scholarly journals Microwave Limb Sounder (MLS) observations of biomass burning products in the stratosphere from Canadian forest fires in August 2017

2021 ◽  
Vol 21 (22) ◽  
pp. 16645-16659
Author(s):  
Hugh C. Pumphrey ◽  
Michael J. Schwartz ◽  
Michelle L. Santee ◽  
George P. Kablick III ◽  
Michael D. Fromm ◽  
...  
Keyword(s):  
British Columbia ◽  
Water Vapour ◽  
Biomass Burning ◽  
Forest Fires ◽  
Plume Dispersion ◽  
Water Vapour Content ◽  
Air Mass ◽  
Back Trajectories ◽  
Mixing Ratios ◽  
Elevated Water

Abstract. Forest fires in British Columbia in August 2017 caused a pyrocumulonimbus event that injected a polluted air mass into the lower stratosphere. The Microwave Limb Sounder (MLS) on the Aura satellite first observed the polluted air mass on 14 August 2017 and continued to observe it for 60 d (100 d in water vapour). We estimate the mass of CO injected into the stratosphere to be 2400 Gg. Events in which a fire injects its burning products directly into the stratosphere are rare: this is the third of four such events in the 16 years since the launch of Aura, the second largest of the four events, and the only one in the Northern Hemisphere. The other three events occurred in Australia in December 2006, February 2009 and from December 2019 to January 2020. Unlike the 2006 and 2009 events, but like the 2019–2020 event, the polluted air mass described here had a clearly elevated water vapour content: between 2.5 and 5 times greater than that in the surrounding atmosphere. We describe the evolution of the polluted air mass, showing that it rose to an altitude of about 24 km (31 hPa) and divided into several identifiable parts. In addition to CO and H2O, we observe enhanced amounts of HCN, CH3CN, CH3Cl and CH3OH with mixing ratios in the range to be expected from a variety of measurements in other biomass burning plumes. We use back trajectories and plume-dispersion modelling to demonstrate that the pollutants observed by MLS originated in the British Columbia fires, the likeliest source being at 53.2∘ N, 121.8∘ W at 05:20 UTC on 13 August 2017.

scholarly journals Stratospheric pollution from Canadian forest fires

2020 ◽  
Author(s):  
Hugh C. Pumphrey ◽  
Michael J. Schwartz ◽  
Michelle L. Santee ◽  
George P. Kablick III ◽  
Michael D. Fromm ◽  
...  
Keyword(s):  
British Columbia ◽  
Water Vapour ◽  
Northern Hemisphere ◽  
Forest Fires ◽  
Lower Stratosphere ◽  
High Water ◽  
Recent Event ◽  
Water Vapour Content ◽  
The Third

Abstract. Forest fires in British Columbia in August 2017 caused a pyrocumulonimbus event that injected a polluted airmass into the lower stratosphere. The Microwave Limb Sounder (MLS) on the Aura satellite first observed the polluted airmass on 14 August 2017 and continued to observe it for 60 days (100 days in water vapour). We estimate the mass of CO injected into the stratosphere to be 2–3 Tg. Events such as this are rare: this is the third of four such events in the 16 years since the launch of Aura, the second-largest of the four events, and the only one in the Northern Hemisphere. Unlike the preceding two events, but like the most recent event, the polluted airmass described here had an unusually high water vapour content.

scholarly journals Saharan dust and biomass burning aerosols during ex-hurricane Ophelia: observations from the new UK lidar and sun-photometer network

2019 ◽  
Vol 19 (6) ◽  
pp. 3557-3578 ◽  
Author(s):  
Martin Osborne ◽  
Florent F. Malavelle ◽  
Mariana Adam ◽  
Joelle Buxmann ◽  
Jaqueline Sugier ◽  
...  
Keyword(s):  
Biomass Burning ◽  
Forest Fires ◽  
Complex Structure ◽  
Atmospheric Dispersion ◽  
Saharan Dust ◽  
Atmospheric Concentration ◽  
Back Trajectories ◽  
Sun Photometer ◽  
The Uk ◽  
Aerosol Types

