Sunphotometer Data Research Articles

Influence of Filter Band Function on Retrieval of Aerosol Optical Depth from Sunphotometer Data

Abstract Beer’s attenuation law is the basis for the retrieval of aerosol optical depth (AOD) from sunphotometer data. However, the filter band function causes uncertainty during the retrieval of AOD from sunphotometer data, particularly for channels covering spectral regions of strong gas absorption. In this work, the uncertainty in AOD retrieval due to the filter band function is systematically analyzed by employing fine spectral absorption cross sections obtained from the Molecular Spectroscopy and Chemical Kinetics Group and the line-by-line radiative transfer model (LBLRTM). The uncertainty in AOD retrieval includes the uncertainty due to the wings of the filter band function in the ultraviolet (UV) region and errors in the optical depth calculation for Rayleigh scattering and absorption of O3, NO2, H2O, CH4, and CO2. The results showed that 1) the uncertainty of AOD retrieval by this method, which is called the approximate AOD retrieval method, might become large when the filter band function is not well designed, particularly in the UV region; 2) in the case of a large zenith observation condition, the errors will be nonnegligible if the Rayleigh scattering optical depth is calculated at a central wavelength without including filter band function; 3) the band-weighted absorption coefficients of O3 and NO2 remain nearly constant when the gas amounts change, except in the case of questionably designed band filters; and 4) these weak-absorption optical depths for H2O, CH4, and CO2 cannot be ignored in the 1020- or 1640-nm channels, where an optical depth error of 0.01−0.02 may be introduced.

Improving Langley calibrations by reducing diurnal variations of aerosol Ångström parameters

Abstract. Errors in the sun photometer calibration constant lead to artificial diurnal variations, symmetric around solar noon, of the retrieved aerosol optical depth (AOD) and the associated Ångström exponent α and its curvature γ. We show in simulations that within the uncertainty of state-of-the-art Langley calibrations, these diurnal variations of α and γ can be significant in low AOD conditions, while those of AOD are negligible. We implement a weighted Monte Carlo method of finding an improved calibration constant by minimizing the diurnal variations in α and γ and apply the method to sun photometer data of a clear day in Innsbruck, Austria. The results show that our method can be used to improve the calibrations in two of the four wavelength channels by up to a factor of 3.6.

Atmospheric turbidity determined by the annual cycle of the aerosol optical depth over north-center Spain from ground (AERONET) and satellite (MODIS)

The present study focuses on the characterization of aerosol seasonal variability over the northern continental region of the Iberian Peninsula based on remotely sensed aerosol optical properties, and in particular the aerosol optical depth (AOD) and Ångström exponent (Alpha) parameters. For this region, a representative annual cycle of these parameters was built based on data from the AERONET-RIMA station of Palencia (Spain, 42N, 4.5W) for the period 2003–2011. The results also examine in details the interannual variability during the whole period. The ability of satellite to reproduce the seasonal patterns and anomalies, is investigated using the MODIS (Moderate Resolution Imaging Radiometer) for the same period. MODIS instantaneous fields are validated against ground-based sunphotometer data, and the differences between monthly values are estimated. The site of Palencia is characterized by a daily AOD (440 nm) of 0.15 ± 0.10 and Alpha (440–870 nm) of 1.29 ± 0.35 on average, thus presenting values typical of a clean continental background. The seasonal pattern corresponds to mid-high turbidity values of the AOD during a period starting in mid-spring to the end of the summer (max 0.19), and a lower AOD during fall and winter months (min 0.09). When using MODIS data, the overall results for Palencia give a higher (by ∼25%) AOD (470 nm) with 0.19 ± 0.15 and a much lower (by ∼50%) Alpha (470–660 nm) of 0.70 ± 0.20. These numbers reflect substantial differences, with overestimations of the monthly means that can be almost double those of AERONET in the summer months. However MODIS satisfactorily reproduces the increase–decrease cycle in the AOD. These large differences tend to be more attributed to the aerosol models used in the MODIS algorithm rather than to the sampling difference between ground and satellite in this season. Despite the poor sampling in winter and the small AOD (<0.1) observed over the area, the best agreement between satellite and ground is found during this period. The seasonal pattern of the Ångström exponent derived from MODIS was found to be very different from that of AERONET, the former showing apparently no consistency with the latter. Given the aforementioned values and the fact that the AERONET Alpha value for 470–660 nm is 1.41 ± 0.37 (wavelengths used in the comparison with MODIS), we can conclude that the Alpha derived from MODIS is not representative of the aerosol type characterizing this region of the globe.

