Columnar Aerosol Research Articles

Column aerosol optical properties and aerosol radiative forcing during a serious haze-fog month over North China Plain in 2013 based on ground-based sunphotometer measurements

Abstract. In January 2013, North China Plain experienced several serious haze events. Cimel sunphotometer measurements at seven sites over rural, suburban and urban regions of North China Plain from 1 to 30 January 2013 were used to further our understanding of spatial-temporal variation of aerosol optical parameters and aerosol radiative forcing (ARF). It was found that Aerosol Optical Depth at 500 nm (AOD500 nm) during non-pollution periods at all stations was lower than 0.30 and increased significantly to greater than 1.00 as pollution events developed. The Angstrom exponent (Alpha) was larger than 0.80 for all stations most of the time. AOD500 nm averages increased from north to south during both polluted and non-polluted periods on the three urban sites in Beijing. The fine mode AOD during pollution periods is about a factor of 2.5 times larger than that during the non-pollution period at urban sites but a factor of 5.0 at suburban and rural sites. The fine mode fraction of AOD675 nm was higher than 80% for all sites during January 2013. The absorption AOD675 nm at rural sites was only about 0.01 during pollution periods, while ~0.03–0.07 and 0.01–0.03 during pollution and non-pollution periods at other sites, respectively. Single scattering albedo varied between 0.87 and 0.95 during January 2013 over North China Plain. The size distribution showed an obvious tri-peak pattern during the most serious period. The fine mode effective radius in the pollution period was about 0.01–0.08 μm larger than during non-pollution periods, while the coarse mode radius in pollution periods was about 0.06–0.38 μm less than that during non-pollution periods. The total, fine and coarse mode particle volumes varied by about 0.06–0.34 μm3, 0.03–0.23 μm3, and 0.03–0.10 μm3, respectively, throughout January 2013. During the most intense period (1–16 January), ARF at the surface exceeded −50 W m−2, −180 W m−2, and −200 W m−2 at rural, suburban, and urban sites, respectively. The ARF readings at the top of the atmosphere were approximately −30 W m−2 in rural and −40–60 W m−2 in urban areas. Positive ARF at the top of the atmosphere at the Huimin suburban site was found to be different from others as a result of the high surface albedo due to snow cover.

The contribution of Saharan dust in PM10 concentration levels in Anatolian Peninsula of Turkey

Sahara-originated dust is the most significant natural source of particulate matter; however, this contribution is still unclear in the Eastern Mediterranean especially in Western Turkey, where significant industrial sources and metropolitan areas are located. The Real-time Air Quality Modeling System (RAQMS) is utilized to explore the possible effects of Saharan dust on high levels of PM10 measured in Turkey. RAQMS model is compared with 118-air quality stations distributed throughout Turkey (81 cities) for April 2008. MODIS aerosol product (MOD04 for Terra and MYD04 for Aqua) is used to see columnar aerosol loading of the atmosphere at 550nm (Aerosol optical depth (AOD) values found to be between 0.6 and 0.8 during the episode). High-resolution vertical profiles of clouds and aerosols are provided from CALIOP, on board of CALISPO satellite. The results suggest a significant contribution of Sahara dust to high levels of PM10 in Turkey with RAQMS and in situ time series showing similar patterns. The two data sets are found to be in agreement with a correlation of 0.87.

Aerosol Optical Properties and Determination of Aerosol Size Distribution in Wuhan, China

Columnar aerosol volume size distributions from March 2012 to February 2013 in Wuhan, China, were investigated with a focus on monthly and seasonal variations in the aerosol optical depths (AODs) and Ångström exponents. AOD is wavelength dependent, and for AOD at, for example, 500 nm, the seasonal averaged AOD value decreased in the order of winter (~0.84), spring (~0.83), summer (~0.76) and autumn (~0.55). The Ångström exponent suggested that the aerosol sizes in summer (~1.22), winter (~1.14), autumn (~1.06) and spring (~0.99) varied from fine to coarse particles. The Ångström exponent and AOD could provide a qualitative evaluation of ASD. Moreover, aerosol size distribution (ASD) was larger in winter than the other three seasons, especially from 1.0 µm to 15 µm due to heavy anthropogenic aerosol and damp climate. The ASD spectral shape showed a bimodal distribution in autumn, winter, and spring, with one peak (<0.1) in the fine mode range and the other (>0.14) in the coarse mode range. However, there appeared to be a trimodal distribution during summer, with two peaks in the coarse mode, which might be due to the hygroscopic growth of the local particles and the generation of aerosol precursor resulting from the extreme-high temperature and relative humidity.

