Terry Deshler

A global climatology of stratospheric aerosol surface area density deduced from Stratospheric Aerosol and Gas Experiment II measurements: 1984–1994

A global climatology of stratospheric aerosol surface area density has been developed using the multiwavelength aerosol extinction measurements of the Stratospheric Aerosol and Gas Experiment (SAGE) II for 1984–1994. The spatial and temporal variability of aerosol surface area density at 15.5, 20.5, and 25.5 km are presented as well as cumulative statistical distributions as a function of altitude and latitude. During this period, which encompassed the injection and dissipation of the aerosol associated with the June 1991 Mount Pinatubo eruption as well as the low loading period of 1989–1991, aerosol surface area density varied by more than a factor 30 at some altitudes. Aerosol surface area density derived from SAGE II and from the University of Wyoming optical particle counters are compared for 1991–1994 and are shown to be in generally good agreement though some differences are noted. An extension of the climatology using single-wavelength measurements by the Stratospheric Aerosol Measurement II (1978–1994) and SAGE (1979–1981) instruments is also presented.

Balloonborne measurements of Pinatubo aerosol during 1991 and 1992 at 41°N: Vertical profiles, size distribution, and volatility

Vertical profiles of aerosol have been measured approximately biweekly since June 1991 at Laramie, Wyoming (41°N). Both the total number concentration and concentration of particles >0.15 to 10.0 µm were measured using balloonborne instruments. The aerosol size distributions were best represented when bimodal lognormal distributions were fit to the data. After an early short-lived intense aerosol layer, the stratospheric maximum surface area and mass (40 µm² cm−3, 160 ppbm) was observed to occur approximately 180 days after the eruption. The aerosol was then observed to remain relatively homogeneous both in altitude and time during 1992, with the maximum surface area and mass remaining relatively constant between 20 to 30 µm² cm−3 and 30 to 60 ppbm.

Global to Microscale Evolution of the Pinatubo Volcanic Aerosol Derived from Diverse Measurements and Analyses

We assemble data on the Pinatubo aerosol from space, air, and ground measurements, develop a composite picture, and assess the consistency and uncertainties of measurement and retrieval techniques. Satellite infrared spectroscopy, particle morphology, and evaporation temperature measurements agree with theoretical calculations in showing a dominant composition of H2SO4-H2O mixture, with H2SO4 weight fraction of 65–80% for most stratospheric temperatures and humidities. Important exceptions are (1) volcanic ash, present at all heights initially and just above the tropopause until at least March 1992, and (2) much smaller H2SO4 fractions at the low temperatures of high-latitude winters and the tropical tropopause. Laboratory spectroscopy and calculations yield wavelength- and temperature-dependent refractive indices for the H2SO4-H2O droplets. These permit derivation of particle size information from measured optical depth spectra, for comparison to impactor and optical-counter measurements. All three techniques paint a generally consistent picture of the evolution of Reff, the effective radius. In the first month after the eruption, although particle numbers increased greatly, Reff outside the tropical core was similar to preeruption values of ∼0.1 to 0.2 μm, because numbers of both small (r 0.6 μm) particles increased. In the next 3–6 months, extracore Reff increased to ∼0.5 μm, reflecting particle growth through condensation and coagulation. Most data show that Reff continued to increase for ∼1 year after the eruption. Reff values up to 0.6–0.8 μm or more are consistent with 0.38–1 μm optical depth spectra in middle to late 1992 and even later. However, in this period, values from in situ measurements are somewhat less. The difference might reflect in situ undersampling of the very few largest particles, insensitivity of optical depth spectra to the smallest particles, or the inability of flat spectra to place an upper limit on particle size. Optical depth spectra extending to wavelengths λ > 1 μm are required to better constrain Reff, especially for Reff > 0.4 μm. Extinction spectra computed from in situ size distributions are consistent with optical depth measurements; both show initial spectra with λmax ≤ 0.42 μm, thereafter increasing to 0.78 ≤ λmax ≤ 1 μm. Not until 1993 do spectra begin to show a clear return to the preemption signature of λmax ≤ 0.42 μm. The twin signatures of large Reff (>0.3 μm) and relatively flat extinction spectra (0.4–1 μm) are among the longest-lived indicators of Pinatubo volcanic influence. They persist for years after the peaks in number, mass, surface area, and optical depth at all wavelengths ≤1 μm. This coupled evolution in particle size distribution and optical depth spectra helps explain the relationship between global maps of 0.5- and 1.0-μm optical depth derived from the Advanced Very High Resolution Radiometer (AVHRR) and Stratospheric Aerosol and Gas Experiment (SAGE) satellite sensors. However, there are important differences between the AVHRR and SAGE midvisible optical thickness products. We discuss possible reasons for these differences and how they might be resolved.