Estimation of aerosol absorption under summer conditions of Western Siberia from sun photometer data

We tested the methods for retrieving the aerosol single scattering albedo and scattering phase function (asymmetry factor), developed by the authors earlier, from measurements of clear-sky radiance in the solar almucantar at the Tomsk station of the AERONET photometric network in 2003–2009. It is shown that, under conditions of strong atmospheric turbidity (fires), the results obtained using the suggested approach and the Dubovik-King algorithm agree well. Under typical summer conditions of Western Siberia, the average single scattering albedo in the blue and green wavelength regions are ∼0.90–0.92 and close to those presented in the WCP and OPAC models for continental aerosol.

Seasonal variation and classification of aerosols over an inland station in India

Aerosols are capable of interfering with the radiation balance and cloud properties of the Earth-atmosphere system. The present study analyses and classifies the aerosol properties over a station, Kanpur, located in the Gangetic plain, which is the area with maximum aerosol loading in Indian subcontinent. Kanpur is one of the highly populated and industrialized cities in north India and is a good source of natural and anthropogenic aerosols. The analysis of aerosol characteristics reveals that aerosol loading is highest during the months of May to June and November to January. Accumulation mode aerosols (mainly of anthropogenic origin) dominate over coarse mode aerosols in the winter and post-monsoon seasons. During the summer, coarse mode aerosols dominate in the atmosphere. A comparison of sunphotometer data with satellite data shows lowest correlation during monsoon season. The satellite derived aerosol optical depth is overestimated during the monsoon season. Atmospheric aerosols are grouped and classified into different clusters according to their optical and microphysical properties. Three clusters were identified and defined according to features of the location and aerosol microphysical properties over the station. The classified clusters are defined as: (1) urban fine aerosols; (2) heavy pollution, and, (3) urban mixed aerosols. Of these heavy pollution episodes occur least and urban fine aerosols are observed at the highest number of occurrence. Wind has significant impact in the determination of local aerosol properties by advection of the particles from other locations and enhancing dust storm activities. Copyright © 2012 Royal Meteorological Society

Examination of aerosol distributions and radiative effects over the Bay of Bengal and the Arabian Sea region during ICARB using satellite data and a general circulation model

