Variability Of Aerosol Optical Properties Research Articles

Variations of aerosol optical properties during the extreme solar event in January 2005

We present the results of analysis of the aerosol optical depth variations for January 2005 when an extreme solar energetic particle event occurred leading to a greatly enhanced flux of energetic particles penetrating into the atmosphere. An increase of the concentration of sulfate or nitrate aerosol was found on the second day after the solar energetic particle event in the south magnetic pole region with the maximum penetration of anisotropic solar cosmic rays. This suggests that an enhanced flux of solar energetic particles can lead to notable changes in the chemical and physical properties of the polar troposphere. A statistical test confirms that the observed change of the aerosol index is significant and is unlikely to be related to a spatial or temporal independent fluctuation of the aerosol content. Thus, the results of the present work provide evidence of a direct influence of cosmic rays on physical‐chemical properties of the atmosphere.

Latitudinal and longitudinal variation in aerosol characteristics from Sun photometer and MODIS over the Bay of Bengal and Arabian Sea during ICARB

Spatial variations in aerosol optical properties as function of latitude and longitude are analysed over the Bay of Bengal and Arabian Sea during ICARB cruise period of March–May 2006 from in situ sun photometer and MODIS (Terra, Aqua) satellite measurements. Monthly mean 550 nm aerosol optical depths (AODs) over the Bay of Bengal and Arabian Sea show an increase from March to May both in spatial extent and magnitude. AODs are found to increase with latitude from 4°N to 20°N over the Bay of Bengal while over Arabian Sea, variations are not significant. Sun photometer and MODIS AODs agree well within ±1σ variation. Bay of Bengal AOD (0.28) is higher than the Arabian Sea (0.24) latitudinally. Aerosol fine mode fraction (FMF) is higher than 0.6 over Bay of Bengal, while FMF in the Arabian Sea is about 0.5. Bay of Bengal α(∼1) is higher than the Arabian Sea value of 0.7, suggesting the dominance of fine mode aerosols over Bay of Bengal which is corroborated by higher FMF values over Bay of Bengal. Air back trajectory analyses suggest that aerosols from different source regions contribute differently to the optical characteristics over the Bay of Bengal and Arabian Sea.

Spatial and temporal variability in aerosol properties over the Mediterranean basin based on 6‐year (2000–2006) MODIS data

The temporal variability of aerosol optical properties is investigated over the broader Mediterranean basin, with emphasis on aerosol optical depth (AOD) that is an effective measure of aerosol load. The study is performed using Collection 005 Level‐3 mean daily spectral aerosol data from the Moderate Resolution Imaging Spectroradiometer (MODIS) instrument on board the Terra and Aqua satellites, which cover the 6‐year period from 2000 to 2006. The results of our analysis reveal a significant interannual variability of AOD in the study region. Specifically, the regional mean visible AOD over land and ocean has decreased over the period 2000–2006 by 20% in relative percentage terms (or by 0.04 in absolute terms). This tendency is statistically significant according to the Man‐Kendall test. However, the decreasing tendency of AOD is not uniform over the whole basin. It appears mainly in the western parts of Iberian, Italian, and Balkan peninsulas (and coastal areas), as well as in the southern Anatolian peninsula. The analysis for summer (June to September) and winter (November to March) seasons revealed different tendencies in both AOD and precipitation. The summer‐period AOD has decreased by 0.04 (or by 14%) probably due to decreased emission rates of anthropogenic pollution. In contrast, the winter AOD has increased by 0.03 (or 19%) mainly related to decreased precipitation (associated with an increasing tendency in the NAO index). The decreasing tendency in MODIS AOD is in good agreement with corresponding AOD tendencies based on data from Aerobot Robotic Network (AERONET) stations in the study region and ground based PM10 measurements at selected stations.

Correction of aerosol effects on multi-temporal images acquired with constant viewing angles: Application to Formosat-2 images

