The CALIPSO version 4 automated aerosol classification and lidar ratio selection algorithm

Abstract. The Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) version 4.10 (V4) level 2 aerosol data products, released in November 2016, include substantial improvements to the aerosol subtyping and lidar ratio selection algorithms. These improvements are described along with resulting changes in aerosol optical depth (AOD). The most fundamental change in the V4 level 2 aerosol products is a new algorithm to identify aerosol subtypes in the stratosphere. Four aerosol subtypes are introduced for stratospheric aerosols: polar stratospheric aerosol (PSA), volcanic ash, sulfate/other, and smoke. The tropospheric aerosol subtyping algorithm was also improved by adding the following enhancements: (1) all aerosol subtypes are now allowed over polar regions, whereas the version 3 (V3) algorithm allowed only clean continental and polluted continental aerosols; (2) a new “dusty marine” aerosol subtype is introduced, representing mixtures of dust and marine aerosols near the ocean surface; and (3) the “polluted continental” and “smoke” subtypes have been renamed “polluted continental/smoke” and “elevated smoke”, respectively. V4 also revises the lidar ratios for clean marine, dust, clean continental, and elevated smoke subtypes. As a consequence of the V4 updates, the mean 532 nm AOD retrieved by CALIOP has increased by 0.044 (0.036) or 52 % (40 %) for nighttime (daytime). Lidar ratio revisions are the most influential factor for AOD changes from V3 to V4, especially for cloud-free skies. Preliminary validation studies show that the AOD discrepancies between CALIOP and AERONET–MODIS (ocean) are reduced in V4 compared to V3.

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The CALIPSO Version 4 Automated Aerosol Classification and Lidar Ratio Selection Algorithm

10.5194/amt-2018-166 ◽

2018 ◽

Cited By ~ 3

Author(s):

Man-Hae Kim ◽

Ali H. Omar ◽

Jason L. Tackett ◽

Mark A. Vaughan ◽

David M. Winker ◽

...

Keyword(s):

Stratospheric Aerosol ◽

Influential Factor ◽

Orthogonal Polarization ◽

Polar Regions ◽

Marine Aerosols ◽

Preliminary Validation ◽

Tropospheric Aerosol ◽

Lidar Ratio ◽

Selection Algorithms ◽

Level 2

Abstract. The Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) version 4.10 (V4) level 2 aerosol data products, released in November 2016, include substantial improvements to the aerosol subtyping and lidar ratio selection algorithms. These improvements are described along with resulting changes in aerosol optical depth (AOD). The most fundamental change in V4 level 2 aerosol products is a new algorithm to identify aerosol subtypes in the stratosphere. Four aerosol subtypes are introduced for the stratospheric aerosols: polar stratospheric aerosol (PSA), volcanic ash, sulfate/other, and smoke. The tropospheric aerosol subtyping algorithm was also improved by adding the following enhancements: (1) all aerosol subtypes are now allowed over polar regions, whereas the version 3 (V3) algorithm allowed only clean continental and polluted continental aerosols; (2) a new “dusty marine” aerosol subtype is introduced, representing mixtures of dust and marine aerosols near the ocean surface; and (3) the “polluted continental” and “smoke” subtypes have been renamed “polluted continental/smoke” and “elevated smoke”, respectively. V4 also revises the lidar ratios for clean marine, dust, clean continental, and elevated smoke subtypes. As a consequence of the V4 updates, the mean 532 nm AOD retrieved by CALIOP has increased by 0.044 (0.036) or 52 % (40 %) for nighttime (daytime). Lidar ratio revisions are the most influential factor for AOD changes from V3 to V4, especially for cloud-free skies. Preliminary validation studies show that the AOD discrepancies between CALIOP and AERONET/MODIS (ocean) are reduced in V4 compared to V3.

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Enhancements to the caliop aerosol subtyping and lidar ratio selection algorithms for level II version 4

EPJ Web of Conferences ◽

10.1051/epjconf/201817602006 ◽

2018 ◽

Vol 176 ◽

pp. 02006 ◽

Cited By ~ 4

Author(s):

A. Omar ◽

J. Tackett ◽

M-H. Kim ◽

M. Vaughan ◽

J. Kar ◽

...