Abstract. On 15–16 October 2017, ex-hurricane Ophelia passed to the west of the British Isles, bringing dust from the Sahara and smoke from Portuguese forest fires that was observable to the naked eye and reported in the UK's national press. We report here detailed observations of this event using the UK operational lidar and sun-photometer network, established for the early detection of aviation hazards, including volcanic ash. We also use ECMWF ERA5 wind field data and MODIS imagery to examine the aerosol transport. The observations, taken continuously over a period of 30 h, show a complex picture, dominated by several different aerosol layers at different times and clearly correlated with the passage of different air masses associated with the intense cyclonic system. A similar evolution was observed at several sites, with a time delay between them explained by their different location with respect to the storm and associated meteorological features. The event commenced with a shallow dust layer at 1–2 km in altitude and culminated in a deep and complex structure that lasted ∼12 h at each site over the UK, correlated with the storm's warm sector. For most of the time, the aerosol detected was dominated by mineral dust mixtures, as highlighted by depolarisation measurements, but an intense biomass burning aerosol (BBA) layer was observed towards the end of the event, lasting around 3 h at each site. The aerosol optical depth at 355 nm (AOD355) during the whole event ranged from 0.2 to 2.9, with the larger AOD correlated to the intense BBA layer. Such a large AOD is unprecedented in the UK according to AERONET records for the last 20 years. The Raman lidars permitted the measurement of the aerosol extinction coefficient at 355 nm, the particle linear depolarisation ratio (PLDR), and the lidar ratio (LR) and made the separation of the dust (depolarising) aerosol from other aerosol types possible. A specific extinction has also been computed to provide an estimate of the atmospheric concentration of both aerosol types separately, which peaked at 420±200 µg m−3 for the dust and 558±232 µg m−3 for the biomass burning aerosols. Back trajectories computed using the Numerical Atmospheric-dispersion Modelling Environment (NAME) were used to identify the sources and strengthen the conclusions drawn from the observations. The UK network represents a significant expansion of the observing capability in northern Europe, with instruments evenly distributed across Great Britain, from Camborne in Cornwall to Lerwick in the Shetland Islands, and this study represents the first attempt to demonstrate its capability and validate the methods in use. Its ultimate purpose will be the detection and quantification of volcanic plumes, but the present study clearly demonstrates the advanced capabilities of the network.

scholarly journals The impact of deforestation in the Amazonian atmospheric radiative balance: a remote sensing assessment

2012 ◽  
Vol 12 (6) ◽  
pp. 14837-14874 ◽  
Author(s):  
E. T. Sena ◽  
P. Artaxo ◽  
A. L. Correia
Keyword(s):  
Water Vapour ◽  
Biomass Burning ◽  
Radiative Forcing ◽  
Surface Albedo ◽  
Radiative Effect ◽  
Water Vapour Content ◽  
Albedo Change ◽  
Direct Radiative Forcing ◽  
The Mean ◽  
The Impact

Abstract. This paper addresses the Amazonian radiative budget after considering three aspects of deforestation: (i) the emission of aerosols from biomass burning due to forest fires; (ii) changes in surface albedo after deforestation and (iii) modifications in the column water vapour amount over deforested areas. Simultaneous Clouds and the Earth's Radiant Energy System (CERES) shortwave fluxes and aerosol optical depth (AOD) retrievals from the Moderate Resolution Imaging SpectroRadiometer (MODIS) were analysed during the peak of the biomass burning seasons (August and September) from 2000 to 2009. A discrete-ordinate radiative transfer (DISORT) code was used to extend instantaneous remote sensing radiative forcing assessments into 24-h averages. The mean direct radiative forcing of aerosols at the top of the atmosphere (TOA) during the biomass burning season for the 10-yr studied period was −5.6 ± 1.7 W m−2. Furthermore, the spatial distribution of the direct radiative forcing of aerosols over Amazon was obtained for the biomass burning season of each year. It was observed that for high AOD (larger than 1 at 550 nm) the imbalance in the radiative forcing at the TOA may be as high as −20 W m−2 locally. The surface reflectance plays a major role in the aerosol direct radiative effect. The study of the effects of biomass burning aerosols over different surface types shows that the direct radiative forcing is systematically more negative over forest than over savannah-like covered areas. Values of −15.7 ± 2.4 W m−2/τ550 nm and −9.3 ± 1.7 W m−2/τ550 nm were calculated for the mean daily aerosol forcing efficiencies over forest and savannah-like vegetation respectively. The overall mean annual albedo-change radiative forcing due to deforestation over the state of Rondônia, Brazil, was determined as −7.3 ± 0.9 W m−2. Biomass burning aerosols impact the radiative budget for approximately two months per year, whereas the surface albedo impact is observed throughout the year. Because of this difference, the estimated impact in the Amazonian annual radiative budget due to surface albedo-change is approximately 6 times higher than the impact due to aerosol emissions. The influence of atmospheric water vapour content in the radiative budget was also studied using AERONET column water vapour. It was observed that column water vapour is in average smaller by about 0.35 cm over deforested areas compared to forested areas. Our results indicate that this drying impact contributes to an increase in the shortwave radiative effect that varies from 0.4 W m−2 to 1.2 W m−2, depending on the column water vapour content before deforestation. The large radiative forcing values presented in this study point out that deforestation has strong implications in convection, cloud development and photosynthesis rate over the Amazon region.

