurdo during the spring of 1987, J. Geophys. Res., 94(D14), 16,615 16,619. Notholt, J. (1994), The moon as a light source for FTIR measurements of stratospheric trace gases during the polar night: Application for HNO 3 in the Arctic, J. Geophys. Res., 99(D2), Notholt, J., and R. Lehmann (2003), The moon as a light source for atmospheric trace gas observations: Measurement technique and analysis method, J. Quant. Spectros. Radiat. Transfer, 76, Notholt, J., G. C. Toon, F. Stordal, S. Solberg, N. Schmidbauer, E. Becker, A. Meier, and B. Sen (1997), Seasonal variations of atmospheric trace gases in the high Arctic at 79 N, J. Geophys. Res., 102, 12,855 12,861. Pougatchev, N. S., B. J. Connor, and C. P. Rinsland (1995), Infrared measurements of the ozone vertical distribution above Kitt Peak, J. Geophys. Res., 100(D8), 16,689 16,698. 8of9

9 Rinsland, C. P., et al. (1998), Northern and Southern Hemisphere groundbased infrared spectroscopic measurements of tropospheric carbon monoxide and ethane, J. Geophys. Res., 103(D21), 28,197 28,217. Roche, A. E., J. B. Kumer, J. L. Mergenthaler, G. A. Ely, W. G. Uplinger, J. F. Potter, T. C. James, and L. W. Sterritt (1993a), The Cryogenic Limb Array Etalon Spectrometer (CLAES) on UARS: Experiment description and performance, J. Geophys. Res., 98(D6), 10,763 10,775. Roche, A. E., J. B. Kumer, and J. L. Mergenthaler (1993b), CLAES observations of ClONO 2 and HNO 3 in the Antarctic stratosphere, between June 15 and September 17, 1992, Geophys. Res. Lett., 20(12), Rodgers, C. D. (1990), Characterization and error analysis of profiles retrieved from remote sounding measurements, J. Geophys. Res., 95(D5), Rodgers, C. D. (2000), Inverse Methods for Atmospheric Sounding: Theory and Practice, World Sci., River Edge, N. J. Rothman, L. S., et al. (2003), The HITRAN molecular spectroscopic database: Edition of 2000 including updates through 2001, J. Quant. Spectros. Radiat. Transfer, 82, Santee, M. L., G. L. Manney, L. Froidevaux, W. G. Read, and J. W. Waters (1999), Six years of UARS Microwave Limb Sounder HNO 3 observations: Seasonal, interhemispheric, and interannual variations in the lower stratosphere, J. Geophys. Res., 104(D7), Santee, M. L., G. L. Manney, N. J. Livesey, and J. W. Waters (2000), UARS Microwave Limb Sounder observations of dentrification and ozone loss in the 2000 Arctic late winter, Geophys. Res. Lett., 27(19), Solomon, S., R. R. Garcia, F. S. Rowland, and D. J. Wuebbles (1986), On the depletion of Antarctic ozone, Nature, 321, Tabazadeh, A., M. L. Santee, M. Y. Danilin, H. C. Pumphrey, P. A. Newman, P. J. Hamill, and J. L. Mergenthaler (2000), Quantifying denitrification and its effect on ozone recovery, Science, 288, Toth, R. L., L. R. Brown, and E. A. Cohen (2004), Line strengths of nitric acid from 850 to 920 cm 1, J. Mol. Spectrosc., 218, Wood, S. W., et al. (2002), Validation of version 5.20 ILAS HNO 3,CH 4, N 2 O, O 3, and NO 2 using ground-based measurements at Arrival Heights and Kiruna, J. Geophys. Res., 107(D24), 8208, doi: / 2001JD World Meteorological Organization (WMO) (2003), Scientific Assessment of Ozone Depletion: 2002, edited by D. L. Albritton et al., World Meteorol. Organ., Geneva. R. L. Batchelor, B. J. Connor, and S. W. Wood, National Institute of Water and Atmospheric Research, Lauder, Private Bag 50061, Central Otago, Omakau 9182, New Zealand. (r.batchelor@niwa.co.nz; b.connor@ niwa.co.nz; s.wood@niwa.co.nz) A. Goldman and F. J. Murcray, Department of Physics, University of Denver, 2112 E. Wesley Avenue, Denver, CO 80208, USA. (goldman@ acd.ucar.edu; fmurcray@du.edu) D. N. Heuff, Department of Physics and Astronomy, University of Canterbury, Private Bag 4800, Christchurch, Canterbury 8020, New Zealand. (d.heuff@canterbury.ac.nz) C. P. Rinsland, NASA Langley Research Center, Mail Stop 401A, 21 Langley Boulevard, Hampton, VA , USA. (c.p.rinsland@larc. nasa.gov) 9of9