Estimating global black carbon emissions using a top‐down Kalman Filter approach

AbstractBlack carbon (BC) is an important aerosol constituent in the atmosphere and climate forcer. A good understanding of the radiative forcing of BC and associated climate feedback and response is critical to minimize the uncertainty in predicting current and future climate influenced by anthropogenic aerosols. One reason for this uncertainty is that current emission inventories of BC are mostly obtained from the so‐called bottom‐up approach, an approach that derives emissions based on categorized emitting sources and emission factors used to convert burning mass to emissions. In this work, we provide a first global‐scale top‐down estimation of global BC emissions, as well as an estimated error range, by using a Kalman Filter. This method uses data of both column aerosol absorption optical depth and surface concentrations from global and regional networks to constrain our fully coupled climate‐aerosol‐urban model and thus to derive an optimized estimate of BC emissions as 17.8 ± 5.6 Tg/yr, a factor of more than 2 higher than commonly used global BC emissions data sets. We further perform 22 additional optimization simulations that incorporate the known uncertain ranges of various important physical, model, and measurement parameters and still yield an optimized value within the above given range, from a low of 14.6 Tg/yr to a high of 22.2 Tg/yr. Furthermore, we show that the emissions difference between our optimized and a priori estimation is not uniform, with East Asia, Southeast Asia, and Eastern Europe underestimated, while North America is overestimated in the a priori inventory.

Synergistic Approach for the Aerosol Monitoring and Identification of Types over Indo-Gangetic Basin in Pre-Monsoon Season

Aerosol optical and microphysical properties were studied at two major industrial cities (Delhi and Kanpur) of the Indo-Gangetic Basin (IGB) during three (2007–2009) consecutive pre-monsoon (PrM: March–May) seasons, using synergetic analyses of CALIOP, MODIS, AERONET and PARASOL observations. CALIOP-derived aerosol properties show vertically elevated aerosol profiles (up to 4 km altitude), majorly consisting of dust particles during all three PrM seasons with maximum loading in May, 2008. The above inference is well corroborated with columnar aerosol properties from MODIS and AERONET observations. The results also show higher aerosol optical depth (AOD) over Delhi as compared to Kanpur. The high aerosol loading found during the late PrM season at both locations can be attributed to the fact that dust/burning activities are at their peak in May during the PrM season over the IGB. The analyses of optical and microphysical parameters coupled with backward trajectory analyses indicate the presence of three different aerosol types (Type I, Type II and Type III) over both cities during PrM 2009. Type I is characterized as dust-dominated aerosols coming via long-range transportation from major dust sources (desert of Sahara, Iran, Afghanistan and western India), whereas a mixture of absorbing aerosols and dust coming from the Arabian Peninsula and the Thar Desert is characterized as Type II aerosols. The presence of highly absorbing, fine mode dominated Type III aerosols are categorized as urban-industrial/biomass burning aerosols, mainly locally originated and/or influenced by agriculture fires in the Himalayan foothills. A combination of CALIOP and PARASOL observations with ground-based measurements also highlights the dominance of biomass burning smoke (mixed with polluted continental) over the IGB during the middle of May in 2009. The variability in aerosol types found during the PrM season indicates the significant effect of natural/human activities, associated with different meteorological conditions, on aerosol behavior over the region.

