J. M. Rosen

Thirty years of in situ stratospheric aerosol size distribution measurements from Laramie, Wyoming (41°N), using balloon‐borne instruments

[1]xa0Vertical profiles of size-resolved aerosol concentrations above Laramie, Wyoming (41°N), have been measured for the past thirty years, 1971-2001. During this period, two somewhat different optical particle counters have been used to measure particles with radii ≥0.15 μm, whereas the instrument to measure condensation nuclei (CN) has not changed significantly since the late 1970s. The two optical particle counters measure aerosols ≥0.15, 0.25 μm and aerosols ≥0.15–2.0 μm in twelve size classes. These measurements have concentration (N) uncertainties ∝ ±N−0.5, but with a minimum of ±10%. Sizing uncertainties are about ±10%. The impact of these uncertainties on size distribution fitting parameters and aerosol moments are approximately ±30% and ±40%. The long-term record from these measurements indicates that volcanoes have controlled stratospheric aerosol abundance for 20 of the past 30 years. The present period, beginning in 1997, represents the longest volcanically quiescent period in the record. These and other measurements clearly show that stratospheric aerosol are now in a background state, a state rarely occurring in recent times, and that this background state is not significantly different than observations in 1979. Aerosol volumes and surface areas, inferred from size distributions fit to the measurements, are compared with SAGE II satellite estimates of surface area and volume. For volume the measurements are in agreement within measurement error throughout the record. For surface area there is good agreement for a volcanic aerosol laden stratosphere, but for background aerosol conditions the SAGE II estimates are about 40% less than the in situ measurements. Present aerosol surface areas are ∼1.0 (0.6) μm2 cm−3 in the 15–20 (20–25) km layer based on in situ measurements. The Laramie size distribution record is now available to the community over the internet.

The evolution of the dehydration in the Antarctic stratospheric vortex

In 1994 an intensive program of balloon-borne frost point measurements was performed at McMurdo, Antarctica. During this program a total of 19 frost point soundings was obtained between February 7 and October 5, which cover a wide range of undisturbed through strongly dehydrated situations. Together with several soundings from South Pole station between 1990 and 1994, they give a comprehensive picture of the general development of the dehydration in the Antarctic stratospheric vortex. The period of dehydration typically starts around the middle of June, and a rapid formation of large particles leads to a fast dehydration of the vortex. The evaporation of falling particles leads to rehydration layers, which have significantly higher water vapor concentrations than the undisturbed stratosphere. Through the formation of these rehydration layers in the early stages of the dehydration we can estimate a particle fall speed of ⅓ km/d and thus a mean particle size of 4 μm. Ice saturation was observed over McMurdo in only two cases and only well after the onset of the dehydration. From the inspection of synoptic maps it then follows that a small cold region inside the vortex seems to be sufficient to dehydrate the entire vortex. Above 20 km the dehydration is completed by the end of July. From the descent of the upper dehydration edge we can estimate a mean descent rate inside the vortex of 1.5 km/month. In McMurdo we observed occasional penetration of the vortex edge in cases where the vortex edge was close to McMurdo, however, these cases seem to have little effect on the bulk of the vortex. A sounding from November 3, 1990, at South Pole shows that the dehydration may persist into November and indicates that there is no significant transport into the vortex throughout winter and early spring.

Sulfuric Acid Droplet Formation and Growth in the Stratosphere After the 1982 Eruption of El Chichón

The eruption of El Chich�n Volcano in March and April 1982 resulted in the nucleation of large numbers of new sulfuric acid droplets and an increase by nearly an order of magnitude in the size of the preexisting particles in the stratosphere. Nearly 107 metric tons of sulfuric acid remained in the stratosphere by the end of 1982, about 40 times as much as was deposited by Mount St. Helens in 1980.

Dehydration and sedimentation of ice particles in the Arctic stratospheric vortex

Balloon borne frost-point hygrometers and backscatter sondes were launched at Sodankyla, Finland in January and February of 1996. These instruments measure water vapor and the backscatter ratio of light due to polar stratospheric clouds in the Arctic stratospheric vortex. Here we report the results of a hygrometer sonde and a backscatter sonde launched within 3.5 hours of each other on January 22/23. Together these soundings show a strong loss of water vapor due to the formation of ice clouds as a result of record cold temperatures in the Arctic stratosphere. The separation of the upper edge of the layer showing water vapor loss and the upper edge of the PSC layer indicates sedimentation of the ice particle layer, possibly leading to a permanent dehydration in the upper part of the layer exhibiting water vapor loss.

Measured Light-Scattering Properties of Individual Aerosol Particles Compared to Mie Scattering Theory

Monodispersed spherical aerosols of 0.26-2-micro diameter with approximate range of indexes of refraction of atmospheric aerosols have been produced in the laboratory by atomization of liquids with a vibrating capillary. Integrated light scattered 8 through 38 degrees from the direction of forward scattering has been measured with a photoelectric particle counter and compared to Mie theory calculations for particles with complex indexes of refraction 1.4033-0i, 1.592-0i, 1.67-0.26i, and 1.65-0.069i. The agreement is good. The calculations take into account the particle counter white light illumination with color temperature 3300 K, the optical system geometry, and the phototube spectral sensitivity. It is shown that for aerosol particles of unknown index of refraction the particle counter size resolution is poor for particle size greater than 0.5micro, but good for particles in the 0.26-0.5-micro size range.

