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soumis a JGR Atmospheres - First posted online: Fri, 8 Oct 2021; International audience; During the extended activity of Mount Etna volcano in February-April 2021, three distinct paroxysmal events took place from 21 to 26 February, which were associated with a very uncommon transport of the injected upper-tropospheric plumes towards the north. Using a synergy of observations and modelling, we characterised the emissions and three-dimensional dispersion for these three plumes, we monitor their downwind distribution and optical properties, and we estimate their radiative impacts at selected locations. With a satellite-based source inversion, we estimate the emitted sulphur dioxide (SO2) mass at an integrated value of 55 kt and plumes injections at up to 12 km altitudes, which qualifies this series as an extreme event for Mount Etna. Then, we combine Lagrangian dispersion modelling, initialised with measured temporally-resolved SO2 emission fluxes and altitudes, with satellite observations to track the dispersion of the three individual plumes. The transport towards the north allowed the height-resolved downwind monitoring of the plumes at selected observatories in France, Italy and Israel, using LiDARs and photometric aerosol observations. Volcanic-specific aerosol optical depths in the visible spectral range ranging from about 0.004 to 0.03 and local daily average shortwave radiative forcing ranging from about -0.2 to -1.2 W/m2 (at the top of atmosphere) and from about -0.2 to -3.0 W/m2 (at the surface) are found. The composition (possible presence of ash), aerosol optical depth and radiative forcing of the plume has a large inter- and intra-plume variability and thus depend strongly on the position of the sampled section of the plumes.

Accurate information about aerosol vertical distribution is needed to reduce uncertainties in aerosol radiative forcing and its effect on atmospheric dynamics. The present study deals with synergistic analyses of aerosol vertical distribution and aerosol optical depth (AOD) with meteorological variables using multisatellite and ground-based remote sensors over Kanpur in central Indo-Gangetic Plain (IGP). Micro-Pulse Lidar Network-derived aerosol vertical extinction (sigma) profiles are analyzed to quantify the interannual and daytime variations during monsoon onset period (May-June) for 2009-2011. The mean aerosol profile is broadly categorized into two layers viz., a surface layer (SL) extending up to 1.5 km (where sigma decreased exponentially with height) and an elevated aerosol layer (EAL) extending between 1.5 and 5.5 km. The increase in total columnar aerosol loading is associated with relatively higher increase in contribution from EAL loading than that from SL. The mean contributions of EALs are about 60%, 51%, and 50% to total columnar AOD during 2009, 2010, and 2011, respectively. We observe distinct parabolic EALs during early morning and late evening but uniformly mixed EALs during midday. The interannual and daytime variations of EALs are mainly influenced by long-range transport and convective capacity of the local emissions, respectively. Radiative flux analysis shows that clear-sky incoming solar radiation at surface is reduced with increase in AOD, which indicates significant cooling at surface. Collocated analysis of atmospheric temperature and aerosol loading reveals that increase in AOD not only resulted in surface dimming but also reduced the temperature (approximately 2-3 C) of lower troposphere (below 3 km altitude). Radiative transfer simulations indicate that the reduction of incoming solar radiation at surface is mainly due to increased absorption by EALs (with increase in total AOD). The observed cooling in lower troposphere in high aerosol loading scenario could be understood as a dynamical feedback of EAL-induced stratification of lower troposphere. Further, the observed radiative effect of EALs increases the stability of the lower troposphere, which could modulate the large-scale atmospheric dynamics during monsoon onset period. These findings encourage follow-up studies on the implication of EALs to the Indian summer monsoon dynamics using numerical models.

Continuous lidar observations of the planetary boundary layer (PBL) depth have been made at the Micropulse Lidar Network (MPLNET) site in Greenbelt, MD since April 2001. However, because of issues with the operational PBL depth algorithm, the data is not reliable for determining seasonal and diurnal trends. Therefore, an improved PBL depth algorithm has been developed which uses a combination of the wavelet technique and image processing. The new algorithm is less susceptible to contamination by clouds and residual layers, and in general, produces lower PBL depths. A 2010 comparison shows the operational algorithm overestimates the daily mean PBL depth when compared to the improved algorithm (1.85 and 1.07 km, respectively). The improved MPLNET PBL depths are validated using radiosonde comparisons which suggests the algorithm performs well to determine the depth of a fully developed PBL. A comparison with the Goddard Earth Observing System-version 5 (GEOS-5) model suggests that the model may underestimate the maximum daytime PBL depth by 410 m during the spring and summer. The best agreement between MPLNET and GEOS-5 occurred during the fall and they diered the most in the winter.