Novel Technique for Profiling of Aerosol, Ozone and Water Vapor during Winter Using Mobile Radiometers over a Hilltop Station

Following a novel approach, the vertical distributions of columnar aerosol optical depth (AOD), precipitable water content (PWC) and ozone (TCO) have been determined using compact, multi-filter, solar radiometers during winter period of 2002-2003. These profiles were obtained by making measurements at different altitudes while ascending to/descending from a rural site, elevated up to an altitude of about 1450 m above mean sea level (AMSL). Besides the wavelength dependency, the profiles depict significant layer structures which are explained on the basis of concurrent atmospheric stability parameters. The aerosol size distributions which were obtained from the inversion of spectral dependence of AOD at different altitudes show a mixture of power-law, mono- and bi-model distributions indicating the influence of aerosols originating from both anthropogenic and natural sources. The present results, representing a rural high-altitude station, are compared with those reported over a few selected similar high-altitude stations in North India, and also observed at a nearby urban station (Pune) in Central India to infer modulation of terrain-induced meteorological parameters on aerosol source strength. The importance of the experimental approach and profiles of AOD and pre-cursor gases over areas with scarce alternative measurements, networks with sporadic presence of ground sites and limited number of satellite retrievals is highlighted.

Long term (2003–2012) spatio-temporal MODIS (Terra/Aqua level 3) derived climatic variations of aerosol optical depth and cloud properties over a semi arid urban tropical region of Northern India

Aerosol optical depth (AOD) values at 550 nm derived from the Moderate Resolution Imaging Spectroradiometer (MODIS) sensor onboard NASA's Terra and Aqua satellites, for the 10 years period of 2003–2012 have been analyzed in the present study. The retrieved satellite data (Terra/Aqua) has been used to investigate the temporal heterogeneity in columnar aerosol characteristics over a semi arid urban tropical Delhi zone (28° 34′, 77° 07′, 233 m ASL) of Northern India with a resolution of 1° × 1° grid in magnitude. Aerosol optical depths have found to be increased >25% across Delhi region of India during the study period of 2003–2012. Yearly mean Terra/Aqua AOD values have shown an increasing trend at a rate of 0.005/0.009 per year respectively. However, seasonally winter means Terra/Aqua AOD values exhibit an increasing trend at a rate of ∼0.012/0.007 per year respectively. In order to provide a better understanding of aerosol–cloud interaction, a single paired one tailed distribution student's t-Test has been applied to the Terra AOD values and cloud parameters. The results extracted in the present study are compared with the earlier studies as well as with the AOD values over various other Indian regions.

The effect of aerosol representation on cloud microphysical properties in Northeast Asia

This study performed a three-dimensional regional-scale simulation of aerosol and cloud fields using a meso-scale non-hydrostatic model with a bin-based cloud microphysics. The representation of aerosols in the model has been improved to account for more realistic multi-modal size distribution and multiple chemical compositions. Two case studies for shallow stratocumulus over Northeast Asia in March 2005 were conducted with different aerosol conditions to evaluate model performance. Improved condensation nuclei (CN) and cloud condensation nuclei (CCN) are attributable to the newly constructed aerosol size distribution. The simulated results of cloud microphysical properties (cloud droplet effective radius, liquid water path, and optical thickness) with improved CN/CCN number are close to the retrievals from satellite-based observation. The effects of aerosol on the microphysical properties of shallow stratocumulus are investigated by model simulation, in terms of columnar aerosol number concentration. Enhanced aerosol number concentration results in increased liquid water path in humid case, but invariant liquid water path in dry case primarily due to precipitation occurrence. The changes of cloud microphysical properties are more predominant for small aerosol burden than for large aerosol burden with the retarded changes in cloud mass and size due to inactive condensation and collision-coalescence processes. Quantitative evaluation of sensitivity factor between aerosol and cloud microphysical properties indicates a strong aerosol-cloud interaction in Northeast Asian region.

Temporal variation of aerosol optical depth and associated shortwave radiative forcing over a coastal site along the west coast of India