Abstract. In this paper we analyse aerosol loading and its direct radiative effects over the Bay of Bengal (BoB) and Arabian Sea (AS) regions for the Integrated Campaign on Aerosols, gases and Radiation Budget (ICARB) undertaken during 2006, using satellite data from the MODerate Resolution Imaging Spectroradiometer (MODIS) on board the Terra and Aqua satellites, the Aerosol Index from the Ozone Monitoring Instrument (OMI) on board the Aura satellite, and the European-Community Hamburg (ECHAM5.5) general circulation model extended by Hamburg Aerosol Module (HAM). By statistically comparing with large-scale satellite data sets, we firstly show that the aerosol properties measured during the ship-based ICARB campaign and simulated by the model are representative for the BoB and AS regions and the pre-monsoon season. In a second step, the modelled aerosol distributions were evaluated by a comparison with the measurements from the ship-based sunphotometer, and the satellite retrievals during ICARB. It is found that the model broadly reproduces the observed spatial and temporal variability in aerosol optical depth (AOD) over BoB and AS regions. However, AOD was systematically underestimated during high-pollution episodes, especially in the BoB leg. We show that this underprediction of AOD is mostly because of the deficiencies in the coarse mode, where the model shows that dust is the dominant component. The analysis of dust AOD along with the OMI Aerosol Index indicate that missing dust transport that results from too low dust emission fluxes over the Thar Desert region in the model caused this deficiency. Thirdly, we analysed the spatio-temporal variability of AOD comparing the ship-based observations to the large-scale satellite observations and simulations. It was found that most of the variability along the track was from geographical patterns, with a minor influence by single events. Aerosol fields were homogeneous enough to yield a good statistical agreement between satellite data at a 1° spatial, but only twice-daily temporal resolution, and the ship-based sunphotometer data at a much finer spatial, but daily-average temporal resolution. Examination of the satellite data further showed that the year 2006 is representative for the five-year period for which satellite data were available. Finally, we estimated the clear-sky solar direct aerosol radiative forcing (DARF). We found that the cruise represents well the regional-seasonal mean forcings. Constraining simulated forcings using the observed AOD distributions yields a robust estimate of regional-seasonal mean DARF of −8.6, −21.4 and +12.9 W m−2 at the top of the atmosphere (TOA), at the surface (SUR) and in the atmosphere (ATM), respectively, for the BoB region, and over the AS, of, −6.8, −12.8, and +6 W m−2 at TOA, SUR, and ATM, respectively.

A regional model of European aerosol transport: Evaluation with sun photometer, lidar and air quality data

Aerosol transport simulations within Europe were performed with the regional transport model COSMO-MUSCAT for two different time periods, July 19–26, 2006 and February 16–26, 2007. Simulated PM 2.5 , backscatter profiles and aerosol optical depths (AODs) were compared to observations, showing good agreements in magnitude, shape and day-to-day variations. Maximum AODs (>0.4) were found over Middle Europe and minimum AODs (<0.13) over the ocean during both time periods, corresponding to regions of high ( PM 2.5 > 10 μg m −3 ) and low ( PM 2.5 < 4.0 μg m −3 ) concentration near the surface. Vertical aerosol distributions were evaluated with lidar measurements from the EARLINET ground network and CALIPSO satellite retrievals. The characteristic vertical distribution and the differences for the summer and the winter cases were represented well by the regional model. Mean differences between −5.0 × 10 −7 to−2.0 × 10 −7 m −1 sr −1 (summer case) and −2.3 × 10 −6 to 1.0 × 10 −6 m −1 sr −1 (winter case) from 0.0 to 2.5 km altitude were found between observed (space-based lidar) and simulated backscatter coefficients. For the cases that were investigated in this study different prescriptions of the vertical distribution at the lateral model boundaries resulted in only small differences in aerosol distributions within the interior of the model region. ► Simulation of European aerosol for two different time periods. ► Evaluation with observed PM2.5, AOD and backscatter coefficients. ► Lidar profiles were used at the lateral model boundaries. ► Observed AOD values were used to adjust the vertical distributions. ► Usage of a regional transport model.

Optical properties of aerosol over a South European urban environment

Aerosol optical depths (AODs) and single scattering albedos (SSAs) were derived from in situ aircraft measurements of scattering and absorption coefficients at 550 nm. These values were compared to the AODs and SSAs derived from the sun photometer (CIMEL) data of the Aerosol Robotic Network (AERONET) site, in Thessaloniki, Greece, between 18 and 21 July 2006. The aircraft-obtained AODs were lower than the corresponding columnar values. However, aircraft SSAs were found to be in good agreement with the columnar values retrieved from the CIMEL. Aircraft aerosol size distributions measured by means of an optical particle counter (OPC) were also in good agreement with the respective distributions derived from the AERONET site. Filter samples were collected on board the aircraft at different altitudes, to provide detailed information on the chemical composition of tropospheric aerosol. The concentrations of identified chemical species were used to calculate particle refractive indices (RIs) for comparison with the AERONET calculated values.