Keyword(s):

Boundary Layer ◽

Marine Boundary Layer ◽

Orthogonal Polarization ◽

Marine Aerosols ◽

Model Studies ◽

Level Ii ◽

Data Products ◽

Lidar Ratio ◽

Selection Algorithms ◽

Aerosol Type

Several enhancements have been implemented for the version 4 aerosol subtyping and lidar ratio selection algorithms of Cloud Aerosol Lidar with Orthogonal Polarization (CALIOP). Version 4 eliminates the confusion between smoke and clean marine aerosols seen in version 3 by modifications to the elevated layer flag definitions used to identify smoke aerosols over the ocean. To differentiate between mixtures of dust and smoke, and dust and marine aerosols, a new aerosol type will be added in the version 4 data products. In the marine boundary layer, moderately depolarizing aerosols are no longer modeled as mixtures of dust and smoke (polluted dust) but rather as mixtures of dust and seasalt (dusty marine). Some lidar ratios have been updated in the version 4 algorithms. In particular, the dust lidar ratios have been adjusted to reflect the latest measurements and model studies.

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CALIPSO level 3 stratospheric aerosol profile product: version 1.00 algorithm description and initial assessment

Atmospheric Measurement Techniques ◽

10.5194/amt-12-6173-2019 ◽

2019 ◽

Vol 12 (11) ◽

pp. 6173-6191 ◽

Cited By ~ 7

Author(s):

Jayanta Kar ◽

Kam-Pui Lee ◽

Mark A. Vaughan ◽

Jason L. Tackett ◽

Charles R. Trepte ◽

...

Keyword(s):

Space Station ◽

Volcanic Eruptions ◽

Satellite Observation ◽

Stratospheric Aerosol ◽

Initial Assessment ◽

Orthogonal Polarization ◽

Aerosol Layer ◽

Stratospheric Dynamics ◽

Level 3 ◽

Lidar Ratio

Abstract. In August 2018, the Cloud–Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) project released a new level 3 stratospheric aerosol profile data product derived from nearly 12 years of measurements acquired by the spaceborne Cloud–Aerosol Lidar with Orthogonal Polarization (CALIOP). This monthly averaged, gridded level 3 product is based on version 4 of the CALIOP level 1B and level 2 data products, which feature significantly improved calibration that now makes it possible to reliably retrieve profiles of stratospheric aerosol extinction and backscatter coefficients at 532 nm. This paper describes the science algorithm and data handling techniques that were developed to generate the CALIPSO version 1.00 level 3 stratospheric aerosol profile product. Further, we show that the extinction profiles (retrieved using a constant lidar ratio of 50 sr) capture the major stratospheric perturbations in both hemispheres over the last decade resulting from volcanic eruptions, extreme smoke events, and signatures of stratospheric dynamics. Initial assessment of the product by intercomparison with the stratospheric aerosol retrievals from the Stratospheric Aerosol and Gas Experiment III (SAGE III) on the International Space Station (ISS) indicates good agreement in the tropical stratospheric aerosol layer (30∘ N–30∘ S), where the average difference between zonal mean extinction profiles is typically less than 25 % between 20 and 30 km (CALIPSO biased high). However, differences can exceed 100 % in the very low aerosol loading regimes found above 25 km at higher latitudes. Similarly, there are large differences (≥100 %) within 2 to 3 km above the tropopause that might be due to cloud contamination issues.

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Extinction and optical depth retrievals for CALIPSO's Version 4 data release

Atmospheric Measurement Techniques ◽

10.5194/amt-11-5701-2018 ◽

2018 ◽

Vol 11 (10) ◽

pp. 5701-5727 ◽

Cited By ~ 39

Author(s):

Stuart A. Young ◽

Mark A. Vaughan ◽

Anne Garnier ◽

Jason L. Tackett ◽

James D. Lambeth ◽

...

Keyword(s):

