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Featured researches published by P. V. Johnston.


Journal of Geophysical Research | 1997

Temperature dependent NO2 cross sections at high spectral resolution

Jerald W. Harder; James W. Brault; P. V. Johnston; George H. Mount

The importance of nitrogen dioxide in both the troposphere and the stratosphere has been known for some years, and since the early 1970s, spectroscopic determinations have played an important role in understanding NOX chemistry. Spectroscopic measurements of the atmosphere have improved in quality in recent years to the point that an accurate determination of the NO2 absorption cross section is essential to accurate retrievals of not only NO2 but also less abundant species in the troposphere and stratosphere. NO2 is such a large absorber (approximately 1% at large air mass) in the stratosphere at twilight or in the troposphere under even mildly polluted conditions, that if it is not properly removed from observed spectra, the spectra of the more subtle species are masked and cannot be measured at all. We present cross sections of NO2 in the spectral region 350–585 nm at four temperatures between 217 and 298 K and total pressures between 100 and 600 torr at a mixing ratio of 84.1 ppmv and at a spectral resolution sufficient for accurate convolution with instruments typically used to measure atmospheric NO2. Data will be presented to demonstrate the presence of NO2 pressure dependence in high resolution. A detailed comparison with commonly used literature cross sections is made to show how such instrument parameters as wavelength accuracy, resolution, spectrograph scattered light, and data sampling affect the usefulness of the observed cross section.


web science | 2003

International Photolysis Frequency Measurement and Model Intercomparison (IPMMI): Spectral actinic solar flux measurements and modeling

A. F. Bais; Sasha Madronich; J. H. Crawford; Samuel R. Hall; Bernhard Mayer; M. van Weele; Jacqueline Lenoble; Jack G. Calvert; C. A. Cantrell; Richard E. Shetter; Andreas Hofzumahaus; Peter Koepke; Paul S. Monks; G. J. Frost; Richard McKenzie; N. Krotkov; Arve Kylling; William H. Swartz; Steven A. Lloyd; G. G. Pfister; T. J. Martin; E.‐P. Roeth; Erik Griffioen; Ansgar Ruggaber; Maarten C. Krol; Alexander Kraus; Gavin D. Edwards; M. Mueller; Barry Lefer; P. V. Johnston

[1] The International Photolysis Frequency Measurement and Model Intercomparison (IPMMI) took place in Boulder, Colorado, from 15 to 19 June 1998, aiming to investigate the level of accuracy of photolysis frequency and spectral downwelling actinic flux measurements and to explore the ability of radiative transfer models to reproduce the measurements. During this period, 2 days were selected to compare model calculations with measurements, one cloud-free and one cloudy. A series of ancillary measurements were also performed and provided parameters required as input to the models. Both measurements and modeling were blind, in the sense that no exchanges of data or calculations were allowed among the participants, and the results were objectively analyzed and compared by two independent referees. The objective of this paper is, first, to present the results of comparisons made between measured and modeled downwelling actinic flux and irradiance spectra and, second, to investigate the reasons for which some of the models or measurements deviate from the others. For clear skies the relative agreement between the 16 models depends strongly on solar zenith angle (SZA) and wavelength as well as on the input parameters used, like the extraterrestrial (ET) solar flux and the absorption cross sections. The majority of the models (11) agreed to within about +/-6% for solar zenith angles smaller than similar to60degrees. The agreement among the measured spectra depends on the optical characteristics of the instruments (e.g., slit function, stray light rejection, and sensitivity). After transforming the measurements to a common spectral resolution, two of the three participating spectroradiometers agree to within similar to10% for wavelengths longer than 310 nm and at all solar zenith angles, while their differences increase when moving to shorter wavelengths. Most models agree well with the measurements (both downwelling actinic flux and global irradiance), especially at local noon, where the agreement is within a few percent. A few models exhibit significant deviations with respect either to wavelength or to solar zenith angle. Models that use the Atmospheric Laboratory for Applications and Science 3 (ATLAS-3) solar flux agree better with the measured spectra, suggesting that ATLAS-3 is probably more appropriate for radiative transfer modeling in the ultraviolet.


Geophysical Research Letters | 1997

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

K. Kreher; P. V. Johnston; S. W. Wood; Bruno Nardi; U. Platt

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.