Tropical cirrus cloud contamination in sun photometer data

Abstract Cirrus clouds are endemic to Southeast Asia and are a source of potential bias in regional passive aerosol remote sensing datasets. Here, performance of the cloud-screening algorithm for the ground-based Aerosol Robotic Network (AERONET) sun photometer data is evaluated for cirrus cloud contamination at Singapore (1.30° N, 103.77° E). Using twelve months of concurrent AERONET Level 1.5 and 2.0 cloud-screened aerosol optical depth (AOD) data, and collocated Level 1.0 Micro-Pulse Lidar Network (MPLNET) measurements, we investigate the baseline AOD bias due to cirrus cloud presence. Observations are considered for a primary sample of all data and a secondary sample where AERONET data are restricted to a zenith viewing angle ≤ 45°. Cirrus clouds are present in zenith-viewing MPL profiles for 34% and 23% of these samples respectively. Based on approximations of cirrus cloud optical properties necessary to estimate cloud optical depth from the single-channel lidar signal, and assuming partial forward scattering of diffuse light by cirrus clouds into the sun photometer’s field of view, we estimate a range in AOD bias due to unscreened cloud presence of 0.034 to 0.060 and 0.031 to 0.055 ± 0.01 for the primary and secondary sample respectively. From the analysis of AERONET AOD for the angle-limited subset alone, we also derive a positive AOD bias of 0.034, which is comparable to the lower bounds for the estimated cloud bias reported for our datasets. These findings, which we attribute to the prevalence of cirrus clouds present from regional convection, are higher than previous reports of global AOD bias in the Moderate Resolution Infrared Spectroradiometer (MODIS) satellite-borne measurements due to residual cirrus cloud presence.

Remote sensing of tropospheric aerosol using UV MAX-DOAS during hazy conditions in winter: Utilization of O4 Absorption bands at wavelength intervals of 338–368 and 367–393 nm

Abstract Multi Axis Differential Optical Absorption Spectroscopy (MAX-DOAS) measurements were taken during 12 hazy days in December 2006 in Fresno, USA. We obtained vertical distributions of aerosol extinction coefficients (AECs) and aerosol optical depths (AODs) at UV wavelengths of 353 and 380 nm by applying a recently developed aerosol retrieval algorithm based on O 4 slant column densities (SCDs) from MAX-DOAS measurements. A linear correlation coefficient (R) of 0.74 (0.73) was obtained between surface PM 2.5 concentrations and AECs at 353 (380) nm within the 0–1 km layer under hazy conditions. AODs obtained from MAX-DOAS measurements were validated by comparison with those measured by a collocated sunphotometer. AODs derived at both wavelengths were overestimated under the hazy atmospheric conditions compared with those retrieved from sunphotometer measurements. A coefficient of determination ( R 2 ) of 0.58 (0.79) was obtained for AODs at 353 (380) nm from MAX-DOAS and sunphotometer during the hazy period, and AODs obtained using MAX-DOAS agreed with sunphotometer data within 50% and 40% for the different wavelengths, respectively.

Volcanic ash from Iceland over Munich: mass concentration retrieved from ground-based remote sensing measurements