This paper presents a new method developed for the atmospheric correction of the images that will be acquired by the Venμs satellite after its launch expected in early 2010. Every two days, the Venμs mission will provide 10 m resolution images of 50 sites, in 12 narrow spectral bands ranging from 415 nm to 910 nm. The sun-synchronous Venμs orbit will have a 2-day repeat cycle, and the images of a given site will always be acquired from the same place, at the same local hour, with constant observation angles. Thanks to these characteristics, the directional effects will be considerably reduced since only the solar angles will slowly vary with time. The algorithm that will be implemented for the atmospheric correction of Venμs data is being developed using both radiative transfer simulations and the actual data acquired by the Formosat-2 satellite. Because of its one-day sun-synchronous repeat cycle, Formosat-2 acquires images with a sun-viewing geometry close to the one Venμs will offer. With this geometry, reflectance time series are free from directional effects on the short term, a feature which reduces the number of unknowns to retrieve. The atmospheric corrections algorithm exploits this feature and the two following assumptions: – Aerosol optical properties vary quickly with time but slowly with location. – Surface reflectances vary quickly with location but slowly with time. Consequently, the top of atmosphere reflectance short term variations (10 to 15 days) are mainly due to the variations of aerosol optical properties, and it is thus possible to use these variations to characterise the atmospheric aerosols and to retrieve surface reflectances. This paper first describes the aerosol inversion method we developed and its results when applied to simulations. In the second part, we show the first tests of the method against three data sets acquired by Formosat-2 images with constant observation angles. Aeronet sun photometers measurements were available on all sites. Formosat-2 estimates of optical thickness compare favourably with Aeronet in situ measurements, leading to a noticeable improvement of the smoothness of time series of surface reflectances after atmospheric correction.

Seasonal asymmetry in diurnal variation of aerosol optical characteristics over Pune, western India

There is a large day‐to‐day variability in aerosol optical properties due to varied meteorological conditions that exhibit different diurnal asymmetry in different synoptic seasons. Aerosol optical depth (AOD) and Ångstrom exponent (AE) measured for a period of 3 years at Pune, an urban site in western India, were analyzed to study the diurnal variation and its seasonal dependence. Mean AOD at 500 nm in the premonsoon (March–May) period was about 0.42, while in the winter (December, January, and February) it was ∼0.38. AOD departures from the daily mean showed ±20% variation (which is equivalent to AOD of 0.08) in both seasons. A systematic seasonal asymmetry in diurnal variation was found. AOD is higher in the morning and lower in the afternoon during winter, mainly because of higher relative humidities, calm winds, and associated ground‐based inversions that are conducive for haze, which persists till noon. However, in premonsoon, AODs are lower in the forenoon hours and higher in the afternoon hours because of higher air temperatures, strong convection, and stronger winds raising more dust locally and transporting it from Saudi Arabia and Afghanistan regions. The difference between Moderate Resolution Imaging Spectroradiometer (MODIS) Terra and Aqua derived AOD showed similar seasonal asymmetry in forenoon‐afternoon difference in aerosol loading as observed at the experimental site.

Dust and pollution aerosols over the Negev desert, Israel: Properties, transport, and radiative effect

The complex spatial, temporal, and optical characteristics of atmospheric aerosols cause large uncertainties in the estimation of aerosol effects on climate. Analysis of long‐term measurements from key regions can provide a better understanding of the role of atmospheric aerosols in the climate system. In the current study, observations of aerosol optical properties and mass concentrations were carried out during 1995–2003 in the Israeli Negev desert. The measurement site is relatively remote from local pollution sources; however, it lies at the crossroad between dust from the Sahara and the Arabian peninsula and pollution from Europe. The instruments employed were a Sun/sky photometer, a stacked filter unit sampler, and an integrating nephelometer. We analyzed the data for seasonal variability, general vertical aerosol structure, and radiative climate effect by dust and anthropogenic aerosol. The intra‐annual variability of aerosol optical properties was found to be closely related to seasonally varying synoptic conditions. Two seasonal peaks of aerosol optical thickness were noted: The first maximum related to dust particle activity and the second to anthropogenic aerosol. Similar maximums were noted in aerosol light scattering at the surface; however, their relative importance is reversed and is related to differences in the vertical distribution of dust and anthropogenic aerosols. The calculated aerosol radiative effect shows cooling both at the top of the atmosphere and at the surface during the whole year. The radiative effect of the airborne dust is the dominating forcing component during most of the time in the study area.

