Bruno Nardi

Ground‐based measurements of tropospheric and stratospheric BrO at Arrival Heights, Antarctica

Ground-based measurements of BrO slant column densities (SCDs) were performed using zenith sky DOAS (Differential Optical Absorption Spectroscopy) during autumn (February to May) and spring (August to October) of 1995 at Arrival Heights (77.8°S, 166.7°E). In both August and September, single episodes of sudden large BrO column enhancement (of magnitude 3.5 and 3.2 × 1014 molec. cm−2 respectively) were observed. The episode in August did not coincide with changes of other stratospheric parameters (OClO, NO2 and temperature). Furthermore, the diurnal variation in the SCD during these events was indicative of a tropospheric rather than a stratospheric absorber. The tropospheric BrO mixing ratios deduced from the data are similar to those observed by ground-based measurements in the Arctic boundary layer (∼30 ppt). Simultaneous balloon soundings, one during each of the two events, showed statistically significant (2 σ) tropospheric ozone depletion between 0.5 and 2 km in August and 1.5 and 2.8 km in September. Our results strongly suggest that halogen catalysed boundary layer ozone depletion not only occurs in the Arctic but also in Antarctica. This has the implication that Arctic Haze and anthropogenic influence is unlikely as a cause for this phenomenon.

Determining the index of refraction of polar stratospheric clouds above Andoya (69°N) by combining size‐resolved concentration and optical scattering measurements

Observations within two polar stratospheric clouds (PSCs) of aerosol scattering and size-resolved aerosol concentration were compared to infer the index of refraction of the PSC particles. The observations were completed in situ with balloon-borne aerosol counters and a laser scatterometer (692, 830 nm) and remotely with an ozone (308, 353 nm) and Rayleigh (532, 1064 nm) lidar. A Monte Carlo analysis, accounting for the errors of the individual measurements, indicates the comparison method has a precision of ±0.03 for an index of refraction range of 1.30–1.60. Measurements from all instruments were obtained in one PSC with relatively little vertical or horizontal structure. The comparison suggested that the index of refraction of the PSC particles was near 1.47±0.01 in the nondepolarizing region of the cloud and 1.52–1.56±0.04 in the depolarizing region. These values were consistent for the observations at 308, 353, 692, and 830 nm. The comparisons with the Rayleigh lidar were not as consistent. Aerosol volumes inferred from the particle measurements agree closely with volumes expected for liquid ternary aerosol (LTA) at the base of the cloud, with nitric acid trihydrate (NAT) above 23 km, in the depolarizing region, and with both LTA and NAT in the bulk of the nondepolarizing portion of the cloud. A much more limited set of measurements was obtained in a second PSC with strong vertical structure, evident in the temperature and aerosol profiles. Comparisons in this cloud were difficult because of the inherent problems in comparing in situ and remote measurements in clouds with strong vertical and horizontal structure. In this PSC the comparisons of in situ aerosol size distribution and remote aerosol scattering did not converge to a clear index of refraction.

Type I PSC-particle properties : Measurements at ALOMAR 1995 to 1997

Polar stratospheric clouds (PSC) were observed above Andenes, Norway (69°N, 16°E) with the ALOMAR Rayleigh/Mie/Raman-Lidar during three winters and with an optical particle counter on two occasions. Multiwave-lengths measurements allow the derivation of the parameters of a monomodal particle size distribution and the derivation of volume densitiespaper if the refractive index is known. The particle counter results are used to determine the best refractive index value. The relation of volume density to temperature is compared to thermodynamic models. This confirms that the PSCs observed consist mainly of a ternary solution with low water content. PSC Type I a and I b can be distinguished by backscatter ratio, color ratio and cooling rate.

Correlations between ozone loss and volcanic aerosol at altitudes below 14 km over McMurdo Station, Antarctica

Ozone and aerosol profiles over McMurdo Station, Antarctica (78°S), have been measured August–October for the years 1986–1995. This spans the development and decay of the recent perturbation to stratospheric aerosol caused by Pinatubo. Volcanic aerosol surface areas, in the 11–14 km region, peaked near 100 µm² cm−3 in 1991, decaying to 20–30 µm² cm−3 in 1992, 15–25 µm² cm−3 in 1993, and to background levels of 4–8 µm² cm−3 in 1994. Based on these measurements the volcanic aerosol signal persisted over Antarctica for three austral springs, implying an exponential decay rate of about 14 months. The aerosol below 14 km was correlated with previously unobserved ozone loss at these altitudes. Ozone loss rates of 5–15 ppb dy−1 (0.3–0.5 DU dy−1) were observed in the 10–12 and 12–14 km layers. Beginning in 1994, when the aerosol approached its pre-Pinatubo level, ozone loss diminished in the 12–14 km layer, and was not observed in the 10–12 km layer.