Abstract. Volcanic ash plumes, emitted by the Eyjafjallajökull volcano (Iceland) in spring 2010, were observed by the lidar systems MULIS and POLIS in Maisach (near Munich, Germany), and by a CIMEL Sun photometer and a JenOptik ceilometer in Munich. We retrieve mass concentrations of volcanic ash from the lidar measurements; spectral optical properties, i.e. extinction coefficients, backscatter coefficients, and linear depolarization ratios, are used as input for an inversion. The inversion algorithm searches for model aerosol ensembles with optical properties that agree with the measured values within their uncertainty ranges. The non-sphericity of ash particles is considered by assuming spheroids. Optical particle properties are calculated using the T-matrix method supplemented by the geometric optics approach. The lidar inversion is applied to observations of the pure volcanic ash plume in the morning of 17 April 2010. We find 1.45 g m−2 for the ratio between the mass concentration and the extinction coefficient at λ = 532 nm, assuming an ash density of 2.6 g cm−3. The uncertainty range for this ratio is from 0.87 g m−2 to 2.32 g m−2. At the peak of the ash concentration over Maisach the extinction coefficient at λ = 532 nm was 0.75 km−1 (1-h-average), which corresponds to a maximum mass concentration of 1.1 mg m−3 (0.65 to 1.8 mg m−3). Model calculations show that particle backscatter at our lidar wavelengths (λ ≤ 1064 nm), and thus the lidar retrieval, is hardly sensitive to large particles (r ≳ 3 μm); large particles, however, may contain significant amounts of mass. Therefore, as an independent cross check of the lidar retrieval and to investigate the presence of large particles in more detail, we model ratios of sky radiances in the aureole of the Sun and compare them to measurements of the CIMEL. These ratios are sensitive to particles up to r ≈ 10 μm. This approach confirms the mass concentrations from the lidar retrieval. We conclude that synergistic utilization of high quality lidar and Sun photometer data, in combination with realistic aerosol models, is recommended for improving ash mass concentration retrievals.

Aerosol single scattering albedo retrieved from ground-based measurements in the UV and visible region

Abstract. Estimates of Aerosol Single Scattering Albedo (SSA) from ground-based spectral measurements in the UV-visible are conducted at Villeneuve d'Ascq (VdA) in France. In order to estimate this parameter, measurements of global and diffuse UV-visible solar irradiances performed under cloud-free conditions since 2003 with a spectroradiometer operated by the Laboratoire d'Optique Atmosphérique (LOA) are used. The technique consists in comparing the measured irradiance values to modelled irradiances computed for various SSA. The retrieval is restricted to the 330–450 nm range to avoid ozone influence. For validation purpose, the retrieved values of SSA at 440 nm are compared to the ones obtained from sunphotometer measurements of the AERONET/PHOTONS network available on the LOA site. The results are rather satisfying: for the period 2003–2006 the Root Mean Square (RMS) of the differences is about 0.05, this value is within the uncertainty domain of retrieval of both products. Distinction between days characterized by different aerosol content, by means of the aerosol optical thickness (AOT) retrieved from ground-based measurements at the same wavelength, shows that the comparisons between both products are better when AOT are higher. Indeed in case AOT are greater than 0.2, the RMS is 0.031 compared to 0.060 for days with an AOT lower than 0.2. The SSA estimated at 340 and 380 nm from ground-based spectra are also studied, though no validation can be carried out with sunphotometer data (440 nm is the shortest wavelength at which the SSA is provided by the network). The good comparisons observed at 440 nm can let assume that the SSA retrieved from spectroradiometer measurements at the two other wavelengths are also obtained with a good confidence level. Thus these values in the UV range can be used to complete aerosol data provided by AERONET/PHOTONS at VdA. Moreover they can be used for a best knowledge of the aerosol absorption that is necessary to quantify the error on surface UV irradiances estimated from satellites.

Improved retrieval of aerosol optical thickness from MODIS measurements through derived surface reflectance over Nanjing, China