The Aerosols99 cruise (January 14 to February 8, 1999) went between Norfolk, Virginia, and Cape Town, South Africa. A Micropulse lidar system was used almost continually during this cruise to profile the aerosol vertical structure. Inversions of this data illustrated a varying vertical structure depending on the dominant air mass. In clean maritime aerosols in the Northern and Southern Hemispheres the aerosols were capped at 1 km. When a dust event from Africa was encountered, the aerosol extinction increased its maximum height to above 2 km. During a period in which the air mass was dominated by biomass burning from southern Africa, the aerosol layer extended to 4 km. Comparisons of the aerosol optical depth (AOD) derived from lidar inversion and surface Sun photometers showed an agreement within 0.05 RMS. Similar comparisons between the extinction measured with a nephelometer and particle soot absorption photometer (at 19 m altitude) and the lowest lidar measurement (75 m) showed good agreement (0.014 km 1 ). The lidar underestimated surface extinction during periods when an elevated aerosol layer (total AOD 0.10) was present over a relatively clean (aerosol extinction 0.05 km 1 ) surface layer, but otherwise gave accurate results.

The Aerosol Robotic Network (AERONET) of automatic Sun/sky radiometers collected data on Tenerife, Canary Islands, in June-July 1997 during the second Aerosol Characterization Experiment (ACE 2). Initially, two instruments were deployed at Izana observatory (2360 m above sea level) and one at a mountain station Teide (3570 m above sea level). Repeatability of the calibration constants (Langley method) for all instruments was less than 0.5%. Aerosol optical depths measured by colocated sunphotometers and column size distributions, retrieved from spectral sky radiance data, were in good agreement. Later, one of the instruments was relocated at sea level. On July 8, 17, and 25, Saharan dust outbreaks were observed. Diurnal variations of spectral aerosol optical depth are presented. Relative diurnal stability of Saharan dust optical properties has been observed. Volume size distributions at various heights (sea level and 2360 m above sea level) show that the main portion of coarse particles is situated above 2360 m level. Measurements on July 25 showed how incoming dust has changed the magnitude and spectral dependence of aerosol optical depth and volume spectra of columnar aerosol. Mean optical depth and Angstrom parameter values for Saharan dust outbreaks during the ACE 2 experiment agree well with the Atlantic Ocean and Bermuda data obtained during the Tropospheric Aerosol Radiative Forcing Observational Experiment (TARFOX) in July 1996, as well as with previously reported Atlantic Ocean results. Also, there is a good agreement between ACE 2 data for Saharan air masses and data obtained on certain sites of the AERONET network.

Cirrus clouds, particularly sub visual high thin cirrus with low optical thickness, are difficult to be screened in operational aerosol retrieval algorithms. Collocated aerosol and cirrus observations from ground measurements, such as the Aerosol Robotic Network (AERONET) and the Micro-Pulse Lidar Network (MPLNET), provide us with an unprecedented opportunity to examine the susceptibility of operational aerosol products to thin cirrus contamination. Quality assured aerosol optical thickness (AOT) measurements were also tested against the CALIPSO vertical feature mask (VFM) and the MODIS-derived thin cirrus screening parameters for the purpose of evaluating thin cirrus contamination. Key results of this study include: (1) Quantitative evaluations of data uncertainties in AERONET AOT retrievals are conducted. Although AERONET cirrus screening schemes are successful in removing most cirrus contamination, strong residuals displaying strong spatial and seasonal variability still exist, particularly over thin cirrus prevalent regions during cirrus peak seasons, (2) Challenges in matching up different data for analysis are highlighted and corresponding solutions proposed, and (3) Estimation of the relative contributions from cirrus contamination to aerosol retrievals are discussed. The results are valuable for better understanding and further improving ground aerosol measurements that are critical for aerosol-related climate research.