scholarly journals Spatial variability of the direct radiative forcing of biomass burning aerosols and the effects of land use change in Amazonia

2013 ◽  
Vol 13 (3) ◽  
pp. 1261-1275 ◽  
Author(s):  
E. T. Sena ◽  
P. Artaxo ◽  
A. L. Correia
Keyword(s):  
Land Use ◽  
Land Use Change ◽  
Water Vapour ◽  
Biomass Burning ◽  
Radiative Forcing ◽  
Surface Albedo ◽  
Water Vapour Content ◽  
Direct Radiative Forcing ◽  
The Mean ◽  
The Impact

Abstract. This paper addresses the Amazonian shortwave radiative budget over cloud-free conditions after considering three aspects of deforestation: (i) the emission of aerosols from biomass burning due to forest fires; (ii) changes in surface albedo after deforestation; and (iii) modifications in the column water vapour amount over deforested areas. Simultaneous Clouds and the Earth's Radiant Energy System (CERES) shortwave fluxes and aerosol optical depth (AOD) retrievals from the Moderate Resolution Imaging SpectroRadiometer (MODIS) were analysed during the peak of the biomass burning seasons (August and September) from 2000 to 2009. A discrete-ordinate radiative transfer (DISORT) code was used to extend instantaneous remote sensing radiative forcing assessments into 24-h averages. The mean direct radiative forcing of aerosols at the top of the atmosphere (TOA) during the biomass burning season for the 10-yr studied period was −5.6 ± 1.7 W m−2. Furthermore, the spatial distribution of the direct radiative forcing of aerosols over Amazonia was obtained for the biomass burning season of each year. It was observed that for high AOD (larger than 1 at 550 nm) the maximum daily direct aerosol radiative forcing at the TOA may be as high as −20 W m−2 locally. The surface reflectance plays a major role in the aerosol direct radiative effect. The study of the effects of biomass burning aerosols over different surface types shows that the direct radiative forcing is systematically more negative over forest than over savannah-like covered areas. Values of −15.7 ± 2.4 W m−2τ550 nm and −9.3 ± 1.7 W m−2τ550 nm were calculated for the mean daily aerosol forcing efficiencies over forest and savannah-like vegetation respectively. The overall mean annual land use change radiative forcing due to deforestation over the state of Rondônia, Brazil, was determined as −7.3 ± 0.9 W m−2. Biomass burning aerosols impact the radiative budget for approximately two months per year, whereas the surface albedo impact is observed throughout the year. Because of this difference, the estimated impact in the Amazonian annual radiative budget due to surface albedo-change is approximately 6 times higher than the impact due to aerosol emissions. The influence of atmospheric water vapour content in the radiative budget was also studied using AERONET column water vapour. It was observed that column water vapour is on average smaller by about 0.35 cm (around 10% of the total column water vapour) over deforested areas compared to forested areas. Our results indicate that this drying contributes to an increase in the shortwave radiative forcing, which varies from 0.4 W m−2 to 1.2 W m−2 depending on the column water vapour content before deforestation. The large radiative forcing values presented in this study point out that deforestation could have strong implications in convection, cloud development and the ratio of direct to diffuse radiation, which impacts carbon uptake by the forest.

scholarly journals Microwave Limb Sounder observations of biomass-burning products from the Australian bush fires of February 2009