Geophysical Research Letters | 1994

Impact of Pinatubo aerosols on the partitioning between NO2 and HNO3

M. Koike; Nicholas Jones; W. A. Matthews; P. V. Johnston; Richard McKenzie; Douglas E. Kinnison; José F. Rodríguez

Ground based infrared observations of HNO3 column amount at Lauder, New Zealand (45°S, 170°E), show significant increases in HNO3 following the arrival of Pinatubo volcanic aerosols. The increase first became apparent in September 1991 when the HNO3 amount was higher than that expected from the regular seasonal variation by 2.3 × 1015 cm−2, which corresponds to 16% of the unperturbed value. Between September 1991 and May 1993, the observed HNO3 amounts were systematically higher, typically by 1.5 to 3.5 × 1015 cm−2 or 10 to 30%. After June 1993, the observed HNO3 amounts were close to the unperturbed values. The decrease in stratospheric NO2 due to the volcanic aerosols was observed at Lauder after August 1991 [Johnston et al., 1992, 1993]. The start of the HNO3 increase and the general trend are in agreement with those expected from the NO2 decrease when heterogeneous reactions on the sulfate aerosols are considered. Model calculations, in which observed aerosol fields have been used, qualitatively agree with these observed trends, although the magnitude of the changes have been underestimated. These results provide further understanding of the impact of heterogeneous chemistry on the partitioning between NO2 and HNO3.


Journal of Geophysical Research | 1991

Altitude distributions of stratospheric constituents from ground‐based measurements at twilight

Richard McKenzie; P. V. Johnston; C. T. McElroy; J. B. Kerr; Susan Solomon

A technique for extracting height profiles from ground-based column measurements at twilight is introduced. Its sensitivities to chemical processes, initialization, and air mass factors are investigated. The method is applied to observations made at Lauder, New Zealand, in 1987. The technique provides information on the vertical structure of atmospheric absorbers such as ozone or NO2 from the surface to about 50 km and is particularly valuable for identifying the influence of pollution on such measurements. When tropospheric pollution is low, it yields profiles in reasonable agreement with model predictions and with satellite measurements.


Advances in Space Research | 2002

Intercomparison of BrO measurements from ERS-2 GOME, ground-based and balloon platforms

M. Van Roozendael; Thomas Wagner; Andreas Richter; Irene Pundt; D. W. Arlander; J. P. Burrows; M. P. Chipperfield; C. Fayt; P. V. Johnston; J.-C. Lambert; K. Kreher; K. Pfeilsticker; U. Platt; J.-P. Pommereau; Björn-Martin Sinnhuber; K. K. Tørnkvist; F. Wittrock

The consistency of BrO column amounts derived from GOME spectra and from correlative ground-based and balloon measurements performed in 1998-1999 during the Third European Stratospheric Experiment on Ozone (THESEO) has been investigated. The study relies on W-visible observations at several mid- and high latitude ground-based stations in both hemispheres, complemented by balloon-borne solar occultation profile measurements and 3D chemical transport model simulations. Previous investigations have reported GOME BrO columns systematically larger than those deduced from balloon, suggesting BrO being present, possibly ubiquitously, in the free troposphere. The robustness of this hypothesis has been further tested based on the presently available correlative data set. It is shown that when accounting for the BrO diurnal variation and the solar zenith angle dependency of the sensitivity of correlative data to the troposphere, measurements from all platforms are consistent with the presence of a tropospheric BrO background of 1-3 ~10’~ mole&m’ extending over mid- and high


Applied Optics | 1992

Solar ultraviolet spectroradiometry in New Zealand: instrumentation and sample results from 1990.

Richard McKenzie; P. V. Johnston; Michael Kotkamp; A. Bittar; J. D. Hamlin

In 1988 the New Zealand Department of Scientific and Industrial Research initiated a program to characterize the spectrum of solar ultraviolet radiation reaching the ground in New Zealand and to identify the extent and causes of its variability. Routine measurements began at Lauder (45 degrees S 170 degrees E) in December 1989. The instrumentation, measurement strategy, and calibration procedures are discussed and uncertainties in the measurements are analyzed. With the present system useful measurements at 1-nm resolution are limited to irradiances greater than 10(-3) microW cm(-2) nm(-1), which corresponds to a lower limit in wavelength in the region 290-295 nm (depending on the Sun angle and ozone amount). This is a useful lower limit for many applications of relevance to the biosphere. Results from the first year of operation are presented and discussed.


Journal of Geophysical Research | 2001

Altitude effects on UV spectral irradiance deduced from measurements at Lauder, New Zealand, and at Mauna Loa Observatory, Hawaii