Tiny suspensions of solid particles or liquid droplets, called aerosols, hover in earth's atmosphere and can be found over just about anywhere including oceans, deserts, vegetated areas, and other global regions. Aerosols come in a variety of sizes, shapes, and compositions which depend on such factors as their origin and how long they have been in the atmosphere (i.e., their residence time). Some of the more common types of aerosols include mineral dust and sea salt which get lifted from the desert and ocean surfaces, respectively by mechanical forces such as strong winds. Depending on their size, aerosols will either fall out gravitationally, as in the case of larger particles, or will remain resident in the atmosphere where they can undergo further change through interactions with other aerosols and cloud particles. Not only do aerosols affect air quality where they pose a health risk, they can also perturb the distribution of radiation in the earth-atmosphere system which can inevitably lead to changes in our climate. One aerosol that has been in the forefront of many recent studies, particularly those examining its radiative effects, is mineral dust. The large spatial coverage of desert source regions and the fact that dust can radiatively interact with such a large part of the electromagnetic spectrum due to its range in particle size, makes it an important aerosol to study. Dust can directly scatter and absorb solar and infrared radiation which can subsequently alter the amount of radiation that would otherwise be present in the absence of dust at any level of the atmosphere like the surface. This is known as radiative forcing. At the surface dust can block incoming solar energy, however at infrared wavelengths, dust acts to partially compensate the solar losses. Evaluating the solar radiative effect of dust aerosols is relatively straightforward due in part to the relatively large signal-to-noise ratio in the measurements. At infrared wavelengths, on the other hand, the effect is rather difficult to ascertain since the measured dust signal level is on the same order as the instrumental uncertainties. Although the radiative impact of dust is much smaller in the infrared, it can still have a noticeable influence on the distribution of energy in the Earth-atmosphere system. This is mainly attributed to the strong light-absorptive properties commonly found in many earth minerals.

[1] We synthesize multiple ground-based and satellite measurements to track the physical and chemical evolution of biomass burning smoke plumes transported from western Canada to the northeastern U.S. This multiple data set case study is an advantageous methodology compared with using individual or small groups of data sets, each with their own limitations. The case study analyzed is a Canadian boreal forest fire event on July 4, 2006 with carbonaceous aerosol smoke emission magnitudes comparable to those during the summer fire seasons of the previous decade. We track long-range transport of these aerosol plumes with data from space-borne remote sensing satellite instruments (MODIS, OMI, MISR, CALIOP lidar, AIRS) and ground-based in situ and remote aerosol observations (AERONET CIMEL sky/Sun photometer, MPLNET lidar, IMPROVE, EPA AirNow). Convective lofting elevated smoke emissions above the boundary layer into the free troposphere, where high speed winds aloft led to rapid, long-range transport. Aerosol layer subsidence occurred during transport due to a region of surface high pressure. Smoke aerosols reaching the boundary layer led to surface fine particulate matter (PM2.5) enhancements accompanied by changes in aerosol composition as the plume mixed with anthropogenic aerosols over the northeastern U.S. The extensive coverage of this smoke plume over the northeastern U.S. affected regional air quality, with increases of 10–20 μg m−3 PM2.5 attributable to biomass burning smoke aerosols and EPA 24-hour PM2.5 standard exceedances along the U.S. East Coast. Although each data set individually provides a limited view of the transport of smoke emissions, we demonstrate that a multi–data set approach results in a more comprehensive view of potential impacts due to long-range transport of smoke from a less extreme fire event. Our case study demonstrates that fires emit smoke aerosols that under certain meteorological conditions can degrade regional air quality 3000 km from the source region, with additional implications for aerosol radiative forcing and regional haze over the northeastern U.S.

[1] We used a combination of ground measurements (Aerosol Robotic Network, AERONET; Micro-Pulse Lidar Network, MPLNET) and satellite data (Moderate Resolution Imaging Spectroradiometer, MODIS; Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation, CALIPSO) to examine the susceptibility of ground and satellite aerosol retrievals to thin cirrus contamination at Phimai, Thailand (102.56°E, 15.18°N, also known as Pimai), during the Biomass-burning Aerosols in South East-Asia: Smoke Impact Assessment (BASE-ASIA) campaign (February–May 2006). Using the strengths of spaceborne or ground lidars to detect cirrus clouds, we conducted statistical analyses for four different scenarios: MPLNET versus AERONET, MPLNET versus MODIS, CALIPSO versus AERONET, and CALIPSO versus MODIS. Cirrus identifications from MPLNET or CALIPSO were paired up with concurrent aerosol optical thickness (AOT) measurements from AERONET or MODIS. Results from the BASE-ASIA campaign suggest that current operational AERONET and MODIS AOT products are influenced by thin cirrus contamination featuring strong seasonality. Concurrent AERONET and MPLNET observations indicate that additional thin cirrus screening changes AOT monthly means by 5%, with 20% of the AERONET aerosol data at Phimai being cirrus contaminated in boreal spring. From noncirrus cases to cirrus-contaminated cases, AERONET AOT increases along with larger particle sizes. We further evaluated the performance of eight MODIS-derived cirrus screening parameters for their effectiveness on thin cirrus screening: apparent reflectance at 1.38 μm (R1.38), cirrus reflectance at 0.66 μm (CR0.66), CR0.66 cirrus flag, reflectance ratio between 1.38 μm and 0.66 μm (RR1.38/0.66), reflectance ratio between 1.38 μm and 1.24 μm (RR1.38/1.24), brightness temperature difference between 8.6 μm and 11 μm (BTD8.6–11), brightness temperature difference between 11 μm and 12 μm (BTD11–12), and cloud phase infrared approach. Correlation analysis with the MPLNET cirrus flag indicates that RR1.38/0.66 is slightly preferable for high thin cirrus screening for the AERONET AOT measurements. The quantitative findings from this study suggest particular caution and careful evaluation of thin cirrus contamination in the satellite and ground AOT measurements before they are used for aerosol-related climatic forcing studies.