2011 ◽  
Vol 11 (13) ◽  
pp. 6285-6296 ◽  
Author(s):  
H. C. Pumphrey ◽  
M. L. Santee ◽  
N. J. Livesey ◽  
M. J. Schwartz ◽  
W. G. Read
Keyword(s):  
New Zealand ◽  
Biomass Burning ◽  
Processing System ◽  
Combustion Products ◽  
Vertical Resolution ◽  
Data Processing System ◽  
Back Trajectories ◽  
Mixing Ratios ◽  
The North ◽  
Vertical Range

Abstract. The large bush fires which occurred in southeast Australia in February 2009 were unusually destructive. However, they were also unusual in the amounts of various combustion products which were injected directly into the stratosphere. We report the observations by the Microwave Limb Sounder (MLS) instrument on the Aura satellite of some of these combustion products. The highest quality observations are of CO; these clearly show a large region of enhanced mixing ratios to the north of New Zealand which remains in that region for about ten days before drifting westwards and finally dissipating over the Atlantic about a month after the fire. The region of enhanced CO ascends from the tropopause to 46 hPa during this period. Back trajectories run from the points where MLS observes enhanced CO pass close to the site of the fire. The MLS observations of CH3CN and HCN resemble those of CO except for their poorer vertical resolution and more limited vertical range. An apparent enhancement in ClO is also observed by MLS, but detailed analysis of the measured radiances reveals this feature to be a signature of CH3OH, which is not currently retrieved by the MLS data processing system. The fires of February 2009 are the only event of this type and magnitude in the 7-yr MLS record.

scholarly journals ACE-FTS observations of pyrogenic trace species in boreal biomass burning plumes during BORTAS

2012 ◽  
Vol 12 (12) ◽  
pp. 31629-31661 ◽  
Author(s):  
K. A. Tereszchuk ◽  
G. González Abad ◽  
C. Clerbaux ◽  
J. Hadji-Lazaro ◽  
D. Hurtmans ◽  
...  
Keyword(s):  
Boreal Forest ◽  
Biomass Burning ◽  
Atmospheric Chemistry ◽  
Forest Fires ◽  
Atmospheric Composition ◽  
Fire Season ◽  
Satellite Measurement ◽  
Mixing Ratios ◽  
Trace Species ◽  
The Impact

Abstract. To further our understanding of the effects of biomass burning emissions on atmospheric composition, the Quantifying the impact of BOReal forest fires on Tropospheric oxidants over the Atlantic using Aircraft and Satellites (BORTAS) campaign was conducted on 12 July to 3 August 2011 during the Boreal forest fire season in Canada. The simultaneous aerial, ground and satellite measurement campaign sought to record instances of Boreal biomass burning to measure the tropospheric volume mixing ratios (VMRs) of short- and long-lived trace molecular species from biomass burning emissions. The goal was to investigate the connection between the composition and the distribution of these pyrogenic outflows and their resulting perturbation to atmospheric chemistry, with particular focus on oxidant species to determine the overall impact on the oxidizing capacity of the free troposphere. Measurements of pyrogenic trace species in Boreal biomass burning plumes were made by the Atmospheric Chemistry Experiment Fourier Transform Spectrometer (ACE-FTS) onboard the Canadian Space Agency (CSA) SCISAT-1 satellite during the BORTAS campaign. Even though most biomass burning smoke is typically confined to the boundary layer, emissions are often injected directly into the upper troposphere via fire-related convective processes, thus allowing space-borne instruments to measure these pyrogenic outflows. An extensive set of 15 molecules, CH3OH, CH4, C2H2, C2H6, C3H6O, CO, HCN, HCOOH, HNO3, H2CO, NO, NO2, OCS, O3 and PAN have been analyzed. Included in this analysis is the calculation of age-dependent sets of enhancement ratios for each of the species.

scholarly journals Microwave Limb Sounder observations of biomass-burning products from the Australian bush fires of February 2009

2011 ◽  
Vol 11 (2) ◽  
pp. 6531-6554 ◽  
Author(s):  
H. C. Pumphrey ◽  
M. L. Santee ◽  
N. J. Livesey ◽  
M. J. Schwartz ◽  
W. G. Read
Keyword(s):  
New Zealand ◽  
Biomass Burning ◽  
Processing System ◽  
Combustion Products ◽  
Vertical Resolution ◽  
Data Processing System ◽  
Back Trajectories ◽  
Mixing Ratios ◽  
The North ◽  
Vertical Range