Determination of surface reflectance in the red and blue channels is a critical step in retrieving aerosol optical thickness (AOT) from Moderate Resolution Imaging Spectroradiometer (MODIS) measurements. The MODIS Collection 005 (C005) aerosol algorithm uses a ratio method to determine the surface reflectance in the red (0.66 μm) and blue (0.47 μm) channels from the surface reflectance in the 2.1 μm channel using global surface reflectance relationships. In this study, we attempted to improve the retrieval of AOT from MODIS measurements using a new surface parameterization derived using ground-based sunphotometer data and 6S radiative transfer code. The estimated surface reflectance in the red, blue and near-IR channel were used to derive ratio between them for use in the new retrieval from MODIS data. Our results demonstrate that the ratio of surface reflectance in the red and blue channels to the surface reflectance in the 2.1 μm channel varies seasonally in the Xianlin district of Nanjing City, China. These ratios are different from those assumed by the MODIS aerosol algorithm for the retrieval of AOT over land. The use of the appropriate ratio for the study area in a given season significantly improves the accuracy with the absolute error decreased from 0.15 to 0.08 and the relative error reduced from 31% to 17% in retrieving AOT from MODIS data.

Depolarization Ratio Retrievals Using AERONET Sun Photometer Data

We present linear particle depolarization ratios (LPDRs) retrieved from measurements with an AERONET Sun photometer at the Gwangju Institute of Science and Technology (GIST), Korea (lTEXg$35.10^{/circ}N$l/TEXg, lTEXg$126.53^{\circ}E$l/TEXg) between 19 October and 3 November 2009. The Sun photometer data were classified into three categories according to lTEXg${\AA}$l/TEXgngstrlTEXg$\ddot{o}$l/TEXg exponent and size distribution: 1) pure Asian dust (19 October 2009), 2) Asian dust mixed with urban pollution observed in the period from 20-26 October 2009, and 3) clean conditions (3 November). We show that the LPDRs can be used to distinguish among Asian dust, mixed aerosol, and non-Asian dust in the atmosphere. The mean LPDR of the pure Asian dust case is 23 %. Mean LPDRs are 13 % for the mixed case. The lowest mean LPDR is 6 % in the clean case. We compare our results to vertically resolved LPDRs (at 532 nm) measured by a Raman LIDAR system at the same site. In most cases, we find good agreement between LPDRs derived with Sun photometer and measured by LIDAR.

Precipitable water vapor characterization in the Gulf of Cadiz region (southwestern Spain) based on Sun photometer, GPS, and radiosonde data

Total precipitable water vapor (PWV) is characterized for the first time over southwestern Europe by means of ground‐based measurements during the period 2001–2005. Existing data from three sites located in the Cadiz Gulf region, El Arenosillo, San Fernando, and Gibraltar, using three different techniques, Sun photometer (SP), GPS, and radiosondes, are used for the analysis. The 5 year data series gives a mean value of about 2 cm (SD = 0.7 cm) and a clear seasonal pattern. In the multiannual monthly means basis, the highest values are reached in August–September, with a mean value of 2.5–2.6 cm, whereas the lowest are obtained in January–February, with an average of 1.4–1.5 cm. The data in the three sites have been compared in order to assess regional variability. Differences could be due to real local variability but also could arise from the differences in the measurement techniques. From daily to monthly bases, water vapor behavior is similar in the three sites, with the largest differences ranging from 3% in summer to 14% in winter. Outstanding results from these analyses are the observed local minimum in July, occurring during the maximum of desert dust intrusions in the southern Iberian Peninsula, and the significant differences found between the El Arenosillo (SP) and San Fernando (GPS) measurements, related to the periodical replacement of the SP instrument at El Arenosillo. The observed differences highlight the importance of drift in each SP because of filter aging or other calibration problems. Finally, the Moderate Resolution Imaging Spectroradiometer (MODIS) near‐infrared water vapor product has been compared to the data from the GPS station (San Fernando). MODIS retrieval slightly overestimates PWV in summer (5%–8%) and significantly underestimates in winter (−23%).

