Terry Deshler

In situ measurements of stratospheric aerosol are the only measurements that provide sufficient detail to determine aerosol number, size, surface area, volume/mass, and effective radius; however, these measurements are limited in space and time. Remote sensing of stratospheric aerosol is both global (from satellite) and more frequent (from LiDAR), but not as detailed. The challenge is to use remote sensing data to infer geophysical quantities of interest. Here, over 35 years of in situ stratospheric aerosol measurements in the mid‐latitudes are used to derive ratios of extinction coefficient ( E ) to backscatter coefficient ( B ), mass ( m ) to backscatter coefficient, surface area ( s ) to backscatter coefficient, and mass and surface area to extinction coefficient. These ratios provide an avenue to convert remotely sensed aerosol measurements to geophysical quantities of interest. The results indicate that from 10 to 30 km m : B and m : E are nearly independent of altitude, while E : B and s : B have some altitude dependence. Still, all ratios have a nearly linear relationship in log space, independent of altitude, implying that these relationships can nearly be represented by a single value for the ratio. The results for: E : B , m : B , and m : E are 49.9, 17.6, and 0.39 for a wavelength of 532 nm. The ratios involving s cannot be represented by a single number, instead requiring an exponential relationship. More complicated dependencies in the form of exponential functions and for different altitude layers are derived for all ratios.

Aerosol from the Hunga Tonga‐Hunga Ha'apai (HT‐HH) volcanic eruption (20.6°S) in January 2022 were not incorporated into the austral polar vortex until the following year, March 2023. Within the polar vortex in situ profiles of aerosol size spectra were completed in the austral autumns of 2019 and 2023, from McMurdo Station, Antarctica (78˚S), 30 months prior to and 15 months after the HT‐HH eruption. The measurements indicate that the HT‐HH impact on aerosol size was primarily confined to particles with diameters >0.5 μm leading to differences in aerosol mass, surface area, and extinction from factors of 2–4 at the volcanic layer's peak below 20 km, increasing to ratios of 5–10 above 20 km. Effective radius, with radiative and microphysical implications, increased from ∼0.2 to ∼0.3 μm. An Earth system model with a modal aerosol package compares favorably with the in situ measurements of the HT‐HH aerosol impact.

The Asian Summer Monsoon (ASM) convection transports aerosols and their precursors from the boundary layer to the upper troposphere and lower stratosphere (UTLS). This process forms an annually recurring aerosol layer near the tropopause. Recent observations have revealed a distinct property of the aerosol layer over the ASM region, it is nitrate‐rich. We present a newly implemented aerosol formation algorithm that enhances the representation of nitrate aerosol in the Community Aerosol and Radiation Model for Atmospheres (CARMA) coupled with the Community Earth System Model (CESM). The simulated aerosol chemical composition, as well as vertical distributions of aerosol size and mass, are evaluated using in situ and remote sensing observations. The simulated concentrations (ammonium, nitrate, and sulfate) and size distributions are generally within the error bars of data. We find nitrate, organics, and sulfate contribute significantly to the UTLS aerosol concentration between 15°–45°N and 0°–160°E. The two key formation mechanisms of nitrate‐containing aerosols in the ATAL are ammonium neutralization to form ammonium nitrate in regions where convection is active, and condensation of nitric acid in regions of cold temperature. Furthermore, including nitrate formation in the model doubles the surface area density in the tropical tropopause region between 15°–45°N and 0°–160°E, which alters the chlorine partitioning and subsequently impacts the rate of ozone depletion.

The Stratospheric Total Aerosol Counter (STAC) is a lightweight balloon‐borne instrument that utilizes condensational growth techniques to measure the total aerosol concentration. STAC is a miniaturized version of the legacy Wyoming condensation particle counter that operated from 1974 through 2020 in the middle latitudes and polar regions, with a few measurements in the tropics. Here we provide a description of the STAC instrument and the total aerosol measurement record, demonstrating that typical total aerosol profiles exhibit a peak in number mixing ratio, with values between 800 and 2,000 particles per mg of air (mg⁻¹), just below the lapse rate tropopause (LRT). In the tropics and middle latitudes, mixing ratios decrease above the LRT likely due to coagulation and scavenging that results in a transfer of mass to the fewer but larger aerosol particles of the Junge layer. Exceptions to this occur in the spring time in the middle latitudes where a new particle layer between 20 and 25 km is frequently observed. In the poles, total aerosol profiles exhibit two distinct features: new particle formation in austral spring, and an increasing mixing ratio above 17 km likely due to the presence of meteoric smoke that has been concentrated within the polar vortex. High observed stratospheric particle mixing ratios, in excess of 2,000 mg⁻¹, are observed in the polar new particle layer and at the top of polar profiles.