Richard McKenzie; P. V. Johnston; Dan Smale; Barry A. Bodhaine; Sasha Madronich

Measurements from Lauder, New Zealand, and from the high-altitude Mauna Loa Observatory, Hawaii, are used to determine the altitude effects on spectral UV irradiance and to relate these altitude differences to other factors that influence UV radiation. The measured ratios UVMauna Loa/UVLauder are complex functions of both wavelength and solar zenith angle (SZA). Spectrally, the ratios tend to increase toward shorter wavelengths through most of the UV-A region. For small SZA (SZA ∼80°, local minima in the ratios are seen at shorter wavelengths in the UV-B region. For biologically weighted irradiances, the peak ratios occur near SZA = 70°, where UV-A, erythemally weighted UV, UV-B, and DNA-weighted UV irradiances at Mauna Loa Observatory exceeded those at Lauder by ∼17%, 26%, 27%, and 29% respectively. The ratios of irradiances at the two altitudes, as functions of SZA and wavelength, were related to differences expected from radiative transfer calculations. For small SZA, modeled and measured ratios agreed within the limits of experimental uncertainty without taking differences in altitude distributions of ozone and temperature into account. However, for larger SZA and shorter wavelengths these profile shapes had a significant effect. In the model calculations, satisfactory agreement with the measurements was achieved only when the contribution from radiation scattered from air or cloud tops below the observation height at Mauna Loa Observatory was included. To model this accurately, a three-dimensional radiative transfer code should be used in conjunction with a topographical model of the surrounding terrain.


Journal of Geophysical Research | 1995

Intercomparison of UV/visible spectrometers for measurements of stratospheric NO2 for the Network for the Detection of Stratospheric Change

D. J. Hofmann; Paolo Bonasoni; Martine De Mazière; Franco Evangelisti; Giorgio Giovanelli; Aaron Goldman; Florence Goutail; Jerald W. Harder; R. O. Jakoubek; P. V. Johnston; Jim Kerr; W. Andrew Matthews; Tom Mcelroy; Richard McKenzie; George H. Mount; U. Platt; Jean-Pierre Pommereau; Alain Sarkissian; Paul C. Simon; Susan Solomon; J. Stutz; Alan Thomas; Michel Van Roozendael; Edmund Wu

During the period May 12–23, 1992, seven groups from seven countries met in Lauder, New Zealand, to intercompare their remote sensing instruments for the measurement of atmospheric column NO2 from the surface. The purpose of the intercomparison was to determine the degree of intercomparability and to qualify instruments for use in the Network for the Detection of Stratospheric Change (NDSC). Three of the instruments which took part in the intercomparison are slated for deployment at primary NDSC sites. All instruments were successful in obtaining slant column NO2 amounts at sunrise and sunset on most of the 12 days of the intercomparison. The group as a whole was able to make measurements of the 90° solar zenith angle slant path NO2 column amount that agreed to about ±10% most of the time; however, the sensitivity of the individual measurements varied considerably. Part of the sensitivity problem for these measurements is the result of instrumentation, and part is related to the data analysis algorithms used. All groups learned a great deal from the intercomparison and improved their results considerably as a result of this exercise.


Journal of Geophysical Research | 2002

Comparison of measurements and model calculations of stratospheric bromine monoxide

Björn-Martin Sinnhuber; D. W. Arlander; Heinrich Bovensmann; J. P. Burrows; M. P. Chipperfield; C.-F. Enell; U. Frieß; F. Hendrick; P. V. Johnston; Roger Jones; K. Kreher; N. Mohamed-Tahrin; Richard W. Muller; K. Pfeilsticker; U. Platt; J.-P. Pommereau; Irene Pundt; Andreas Richter; A. M. South; K. K. Tørnkvist; M. Van Roozendael; T. Wagner; F. Wittrock

Ground-based zenith sky UV-visible measurements of stratospheric bromine monoxide (BrO) slant column densities are compared with simulations from the SLIMCAT three-dimensional chemical transport model. The observations have been obtained from a network of 11 sites, covering high and midlatitudes of both hemispheres. This data set gives for the first time a near-global picture of the distribution of stratospheric BrO from ground-based observations and is used to test our current understanding of stratospheric bromine chemistry. In order to allow a direct comparison between observations and model calculations, a radiative transfer model has been coupled to the chemical model to calculate simulated slant column densities. The model reproduces the observations in general very well. The absolute amount of the BrO slant columns is consistent with a total stratospheric bromine loading of 20 ± 4 ppt for the period 1998-2000, in agreement with previous estimates. The seasonal and latitudinal variations of BrO are well reproduced by the model. In particular, the good agreement between the observed and modeled diurnal variation provides strong evidence that the BrO-related bromine chemistry is correctly modeled. A discrepancy between observed and modeled BrO at high latitudes during events of chlorine activation can be resolved by increasing the rate constant for the reaction BrO + ClO → BrCl + O 2 to the upper limit of current recommendations. However, other possible causes of the discrepancy at high latitudes cannot be ruled out.

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K. Kreher

National Institute of Water and Atmospheric Research

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Richard McKenzie

National Institute of Water and Atmospheric Research

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A. Thomas

National Institute of Water and Atmospheric Research

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S. W. Wood

National Institute of Water and Atmospheric Research

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F. Hendrick

Belgian Institute for Space Aeronomy

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Michael Kotkamp

National Institute of Water and Atmospheric Research

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M. Van Roozendael

Belgian Institute for Space Aeronomy

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U. Platt

Heidelberg University

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