Mineral dust observed with AERONET Sun photometer, Raman lidar, and in situ instruments during SAMUM 2006: Shape‐dependent particle properties

Nearly pure Saharan dust was observed with an Aerosol Robotic Network (AERONET) Sun photometer, several Raman and high spectral resolution lidars, and airborne in situ instruments during the Saharan Mineral Dust Experiment (SAMUM) 2006 in Morocco. In the framework of a case study we present particle shape‐dependent dust properties, i.e., backscatter coefficients, extinction‐to‐backscatter (lidar) ratios, and linear particle depolarization ratios. These parameters can be inferred from AERONET's latest version of the mineral dust retrieval algorithm. The parameters can be measured with multiwavelength Raman/depolarization lidar without critical assumptions on particle shape. Lidar ratios inferred from the AERONET Sun photometer data tend to be larger than lidar ratios measured directly with lidar. Linear dust depolarization ratios were derived for several measurement wavelengths from the data products of the AERONET Sun photometer. The depolarization ratios tend to be smaller than the depolarization ratios measured with lidar. The largest differences exist in the near‐ultraviolet wavelength range. Particle axis ratios were determined with scanning electron microscopy. The axis ratio distribution differs significantly from the axis ratio distribution that is assumed in the AERONET retrievals. If the axis ratio distributions measured during SAMUM are used, the reproducibility of the lidar data products improves. The differences may in part be caused by an insufficient understanding of the light‐scattering model that is used in the AERONET algorithm. The results of the present study will be used to develop a dust light‐scattering model that will serve as the theoretical basis for the inversion of optical data into dust microphysical properties.

A comparison of total precipitable water measurements from radiosonde and sunphotometers

Atmospheric water vapour is an essential component of the terrestrial atmosphere and must be known precisely in a wide range of applications such as radiative transfer modelling or weather forecasting to mention just a few examples. Vertically integrated measurements, or total precipitable water (TPW) equivalent amounts traditionally derived from radiosonde measurements, are needed in many of these applications and can also be obtained from other methodologies such as sunphotometers or GPS-based techniques. This paper presents a study comparing different measurements of TPW from radiosonde and sunphotometer data recorded from 2001 to 2004 in Barcelona, Spain. Three collocated instruments were employed in this study: RS-80A Vaisala sondes and two types of commonly used sunphotometers (Cimel 318N-VBS7 and Microtops II). A cloud screening filter was applied to photometer data based on the quality control procedure of the AERONET database. A systematic comparison among the measurements indicates that bivariate correlations between different instruments were high, with correlation factors ( r 2) above 0.8 in all cases. Measurements covered all seasons allowing examining intra-annual variability, which generally did not exhibit statistically significant differences. Examination of 57 concurrent measurements of the three instruments indicated that radiosonde TPW measurements were the highest (15 mm on average) and Cimel and Microtops presented similar values (12 mm and 11 mm respectively).

Aerosol optical properties based on ground measurements over the Chinese Yangtze Delta Region