Optical characterization of aerosol was performed by assessing the columnar aerosol optical depth (AOD) and angstrom wavelength exponent (α) using data from the Microtops II Sunphotometer. The data were collected on cloud free days over Goa, a coastal site along the west coast of India, from January to December 2008. Along with the composite aerosol, the black carbon (BC) mass concentration from the Aethalometer was also analyzed. The AOD0.500μm and angstrom wavelength exponent (α) were in the range of 0.26 to 0.7 and 0.52 to 1.33, respectively, indicative of a significant seasonal shift in aerosol characteristics during the study period. The monthly mean AOD0.500μm exhibited a bi-modal distribution, with a primary peak in April (0.7) and a secondary peak in October (0.54), whereas the minimum of 0.26 was observed in May. The monthly mean BC mass concentration varied between 0.31μg/m3 and 4.5μg/m3, and the single scattering albedo (SSA), estimated using the OPAC model, ranged from 0.87 to 0.97. Modeled aerosol optical properties were used to estimate the direct aerosol shortwave radiative forcing (DASRF) in the wavelength range 0.25μm4.0μm. The monthly mean forcing at the surface, at the top of the atmosphere (TOA) and in the atmosphere varied between −14.1Wm−2 and −35.6Wm−2, −6.7Wm−2 and −13.4Wm−2 and 5.5Wm−2 to 22.5Wm−2, respectively. These results indicate that the annual SSA cycle in the atmosphere is regulated by BC (absorbing aerosol), resulting in a positive forcing; however, the surface forcing was governed by the natural aerosol scattering, which yielded a negative forcing. These two conditions neutralized, resulting in a negative forcing at the TOA that remains nearly constant throughout the year.

North-south cross sections of the vertical aerosol distribution over the Atlantic Ocean from multiwavelength Raman/polarization lidar during Polarstern cruises.

Shipborne aerosol lidar observations were performed aboard the research vessel Polarstern in 2009 and 2010 during three north-south cruises from about 50°N to 50°S. The aerosol data set provides an excellent opportunity to characterize and contrast the vertical aerosol distribution over the Atlantic Ocean in the polluted northern and relatively clean southern hemisphere. Three case studies, an observed pure Saharan dust plume, a Patagonian dust plume east of South America, and a case of a mixed dust/smoke plume west of Central Africa are exemplarily shown and discussed by means of their optical properties. The meridional transatlantic cruises were used to determine the latitudinal cross section of the aerosol optical thickness (AOT). Profiles of particle backscatter and extinction coefficients are presented as mean profiles for latitudinal belts to contrast northern- and southern-hemispheric aerosol loads and optical effects. Results of lidar observations at Punta Arenas (53°S), Chile, and Stellenbosch (34°S), South Africa, are shown and confirm the lower frequency of occurrence of free-tropospheric aerosol in the southern hemisphere than in the northern hemisphere. The maximum latitudinal mean AOT of 0.27 was found in the northern tropics (0– 15°N) in the Saharan outflow region. Marine AOT is typically 0.05 ± 0.03. Particle optical properties are presented separately for the marine boundary layer and the free troposphere. Concerning the contrast between the anthropogenically influenced midlatitudinal aerosol conditions in the 30– 60°N belt and the respective belt in the southern hemisphere over the remote Atlantic, it is found that the AOT and extinction coefficients for the vertical column from 0–5km (total aerosol column) and 1–5km height (lofted aerosol above the marine boundary layer) are a factor of 1.6 and 2 higher at northern midlatitudes than at respective southern midlatitudes, and a factor of 2.5 higher than at the clean marine southern-hemispheric site of Punta Arenas. The strong contrast is confined to the lowermost 3km of the atmosphere.

Absorption characteristics of aerosols over the northwestern region of India: Distinct seasonal signatures of biomass burning aerosols and mineral dust

Continuous measurements of aerosol black carbon (BC) mass concentrations made over a period of 3 years from a semi-arid, near-coastal, remote and sparsely inhabited location along with satellite-based data of aerosol absorption index, optical depth and extinction profiles in western India are used to characterize the distinct nature of aerosols near the surface and in the free troposphere and their seasonality. Despite being far remote and sparsely inhabited, significant levels of BC are observed in the ambient during winter (1.45 ± 0.71 μg m−3) attributed to biomass burning aerosols, advected to the site from the north and west; while during summer the concentrations are far reduced (0.23 ± 0.11 μg m−3) and represent the apparent background concentrations. The spectral absorption coefficients suggest the BC during summer be mostly of fossil fuel combustions. The strong convective boundary layer dynamics produces significant diurnal variation during winter and modulates to a lesser extent the seasonal variation. Examination of aerosol (absorption) index from OMI data for the study period showed a seasonal pattern that is almost opposite to that seen at the surface; with high aerosol index in summer, showing a significant difference between the surface and columnar aerosol types in summer. MISR and MODIS-derived columnar AOD follow the OMI pattern. Analysis of the vertical profiles of aerosol extinction and volume depolarization ratio (VDR), derived from CALIPSO data indicates the presence of strong dust layers with VDR ∼ 0.3 in the altitude region 4–6 km, contributing to the high aerosol index in the OMI data, while the surface measurements show absorptive properties representing fossil fuel BC aerosols.

