D. J. Hofmann

Stratospheric Aerosol Measurements III: Optical Model Calculations.

Abstract : Articles in the literature dealing with light scattering properties of atmospheric aerosols are numerous and of great variety. Many fall into two general categories: those in which the effect of aerosols on radiative transfer processes in the atmosphere is studied, and those in which aerosol size distribution or composition information is obtained (or shown possible to be obtained) from light scattering measurements in the atmosphere. The results reported here, although limited to the stratospheric aerosol layer, are believed to be based on the most consistent and globally extensive measurements of the aerosol size distribution made to date. From this comprehensive set of data and other data on aerosol composition, the authors have constructed appropriate optical models which are used in Mie single-scattering calculations of the aerosol total extinction, absorption, 180 degree lidar backscattering, angular scattering, outward (2 pie hemispheric) scattering for a given solar zenith angle and finally, values of the global stratospheric aerosol albedo.

Balloon-borne observations of the development and vertical structure of the Antarctic ozone hole in 1986

A springtime deficit in Antarctic stratospheric ozone has been developing over recent years1,2. Here we describe the vertical distribution of ozone which was measured at McMurdo Station, Antarctica (78 °S), using balloon borne sensors, on 33 occasions during the period 25 August–6 November 1986. These observations suggest a highly structured cavity confined to the 12–20 km altitude region. In the 17–19 km altitude range, the ozone volume mixing ratio declined from about 2 p.p.m. at the end of August to about 0.5 p.p.m. by mid-October. The average decay in this region can be described as exponential with a half life of about 25 days. While total ozone, as obtained from profile integration, declined only about 35%, the integrated ozone between 14 and 18 km declined more than 70%. Vertical ozone profiles in the vortex revealed unusual structure with major features from 1 to 5 km thick which had suffered ozone depletions as great as 90%.

Midlatitude lidar backscatter to mass, area, and extinction conversion model based on in situ aerosol measurements from 1980 to 1987

Balloonborne particle counter data from Laramie, WY are used to define a seasonally averaged stratospheric sulfuric acid aerosol size distribution in three altitude intervals from 15 to 30 km for the 1980-1987 period. This period includes the volcanic eruptions of Mt. St. Helens, Alaid, Nyamuragira, El Chichon, and Nevado el Ruiz and begins and ends at what are believed to be periods of near background (nonvolcanic) stratospheric conditions. The size distributions are used to calculate lidar backscatter to extinction, mass, and area ratios for an appropriate range of particle indices of refraction. These ratios may then be used to infer particle extinction, mass, and area from midlatitude lidar data for this time period.

Stratospheric Aerosol Measurements I: Time Variations at Northern Midlatitudes

Abstract The results of over 70 balloon soundings, by the University of Wyomings Atmospheric Physics Group mostly during 1972 and 1973 from a number of stations, are being utilized in a study of the temporal and spatial distribution of the global stratospheric aerosol. This paper deals with the instrumentation, calibration, etc., and with the results of monthly soundings from the Laramie (41°N) station during the approximately two-year period of measurement. This period comprises an interval apparently free of major volcanic activity just prior to the extensive volcanic contributions to the stratospheric aerosol which occurred in late 1974. It thus may be compared to the pre-Agung era and is perhaps as close to the so-called “natural stratospheric background conditions,” if indeed such conditions ever exist, as will likely be attained in the near future. A simple seasonal variation in the total stratospheric aerosol loading below about 20 km altitude dominates the temporal variation at Laramie, resulting...

Twenty years of balloon-borne tropospheric aerosol measurements at Laramie, Wyoming

vary between 0.01 and 0.04 from winter to summer, the estimated mass scattering cross section is about 3 throuthe troposphere. A distinct anticorrelation exists between the optically active and the condensation nuclei components, resulting in a maximum in the mixing ratio of the latter just below the tropopause where the larger particles generally show a minimum. This relation is due to coagulation of the small, newly nucleated parti- cles with the existing larger particles and to the competition for available condensable vapors presented by the larger particles, resulting in an effective new particle source and reservoir region occurring in the upper troposphere. There is evidence for a decreasing trend of 1.6-1.8% per year in the optically active tropospheric aerosol over the past 20 years which may be related to a similar reduction in SO 2 emissions in the United States over this perioct

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

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.