Balloonborne measurements of the Pinatubo aerosol size distribution and volatility at Laramie, Wyoming during the summer of 1991

Measurements using balloonborne optical particle counters at Laramie, Wyoming during the summer of 1991 are used to study the particle size distribution and volatility of the aerosol which formed in the stratosphere following the mid-June eruptions of Mt. Pinatubo. Enhanced aerosol layers were observed below 20 km as early as 16 July, about 1 month after the eruption. During late July, a transient though substantial particle layer was observed in the 23 km region. High concentrations of large particles in this high altitude layer resulted in aerosol mass mixing ratios as large as 0.5 ppm, considerably larger than observed following the eruption of El Chichon. Aerosol volatility tests indicated that well over 90% of the particles were composed of an H2SO4/H2O solution in all layers observed, indicating rapid conversion of SO2 to H2SO4 and subsequent droplet growth. High concentrations of droplets suggest homogeneous or ion nucleation as the most likely aerosol production mechanism.

Transmission Efficiency of an Aerodynamic Focusing Lens System: Comparison of Model Calculations and Laboratory Measurements for the Aerodyne Aerosol Mass Spectrometer

The size-dependent particle transmission efficiency of the aerodynamic lens system used in the Aerodyne Aerosol Mass Spectrometer (AMS) was investigated with computational fluid dynamics (CFD) calculations and experimental measurements. The CFD calculations revealed that the entire lens system, including the aerodynamic lens itself, the critical orifice which defines the operating lens pressure, and a valve assembly, needs to be considered. Previous calculations considered only the aerodynamic lens. The calculations also investigated the effect of operating the lens system at two different sampling pressures, 7.8 × 104 Pa (585 torr) and 1.0 × 105 Pa (760 torr). Experimental measurements of transmission efficiency were performed with size-selected diethyl hexyl sebacate (DEHS), NH4NO3, and NaNO3 particles on three different AMS instruments at two different ambient sampling pressures (7.8 × 104 Pa, 585 torr and 1.0 × 105 Pa, 760 torr). Comparisons of the measurements and the calculations show qualitative agreement, but there are significant deviations which are as yet unexplained. On the small size end (30 nm to 150 nm vacuum aerodynamic diameter), the measured transmission efficiency is lower than predicted. On the large size end (> 350 nm vacuum aerodynamic diameter) the measured transmission efficiency is greater than predicted at 7.8 × 104 Pa (585 torr) and in good agreement with the prediction at 1.0 × 105 Pa (760 torr).

Large volcanic aerosol load in the stratosphere linked to Asian monsoon transport.

Indirect Injection Aerosols in the stratosphere, especially submicron-hydrated sulfuric acid droplets, are an important factor influencing climate variability. Stratospheric sulfate aerosols can form from sulfur dioxide that has been transported from the underlying troposphere. Large volcanic eruptions can inject sulfur dioxide and other material into the stratosphere, but smaller volcanoes have been thought not to be energetic enough to do so. Bourassa et al. (p. 78) used satellite data to show that sulfur dioxide from the 2011 eruption of the Nabro stratovolcano in Eritrea was lofted into the stratosphere by deep convection associated with the Asian summer monsoon. Even moderate volcanic eruptions can inject sulfur dioxide into the stratosphere with the help of the Asian monsoon. The Nabro stratovolcano in Eritrea, northeastern Africa, erupted on 13 June 2011, injecting approximately 1.3 teragrams of sulfur dioxide (SO2) to altitudes of 9 to 14 kilometers in the upper troposphere, which resulted in a large aerosol enhancement in the stratosphere. The SO2 was lofted into the lower stratosphere by deep convection and the circulation associated with the Asian summer monsoon while gradually converting to sulfate aerosol. This demonstrates that to affect climate, volcanic eruptions need not be strong enough to inject sulfur directly to the stratosphere.