Abstract. The large bush fires which occurred in southeast Australia in February 2009 were unusually destructive. However, they were also unusual in the amounts of various combustion products which were injected directly into the stratosphere. We report the observations by the Microwave Limb Sounder (MLS) instrument on the Aura satellite of some of these combustion products. The highest quality observations are of CO; these clearly show a large region of enhanced mixing ratios to the north of New Zealand which remains in that region for about ten days before drifting westwards and finally dissipating over the Atlantic about a month after the fire. Back trajectories run from the points where MLS observes enhanced CO pass close to the site of the fire. The MLS observations of CH3CN and HCN resemble those of CO except for their poorer vertical resolution and more limited vertical range. An apparent enhancement in ClO is also observed by MLS, but detailed analysis of the measured radiances reveals this feature to be a signature of CH3OH, which is not currently retrieved by the MLS data processing system. The fires of February 2009 are the only event of this type and magnitude in the 6-yr MLS record.

scholarly journals Evaluation of ECMWF water vapour fields by airborne differential absorption lidar measurements: a case study between Brazil and Europe

2007 ◽  
Vol 7 (19) ◽  
pp. 5033-5042 ◽  
Author(s):  
H. Flentje ◽  
A. Dörnbrack ◽  
A. Fix ◽  
G. Ehret ◽  
E. Hólm
Keyword(s):  
Water Vapour ◽  
Hadley Circulation ◽  
Convective Transport ◽  
Differential Absorption ◽  
Short Term ◽  
Differential Absorption Lidar ◽  
Air Mass ◽  
Back Trajectories ◽  
Air Masses ◽  
Upper Troposphere

Abstract. Three extended airborne Differential Absorption Lidar (DIAL) sections of tropospheric water vapour across the tropical and sub-tropical Atlantic in March 2004 are compared to short-term forecasts of the European Centre for Medium Range Weather Forecasts (ECMWF). The humidity fields between 28° S and 36° N exhibit large inter air-mass gradients and reflect typical transport patterns of low- and mid-latitudes like convection (e.g. Hadley circulation), subsidence and baroclinic development with stratospheric intrusion. These processes re-distribute water vapour vertically such that locations with extraordinary dry/moist air-masses are observed in the lower/upper troposphere, respectively. The mixing ratios range over 3 orders of magnitude. Back-trajectories are used to trace and characterize the observed air-masses. Overall, the observed water vapour distributions are largely reproduced by the short-term forecasts at 0.25° resolution (T799/L91), the correlation ranges from 0.69 to 0.92. Locally, large differences occur due to comparably small spatial shifts in presence of strong gradients. Systematic deviations are found associated with specific atmospheric domains. The planetary boundary layer in the forecast is too moist and to shallow. Convective transport of humidity to the middle and upper troposphere tends to be overestimated. Potential impacts arising from data assimilation and model physics are considered. The matching of air-mass boundaries (transport) is discussed with repect to scales and the representativity of the 2-D sections for the 3-D humidity field. The normalized bias of the model with respect to the observations is 6%, 11% and 0% (moist model biases) for the three along-flight sections, whereby however the lowest levels are excluded.

scholarly journals Ozone photochemistry in boreal biomass burning plumes

2013 ◽  
Vol 13 (15) ◽  
pp. 7321-7341 ◽  
Author(s):  
M. Parrington ◽  
P. I. Palmer ◽  
A. C. Lewis ◽  
J. D. Lee ◽  
A. R. Rickard ◽  
...  
Keyword(s):  
Biomass Burning ◽  
Forest Fires ◽  
Dynamic Range ◽  
Ozone Production ◽  
The Arctic ◽  
Aircraft Measurements ◽  
Alkyl Nitrates ◽  
Mixing Ratios ◽  
Aerosol Loading ◽  
The Impact