Optical Depth ◽

Satellite Observations ◽

Orthogonal Polarization ◽

Ice Clouds ◽

Extinction Coefficients ◽

Retrieval Algorithms ◽

Data Release ◽

Data Products ◽

Lidar Ratio ◽

Level 2

Abstract. The Cloud–Aerosol Lidar with Orthogonal Polarization (CALIOP) on board the Cloud–Aerosol Lidar Infrared Pathfinder Satellite Observations (CALIPSO) satellite has been making near-global height-resolved measurements of cloud and aerosol layers since mid-June 2006. Version 4.10 (V4) of the CALIOP data products, released in November 2016, introduces extensive upgrades to the algorithms used to retrieve the spatial and optical properties of these layers, and thus there are both obvious and subtle differences between V4 and previous data releases. This paper describes the improvements made to the extinction retrieval algorithms and illustrates the impacts of these changes on the extinction and optical depth estimates reported in the CALIPSO lidar level 2 data products. The lidar ratios for both aerosols and ice clouds are generally higher than in previous data releases, resulting in generally higher extinction coefficients and optical depths in V4. A newly implemented algorithm for retrieving extinction coefficients in opaque layers is described and its impact examined. Precise lidar ratio estimates are also retrieved in these opaque layers. For semi-transparent cirrus clouds, comparisons between CALIOP V4 optical depths and the optical depths reported by MODIS collection 6 show substantial improvements relative to earlier comparisons between CALIOP version 3 and MODIS collection 5.

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Spaceborne observations of the lidar ratio of marine aerosols

Atmospheric Chemistry and Physics ◽

10.5194/acp-15-3241-2015 ◽

2015 ◽

Vol 15 (6) ◽

pp. 3241-3255 ◽

Cited By ~ 18

Author(s):

K. W. Dawson ◽

N. Meskhidze ◽

D. Josset ◽

S. Gassó

Keyword(s):

Optical Depth ◽

Surface Wind ◽

Spatiotemporal Variability ◽

Retrieval Algorithm ◽

Orthogonal Polarization ◽

Aerosol Layer ◽

Marine Aerosol ◽

Marine Aerosols ◽

Lidar Ratio ◽

The Mean

Abstract. Retrievals of aerosol optical depth (AOD) from the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) satellite sensor require the assumption of the extinction-to-backscatter ratio, also known as the lidar ratio. This paper evaluates a new method to calculate the lidar ratio of marine aerosols using two independent sources: the AOD from the Synergized Optical Depth of Aerosols (SODA) project and the integrated attenuated backscatter from CALIOP. With this method, the particulate lidar ratio can be derived for individual CALIOP retrievals in single aerosol layer, cloud-free columns over the ocean. Global analyses are carried out using CALIOP level 2, 5 km marine aerosol layer products and the collocated SODA nighttime data from December 2007 to November 2010. The global mean lidar ratio for marine aerosols was found to be 26 sr, roughly 30% higher than the current value prescribed by the CALIOP standard retrieval algorithm. Data analysis also showed considerable spatiotemporal variability in the calculated lidar ratio over the remote oceans. The calculated marine aerosol lidar ratio is found to vary with the mean ocean surface wind speed (U10). An increase in U10 reduces the mean lidar ratio for marine regions from 32 ± 17 sr (for 0 < U10 < 4 m s−1) to 22 ± 7 sr (for U10 > 15 m s−1). Such changes in the lidar ratio are expected to have a corresponding effect on the marine AOD from CALIOP. The outcomes of this study are relevant for future improvements of the SODA and CALIOP operational product and could lead to more accurate retrievals of marine AOD.

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Looking through the haze: evaluating the CALIPSO level 2 aerosol optical depth using airborne high spectral resolution lidar data

Atmospheric Measurement Techniques Discussions ◽

10.5194/amtd-7-6141-2014 ◽

2014 ◽

Vol 7 (6) ◽

pp. 6141-6204 ◽

Cited By ~ 2

Author(s):

R. R. Rogers ◽

M. A. Vaughan ◽

C. A. Hostetler ◽

S. P. Burton ◽

R. A. Ferrare ◽

...

Keyword(s):