Retrieval of aerosol optical and microphysical characteristics according to data from ground-based spectral measurements of direct and diffuse solar radiation. Part 2. Algorithm testing

We present the results of the study of sensitivity of our algorithm for retrieving the columnar aerosol optical and microphysical characteristics to measurement errors. The algorithm testing against data from field measurements under conditions of both increased and moderate aerosol turbidity of the clear sky atmosphere is discussed. We present the average values of the aerosol optical and microphysical characteristics retrieved according to data from photometric measurements at the AERONET station in Tomsk during 2004–2009 for situations when the aerosol optical depth in the spectral channel of 440 nm did not exceed 0.4.

Satellite derived aerosol optical depth climatology over Bangalore, India

Climatological aerosol optical depths (AOD) over Bangalore, India have been examined to bring out the temporal heterogeneity in columnar aerosol characteristics. AOD values at 550nm derived from the Moderate Resolution Imaging Spectroradiometer (MODIS) sensor onboard NASA’s Terra and Aqua satellites, for the period of 2002–2011 have been analyzed (independently) for the purpose. Frequency distributions of the AOD values are examined to infer the monthly mean values. Monthly and seasonal variations of AOD are investigated in the light of regional synoptic meteorology. Climatological monthly and seasonal mean Terra and Aqua AOD values exhibited similar temporal variation patterns. Monthly mean AOD values increased from January, peaks during May and thereafter (except for a secondary peak during July) fall off to reach a minimum during December. Monsoon season recorded the highest climatological seasonal mean AOD, while winter season recorded the lowest. AOD values show an overall increasing trend on a yearly basis, which was found mainly due to sustained increase in the seasonal averaged AOD during summer. The results obtained in the present study are compared with that of the earlier studies over the same location and also with AOD over various other Indian locations. Finally, the radiative and climatic impacts are discussed.

Spatio-temporal variations in columnar aerosol optical properties over Bay of Bengal: Signatures of elevated dust

Multiyear all-season characterization of the columnar optical properties of aerosols over Bay of Bengal (BoB) has been carried out using simultaneous data from MODIS and TOMS satellite sensors. Significant spatio-temporal heterogeneity has been observed in the columnar aerosol optical properties over BoB. Latitudinally, MODIS derived Aerosol optical depths (AOD), its Small mode fraction (SMF, except pre-monsoon) has shown increasing trends towards northern latitudes of BoB. A pocket of high Aerosol Index (AI) values has been observed over the north-western part of BoB during pre-monsoon. Longitudinally, the variations in the aerosol properties are subdued comparatively. SMF values tend to increase weakly towards eastern longitudes during all the seasons. Variations in the aerosol properties are parameterised using best fit analytical functions. By simultaneous examination of AI, AOD and SMF, it has been inferred that the aerosol load responsible for the higher values of AI over BoB during pre-monsoon is dominated by the transported UV-absorbing dust.

Simulation on the spatio-temporal distribution and emission flux of dust aerosol over East Asia in 2000-2009

A regional climate model (RegCM3), coupled with an online dust module, is used to simulate the spatio-temporal distribution and emission flux of dust aerosol (smaller than 20 μm in diameter) over East Asia in the period from 2000 to 2009. The model performance is firstly evaluated against available observations. Simulation results show that the model can capture the characteristics of spatio-temporal distribution of dust aerosol very well over East Asia. There always exist two extremes of dust aerosol optical depth (AOD) and column burden (CB), one is in the Taklimakan Desert of Xinjiang Uygur Autonomous Region, China, and the other is in the Badain Jaran Desert of Inner Mongolian Autonomous Region, China. The maximum value of CB appears in spring, secondary maximum in winter and minimum in autumn. To the east of 110°E, dust is transported eastward from a maximum center at a height of 700 hPa over the East Asian continent. Dust emission sources are mainly located in the Taklimakan Desert, Badain Jaran Desert, North Qinghai-Tibetan Plateau and Southwest Mongolia. There is also an obvious seasonal variation of dust emission flux (EF). Annual mean dust EF is 1,015.34 mg/(m 2 ·d), of which 62.4% and 2.3% are re-deposited onto the East Asian continent through a dry and wet deposition process, respectively, and the remaining 35.3% is injected into the atmosphere or subject to long-range transport.