Investigation of aerosol components influencing atmospheric transfer of UV radiation in Baltic Sea region

Linking of atmospheric aerosol size distributions and optical properties via predefined aerosol components was investigated. The measured aerosol volume distributions were decomposed to Optical Properties of Aerosols and Clouds (OPAC) components, and aerosol optical properties were calculated for a mixture of those components. The obtained aerosol optical properties were then used for modeling the surface UV irradiances with the libRadtran radiative transfer code. The results were verified with the columnar aerosol characteristics obtained from Aerosol Robotic Network (AERONET) station Tõravere (58.26°N, 26.46°E) and clear‐sky surface UV measurements in Pärnu, Estonia (58.38°N, 24.51°E). The best decomposition results were obtained with four OPAC components, when their lookup characteristics varied within ±10%. Variation of aerosol optical properties in 17 days was influenced by the following aerosol components: soot, 1.2 ± 1.4%; insoluble, 23.1 ± 8.3%; water‐soluble, 44.0 ± 10.8%; accumulation mode sea salt, 31.6 ± 6.2% of total aerosol volume. The average refractive index (for λ = 440 nm) of the component mixture was of 1.42 − 0.013i. Interpretation of the soot component was disputable, since similarly high soot concentrations corresponded to the secondary particles in polluted atmosphere and the nucleation bursts in clean atmosphere. The sea‐salt component showed a correlation with the aerosol residence time over sea. The water‐soluble component and the additional “biomass haze” component represented partly the same aerosol volume in the diameter range of 0.18–1.8 μm. The surface UV irradiances modeled with the AERONET data and the fitted aerosol components were highly correlated with each other, but both model results underestimated the UV extinction by aerosol.

Comparison and analysis of two aerosol retrievals over the ocean in the Terra/Clouds and the Earth's Radiant Energy System–Moderate Resolution Imaging Spectroradiometer single scanner footprint data: 2. Regional evaluation

The advanced multichannel Moderate Resolution Imaging Spectroradiometer (MODIS) and simple independent two‐channel Advanced Very High Resolution Radiometer (AVHRR) aerosol retrieval algorithms were compared regionally using the Terra/CERES‐MODIS Single Scanner Footprint (SSF) data. On average, it was found that the two methods tend to overestimate 0.66‐μm aerosol optical thickness (AOT) compared to AERONET surface observations in the original SSF data. If the most cloud‐free data are used, the mean satellite retrievals agree to within ±10% of the AERONET data. The MODIS near‐infrared (1.60‐μm) AOTs are in better agreement with the surface data than the AVHRR‐type retrievals. The satellite‐derived aerosol size parameters are 20–30% smaller than the surface‐based values with the MODIS values closer to the AERONET values than that of AVHRR‐type. The effects of aerosol model assumptions, cloud contamination, and surface roughness on the two aerosol retrievals were analyzed in detail with the careful classification of clear‐sky and surface roughness conditions. For most of the regions examined, the annual mean AOTs from the MODIS retrieval are 0.03 and 0.02 less than their AVHRR‐type counterparts at 0.66 and 1.60 μm, respectively. However, the MODIS values may exceed the AVHRR‐type values in regions where the prevailing aerosol type varies with season or is under an apparent influence of cloud or surface disturbance. Examination of the surface treatments used by the two retrieval methods indicates the need for improvement over very rough ocean surfaces, especially for the AVHRR method. The results indicate that aerosol model assumptions become important for regional retrievals and the dynamic aerosol models used in the MODIS retrieval are better suited for simultaneously measuring the regional variations in aerosol optical properties and their global mean values.

Comparisons of aerosol optical properties derived from Sun photometry to estimates inferred from surface measurements in Big Bend National Park, Texas

Abstract As a part of the Big Bend Regional Aerosol and Visibility Observational Study (BRAVO, July–October 1999), aerosol physical, optical and chemical properties were measured continuously in an effort to characterize visibility in Big Bend National Park. Surface-based estimates of aerosol extinction coefficients derived from size distribution data were compared to aerosol optical properties retrieved independently from ground-based remote sensing measurements from the United States Department of Agriculture (USDA) UVB radiometer network site. Comparisons suggested that for the majority of the study, surface visibility was a good indicator of the column aerosol optical depth, with a correlation coefficient of r 2 = 0.77 . The average (and one standard deviation) aerosol optical depth from the USDA network was 0.13±0.06 at a wavelength of 500 nm. Angstrom wavelength exponents were computed for both data sets to characterize the spectral variations of aerosol optical properties over the wavelength range of 415–860 nm. Variations in Angstrom exponents corresponded to changing aerosol properties as seen in the ground-based composition and size distribution data sets. The average Angstrom exponent from in situ measurements was 1.5±0.4 compared to column estimates of 1.3±0.4. The root mean square of the difference between the estimates of Angstrom exponents was 0.20 (∼12%) and on average the estimates were highly correlated ( r 2 = 0.89 ). Aerosol optical depths and ambient surface aerosol extinction coefficients were positively correlated with PM2.5 sulfate mass concentrations ( r 2 = 0.68 and r 2 = 0.79 , respectively), suggesting that sulfate aerosols were a major contributor to visibility degradation and column aerosol loading during the study.