Optical Depth ◽

Spectral Resolution ◽

Detection Threshold ◽

High Spectral Resolution ◽

Orthogonal Polarization ◽

Aerosol Layer ◽

Uncertainty Estimates ◽

Lidar Ratio ◽

High Spectral Resolution Lidar ◽

Level 2

Abstract. The Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) instrument onboard the Cloud Aerosol Lidar and Pathfinder Satellite Observations (CALIPSO) spacecraft has provided over 8 years of nearly continuous vertical profiling of Earth's atmosphere. In this paper we investigate the CALIOP 532 nm aerosol layer optical depth (AOD) product, the AOD of individual layers, and the column AOD product, the sum AOD of the complete column, using an extensive database of coincident measurements. The CALIOP AOD measurements and AOD uncertainty estimates are compared with collocated AOD measurements collected with the NASA High Spectral Resolution Lidar (HSRL) in the North American and Caribbean regions. In addition, the CALIOP aerosol lidar ratios are investigated using the HSRL measurements. In general, compared with the HSRL values, the CALIOP layer AOD are biased high by less than 50% for AOD < 0.3 with higher errors for higher AOD. Less than 60% of the HSRL AOD measurements are encompassed within the CALIOP layer one-standard-deviation uncertainty range (around the CALIOP layer AOD), so an error estimate is created to encompass 68% of the HSRL data. Using this new metric, the CALIOP layer AOD error is estimated using the HSRL layer AOD as ± 0.035 ± 0.05 · (HSRL layer AOD) at night and ±0.05 ± 0.05 · (HSRL layer AOD) during the daytime. Furthermore, the CALIOP layer AOD error is found to correlate with aerosol loading as well as aerosol subtype, with the AODs in marine and dust layers agreeing most closely with the HSRL values. The lidar ratios used by CALIOP for polluted dust, polluted continental, and biomass burning layers are larger than the values measured by the HSRL in the CALIOP layers, and, therefore, the AODs for these types retrieved by CALIOP were generally too large. We estimated the CALIOP column AOD error can be expressed as ± 0.05 ± 0.07 · (HSRL column AOD) at night and ± 0.08 ± 0.1 · (HSRL column AOD) during the daytime. Multiple sources of error contribute to both positive and negative errors in the CALIOP column AOD, including multiple layers in the column of different aerosol types, lidar ratio errors, cloud misclassification, and undetected aerosol layers. The undetected layers were further investigated and we found that the layer detection algorithm works well at night, although undetected aerosols in the free troposphere introduce a mean underestimate of 0.02 in the column AOD in the dataset examined. The decreased SNR during the daytime led to poorer performance of the layer detection. This caused the daytime CALIOP column AOD to be less accurate than during the nighttime because CALIOP frequently does not detect optically thin aerosol layers with AOD < 0.1. Given that the median vertical extent of aerosol detected within any column was 1.6 km during the nighttime and 1.5 km during the daytime we can estimate the minimum extinction detection threshold to be 0.012 km−1 at night and 0.067 km−1 during the daytime in a layer median sense. This extensive validation of level 2 CALIOP aerosol layer optical depth products extends previous validation studies to nighttime lighting conditions and provides independent measurements of the lidar ratio, thus allowing the assessment of the effect on the CALIOP AOD of using inappropriate lidar ratio values in the extinction retrieval.

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Stratospheric aerosols from the Sarychev volcano eruption in the 2009 Arctic summer

Atmospheric Chemistry and Physics ◽

10.5194/acp-13-6533-2013 ◽

2013 ◽

Vol 13 (13) ◽

pp. 6533-6552 ◽

Cited By ~ 22

Author(s):

F. Jégou ◽

G. Berthet ◽

C. Brogniez ◽

J.-B. Renard ◽

P. François ◽

...

Keyword(s):

Lower Stratosphere ◽

Imaging System ◽

Stratospheric Aerosol ◽

Atmospheric Composition ◽

The Arctic ◽

Orthogonal Polarization ◽

Volcano Eruption ◽

A Value ◽

Sulphate Aerosols ◽

Tropospheric Aerosol

Abstract. Aerosols from the Sarychev volcano eruption (Kuril Islands, northeast of Japan) were observed in the Arctic lower stratosphere a few days after the strongest SO2 injection which occurred on 15 and 16 June 2009. From the observations provided by the Infrared Atmospheric Sounding Interferometer (IASI) an estimated 0.9 Tg of sulphur dioxide was injected into the upper troposphere and lower stratosphere (UTLS). The resultant stratospheric sulphate aerosols were detected from satellites by the Optical Spectrograph and Infrared Imaging System (OSIRIS) limb sounder and by the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) and from the surface by the Network for the Detection of Atmospheric Composition Changes (NDACC) lidar deployed at OHP (Observatoire de Haute-Provence, France). By the first week of July the aerosol plume had spread out over the entire Arctic region. The Sarychev-induced stratospheric aerosol over the Kiruna region (north of Sweden) was measured by the Stratospheric and Tropospheric Aerosol Counter (STAC) during eight balloon flights planned in August and September 2009. During this balloon campaign the Micro Radiomètre Ballon (MicroRADIBAL) and the Spectroscopie d'Absorption Lunaire pour l'Observation des Minoritaires Ozone et NOx (SALOMON) remote-sensing instruments also observed these aerosols. Aerosol concentrations returned to near-background levels by spring 2010. The effective radius, the surface area density (SAD), the aerosol extinction, and the total sulphur mass from STAC in situ measurements are enhanced with mean values in the range 0.15–0.21 μm, 5.5–14.7 μm2 cm−3, 5.5–29.5 × 10−4 km−1, and 4.9–12.6 × 10−10 kg[S] kg−1[air], respectively, between 14 km and 18 km. The observed and modelled e-folding time of sulphate aerosols from the Sarychev eruption is around 70–80 days, a value much shorter than the 12–14 months calculated for aerosols from the 1991 eruption of Mt Pinatubo. The OSIRIS stratospheric aerosol optical depth (AOD) at 750 nm is enhanced by a factor of 6, with a value of 0.02 in late July compared to 0.0035 before the eruption. The HadGEM2 and MIMOSA model outputs indicate that aerosol layers in polar region up to 14–15 km are largely modulated by stratosphere–troposphere exchange processes. The spatial extent of the Sarychev plume is well represented in the HadGEM2 model with lower altitudes of the plume being controlled by upper tropospheric troughs which displace the plume downward and upper altitudes around 18–20 km, in agreement with lidar observations. Good consistency is found between the HadGEM2 sulphur mass density and the value inferred from the STAC observations, with a maximum located about 1 km above the tropopause ranging from 1 to 2 × 10−9 kg[S] kg−1[air], which is one order of magnitude higher than the background level.