A synergistic process was developed to study the vertical distributions of aerosol optical properties and their effects on solar heating using data retrieved from ground-based radiation measurements and radiative transfer simulations. Continuous MPLNET and AERONET observations were made at a rural site in northern Taiwan from 2005 to 2007. The aerosol vertical extinction profiles retrieved from ground-based lidar measurements were categorized into near-surface, mixed, and two-layer transport types, representing 76% of all cases. Fine-mode (Angstrom exponent, alpha, approx.1.4) and moderate-absorbing aerosols (columnar single-scattering albedo approx.0.93, asymmetry factor approx.0.73 at 440 nm wavelength) dominated in this region. The column-integrated aerosol optical thickness at 500 nm (tau(sub 500nm)) ranges from 0.1 to 0.6 for the near-surface transport type, but can be doubled in the presence of upper-layer aerosol transport. We utilize aerosol radiative efficiency (ARE; the impact on solar radiation per unit change of tau(sub 500nm)) to quantify the radiative effects due to different vertical distributions of aerosols. Our results show that the ARE at the top-of-atmosphere (-23 W/ sq m) is weakly sensitive to aerosol vertical distributions confined in the lower troposphere. On the other hand, values of the ARE at the surface are -44.3, -40.6 and -39.7 W/sq m 38 for near-surface, mixed, and two-layer transport types, respectively. Further analyses show that the impact of aerosols on the vertical profile of solar heating is larger for the near-surface transport type than that of two-layer transport type. The impacts of aerosol on the surface radiation and the solar heating profiles have implications for the stability and convection in the lower troposphere.

[1] Global space borne lidar profiling of atmospheric clouds and aerosol began in 2003 following the launch of the Geoscience Laser Altimeter System (GLAS) on the Ice, Cloud and land Elevation Satellite. GLAS obtains nadir profiles through the atmosphere in two wavelength channels, day and night, at a fundamental resolution of 76.8 m vertical and 172 m along track. The 532 nm channel uses photon-counting detectors and resolves profiles of observed backscatter cross sections to 10−7 1/m-sr. The 1064 nm channel employs analog detection adequate to 10−6 1/m-sr and with greater dynamic range. By 2005 approximately seven months of global data are available. Processing algorithms produce data products for the corrected lidar signal, cloud and aerosol layer boundaries and optical thickness and extinction and backscatter cross sections. Operational sensitivity is shown by the frequency distribution for cloud optical thickness peaking at approximately 0.02.

[1] Data from the satellite lidar Geoscience Laser Altimeter System (GLAS) has provided a new means to retrieve height and optical depth of transmissive cloud and aerosol layers globally. We compare data sets from GLAS and an airborne under-flight of the Cloud Physics Lidar (CPL) during a unique smoke opportunity as part of a validation experiment in October 2003. The CPL has known layer identification and optical retrieval performance. GLAS data products, including calibrated attenuated backscatter profiles, layer identification, and optical depth, are compared to simultaneous aircraft lidar retrievals with similar model assumptions with a goal toward discovering algorithm biases in GLAS. The case described here involves heavy smoke layers from large-scale fires in southern California and thin cirrus clouds. The GLAS optical retrievals agree with the CPL data when the GLAS aerosol lidar ratio, S, is reset from default maritime to smoke and in inland urban pollution localities.

