On the complexity of the boundary layer structure and aerosol vertical distribution in the coastal Mediterranean regions: A case study

The planetary boundary layer structure in the coastal areas, and particularly in complex orography regions such as the Mediterranean, is extremely intricate. In this study, we show the evolution of the planetary boundary layer based on in situ airborne measurements and ground-based remote sensing observations carried out during the MORE (Marine Ozone and Radiation Experiment) campaign in June 2010. The campaign was held in a rural coastal Mediterranean region in Southern Italy. The study focuses on the observations made on 17 June. Vertical profiles of meteorological parameters and aerosol size distribution were measured during two flights: in the morning and in the afternoon. Airborne observations were combined with ground-based LIDAR, SODAR, microwave and visible radiometer measurements, allowing a detailed description of the atmospheric vertical structure. The analysis was complemented with data from a regional atmospheric model run with horizontal resolutions of 12, 4 and 1 km, respectively; back-trajectories were calculated at these spatial resolutions. The observations show the simultaneous occurrence of dust transport, descent of mid-tropospheric air and sea breeze circulation on 17 June. Local pollution effects on the aerosol distribution, and a possible event of new particles formation were also observed. A large variability in the thermodynamical structure and aerosol distribution in the flight region, extending by approximately 30km along the coast, was found. Within this complex, environment-relevant differences in the back-trajectories calculated at different spatial resolutions are found, suggesting that the description of several dynamical processes, and in particular the sea breeze circulation, requires high-resolution meteorological analyses. The study also shows that the integration of different observational techniques is needed to describe these complex conditions; in particular, the availability of flights and their timing with respect to the occurring phenomena are crucial

Optical-microphysical Properties of Saharan Dust Aerosols and Composition Relationship Using a Multi-wavelength Raman Lidar, in Situ Sensors and Modelling: a Case Study Analysis

A strong Saharan dust event that occurred over the city of Athens, Greece (37.9° N, 23.6° E) between 27 March and 3 April 2009 was followed by a synergy of three instruments: a 6-wavelength Raman lidar, a CIMEL sun-sky radiometer and the MODIS sensor. The BSC-DREAM model was used to forecast the dust event and to simulate the vertical profiles of the aerosol concentration. Due to mixture of dust particles with low clouds during most of the reported period, the dust event could be followed by the lidar only during the cloud-free day of 2 April 2009. The lidar data obtained were used to retrieve the vertical profile of the optical (extinction and backscatter coefficients) properties of aerosols in the troposphere. The aerosol optical depth (AOD) values derived from the CIMEL ranged from 0.33-0.91 (355 nm) to 0.18-0.60 (532 nm), while the lidar ratio (LR) values retrieved from the Raman lidar ranged within 75-100 sr (355 nm) and 45-75 sr (532 nm). Inside a selected dust layer region, between 1.8 and 3.5 km height, mean LR values were 83 ± 7 and 54 ± 7 sr, at 355 and 532 nm, respectively, while the Ångström-backscatter-related (ABR 355/532) and Ångström-extinction-related (AER 355/532) were found larger than 1 (1.17 ± 0.08 and 1.11 ± 0.02, respectively), indicating mixing of dust with other particles. Additionally, a retrieval technique representing dust as a mixture of spheres and spheroids was used to derive the mean aerosol microphysical properties (mean and effective radius, number, surface and volume density, and mean refractive index) inside the selected atmospheric layers. Thus, the mean value of the retrieved refractive index was found to be 1.49( ± 0.10) + 0.007( ± 0.007)i, and that of the effective radiuses was 0.30 ± 0.18 μm. The final data set of the aerosol optical and microphysical properties along with the water vapor profiles obtained by Raman lidar were incorporated into the ISORROPIA II model to provide a possible aerosol composition consistent with the retrieved refractive index values. Thus, the inferred chemical properties showed 12-40% of dust content, sulfate composition of 16-60%, and organic carbon content of 15-64%, indicating a possible mixing of dust with haze and smoke. PM10 concentrations levels, PM10 composition results and SEM-EDX (Scanning Electron Microscope-Energy Dispersive X-ray) analysis results on sizes and mineralogy of particles from samples during the Saharan dust transport event were used to evaluate the retrieval

Estimation of aerosol water and chemical composition from AERONET Sun–sky radiometer measurements at Cabauw, the Netherlands

