Stratospheric Aerosol Levels Research Articles

Stratospheric aerosol-Observations, processes, and impact on climate

Interest in stratospheric aerosol and its role in climate have increased over the last decade due to the observed increase in stratospheric aerosol since 2000 and the potential for changes in the sulfur cycle induced by climate change. This review provides an overview about the advances in stratospheric aerosol research since the last comprehensive assessment of stratospheric aerosol was published in 2006. A crucial development since 2006 is the substantial improvement in the agreement between in situ and space-based inferences of stratospheric aerosol properties during volcanically quiescent periods. Furthermore, new measurement systems and techniques, both in situ and space based, have been developed for measuring physical aerosol properties with greater accuracy and for characterizing aerosol composition. However, these changes induce challenges to constructing a long-term stratospheric aerosol climatology. Currently, changes in stratospheric aerosol levels less than 20% cannot be confidently quantified. The volcanic signals tend to mask any nonvolcanically driven change, making them difficult to understand. While the role of carbonyl sulfide as a substantial and relatively constant source of stratospheric sulfur has been confirmed by new observations and model simulations, large uncertainties remain with respect to the contribution from anthropogenic sulfur dioxide emissions. New evidence has been provided that stratospheric aerosol can also contain small amounts of nonsulfate matter such as black carbon and organics. Chemistry-climate models have substantially increased in quantity and sophistication. In many models the implementation of stratospheric aerosol processes is coupled to radiation and/or stratospheric chemistry modules to account for relevant feedback processes.

Increase in upper tropospheric and lower stratospheric aerosol levels and its potential connection with Asian pollution.

Satellite observations have shown that the Asian Summer Monsoon strongly influences the upper troposphere and lower stratosphere (UTLS) aerosol morphology through its role in the formation of the Asian Tropopause Aerosol Layer (ATAL). Stratospheric Aerosol and Gas Experiment II solar occultation and Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) lidar observations show that summertime UTLS Aerosol Optical Depth (AOD) between 13 and 18 km over Asia has increased by three times since the late 1990s. Here we present the first in situ balloon measurements of aerosol backscatter in the UTLS from Western China, which confirm high aerosol levels observed by CALIPSO since 2006. Aircraft in situ measurements suggest that aerosols at lower altitudes of the ATAL are largely composed of carbonaceous and sulfate materials (carbon/sulfur elemental ratio ranging from 2 to 10). Back trajectory analysis from Cloud-Aerosol Lidar with Orthogonal Polarization observations indicates that deep convection over the Indian subcontinent supplies the ATAL through the transport of pollution into the UTLS. Time series of deep convection occurrence, carbon monoxide, aerosol, temperature, and relative humidity suggest that secondary aerosol formation and growth in a cold, moist convective environment could play an important role in the formation of ATAL. Finally, radiative calculations show that the ATAL layer has exerted a short-term regional forcing at the top of the atmosphere of −0.1 W/m2 in the past 18 years.Key PointsIncrease of summertime upper tropospheric aerosol levels over Asia since the 1990s Upper tropospheric enhancement also observed by in situ backscatter measurements Significant regional radiative forcing of −0.1 W/m2

Nabro volcano aerosol in the stratosphere over Georgia, South Caucasus from ground-based spectrometry of twilight sky brightness

Abstract. Ground-based spectral measurements of twilight sky brightness were carried out between September 2009 and August 2011 in Georgia, South Caucasus. The algorithm which allowed to retrieve the lower stratospheric and upper tropospheric aerosol extinction profiles was developed. The Monte-Carlo technique was used to correctly represent multiple scattering in a spherical atmosphere. The estimated stratospheric aerosol optical depths at a wavelength of 780 nm were: 6 × 10−3 ± 2 × 10−3 (31 August 2009–29 November 2009), 2.5 × 10−3 ± 7 × 10−4 (20 March 2010–15 January 2011) and 8 × 10−3 ± 3 × 10−3 (18 July 2011–3 August 2011). The optical depth values correspond to the moderately elevated stratospheric aerosol level after the Sarychev eruption in 2009, background stratospheric aerosol layer, and the volcanically disturbed stratospheric aerosol layer after the Nabro eruption in June 2011.

SAGE II measurements of stratospheric aerosol properties at non-volcanic levels

Abstract. Since 2000, stratospheric aerosol levels have been relatively stable and at the lowest levels observed in the historical record. Given the challenges of making satellite measurements of aerosol properties at these levels, we have performed a study of the sensitivity of the product to the major components of the processing algorithm used in the production of SAGE II aerosol extinction measurements and the retrieval process that produces the operational surface area density (SAD) product. We find that the aerosol extinction measurements, particularly at 1020 nm, remain robust and reliable at the observed aerosol levels. On the other hand, during background periods, the SAD operational product has an uncertainty of at least a factor of 2 due to the lack of sensitivity to particles with radii less than 100 nm.