Abstract. We present an analysis of ozone (O3) photochemistry observed by aircraft measurements of boreal biomass burning plumes over eastern Canada in the summer of 2011. Measurements of O3 and a number of key chemical species associated with O3 photochemistry, including non-methane hydrocarbons (NMHCs), nitrogen oxides (NOx) and total nitrogen containing species (NOy), were made from the UK FAAM BAe-146 research aircraft as part of the "quantifying the impact of BOReal forest fires on Tropospheric oxidants over the Atlantic using Aircraft and Satellites" (BORTAS) experiment between 12 July and 3 August 2011. The location and timing of the aircraft measurements put BORTAS into a unique position to sample biomass burning plumes from the same source region in Northwestern Ontario with a range of ages. We found that O3 mixing ratios measured in biomass burning plumes were indistinguishable from non-plume measurements, but evaluating them in relationship to measurements of carbon monoxide (CO), total alkyl nitrates (ΣAN) and the surrogate species NOz (= NOy-NOx) revealed that the potential for O3 production increased with plume age. We used NMHC ratios to estimate photochemical ages of the observed biomass burning plumes between 0 and 10 days. The BORTAS measurements provided a wide dynamic range of O3 production in the sampled biomass burning plumes with ΔO3/ΔCO enhancement ratios increasing from 0.020 ± 0.008 ppbv ppbv−1 in plumes with photochemical ages less than 2 days to 0.55 ± 0.29 ppbv ppbv−1 in plumes with photochemical ages greater than 5 days. We found that the main contributing factor to the variability in the ΔO3/ΔCO enhancement ratio was ΔCO in plumes with photochemical ages less than 4 days, and that was a transition to ΔO3 becoming the main contributing factor in plumes with ages greater than 4 days. In comparing O3 mixing ratios with components of the NOy budget, we observed that plumes with ages between 2 and 4 days were characterised by high aerosol loading, relative humidity greater than 40%, and low ozone production efficiency (OPE) of 7.7 ± 3.5 ppbv ppbv−1 relative to ΣAN and 1.6 ± 0.9 ppbv ppbv−1 relative to NOz. In plumes with ages greater than 4 days, OPE increased to 472 ± 28 ppbv ppbv−1 relative to ΣAN and 155 ± 5 ppbv ppbv−1 relative to NOz. From the BORTAS measurements we estimated that aged plumes with low aerosol loading were close to being in photostationary steady state and O3 production in younger plumes was inhibited by high aerosol loading and greater production of ΣAN relative to O3. The BORTAS measurements of O3 photochemistry in boreal biomass burning plumes were found to be consistent with previous summertime aircraft measurements made over the same region during the Arctic Research of the Composition of the Troposphere (ARCTAS-B) in 2008 and Atmospheric Boundary Layer Experiment (ABLE 3B) in 1990.

scholarly journals Distribution of ozone and its precursors over Bay of Bengal during winter 2009: role of meteorology

2011 ◽  
Vol 29 (9) ◽  
pp. 1613-1627 ◽  
Author(s):  
L. M. David ◽  
I. A. Girach ◽  
P. R. Nair
Keyword(s):  
Bay Of Bengal ◽  
Forest Fires ◽  
Anthropogenic Activities ◽  
Mean Value ◽  
Radiation Budget ◽  
Mixing Ratio ◽  
Ozone Level ◽  
Back Trajectories ◽  
Mixing Ratios ◽  
The North

Abstract. Measurements of ozone and NO2 were carried out in the marine environment of the Bay of Bengal (BoB) during the winter months, December 2008–January 2009, as part of the second Integrated Campaign for Aerosols, gases and Radiation Budget conducted under the Geosphere Biosphere Programme of the Indian Space Research Organization. The ozone mixing ratio was found to be high in the head and the southeast BoB with a mean value of 61 ± 7 ppb and 53 ± 6 ppb, respectively. The mixing ratios of NO2 and CO were also relatively high in these regions. The spatial patterns were examined in the light of airflow patterns, air mass back trajectories and other meteorological conditions and satellite retrieved maps of tropospheric ozone, NO2, CO, and fire count in and around the region. The distribution of these gases was strongly associated with the transport from the adjoining land mass. The anthropogenic activities and forest fires/biomass burning over the Indo Gangetic Plains and other East Asian regions contribute to ozone and its precursors over the BoB. Similarity in the spatial pattern suggests that their source regions could be more or less the same. Most of the diurnal patterns showed decrease of the ozone mixing ratio during noon/afternoon followed by a nighttime increase and a morning high. Over this oceanic region, photochemical production of ozone involving NO2 was not very active. Water vapour played a major role in controlling the variation of ozone. An attempt is made to simulate ozone level over the north and south BoB using the photochemical box model (NCAR-MM). The present observed features were compared with those measured during the earlier cruises conducted in different seasons.

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