The recovery and re-calibration of a dataset of vertical aerosol extinction profiles of the 1963/64 stratospheric aerosol layer measured by a searchlight at 32° N in New Mexico, US, is reported. The recovered dataset consists of 105 aerosol extinction profiles at 550 nm that cover the period from December 1963 to December 1964. It is a unique record of the portion of the aerosol cloud from the March 1963 Agung volcanic eruption that was transported into the Northern Hemisphere subtropics. The data-recovery methodology involved re-digitizing the 105 original aerosol extinction profiles from individual Figures within a research report, followed by the re-calibration. It involves inverting the original equation used to compute the aerosol extinction profile to retrieve the corresponding normalized detector response profile. The re-calibration of the original aerosol extinction profiles used Rayleigh extinction profiles calculated from local soundings. Rayleigh and aerosol slant transmission corrections are applied using the MODTRAN code in transmission mode. Also, a best-estimate aerosol phase function was calculated from observations and applied to the entire column. The tropospheric aerosol phase function from an AERONET station in the vicinity of the searchlight location was applied between 2.76 to 11.7 km. The stratospheric phase function, applied for a 12.2 to 35.2 km altitude range, is calculated from particle-size distributions measured by a high-altitude aircraft in the vicinity of the searchlight in early 1964. The original error estimate was updated considering unaccounted errors. Both the re-calibrated aerosol extinction profiles and the re-calibrated stratospheric aerosol optical depth magnitudes showed higher magnitudes than the original aerosol extinction profiles and the original stratospheric aerosol optical depth, respectively. However, the magnitudes of the re-calibrated variables show a reasonable agreement with other contemporary observations. The re-calibrated stratospheric aerosol optical depth demonstrated its consistency with the tropics-to-pole decreasing trend, associated with the major volcanic eruption stratospheric aerosol pattern when compared to the time-coincident stratospheric aerosol optical depth lidar observations at Lexington at 42° N.

Profiles of stratospheric aerosol size distributions have been measured using balloon‐borne in situ optical particle counters, from Laramie, Wyoming (41°N) since 1971. In 2019, this measurement record transitioned to the Laboratory for Atmospheric and Space Physics (LASP) in Boulder, Colorado (40°N). The new LASP Optical Particle Counter (LOPC), the fourth generation of instruments used for this record, is smaller and lighter (2 kg) than prior instruments, measures aerosols with diameters ≥0.3–30 μm in up to 450 size bins, with a flow rate of 20 L min⁻¹. The improved size resolution enables the complete measurement of size distributions, and calculation of aerosol extinction without fitting a priori distribution shapes. The higher flow provides the sensitivity required to measure super‐micron particles in the stratosphere. The LOPC has been validated against prior Wyoming OPCs, through joint flights, laboratory comparisons, and statistical comparisons with the Wyoming record. The agreement between instruments is generally within the measurement uncertainty of ±10%–20% in sizing and ±10% in concentration, and within ±40% for calculated aerosol moments. The record is being continued with balloon soundings every 2 months from Colorado, coordinated with measurements of aerosol extinction from the SAGE III instrument on the International Space Station. Comparisons of aerosol extinction from the remote and in situ platforms have shown good agreement in the stratosphere, particularly for wavelengths in situ measurements, yet still within the ±40% uncertainty.

Atmospheric waves in the tropical tropopause layer are recognized as a significant influence on processes that impact global climate. For example, waves drive the quasi‐biennial oscillation (QBO) in equatorial stratospheric winds and modulate occurrences of cirrus clouds. However, the QBO in the lower stratosphere and thin cirrus have continued to elude accurate simulation in state‐of‐the‐art climate models and seasonal forecast systems. We use first‐of‐their‐kind profile measurements deployed beneath a long‐duration balloon to provide new insights into impacts of fine‐scale waves on equatorial cirrus clouds and the QBO just above the tropopause. Analysis of these balloon‐borne measurements reveals previously uncharacterized fine‐vertical‐scale waves (1000 km) and multiday periods. These waves affect cirrus clouds and QBO winds in ways that could explain current climate model shortcomings in representing these stratospheric influences on climate. Accurately simulating these fine‐vertical‐scale processes thus has the potential to improve sub‐seasonal to near‐term climate prediction.