Remote sensing of aerosols provides important information on atmospheric aerosol abundance. However, due to the hygroscopic nature of aerosol particles observed aerosol optical properties are influenced by atmospheric humidity, and the measurements do not unambiguously characterize the aerosol dry mass and composition, which complicates the comparison with aerosol models. In this study we derive aerosol water and chemical composition by a modeling approach that combines individual measurements of remotely sensed aerosol properties (e.g., optical thickness, single-scattering albedo, refractive index and size distribution) from an AERONET (Aerosol Robotic Network) Sun–sky radiometer with radiosonde measurements of relative humidity. The model simulates water uptake by aerosols based on the chemical composition (e.g., sulfates, ammonium, nitrate, organic matter and black carbon) and size distribution. A minimization method is used to calculate aerosol composition and concentration, which are then compared to in situ measurements from the Intensive Measurement Campaign At the Cabauw Tower (IMPACT, May 2008, the Netherlands). Computed concentrations show good agreement with campaign-average (i.e., 1–14 May) surface observations (mean bias is 3% for PM₁₀ and 4–25% for the individual compounds). They follow the day-to-day (synoptic) variability in the observations and are in reasonable agreement for daily average concentrations (i.e., mean bias is 5% for PM₁₀ and black carbon, 10% for the inorganic salts and 18% for organic matter; root-mean-squared deviations are 26% for PM₁₀ and 35–45% for the individual compounds). The modeled water volume fraction is highly variable and strongly dependent on composition. During this campaign we find that it is >0.5 at approximately 80% relative humidity (RH) when the aerosol composition is dominated by hygroscopic inorganic salts, and <0.1 when RH is below 40%, especially when the composition is dominated by less hygroscopic compounds such as organic matter. The scattering enhancement factor (f(RH), the ratio of the scattering coefficient at 85% RH and its dry value at 676 nm) during 1–14 May is 2.6 ± 0.5. The uncertainty in AERONET (real) refractive index (0.025–0.05) is the largest source of uncertainty in the modeled aerosol composition and leads to an uncertainty of 0.1–0.25 (50–100%) in aerosol water volume fraction. Our methodology performs relatively well at Cabauw, but a better performance may be expected for regions with higher aerosol loading where the uncertainties in the AERONET inversions are smaller

Estimating Marine Aerosol Particle Volume and Number from Maritime Aerosol Network Data

As well as spectral aerosol optical depth (AOD), aerosol composition and concentration (number, volume, or mass) are of interest for a variety of applications. However, remote sensing of these quantities is more difficult than for AOD, as it is more sensitive to assumptions relating to aerosol composition. This study uses spectral AOD measured on Maritime Aerosol Network (MAN) cruises, with the additional constraint of a microphysical model for unpolluted maritime aerosol based on analysis of Aerosol Robotic Network (AERONET) inversions, to estimate these quantities over open ocean. When the MAN data are subset to those likely to be comprised of maritime aerosol, number and volume concentrations obtained are physically reasonable. Attempts to estimate surface concentration from columnar abundance, however, are shown to be limited by uncertainties in vertical distribution. Columnar AOD at 550 nm and aerosol number for unpolluted maritime cases are also compared with Moderate Resolution Imaging Spectroradiometer (MODIS) data, for both the present Collection 5.1 and forthcoming Collection 6. MODIS provides a best-fitting retrieval solution, as well as the average for several different solutions, with different aerosol microphysical models. The average solution MODIS dataset agrees more closely with MAN than the best solution dataset. Terra tends to retrieve lower aerosol number than MAN, and Aqua higher, linked with differences in the aerosol models commonly chosen. Collection 6 AOD is likely to agree more closely with MAN over open ocean than Collection 5.1. In situations where spectral AOD is measured accurately, and aerosol microphysical properties are reasonably well-constrained, estimates of aerosol number and volume using MAN or similar data would provide for a greater variety of potential comparisons with aerosol properties derived from satellite or chemistry transport model data

Retrievals of Antarctic aerosol characteristics using a Sun-sky radiometer during the 2001-2002 austral summer campaign

In order to characterize the Antarctic aerosol and to analyze the effect of katabatic winds on the properties of suspended particles, measurements of solar direct and diffuse irradiance were carried out at the Italian Terra Nova Bay station in Antarctica, during the 2001-2002 austral summer campaign. Measurements were performed by the ground-based PREDE sky radiometer and processed by using the Skyrad inversion code. Aerosol optical thickness at 500 nm was found to vary between 0.01 and 0.02. The volume size distribution curves showed bimodal features with the two modes located within 0.1-0.3 μm and 5-7 μm radius intervals, respectively. The real part of the refractive index characterizing the Antarctic aerosol was found to have a mean value of 1.40. During the katabatic event the analysis indicated that the advection of larger and drier fresh particles, together with the removal of marine suspended particles, caused the decrease in aerosol optical thickness

Simulation of aerosol optical properties over a tropical urban site in India using a global model and its comparison with ground measurements