Polarized light scattered from monodisperse randomly oriented nonspherical aerosol particles: measurements.

Measurements of polarized light scattered from monodisperse nonspherical randomly oriented aerosol particles are presented along with Mie theoretical results for spheres of approximately the same cross sectional area. For slightly nonspherical particles of sodium chloride and potassium sulfate with size parameter (defined as the ratio of the particle circumference to the wavelength) greater than about five, the intensity of light scattered is generally more than as predicted by Mie theory in the forward scattering lobe, but less at nonforward angles. For particles with size parameter less than five, the Mie results more closely match the measurements. Measured angular scattering patterns for randomly oriented particles are smoother than the Mie theoretical results and are nearly the same for salt and potassium sulfate particles of the same size. Measurements of particle depolarization are nearly independent of scattering angle.

Optical modeling of stratopheric aerosols - Present status

A stratospheric aerosol optical model is developed which is based on a size distribution conforming to direct measurements. Additional constraints are consistent with large data sets of independently measured macroscopic aerosol properties such as mass and backscatter. The period under study covers background as well as highly disturbed volcanic conditions and an altitude interval ranging from the tropopause to approximately 30 km. The predictions of the model are used to form a basis for interpreting and intercomparing several diverse types of stratospheric aerosol measurement.

UV measurements at Mauna Loa : July 1995 to July 1996

A UV spectroradiometer was installed at Mauna Loa Observatory (MLO), Hawaii, in July 1995. This instrument, based on a commercially available double monochromator, uses a diffuser mounted as a horizontal receptor inside a quartz dome and views the whole sky. The instrument scans over the 290–450 nm spectral range with a band pass of about 1 nm for each 5° of solar zenith angle (SZA). The UV irradiances measured at MLO are much more intense than at low-altitude midlatitude locations. For observations at SZA 45° the erythemally weighted UV irradiances can exceed 21 μW cm−2, which is approximately 15–20% greater than that seen at Lauder, New Zealand, for similar ozone amounts. The difference is primarily due to the higher altitude at MLO (3.4 km). For overhead Sun conditions at MLO the largest value of erythemal UV was 51.3±3.1 μW cm−2, which to our knowledge is the highest recorded any-where at the Earths surface. UV irradiance is strongly correlated (inversely) with Dobson spectrophotometer total ozone measurements at MLO, with higher correlations at shorter wavelengths. The radiative amplification factor (RAF) for erythema at MLO is about 1.33±0.2 at SZA 45°.

Tropospheric ozone during Mauna Loa Observatory Photochemistry Experiment 2 compared to long-term measurements from surface and ozonesonde observations

Continuous surface ozone measurements have been made at Mauna Loa Observatory (MLO) for 20 years. In addition, weekly ozone profile measurements using balloonborne ozonesondes have been carried out from Hilo, Hawaii, since 1985. These long-term records are compared with data obtained during the MLOPEX 2 period from September 1991 to August 1992. Ozone behavior at the observatory level (∼3.4 km) during autumn and winter of 1991–1992 was similar to that found during the period 1980–1990. In spring and summer 1992, however, there were several significant differences from the long-term behavior. During March and April 1992, there was about 10% more ozone than the long-term average, and the variability was less than half of what is seen normally. These characteristics are associated with strong flow from the north and west. Both June and July 1992 saw periods of elevated ozone with the June average 20% higher than normal. During the more limited sampling (weekly profiles) when ozonesonde measurements were made, the 1992 spring enhancement was particularly pronounced at 500 mbar (∼6 km), while during the summer the larger than normal concentrations were at 700 mbar (∼3.5 km). In the upper troposphere, on the other hand, spring ozone amounts in 1992 were much below normal with only about half the ozone usually seen in the 12- to 15-km region. The ozone profiles are discussed in terms of the representativeness of the MLO surface measurements in characterizing the free troposphere ozone behavior at both the altitude of the observatory as well as other heights in the atmosphere. During the winter and spring, the MLO measurements are often representative of behavior over a broad depth of the troposphere (3–10 km). In the summer and autumn the MLO observations are more characteristic of free tropospheric conditions at or near the observatory level.