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Lidar ratios of stratospheric volcanic ash and sulfate aerosols retrieved from CALIOP measurements

10.5194/acp-2016-1173 ◽

2017 ◽

Author(s):

Andrew T. Prata ◽

Stuart A. Young ◽

Steven T. Siems ◽

Michael J. Manton

Keyword(s):

Volcanic Ash ◽

Stratospheric Aerosol ◽

Orthogonal Polarization ◽

Volcanic Aerosol ◽

Time Period ◽

Lidar Ratio ◽

The Mean ◽

Lidar Observations ◽

532 Nm ◽

Sarychev Peak

Abstract. We apply a two-way transmittance constraint to nighttime CALIOP (Cloud-Aerosol Lidar with Orthogonal Polarization) observations of volcanic aerosol layers to retrieve estimates of the particulate lidar ratio (Sp) at 532 nm. This technique is applied to three volcanic eruption case studies that were found to have injected aerosols directly into the stratosphere. Numerous lidar observations permitted characterisation of the optical and geometric properties of the volcanic aerosol layers over a time period of 1–2 weeks. For the volcanic ash layers produced by the Puyehue-Cordón Caulle eruption (June 2011) we obtain mean and median particulate lidar ratios of 72 ± 14 sr and 70 sr, respectively. For the sulfates produced by Kasatochi (August 2008) and Sarychev Peak (June 2009), the mean of the retrieved lidar ratios were 68 ± 21 sr (median 62 sr) and 66 ± 15 sr (median 61 sr), respectively. The layer-integrated volume depolarisation ratios (∂v) observed for the Puyehue ash layers (∂v = 0.28 ± 0.03) were much larger than those found for the sulfate layers produced by the Kasatochi (∂v = 0.08 ± 0.03) and Sarychev (∂v = 0.05 ± 0.04) eruptions. However, for the Sarychev layers we observe an exponential decay (e-folding time of 1 week) in ∂v with time from 0.25 to 0.05. The layer-integrated attenuated colour ratios for the Puyehue ash layers (χ' = 0.54 ± 0.07) were also larger than what was observed for the Kasatochi (χ' = 0.35 ± 0.07) and Sarychev (χ' = 0.32 ± 0.07) sulfate layers, indicating that the Puyehue ash layers were generally composed of larger particles. These observations are particularly relevant to the new stratospheric aerosol subtyping classification scheme, which has been incorporated into version 4 of the level 2 CALIPSO data products.

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Extinction and Optical Depth Retrievals for CALIPSO's Version 4 Data Release

10.5194/amt-2018-182 ◽

2018 ◽

Author(s):

Stuart A. Young ◽

Mark A. Vaughan ◽

Anne Garnier ◽

Jason L. Tackett ◽

James B. Lambeth ◽

...