Relative performance of three SAGE‐II data versions under high aerosol conditions based on comparisons with microwave and ozonesonde profiles measured at two NDSC sites

Intercomparisons between SAGE‐II data versions 5.93, 5.96, and 6.1 with microwave ozone and ozonesonde measurements made at the NDSC primary station at Lauder, New Zealand, and with microwave measurements made at the NDSC complementary station at Table Mountain, California, are reported on here. The focus is on SAGE‐II measurement performance during the period when stratospheric aerosol levels were substantially elevated following the 1991 Mt. Pinatubo eruption. SAGE‐II ozone retrievals are potentially affected by aerosol levels and size distributions because extinction due to aerosol must be estimated and subtracted from the measured total extinction in the 600 nm ozone channel to determine the ozone amount; the microwave and ozonesonde comparison measurements are aerosol insensitive. Around 10–25 hPa, the newer algorithm versions retain a tendency toward extinction‐dependent bias previously reported for version 5.9; this dependence may be larger than previously indicated at moderately high aerosol levels. In the 30–40 hPa range, the extinction dependence of version 6.1 (and, usually, version 5.96) data is a few times less than that of version 5.93. Between about 30 and 80 hPa most points at moderately elevated aerosol extinctions are less affected than in version 5.93, and these are fewer and/or less affected in version 6.1 than in version 5.96. When aerosols are at background levels, the precisions of version 5.93 and 5.96 measurements are at least somewhat better, around 30 hPa, than the errors provided with the data. The errors provided with version 6.1 data are substantially smaller than in previous versions, and small enough that the experimental sensitivity was insufficient to draw conclusions regarding the actual precision in comparison to the error values.

Microwave ozone and lidar aerosol profile observations at Table Mountain, California, following the Pinatubo eruption

Ozone profiles measured with a ground‐based microwave instrument in years from 1989 through 1992 at latitude 34°N. were searched for effects arising from the eruption of Mount Pinatubo in 1991. Between 20 and 24 km, ozone values after November 1991 were systematically less than in earlier years, and the deviation generally grew with time through May 1992. The minimum was 11 to 15% below values in earlier years, and occurred later than minima observed with ozonesondes at higher latitudes or predicted by some models. Although there was a steady increase in ozone levels in the 18–10 hPa range between February and May 1992, the highest values were not significantly above normal. Effects of the quasi‐biennial oscillation on ozone values are much smaller than the observed changes. Colocated lidar observations show that stratospheric aerosol levels were steadily elevated from November 1991 through at least March 1992, with some intermittent activity in late August through October 1991. The main aerosol cloud thus arrived at the same time as depletion first appeared.

Lidar measurements of stratospheric aerosol over Mauna Loa Observatory

Dual‐wavelength aerosol lidar backscatter measurements at Mauna Loa Observatory are used to monitor and characterize the 15–30 km stratospheric aerosol layer. The decay of aerosol loading following the El Chichón, Mexico (17°N) and Pinatubo, Philippine Islands (15°N) volcanic eruptions of 1982 and 1991, respectively, depends on the phase of the quasibiennial oscillation (QBO) in tropical stratospheric winds. Averaged over a 3‐year period, these effects are removed and an exponential decay with a characteristic (e−1) decay time of about 1 year is observed for both eruptions. By the end of 1996, about 5 ½ years after the Pinatubo eruption, stratospheric aerosol levels at Mauna Loa had decayed to pre‐eruption levels, approximately matching the lowest aerosol levels seen here in the past 17 years (about 6 × 10−5 sr−1 at 694 nm integrated between 15.8 and 33 km). However, this background stratospheric aerosol level at Mauna Loa may depend on the QBO, being slightly lower during the westerly phase. Analyses of aerosol backscatter, backscatter wavelength dependence, and trajectories provide evidence for a minor injection of aerosol from the Rabaul eruption in Papua, New Guinea (4°S) in September of 1994.

A comparison of the stratospheric aerosol background periods of 1979 and 1989–1991

A comparison of global stratospheric aerosol levels measured in 1979 by the Stratospheric Aerosol and Gas Experiment (SAGE) and in 1989–1991 by SAGE II is presented. These periods exhibit the lowest stratospheric aerosol levels in the era of modern measurements and are often referred to as background periods. We find that, depending on latitude, the 1‐μm aerosol optical depth in 1989–1991 was 10 to 30% higher than that observed in 1979. We demonstrate that the latter period (prior to the June 1991 eruption of Mount Pinatubo) was characterized by an ongoing global recovery from the eruptions of El Chichón in 1982 and Nevado del Ruiz in 1985, with a further complication introduced by the February 1990 Kelut eruption. Therefore the differences between 1979 and 1989–1991 cannot be completely attributed to nonvolcanic sources.

Balloon observations of the time development of the stratospheric aerosol event of 1974-1975

Approximately monthly balloon soundings, at mid-latitude using an aerosol particle counter, are utilized in a study of the time development of an order of magnitude increase in the stratospheric aerosol level which began in late 1974. A substantial reduction, by a factor of 5, was observed in the total number of stratospheric particles with r ≥ 0.15 μm from the peak observed in February 1975 to the minimum observed in October 1975. The characteristic decay time for the peak particle mixing ratio was about 1 year. Aerosol vaporization measurements show that all the new aerosol having r ≥ 0.15 μm was volatile 3–4 months after the increase began, a result suggesting a relatively rapid gas-to-particle conversion process. If the gas involved was volcanic SO2, then an injection of about 0.7 Mt would be required to explain the aerosol increase.