Stratospheric Sulfuric Acid Layer: Evidence for an Anthropogenic Component

Recent measurements of small aerosol particles in the stratosphere over Laramie, Wyoming, indicate low-concentration background conditions. A comparison of measurements made some 20 years ago with the present background concentration reveals the possibility of an increase of 9 percent per year. Since the aerosol particles are predominantly sulfuric acid droplets which form in the stratosphere from tropospheric sulfur-containing gases, such an increase may be related to man-made sulfur emissions.

Comparison of analyzed stratospheric temperatures and calculated trajectories with long‐duration balloon data

The Polar Vortex Balloon Experiment (POVORBEX) has flown eight long-duration flights 1992 to 1995 in the arctic winter stratosphere at about 50 mbar. European Centre for Medium-Range Weather Forecasts (ECMWF) analyses are compared with the balloon temperatures and winds, which are accurate and independent, and substantial differences are found. For example, the average temperature difference is 2.4 K. These temperature differences have important implications for the potential for polar stratospheric clouds, which play a vital role in the destruction of ozone. Large differences are also found between the observed and calculated trajectories. For the longest flight, which lasted almost 6 days, the calculated trajectory end point is off by 2700 km (23% of the trajectory length). For the other seven POVORBEX flights, which lasted about 1/2-3 days, the calculated end point is off by 156-544 km (5-19% of the trajectory length for six of the flights). The main causes of these differences are discrepancies between real and ECMWF analyzed winds and temperatures and the large distance between the model levels at this altitude.

In situ mountain-wave polar stratospheric cloud measurements: Implications for nitric acid trihydrate formation

0.2 cm 3 , median radii of 1 to 2 mm and volumes up to 1 mm 3 cm 3 . A comparison between optical PSC data and optical simulations based on the measured particle size distribution indicates that the NAT particles were aspherical with an aspect ratio of 0.5. The NAT particle properties have been compared to another PSC observation on 25 January 2000, where NAT particle number densities were about an order of magnitude higher. In both cases, microphysical modeling indicates that the NAT particles have formed on ice particles in the mountain-wave events. Differences in the NAT particle number density can be explained by the meteorological conditions. We suggest that the higher NAT number density on 25 January can be caused by stronger wave activity observed on that day, larger cooling rates and therefore higher NAT supersaturation. INDEX TERMS: 0305 Atmospheric Composition and Structure: Aerosols and particles (0345, 4801); 0320 Atmospheric Composition and Structure: Cloud physics and chemistry; 0340 Atmospheric Composition and Structure: Middle atmosphere— composition and chemistry; KEYWORDS: polar stratospheric cloud (PSC), nitric acid trihydrate (NAT), ozone, gravity wave, PSC formation

Chemical, microphysical, and optical properties of polar stratospheric clouds

A balloonborne gondola for a comprehensive study of polar stratospheric clouds (PSCs) was launched on 25 January 2000 near Kiruna/Sweden. Besides an aerosol composition mass spectrometer, the gondola carried optical particle counters, two backscatter sondes, a hygrometer, and several temperature and pressure sensors. A mountain wave induced PSC was sampled between 20 and 23 km altitude. Strongly correlated PSC particle properties were detected with the different instruments. A large variability of particle types was measured in numerous PSC layers, and PSC development was followed for about two hours. Liquid ternary PSC layers were found at temperatures near the ice frost point. A large fraction of the sampled cloud layers consisted of nitric acid trihydrate (NAT) particles with a molar ratio H 2 O:HNO 3 close to 3 at temperatures near and below the equilibrium temperature T NAT . The median radius of the NAT particle size distribution was between 0.5 and 0.75 μm at concentrations around 0.5 cm -3 . Below the NAT layers and above T NAT , thin cloud layers containing a few large particles with radii up to 3.5 μm coexisted with smaller solid or liquid particles. The molar ratio in this region was found to be close to two.

Correlation of aerosol and carbon monoxide at 45°S: Evidence of biomass burning emissions

Altitude profiles of Carbon Monoxide (CO) and aerosols have been compared from the Network for Stratospheric Change (NDSC) mid-latitude southern hemisphere site at Lauder, New Zealand. The CO mixing ratio profile was derived from infrared spectra recorded with a very high resolution Fourier Transform interferometer using three lines of the (1–0) band between 2057 and 2160 cm−1. The aerosol surface area was derived from balloon-borne backscatter radiation at 940 nm. Both datasets show significant enhancements occurring over the observation site in the austral spring. When displayed together their combined effect illustrates the close correlation between CO and aerosols. Peak concentrations are consistently recorded between September and October over a five year time frame (1994–1999), with the enhancements typically occurring at heights of between 3 to 8 km. The temporal and spatial correlation between the aerosol plumes and enhanced CO concentrations are interpreted in terms of the effect of long range transport of biomass burning plumes in combination with the El Nino-Southern Oscillation (ENSO) cycles influence on southern hemisphere climate dynamics.