Received 6 February 2004; revised 30 July 2004; accepted 3 August 2004; published 24 February 2005. [1] The capability to detect the presence of absorbing aerosols in the atmosphere using space-based near-UV observations has been demonstrated in the last few years, as indicated by the widespread use by the atmospheric sciences community of the Total Ozone Mapping Spectrometer (TOMS) aerosol index as a qualitative representation of aerosol absorption. An inversion procedure has been developed to convert the unique spectral signature generated by the interaction of molecular scattering and particle absorption into a quantitative measure of aerosol absorption. In this work we evaluate the accuracy of the near-UV method of aerosol absorption sensing by means of a comparison of TOMS retrieved aerosol single scattering albedo and extinction optical depth to groundbased measurements of the same parameters by the Aerosol Robotic Network (AERONET) for a 2-month period during the SAFARI 2000 campaign. The availability of collocated AERONET observations of aerosol properties, as well as Micropulse Lidar Network measurements of the aerosol vertical distribution, offered a rare opportunity for the evaluation of the uncertainty associated with the height of the absorbing aerosol layer in the TOMS aerosol retrieval algorithm. Results of the comparative analysis indicate that in the absence of explicit information on the vertical distribution of the aerosols, the standard TOMS algorithm assumption yields, in most cases, reasonable agreement of aerosol optical depth (±30%) and single scattering albedo (±0.03) with the AERONET observations. When information on the aerosol vertical distribution is available, the accuracy of the retrieved parameters improves significantly in those cases when the actual aerosol profile is markedly different from the idealized algorithmic assumption.

[1] Smoke and pollutants from Canadian forest fires are sometimes transported over the United States at low altitudes behind advancing cold fronts. An unusual event occurred in July 2002 in which smoke from fires in Quebec was observed by satellite, lidar, and aircraft to arrive over the Washington, D.C., area at high altitudes. This elevated smoke plume subsequently mixed to the surface as it was entrained into the turbulent planetary boundary layer and had adverse effects on the surface air quality over the region. Trajectory and three-dimensional model calculations confirmed the origin of the smoke, its transport at high altitudes, and the mechanism for bringing the pollutants to the surface. Additionally, the modeled smoke optical properties agreed well with aircraft and remote sensing observations provided the smoke particles were allowed to age by coagulation in the model. These results have important implications for the long-range transport of pollutants and their subsequent entrainment to the surface, as well as the evolving optical properties of smoke from boreal forest fires.

except aerosol optical depth and the vertical profiles of aerosol extinction, are reported at a relative humidity of 55 ± 5%. An overdetermined data set was collected so that measured and calculated aerosol properties could be compared, internal consistency in the data set could be assessed, and sourcesof uncertainty could beidentified. Byadjusting the measured size distribution to take into account nonsphericity of the dust aerosol, calculated and measured aerosol mass and scattering coefficients agreed within overall experimental uncertainties. Differences between measured and calculated aerosol absorption coefficients were not within reasonable uncertainty limits, however, and may indicate the inability of Mie theory and the assumption of internally mixed homogeneous spheres to predict absorption by the ACE-Asia aerosol. Mass scattering efficiencies of non-sea-salt sulfate aerosol, sea salt, submicron particulate organic matter, and dust found for the ACE-Asia aerosol are comparable to values estimated for ACE 1, Aerosols99, and the Indian Ocean Experiment (INDOEX). Unique to the ACE-Asia aerosol were the large mass fractions of dust, the dominance of dust in controlling the aerosol optical properties, and the interaction of dust with soot aerosol. INDEXTERMS: 0305 Atmospheric Composition and Structure: Aerosols and particles (0345, 4801); 0345 Atmospheric Composition and Structure: Pollution—urban and regional (0305); 0365 Atmospheric Composition and Structure: Troposphere—composition and chemistry; KEYWORDS: aerosol optical properties, aerosol chemical composition, ACE-Asia

During the dry season airborne campaign of the Southern African Regional Science Initiative (SAFARI 2000), coordinated observations were made of massive thick aerosol layers. These layers were often dominated by aerosols from biomass burning. We report on airborne Sun photometer measurements of aerosol optical depth (λ = 0.354-1.557 μm), columnar water vapor, and vertical profiles of aerosol extinction and water vapor density that were obtained aboard the University of Washington's Convair-580 research aircraft. We compare these with ground-based AERONET Sun/sky radiometer results, with ground based lidar data (MPL-Net), and with measurements from a downward pointing lidar aboard the high-flying NASA ER-2 aircraft. Finally, we show comparisons between aerosol optical depths from the Sun photometer and those retrieved over land and over water using four spaceborne sensors (TOMS, MODIS, MISR, and ATSR-2).

[1] We simulate Saharan dust transport during the Puerto Rico Dust Experiment (June–July 2000) with a three-dimensional aerosol transport model driven by assimilated meteorology. The model does a reasonable job of locating the dust plume as it emerges from Africa but transports it somewhat farther south in the western North Atlantic Ocean than is seen in satellite imagery. The model is able to simulate low-level, uniformly mixed, and elevated vertical dust layer profiles over Puerto Rico similar to observations made in PRIDE. We determine that the variability in the dust vertical profile across the North Atlantic Ocean is most strongly associated with descent of the dust by sedimentation and downward vertical winds during transit rather than low-level transport directly from source regions. Wet removal plays a key role in modulating this process. Assuming our dust is 3.5% iron by mass, we estimate July 2000 iron deposition into the North Atlantic Ocean to be between 0.71 and 0.88 Tg, which is consistent with estimates derived from observed surface dust mass concentrations. We estimate that if annual dust deposition remains constant at five times our July 2000 estimates, there is an accumulation of 1 m of sediment from Saharan dust over the Florida peninsula every one million years.