ozone loss in the lower stratosphere over the United States in 1992–1993: Evidence for heterogeneous chemistry on the Pinatubo aerosol

Ozone profiles obtained at Boulder, Colorado and Wallops Island, Virginia indicate that ozone was about 25% below normal during the winter and spring of 1992–93 in the 12–22 km region. This large ozone reduction in the lower stratosphere, though sometimes partially compensated by higher than normal ozone above 24 km, was responsible for the low total column ozone values observed across the United States during this period. Normal temperatures throughout the low ozone region suggest that transport-related effects are probably not the most important cause of the ozone deficits. The region of low ozone at Boulder corresponds closely with the location of the enhanced H2SO4/H2O aerosol from the Pinatubo eruption of 1991 as measured near Boulder and at Laramie, Wyoming. Trajectory analyses suggest that except at low altitudes in spring, air parcels on the days of the ozone measurements generally arrived at Boulder from higher latitude, although seldom higher than 60°N, and hence may have been subjected to heterogeneous chemical processing on the surface of Pinatubo aerosol droplets resulting in chlorine-catalyzed ozone destruction, a process which is believed to be more effective under the lower winter temperatures and sunlight levels of higher latitudes.

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.

Stratospheric cloud observations during formation of the Antarctic ozone hole in 1989

The results of six balloon flights at McMurdo Station, Antarctica, under varying temperature conditions, are used in a study of polar stratospheric clouds during September 1989. A new particle counter, with size resolution in the 0.5 μm radius region, indicated that size distributions observed in the clouds were bimodal. Mode radii ranging from 0.05 to 0.10 μm were observed for the small particle mode, representing the sulfate layer or condensational growth enhancements of it. Mode radii generally ranged from 1.5 to 3.5 μm for the large particle mode at concentrations 3 to 4 orders of magnitude lower than the small particle mode. The large particle mode, when observed at temperatures above the water ice point, is believed to be the result of nitric acid trihydrate (NAT) condensation on larger particles of the sulfate layer. In this case the HNO3 condensed mass mixing ratios were 1 to 5 ppbv for most of the cloud layers. Generally, the large particle NAT concentrations were higher in the lower stratosphere, indicating the redistribution of HNO3 through particle sedimentation. On several occasions, distributions were observed with mode radii as high as 7 μm, and correspondingly large inferred mass, indicating water ice clouds in the 12 to 15 km region. On other occasions, absence of such clouds at very low temperatures inferred water vapor mixing ratios of less than 3 ppmv.

Total volcanic stratospheric aerosol optical depths and implications for global climate change

Understanding the cooling effect of recent volcanoes is of particular interest in the context of the post-2000 slowing of the rate of global warming. Satellite observations of aerosol optical depth above 15 km have demonstrated that small-magnitude volcanic eruptions substantially perturb incoming solar radiation. Here we use lidar, Aerosol Robotic Network, and balloon-borne observations to provide evidence that currently available satellite databases neglect substantial amounts of volcanic aerosol between the tropopause and 15 km at middle to high latitudes and therefore underestimate total radiative forcing resulting from the recent eruptions. Incorporating these estimates into a simple climate model, we determine the global volcanic aerosol forcing since 2000 to be −0.19 ± 0.09 Wm−2. This translates into an estimated global cooling of 0.05 to 0.12°C. We conclude that recent volcanic events are responsible for more post-2000 cooling than is implied by satellite databases that neglect volcanic aerosol effects below 15 km.