The characteristics of Aerosol Optical Depth (AOD) and Angstrom exponent were analyzed and compared using Cimel sunphotometer data from 2007 to 2008 at five sites located in the Yangtze River Delta region of China. The simultaneous measurements between Lin’an and ZFU showed a very high consistency of AOD at all wavelengths. The differences are less than 0.02 for Angstrom exponent and AOD at all wavelengths. The mean values of AOD at 440 nm at the Pudong, Taihu and Lin’an were about 0.74 ± 0.43, 0.85 ± 0.46, and 0.89 ± 0.46, respectively. The mean values of Angstrom exponents were about 1.27 ± 0.30, 1.20 ± 0.28 and 1.32 ± 0.35, respectively. The variation of monthly averaged AOD over Pudong showed a single peak distribution, with the maximum value occurring in July (AOD 440nm 1.26 ± 0.61) and minimum in January (AOD 440nm 0.50 ± 0.27). However, the variations of monthly averaged AOD at Taihu and Lin’an showed a bi-modal distribution. There were peak values of AOD occurring in July (AOD 440nm 1.41 ± 0.49) and September (AOD 440nm 1.22 ± 0.52) for Taihu. For Lin’an, the two peak values of AOD occurred in June (AOD 440nm 1.17 ± 0.69) and September (AOD 440nm 1.28 ± 0.46). The AOD accumulated mainly between 0.30–0.90(68%), 0.30–1.20(75%) and 0.30–1.20 (∼75%) at Pudong, Taihu, and Lin’an, respectively. The Angstrom exponent accumulated mainly between 1.10–1.60 (75%), 1.10–1.50 (63%) and 1.20–1.60, 50% (50%) at Pudong, Taihu, and Lin’an, respectively. The synchronized observation showed that the AOD at Pudong was larger than those at Dongtan by 0.03, 0.03, 0.04, 0.07, and 0.08 at wavelengths of 1020 nm, 870 nm, 670 nm, 500 nm and 440 nm, respectively. The synchronized observations at Pudong, Taihu and Lin’an showed that the three stations had high level AOD with means at 440 nm about 0.68, 0.73, and 0.78, respectively. The relationship between MODIS retrieved and ground-based measured AOD shows good agreement with R 2 ranging from 0.68 to 0.79 at Pudong, Taihu, Lin’an and Dongtan. The MODIS results were overestimated comparing the ground measurements at Pudong, Taihu, and Dongtan but exceptional at Lin’an. The analysis results between aerosol optical properties and wind measurement at Pudong showed that the wind speed from the east correlates with the lower observed AOD. The back trajectory analysis indicates that more than 50% airmasses were from the marine area at Pudong, while back trajectories distribution is relatively homogeneous at Lin’an.

Estimation of the microphysical aerosol properties over Thessaloniki, Greece, during the SCOUT‐O3 campaign with the synergy of Raman lidar and Sun photometer data

An experimental campaign was held at Thessaloniki, Greece (40.6°N, 22.9°E), in July 2006, in the framework of the integrated project Stratosphere‐Climate Links with Emphasis on the Upper Troposphere and Lower Stratosphere (SCOUT‐O3). One of the main objectives of the campaign was to determine the local aerosol properties and their impact on the UV irradiance at the Earth's surface. In this article, we present vertically resolved microphysical aerosol properties retrieved from the inversion of optical data that were obtained from a combined one‐wavelength Raman/two‐wavelength backscatter lidar system and a CIMEL Sun photometer. A number of assumptions were undertaken to overcome the limitations of the existing optical input data needed for the retrieval of microphysical properties. We found acceptable agreement with Aerosol Robotic Network retrievals for the fine‐mode particle effective radius, which ranged between 0.11 and 0.19 for the campaign period. It is shown that under complex layering of the aerosols, general assumptions may result in unrealistic retrievals, especially in the presence of aged smoke aerosols. Furthermore, with this instrument setup, the inversion algorithm can also be applied successfully for the complex refractive index in cases of vertically homogeneous layers of continental polluted aerosols. For these inversion cases, the vertically resolved retrievals for the single‐scattering albedo resulted in values around 0.9 at 532 nm, which were in very good agreement with estimates from airborne in situ observations obtained in the vicinity of the lidar site.

On the origin of seasonal variation of aerosol optical thickness in UV range over Belsk, Poland

Aerosol optical thickness (AOT) and seasonal variation of AOT over Belsk, Poland, in the UV wavelength range (310–380 nm) have been analysed using results of measurements by Brewer spectrophotometer No. 064 and Cimel sunphotometer data for the 2002–2007 period. The comparison of AOT derived from direct Sun measurements by Brewer spectrophotometer in the 310–320 nm range and retrieved from Cimel measurements at longer wavelengths shows good correlation (R = 0.96), with overestimation of retrieved values compared to the measured ones by about 6%. Basing on aerosol microphysical properties taken from almucantar retrievals and Mie theory, optical properties of aerosol in the UV range has been calculated. Analysis of seasonal variation of AOT at Belsk reveals two maxima: in April and July–August. Analysis of back-trajectories in conjunction with analysis of fire maps from Fire Information For Resource Management System shows that these seasonal maxima are connected with seasonal biomass burning in Eastern and Southern Europe.