For 26 days in mid-June and July 2000, a research group comprised of U.S. Navy, NASA, and university scientists conducted the Puerto Rico Dust Experiment (PRIDE). In this paper we give a brief overview of mean meteorological conditions during the study. We focus on findings on African dust transported into the Caribbean utilizing Navajo aircraft and AERONET Sun photometer data. During the study midvisible aerosol optical thickness (AOT) in Puerto Rico averaged 0.25, with a maximum less than 0.5 and with clean marine periods of _0.08. Dust AOTs near the coast of Africa (Cape Verde Islands and Dakar) averaged _0.4, 30% less than previous years. By analyzing dust vertical profiles in addition to supplemental meteorology and MPLNET lidar data we found that dust transport cannot be easily categorized into any particular conceptual model. Toward the end of the study period, the vertical distribution of dust was similar to the commonly assumed Saharan Air Layer (SAL) transport. During the early periods of the study, dust had the highest concentrations in the marine and convective boundary layers with only a, weak dust layer in the SAL being present, a state usually associated with wintertime transport patterns. We corroborate the findings of Maring et al. that in most cases, there was an unexpected lack of vertical stratification of dust particle size. We systematically analyze processes which may impact dust vertical distribution and determine and speculate that dust vertical distribution predominately influenced by flow patterns over Africa and differential advection couple with mixing by easterly waves and regional subsidence.

Analyses of aerosol optical depth (AOD) and columnar water vapor (CWV) measurements obtained with the six-channel NASA Ames Airborne Tracking Sunphotometer (AATS-6) mounted on a twin-engine aircraft during the summer 2000 Puerto Rico Dust Experiment are presented. In general, aerosol extinction values calculated from AATS-6 AOD measurements acquired during aircraft profiles up to 5 km ASL reproduce the vertical structure measured by coincident aircraft in-situ measurements of total aerosol number and surface area concentration. Calculations show that the spectral dependence of AOD was small (mean Angstrom wavelength exponents of approximately 0.20) within three atmospheric layers defined as the total column beneath the top of each aircraft profile, the region beneath the trade wind inversion, and the region within the Saharan Air Layer (SAL) above the trade inversion. This spectral behavior is consistent with attenuation of incoming solar radiation by large dust particles or by dust plus sea salt. Values of CWV calculated from profile measurements by AATS-6 at 941.9 nm and from aircraft in-situ measurements by a chilled mirror dewpoint hygrometer agree to within approximately 4% (0.13 g/sq cm). AATS-6 AOD values measured on the ground at Roosevelt Roads Naval Air Station and during low altitude aircraft runs over the adjacent Cabras Island aerosol/radiation ground site agree to within 0.004 to 0.030 with coincident data obtained with an AERONET Sun/sky Cimel radiometer located at Cabras Island. For the same observation times, AERONET retrievals of CWV exceed AATS-6 values by a mean of 0.74 g/sq cm (approximately 21 %) for the 2.9-3.9 g/sq cm measured by AATS-6. Comparison of AATS-6 aerosol extinction values obtained during four aircraft ascents over Cabras Island with corresponding values calculated from coincident aerosol backscatter measurements by a ground-based micro-pulse lidar (MPL-Net) located at Cabras yields a similar vertical structure above the trade inversion. Finally, AATS-6 AOD values measured during low altitude aircraft traverses over the ocean are compared with corresponding AOD values retrieved over water from upwelling radiance measurements by the MODIS, TOMS, and GOES-8 Imager satellite sensors, with mixed results. These exercises highlight the need for continued satellite sensor comparison/validation studies to improve satellite AOD retrieval algorithms, and the usefulness of airborne sunphotometer measurements in the validation process.