This paper quantifies the tropical stratospheric aerosol content as impacted by volcanic events over the 2013–2019 period. We use global model simulations by the Whole Atmosphere Community Climate Model (WACCM) which is part of the Community Earth System Model version 1.0 (CESM1). WACCM is associated with the Community Aerosol and Radiation Model for Atmospheres (CARMA) sectional aerosol microphysics model which includes full sulphur chemical and microphysical cycles with no a priori assumption on particle size. Five main volcanic events (Kelud, Calbuco, Ambae, Raikoke and Ulawun) have been reported and are shown to have significantly influenced the stratospheric aerosol layer in the tropics, either through direct injection in this region or through transport from extra-tropical latitudes. Space-borne data as well as ground-based lidar and balloon-borne in situ observations are used to evaluate the model calculations in terms of aerosol content, vertical distribution, optical and microphysical properties, transport and residence time of the various volcanic plumes. Overall, zonal mean model results reproduce the occurrence and vertical extents of the plumes derived from satellite observations but shows some discrepancies for absolute values of extinction and of stratospheric aerosol optical depth (SAOD). Features of meridional transport of the plumes emitted from extra-tropical latitudes are captured by the model but simulated absolute values of SAOD differ from 6 to 200% among the various eruptions. Simulations tend to agree well with observed in situ vertical profiles for the Kelud and Calbuco plumes but this is likely to depend on the period for which comparison is done. Some explanations for the model–measurement discrepancies are discussed such as the inaccurate knowledge of the injection parameters and the presence of ash not accounted in the simulations.

In this presentation I will explain an analysis of three different recovered remote-sensing measurements of the 1960s Northern Hemisphere mid-latitude stratospheric aerosol layer. Two of the datasets were recovered within student projects on the Leeds MRes in Climate and Atmospheric Science, the 3rd following a collaboration with Dr. Juan-Carlos Antuna Marrero (Univ. Valladolid, Spain) as part of a “data rescue activity” within the World Climate Research Program activity on stratospheric sulphur, SSiRC: http://www.sparc-ssirc.org/data/datarescueactivity.html Two of the datasets are for the 1963-1965 period when the tropical stratospheric reservoir was highly elevated following the two March 1963 Agung major eruptions (e.g. Niemeier et al., 2019): a series of searchlight measurements from White Sands, New Mexico during 1963 and 1964 (Elterman and Campbell, 1964; Elterman, 1966; Elterman et al., 1973), and the first ever multi-annual stratospheric aerosol dataset from the MIT lidar at Lexington, Massachussetts (Grams, 1966; Grams & Fiocco, 1967; Antuna Marrero et al., 2020). The 3rd dataset, from the 1966-67 period (after the Agung aerosol cloud had fully dispersed) is from two types of balloon measurements: a dust-sonde OPC (Rosen, 1964; Rosen, 1968) and solar-extinction-sounder (Rosen, 1969; Pepin, 1970) both balloon instruments measuring during a Sep 1966 field campaign in the tropics (Panama City, Panama) and a sustained set of NH mid-latitude measurements from Minneapolis, Minnesota in 1963-1967. The observations will be compared to interactive stratospheric aerosol model simulations in GA4 UM-UKCA of the Agung aerosol cloud (Dhomse et al., 2020) and new model experiments seeking to constrain the aerosol clouds from two VEI4 eruptions in Sep 1965 (Taal, Phillipines) and Aug 1966 (Awu, Indonesia).

Both observations and recent model results reveal a significant organic component in stratospheric aerosols. However, intrinsic optical properties (i.e., complex refractive index) of this organic component and the mixing state of organic and ubiquitous sulfuric acid components are quite uncertain. We examine the effects of different complex refractive indices and particle mixing states on shortwave radiative forcing (RF) of stratospheric aerosols in northern midlatitudes. In the absence of large volcanic sulfur emissions, our calculations show that organic components may have substantial impacts on stratospheric aerosol optical depth (AOD). Compared to pure sulfuric acid/water aerosol, organic‐containing aerosols could cause ±100% change in shortwave RF (for low AOD conditions) depending on the refractive index and mixing state. The range found here of shortwave RF results, for different scenarios of organic complex refractive index and mixing state, call for better understandings of chemical and transport processes determining aerosol optical properties in the stratosphere.