Variability of Aerosol Parameters Derived from Ground and Satellite Measurements over Varanasi Located in the Indo-Gangetic Basin

Atmospheric aerosol plays a very important role in the Earth’s radiation budget and global climate studies. This study reports the variability of the physical and optical properties of columnar aerosol and water vapour over Varanasi (25.2°N, 82.9°E), located in the heart of the Indo-Gangetic Basin, for the first time. The study was carried out using a MICROTOPS-II sunphotometer to measure the Aerosol Optical Depth (AOD) and metrological parameters. It is observed that the AOD loading is enhanced during the pre-monsoon and winter seasons, while it is decreased during the monsoon season. The variability of the angstrom coefficient (α) and turbidity coefficient (β) shows that in the pre-monsoon season coarse-mode aerosol particles are dominant, while fine-mode aerosol particles are dominant in the winter. We compared our MICROTOPS-II ground-based data with the level-3 data from the Moderate Resolution Imaging Spectroradiometer (MODIS) onboard Terra satellite. A good seasonal correlation (correlation coefficient, R2 ~0.57) was observed between MICROTOPS-II and MODIS AOD data. The results of the variability of water vapor over Varanasi are also discussed. The correlation coefficient (R2) between daily MICROTOPS-II and MODIS water vapor is found to be more than 0.85 for the year 2011.

Ship-Borne Measurements of Columnar and Surface Aerosol Loading over the Bay of Bengal during W-ICARB Campaign: Role of Airmass Transport, Latitudinal and Longitudinal Gradients

The present study analyzes simultaneous and collocated measurements of spectral columnar aerosol optical depth (AOD) and the variations in total mass concentration of near surface aerosols in the marine atmosphere over the Bay of Bengal (BoB) during the winter phase of the Integrated Campaign for Aerosols, gases and Radiation Budget (W-ICARB) expedition of December and January 2008/2009 on board the Oceanographic Research Vessel (ORV) of Sagar Kanya. High AOD500 values (＞ 0.8) are found close to coastal regions in the western and northern BoB due to outflow of aerosols and pollutants from the densely populated Indo-Gangetic Plain (IGP). In these regions, the Angstrom exponent α380–1020 values are also found to be high (~1.2–1.3), indicating relative abundance of accumulation-mode continental aerosols. Low AOD500 (0.1–0.2) values are observed in central and southern BoB, far away from the mainland. The AOD500 and total aerosol mass loading increased along with the latitude. The total mass concentration is found to vary between 15 μg/m3 and 45 μg/m3, with higher loadings near the east coast and northern parts of the BoB. NCEP reanalysis of the data with winds at 925 hPa, along with airmass trajectories calculated using the HYSPLIT model, suggest transport of continental aerosols from central and northern India over the BoB at lower heights, and mineral dust aerosol transport from arid regions in the west of India at higher altitudes; while the increase in aerosol loading over eastern BoB was strongly affected by airmasses originating from Southeast Asia. The spatial correlation map between the MODIS and MISR AOD data is analyzed, revealing a strong correlation between the two datasets. The influence of wind speed and continental airmass on AOD is also examined, which provides information on the effects of the adjoining landmass on the marine aerosol field.

Ground-based in situ measurements of near-surface aerosol mass concentration over Anantapur: Heterogeneity in source impacts

Surface measurements of aerosol physical properties were made at Anantapur (14.62°N, 77.65°E, 331 m a.s.l), a semiarid rural site in India, during August 2008–July 2009. Measurements included the segregated sizes of aerosolsas as well as total mass concentration and size distributions of aerosols measured at low relative humidity (RH 68% to the total aerosol mass concentration. Accumulation aerosol mass fraction, A f (= M a/M t) was highest during winter (mean value of A f ∼ 0.80) and lowest (A f ∼ 0.64) during the monsoon season. The regression analysis shows that both R eff and R m are dependent on coarse-mode aerosols. The relationship between the simultaneous measurements of daily mean aerosol optical depth at 500 nm (AOD500) and PM2.5 mass concentration ([PM2.5]) shows that surface-level aerosol mass concentration increases with the increase in columnar aerosol optical depth over the observation period.