[1] As part of the Indian Ocean Experiment (INDOEX) 1999 Intensive Field Phase, measurements of aerosol properties were made on board the R/V Ronald H. Brown in the Indian Ocean north and south of the Intertropical Convergence Zone (ITCZ) in the Arabian Sea and in the Bay of Bengal. On the basis of air mass trajectories, eight air mass source regions were identified including the southern hemisphere Atlantic; southern hemisphere Indian Ocean; northern hemisphere Indian Ocean; east Indian subcontinent where trajectories came from near Calcutta, through the southern portion of India, and then to the ship; Indian subcontinent where trajectories came from across central India to the ship; Arabia; Arabia/Indian subcontinent, a mixed region where lower-level trajectories came from Arabia and upper-level trajectories came from India; and Arabian Sea/coastal India where trajectories came from along the coast of India to the ship. Properties of the aerosol measured in the marine boundary layer included chemical composition, number size distribution, and scattering and absorption coefficients. In addition, vertical profiles of aerosol backscatter and optical depth were measured. Presented here as a function of air mass source region are the concentrations and mass fractions of the dominant aerosol chemical components, the fraction of the extinction measured at the surface due to each component, mass extinction efficiencies of the individual components, aerosol scattering and absorption coefficients, single scattering albedo, Angstrom exponent, and optical depth. All results except aerosol optical depth are reported at the measurement relative humidity of 55 ± 5%. For air masses that originated from the two southern hemisphere marine regions (southern hemisphere Atlantic and Indian Ocean), sea salt dominated the extinction by sub-1 μm and sub-10 μm aerosol particles. The ratios of sub-1μm to sub-10 μm extinction were the lowest measured of all air mass source regions (mean values of 28 and 40%) due to the dominance of the aerosol mass by supermicron sea salt. In addition, aerosol optical depths were the lowest measured averaging 0.06 ± 0.03. Non-sea-salt (nss) sulfate aerosol concentrations in air masses from the northern hemisphere Indian Ocean were a factor of 6 higher than those in southern hemisphere air masses, while submicron sea-salt concentrations were comparable. Sulfate aerosol made up 40% of the sub-1μm extinction, while sea salt dominated the sub-10 μm extinction. Aerosol optical depths for this source region averaged 0.10 ± 0.03. A mean single scattering albedo near 0.89 and detectable black carbon (BC) concentrations (0.14 ± 0.05 μg m−3) indicated the transport of continentally derived aerosol to the ITCZ. The two regions influenced by low-level (500 m) airflow from Arabia had higher concentrations of submicron nss sulfate, particulate organic matter (POM), and inorganic oxidized material (IOM) than were observed in the marine regions. Concentrations of supermicron IOM were comparable to supermicron sea-salt concentrations. Nss sulfate aerosol dominated the sub-1 μm extinction and made significant contributions to the sub-10 μm extinction. Sea salt dominated the supermicron extinction. Mean BC contributions to submicron extinction were 8 and 12%. Single scattering albedo values averaged 0.93 ± 0.02 and 0.89 ± 0.02 for these two source regions. Aerosol optical depths averaged 0.19 ± 0.12 and 0.38 ± 0.07 with the higher value due to upper-level (2500 m) flow from India. Regions influenced by low-level airflow from the Indian subcontinent had the highest submicron nss sulfate, POM, BC, and IOM concentrations measured during the experiment. Supermicron sea-salt concentrations were lower than or comparable to supermicron nitrate concentrations. Sub-1 μm and sub-10 μm extinction were dominated by nss sulfate aerosol although a burning component consisting of BC, KNO3, and K2SO4 made a nearly equivalent contribution. These regions had a mean single scattering albedo of 0.85 ± 0.01, the lowest measured for any region. Mean aerosol optical depths were highest (0.3 to 0.4) for regions with low-level or upper-level airflow from the Indian subcontinent.