Aerosols have great impacts on atmospheric environment, human health, and earth's climate. Therefore, information on their spatial and temporal distribution is of paramount importance. Despite numerous studies have examined the variation and trends of BC and AOD over India, only very few have focused on their spatial distribution or even correlating the observations with model simulations. In the present study, a three-dimensional aerosol transport-radiation model coupled with a general circulation model. SPRINTARS, simulated atmospheric aerosol distributions including BC and aerosol optical properties, i.e., aerosol optical thickness (AOT), Ångström Exponent (AE), and single scattering albedo (SSA). The simulated results are compared with both BC measurements by aethalometer and aerosol optical properties measured by ground-based skyradiometer and by satellite sensor, MODIS/Terra over Hyderabad, which is a tropical urban area of India, for the year 2008. The simulated AOT and AE in Hyderabad are found to be comparable to ground-based measured ones. The simulated SSA tends to be higher than the ground-based measurements. Both these comparisons of aerosol optical properties between the simulations with different emission inventories and the measurements indicate that, firstly the model uncertainties derived from aerosol emission inventory cannot explain the gaps between the simulations and the measurements and secondly the vertical transport of BC and the treatment of BC-containing particles can be the main issue in the global model to solve the gap

A First Case Study of CCN Concentrations from Spaceborne Lidar Observations

We present here the first cloud condensation nuclei (CCN) concentration profiles derived from measurements with the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) aboard the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO), for different aerosol types at a supersaturation of 0.15%. CCN concentrations, along with the corresponding uncertainties, were inferred for a nighttime CALIPSO overpass on 9 September 2011, with coincident observations with the Facility for Airborne Atmospheric Measurements (FAAM) BAe-146 research aircraft, within the framework of the Evaluation of CALIPSO’s Aerosol Classification scheme over Eastern Mediterranean (ACEMED) research campaign over Thessaloniki, Greece. The CALIPSO aerosol typing is evaluated, based on data from the Copernicus Atmosphere Monitoring Service (CAMS) reanalysis. Backward trajectories and satellite-based fire counts are used to examine the origin of air masses on that day. Our CCN retrievals are evaluated against particle number concentration retrievals at different height levels, based on the ACEMED airborne measurements and compared against CCN-related retrievals from the Moderate Resolution Imaging Spectroradiometer (MODIS) sensors aboard Terra and Aqua product over Thessaloniki showing that it is feasible to obtain CCN concentrations from CALIPSO, with an uncertainty of a factor of two to three

Detecting volcanic sulfur dioxide plumes in the Northern Hemisphere using the Brewer spectrophotometer, other networks, and satellite observations

This paper demonstrates that SO 2 columnar amounts have significantly increased following the five largest volcanic eruptions of the past decade in the Northern Hemisphere. A strong positive signal was detected by all the existing networks either ground based (Brewer, EARLINET, AirBase) or from satellites (OMI, GOME-2). The study particularly examines the adequacy of the existing Brewer network to detect SO 2 plumes of volcanic origin in comparison to other networks and satellite platforms. The comparison with OMI and 45 GOME-2 SO 2 space-borne retrievals shows statistically significant agreement between the Brewer network data and the collocated satellite overpasses. It is shown that the Brewer instrument is capable of detecting significant columnar SO 2 increases following large volcanic eruptions, when SO 2 levels rise well above the instrumental noise of daily observations, estimated to be of the order of 2 DU. A model exercise from the MACC project shows that the large increases of SO 2 over Europe following the Bárðarbunga eruption in Iceland were not caused by local sources or ship emissions but are clearly linked to the eruption. We propose that by combining Brewer data with that from other networks and satellites, a useful tool aided by trajectory analyses and modeling could be created which can be used to forecast high SO 2 values both at ground level and in air flight corridors following future eruptions

Comparison of aerosol products retrieved from AERONET, MICROTOPS and MODIS over a tropical urban city, Pune, India

Aerosol Optical Depth (AOD) measurements from Aerosol Robotic NETwork (AERONET; level 2.0), Microtops - II sun-photometer and MODerate Resolution Imaging Spectroradiometer (MODIS) (Terra and Aqua; level 2, collection 5, dark target) were compared and used to characterize aerosols over Pune, India. AODs from Microtops and MODIS were compared with those measured by AERONET to evaluate the measurement quality. To the best of our knowledge, this is the first systematic comparison of MODIS aerosol products over Pune, India. The results of the analysis show that during 2008-10, 68 to 84 of the MODIS AODs fell within an expected error, as defined by the MODIS science team, and thus the retrievals from this system are validated and accepted. In addition, during pre-monsoon periods MODIS retrievals are better-matched with ground-based measurements. On the seasonal scale, MODIS retrievals corroborate well with ground-based measurements, with correlation coefficients ranging from 0.62 to 0.93. Despite an overall satellite-ground agreement, MODIS tends to under-estimate AOD during winter, and this may be due to improper assumptions of surface reflectance and the incorrect selection of aerosol types. AERONET retrieved single scattering albedo (SSA) values in winter (0.82-0.86), suggesting the dominance of absorbing aerosols, slightly increased (0.87-0.89) in pre-monsoon season, indicating more scattering type of aerosols. These values are about 8.9-1.1 lower than those of the assumed SSA values in the MODIS algorithm