Aerosol, Ozone and Water Vapour Distributions Observed over Land and Sea Regions during INDOEX Field Phases

Extensive land- and ocean-based experiments involving active and passive remote sensing of aerosols and minor constituents have been conducted simultaneously over Pune and over the Indian Ocean and Arabian Sea during the INDian Ocean EXperiment (INDOEX) field phases with an aim to study the interface between the polluted continental air-mass and pristine marine air-mass, and the underlying circulat ion and Inter Tropical Convergence Zone (ITCZ) phenomena, which mixes the polluted air mass from Northern hemisphere with the pristine air mass from the Southern hemisphere. In this communicat ion, we report some important findings of the special observational programs which were carried out during the First Field Phase (FFP) i.e. fro m 17 February to 31 March 1998 and Intense Field Phase (IFP) i.e. fro m 1 January to 31 March 1999 of the INDOEX. These programs involve synchronous measurements of aerosol, ozone and precipitable water vapor using ground-based lidar, sunphotometer / spectroradiometer and ozone / water vapor monitor at the Indian Institute of Tropical Meteorology (IITM ), Pune, India, and aerosol characterization over the Indian Ocean and Arabian Sea utilizing a mult i-channel solar radio meter installed onboard Ocean Research Vessel (ORV) Sagar Kanya ship. Besides significant deviations between aerosol features over the land and oceanic regions, the lidar derived aerosol colu mn content (ACC) in the boundary layer, and radio meter derived height-integrated aerosol optical depth (AOD) show significant trend and characteristic changes in aerosol size distribution over land during both FFP and IFP. Moreover, the IFP measurements indicate higher aerosol ext inction (particularly in the initial period) as co mpared to those of FFP. On some experimental days, the land AODs appear to be of the magnitude similar to those of marine AODs observed close to the Indian subcontinent and adjoining South Asian regions. The variations in AOD over land also show an association with those observed over ocean, particularly near the coast. Besides an increasing trend in total colu mn ozone throughout the IFP, it exhib its a specific relat ionship with those observed in AOD and water vapor over land. Co mparison between the boundary layer and total column AODs reveals that the observed increasing trend in AOD is not confined to the boundary layer alone but extends to higher altitudes with significant contribution fro m the former to the latter.

Retrieval of aerosol microphysical properties by means of sun and star photometry at Granada, Spain

Knowledge of aerosol microphysical properties is essential to understand the role of atmospheric aerosols in radiative forcing. Passive remote-sensing techniques allow the retrieval of columnar aerosol size distribution from spectral measurements of aerosol optical depth by applying numerical methods. These methods are widely applied to sun photometers. For night-time retrievals of aerosol optical depth, the GFAT (Atmospheric Physics Group of the University of Granada), operates a star photometer with channels at 380, 436, 500, 670, 880, and 1020 nm in the city of Granada (southeastern Spain, 36.83° N, 3.58° W, 680 m a.s.l.). The GFAT also operates a Cimel CE-318 sun photometer with filters at the same wavelengths. To our knowledge, the variations between daytime and night-time columnar aerosol microphysical properties have not been studied until now. In this work, the applicability of an inversion code based only on direct irradiance measurements is presented to retrieve aerosol size distributions, for both daytime and night-time. From March 2007 to February 2009, statistical analyses and time evolutions of the effective radius and the properties of the fine mode such as mean radius, width, and particle volume concentration are analysed. Daytime effective radius values are large in summer and small in winter, whereas no clear seasonal pattern emerges at night-time. During daytime, the fine mode radius does not show any remarkable pattern, whereas at night-time, this parameter shows a clear seasonal pattern with large values in summer and small values in winter. On the other hand, fine mode widths do not show any pattern, neither daytime nor night-time, although large values of this parameter are found during daytime in all seasons. The fine-particle volume concentration presents no seasonal pattern during the daytime, whereas at night-time it shows larger values in summer than in other seasons.