Keyword(s):

Optical Depth ◽

Satellite Observations ◽

Orthogonal Polarization ◽

Ice Clouds ◽

Extinction Coefficients ◽

Retrieval Algorithms ◽

Data Release ◽

Data Products ◽

Lidar Ratio ◽

Level 2

Abstract. The Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) on board the Cloud-Aerosol Lidar Infrared Pathfinder Satellite Observations (CALIPSO) satellite has been making near-global height-resolved measurements of cloud and aerosol layers since mid-June 2006. Version 4.10 (V4) of the CALIOP data products, released in November, 2016, introduces extensive upgrades to the algorithms used to retrieve the spatial and optical properties of these layers, and thus there are both obvious and subtle differences between V4 and previous data releases. This paper describes the improvements made to the extinction retrieval algorithms and illustrates the impacts of these changes on the extinction and optical depth estimates reported in the CALIPSO lidar level 2 data products. The lidar ratios for both aerosols and ice clouds are generally higher than in previous data releases, resulting in generally higher extinction coefficients and optical depths in V4. A newly implemented algorithm for retrieving extinction coefficients in opaque layers is described and its impact examined. Precise lidar ratio estimates are also retrieved in these opaque layers. For semi-transparent cirrus clouds, comparisons between CALIOP V4 optical depths and the optical depths reported by MODIS collection 6 show substantial improvements relative to earlier comparisons between CALIOP version 3 and MODIS collection 5.

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Stratospheric aerosols from the Sarychev volcano eruption in the 2009 Arctic summer

Atmospheric Chemistry and Physics Discussions ◽

10.5194/acpd-13-3613-2013 ◽

2013 ◽

Vol 13 (2) ◽

pp. 3613-3662

Author(s):

F. Jégou ◽

G. Berthet ◽

C. Brogniez ◽

J.-B. Renard ◽

P. François ◽

...

Keyword(s):

Lower Stratosphere ◽

Imaging System ◽

Mass Density ◽

Stratospheric Aerosol ◽

The Arctic ◽

Orthogonal Polarization ◽

Volcano Eruption ◽

A Value ◽

Sulphate Aerosols ◽

Tropospheric Aerosol

Abstract. Aerosols from the Sarychev volcano eruption (Kuril Islands, northeast of Japan) were observed in the Arctic lower stratosphere a few days after the strongest SO2 injection which occurred on 15 and 16 June 2009. From the observations provided by the Infrared Atmospheric Sounding Interferometer (IASI) an estimated 0.9 Tg of sulphur dioxide was injected into the Upper Troposphere and Lower Stratosphere (UTLS). The resultant stratospheric sulphate aerosols were detected by the Optical Spectrograph and Infrared Imaging System (OSIRIS) limb sounder and by the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) satellite instruments. By the first week of July the aerosol plume had spread out over the entire Arctic region. The Sarychev-induced stratospheric aerosol over the Kiruna region (north of Sweden) was measured by the Stratospheric and Tropospheric Aerosol Counter (STAC) during eight balloon flights planned in August and September 2009. During this balloon campaign the Micro RADIomètre BALlon (MicroRADIBAL) and the Spectroscopie d'Absorption Lunaire pour l'Observation des Minoritaires Ozone et NOx (SALOMON) remote-sensing instruments also observed these aerosols. Aerosol concentrations returned to near-background levels by spring 2010. The effective radius, the Surface Area Density (SAD), the aerosol extinction, and the total sulphur mass from STAC in situ measurements are enhanced with mean values in the range 0.15–0.21 μm, 5.5–14.7 μm2 cm−3, 5.5–29.5×10−4 km−1, and 4.9–12.6×10−10 kg [S] kg−1 [air], respectively, between 14 km and 18 km. The observed and modelled e-folding time of sulphate aerosols from the Sarychev eruption is around 70–80 days, a value much shorter than the 12–14 months calculated for aerosols from the 1991 eruption of Mt. Pinatubo. The OSIRIS stratospheric Aerosol Optical Depth (AOD) at 750 nm is enhanced by a factor of 6 with a value of 0.02 in late July compared to 0.0035 before the eruption. The HadGEM2 and MIMOSA model outputs indicate that aerosol layers in polar region up to 14–15 km are largely modulated by stratosphere-troposphere exchange processes. The spatial extension of the Sarychev plume is well represented in the HadGEM2 model with lower altitudes of the plume being controlled by upper tropospheric troughs which displace the plume downward and upper altitudes around 18–20 km in agreement with lidar observations. A good consistency is found between the HadGEM2 sulphur mass density and the value inferred from the STAC observations with a maximum located about 1 km above the tropopause ranging from 1 to 2×10−9 kg [S] kg−1 [air], which is one order of magnitude higher than the background level.

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