Micro-pulse lidar systems (MPL) were used to measure aerosol properties during the Indian Ocean Experiment (INDOEX) 1999 field phase. Measurements were made from two platforms: the NOAA ship RN Ronald H. Brown, and the Kaashidhoo Climate Observatory (KCO) in the Maldives. Sunphotometers were used to provide aerosol optical depths (AOD) needed to calibrate the MPL. This study focuses on the height distribution and optical properties (at 523 nm) of aerosols observed during the campaign. The height of the highest aerosols (top height) was calculated and found to be below 4 km for most of the cruise. The marine boundary layer (MBL) top was calculated and found to be less than 1 km. MPL results were combined with air mass trajectories, radiosonde profiles of temperature and humidity, and aerosol concentration and optical measurements. Humidity varied from approximately 80% near the surface to 50% near the top height during the entire cruise. The average value and standard deviation of aerosol optical parameters were determined for characteristic air mass regimes. Marine aerosols in the absence of any continental influence were found to have an AOD of 0.05 +/- 0.03, an extinction-to-backscatter ratio (S-ratio) of 33 +/- 6 sr, and peak extinction values around 0.05/km (near the MBL top). The marine results are shown to be in agreement with previously measured and expected values. Polluted marine areas over the Indian Ocean, influenced by continental aerosols, had AOD values in excess of 0.2, S-ratios well above 40 sr, and peak extinction values approximately 0.20/km (near the MBL top). The polluted marine results are shown to be similar to previously published values for continental aerosols. Comparisons between MPL derived extinction near the ship (75 m) and extinction calculated at ship-level using scattering measured by a nephelometer and absorption using a PSAP were conducted. The comparisons indicated that the MPL algorithm (using a constant S-ratio throughout the lower troposphere) calculates extinction near the surface in agreement with the ship-level measurements only when the MBL aerosols are well mixed with aerosols above. Finally, a review of the MPL extinction profiles showed that the model of aerosol vertical extinction developed during an earlier INDOEX field campaign (at the Maldives) did not correctly describe the true vertical distribution over the greater Indian Ocean region. Using the average extinction profile and AOD obtained during marine conditions, a new model of aerosol vertical extinction was determined for marine atmospheres over the Indian Ocean. A new model of aerosol vertical extinction for polluted marine atmospheres was also developed using the average extinction profile and AOD obtained during marine conditions influenced by continental aerosols.

We assess the consistency (closure) between solar beam attenuation by aerosols and water vapor measured by airborne sunphotometry and derived from airborne in-situ, and ship-based lidar measurements during the April 2001 Asian Pacific Regional Aerosol Characterization Experiment (ACE-Asia). The airborne data presented here were obtained aboard the Twin Otter aircraft. Comparing aerosol extinction o(550 nm) from four different techniques shows good agreement for the vertical distribution of aerosol layers. However, the level of agreement in absolute magnitude of the derived aerosol extinction varied among the aerosol layers sampled. The sigma(550 nm) computed from airborne in-situ size distribution and composition measurements shows good agreement with airborne sunphotometry in the marine boundary layer but is considerably lower in layers dominated by dust if the particles are assumed to be spherical. The sigma(550 nm) from airborne in-situ scattering and absorption measurements are about approx. 13% lower than those obtained from airborne sunphotometry during 14 vertical profiles. Combining lidar and the airborne sunphotometer measurements reveals the prevalence of dust layers at altitudes up to 10 km with layer aerosol optical depth (from 3.5 to 10 km altitude) of approx. 0.1 to 0.2 (500 nm) and extinction-to-backscatter ratios of 59-71 sr (523 nm). The airborne sunphotometer aboard the Twin Otter reveals a relatively dry atmosphere during ACE- Asia with all water vapor columns less than 1.5 cm and water vapor densities w less than 12 g/cu m. Comparing layer water vapor amounts and w from the airborne sunphotometer to the same quantities measured with aircraft in-situ sensors leads to a high correlation (r(sup 3)=0.96) but the sunphotometer tends to underestimate w by 7%.

From March to May 2001, aerosol size distributions and chemical compositions were measured using differential mobility analyzers (DMA), an aerodynamic particle sizer (APS), Micro-Orifice Uniform Deposit Impactors (MOUDI), and denuder samplers onboard the Twin Otter aircraft as part of the Aerosol Characterization Experiment (ACE)-Asia campaign. Of the 19 research flights, measurements on four flights that represented different aerosol characteristics are analyzed in detail. Clear-column radiative closure is studied by comparing aerosol extinctions predicted using in situ aerosol size distribution and chemical composition measurements to those derived from the 14-wavelength NASA Ames Airborne Tracking Sun photometer (AATS-14). In the boundary layer, pollution layers, and free troposphere with no significant mineral dust present, aerosol extinction closure was achieved within the estimated uncertainties over the full range of wavelengths of AATS-14. Aerosol extinctions predicted based on measured size distributions also reproduce the wavelength dependence derived from AATS-14 data. Considering all four flights, the best fit lines yield Predicted/Observed ratios in boundary and pollution layers of 0.97 ± 0.24 and 1.07 ± 0.08 at λ = 525 nm and 0.96 ± 0.21 and 1.08 ± 0.08 at λ = 1059 nm, respectively. In free troposphere dust layers, aerosol extinctions predicted from the measured size distributions were generally smaller than those derived from the AATS-14 data, with Predicted/Observed ratios of 0.65 ± 0.06 and 0.66 ± 0.05 at 525 and 1059 nm, respectively. A detailed analysis suggests that the discrepancy is likely a result of the lack of the knowledge of mineral dust shape as well as variations in aerosol extinction derived from AATS-14 data when viewing through horizontally inhomogeneous layers.