Spring 1996 and 1997 ozonesonde measurements over McMurdo Station, Antarctica

Ozone and temperature profiles were measured with balloon-borne instruments from McMurdo Station, Antarctica (77.85°S, 166.67°E) from late August to late October, on 38 occasions in 1996 and on 26 occasions in 1997. Minimum column ozone values of 147 DU and 141 DU were measured in 1996 and 1997, reduced from initial measurements in late August of 277 DU and 313 DU, respectively. These ozone minima are slightly higher than the lowest seasonal minimum detected in 1993, and are similar to observations of 1994, 1995. The observed recoveries during 1996, and especially 1997, as indicated by increasing temperatures and ozone above 20 km as the polar vortex became unstable, were among the weakest measured since 1986 with low ozone extending into late October. We continue to observe in both 1996 and 1997 almost complete ozone depletion over a layer more than 4 km thick and centered near 17 km, similar to the first such observation over McMurdo in 1995. The maximum observed thickness of this layer has remained relatively constant since 1995.

Ozone measurements over McMurdo Station, Antarctica during spring 1994 and 1995

Ozone and temperature profiles were measured with balloon-borne instruments from McMurdo Station, Antarctica (77.85°S, 166.67°E) from late August to late October, on 39 occasions in 1994 and on 35 occasions in 1995. Minimum column ozone values of 138 DU and 139 DU were measured in 1994 and 1995, reduced from initial measurements in late August of 272 DU and 256 DU, respectively. These minima are higher than the record low measurement from McMurdo taken in October 1993 (130 DU), but lower than all other years since 1986 when ozone measurements were initiated at McMurdo. In 1994 and 1995, as in previous years, the onset of ozone recovery began mid-to-late October above 20 km. Below 14 km ozone concentrations have returned to pre-Pinatubo levels. Between 16–22 km they are near, and in 1995 often lower than, the record low levels set in 1993. In late August of 1995, the total and 12–20 km column ozone were also often observed to be near or lower than that for the same period of all previous years since 1986. By early October 1995, almost complete ozone depletion was experienced uniformly over the altitude region 14.5–19.5 km.

Advances in modeling the obstruction in the HIRDLS optical train and resulting data

A piece of plastic film came loose during launch and blocked most of the optical aperture. The largest remaining problem in correcting the measured radiances is the removal of the signals from this blockage. The present method is briefly described, followed by an outline of a new version, called the (ST)3 method. It relies on more understanding of the behavior of the blockage signals acquired in previous work. The method involves Scaling and Time interpolating the signals, Shifting them to align features, and Translating them to recover the scaled value at the reference angle. The residuals are represented by empirical orthogonal functions, coefficients of which may be Substituted from other channels. Finally, allowance for long-term Temporal changes in the blockage signals are being developed. Results for a day in the middle of the mission are presented, as well as their effects on water vapor retrievals.

HIRDLS field-of-view calibration techniques and results

The techniques used to calibrate the field of view of the High Resolution Dynamics Limb Sounder (HIRDLS) instrument and the results of the calibration are presented. HIRDLS will be flown on the NASA EOS Aura platform. Both in-field and out-of-field calibrations were performed. The calibration results are compared to the requirements and, in the case of out-of-field, mechanisms explaining the results are discussed.

Prelaunch calibration of the NASA AURA HIRDLS instrument

The High Resolution Dynamics Limb Sounder (HIRDLS) instrument is scheduled for launch on the NASA AURA satellite in January 2004; it is a joint project between the UK and USA. HIRDLS is a mid-infrared limb emission sounder which will measure the concentration of trace species and aerosol, and temperature and pressure variations in the Earths atmosphere between about 8 and 100 km altitude on a finer spatial scale than has been achieved before. This will depend upon both a high quality of instrument build, and very precise pre-launch calibration. Proto Flight Model calibration was performed in a purpose-built laboratory at Oxford University during an 13-week period in 2002. The tests were made in vacuum under cryogenic conditions close to the space environment. The measurements were divided into spectral, spatial and radiometric, with the HIRDLS pointing capability being used to control which item of test equipment was viewed. A large degree of automation was achieved, and this combined with 24-hour/7-day working enabled a large quantity of information to be obtained.