Variability of aerosol optical properties derived from in situ aircraft measurements during ACE‐Asia

Airborne measurements of aerosol light scattering (using nephelometers) and absorption (using particle/soot absorption photometers; PSAPs) in the Asian outflow region are presented. Aerosol particles were sampled through a new low turbulence inlet that proved very effective at transmitting coarse‐mode particles. Noise and artifacts are characterized using in‐flight measurements of particle‐free air and measurements with identical instruments operated in parallel. For example, the sensitivities of PSAP noise to changing altitude, changing relative humidity (RH), and particle‐loading on the internal filter are quantified. On the basis of these and previous instrument characterizations, we report averages, variations, and uncertainties of optical properties, focusing on data from approximately 300 level‐leg samples obtained during 19 research flights in the spring of 2001. Several broad patterns emerge from this analysis. Two dominant components, fine‐mode pollution and coarse‐mode mineral dust, were observed to vary independently when separated using a cut point of 1 μm aerodynamic diameter at low RH. Fine‐mode pollution was found to be moderately absorbing (single scatter albedo at low RH and 550 nm, ω = 0.88 ± 0.03; mean and 95% confidence uncertainty) and moderately hygroscopic (relative increase in scattering from 40% to 85% RH, fRH = 1.7 ± 0.2), while coarse‐mode dust was found to have very low absorption (ω = 0.96 ± 0.01) and to be almost nonhygroscopic (fRH = 1.1 ± 0.1). These and other optical properties are intended to serve as constraints on optical models of the Asian aerosol for the purpose of satellite retrievals and calculations of direct radiative effects.

Optical Properties of Aerosols in the Marine Boundary Layer during a Cruise from Tokyo, Japan to Fremantle, Australia.

Shipboard aerosol measurements in the marine boundary layer (MBL) were carried out over the western Pacific Ocean through the eastern Indian Ocean during a cruise of the Antarctic research vessel Shirase from Tokyo, Japan to Fremantle, Australia in November 2000, as part of the 42nd Japanese Antarctic Research Expedition (JARE42) activities. The latitudinal variation of aerosol optical properties is investigated with the origin of aerosols. For elucidation of aerosol properties, a method for retrieving the complex refractive index from combined measurements is proposed using an optical particle counter, an integrating nephelometer and a particle soot absorption photometer. Backward trajectory analyses indicate three regions that are characterized by three distinct aerosol types: anthropogenic/continental aerosols (30°-33.5°N), maritime aerosols (3°-28°N), and biomass-burning aerosols (12°-2°S). Retrieval of the imaginary part of the complex refractive index is successfully achieved by the proposed method, resulting in mean values of 0.0056, 0.0003, and 0.0036 for anthropogenic/continental, maritime, and biomass-burning aerosols, respectively.

Variability of Aerosol Optical Properties at Four North American Surface Monitoring Sites

Aerosol optical properties measured over several years at surface monitoring stations located at Bondville, Illinois (BND); Lamont, Oklahoma (SGP); Sable Island, Nova Scotia (WSA); and Barrow, Alaska (BRW), have been analyzed to determine the importance of the variability in aerosol optical properties to direct aerosol radiative forcing calculations. The amount of aerosol present is of primary importance and the aerosol optical properties are of secondary importance to direct aerosol radiative forcing calculations. The mean aerosol light absorption coefficient (σap) is 10 times larger and the mean aerosol scattering coefficient (σsp) is 5 times larger at the anthropogenically influenced site at BND than at BRW. The aerosol optical properties of single scattering albedo (ωo) and hemispheric backscatter fraction (b) have variability of approximately ± 3% and ± 8%, respectively, in mean values among the four stations. To assess the importance of the variability in ωo and b on top of the atmosphere aerosol radiative forcing calculations, the aerosol radiative forcing efficiency (ΔF/δ) is calculated. The ΔF/δ is defined as the aerosol forcing (ΔF) per unit optical depth (δ) and does not depend explicitly on the amount of aerosol present. Based on measurements at four North American stations, radiative transfer calculations that assume fixed aerosol properties can have errors of 1%–6% in the annual average forcing at the top of the atmosphere due to variations in average single scattering albedo and backscatter fraction among the sites studied. The errors increase when shorter-term variations in aerosol properties are considered; for monthly and hourly timescales, errors are expected to be greater than 8% and 15%, respectively, approximately one-third of the time. Systematic relationships exist between various aerosol optical properties [σap, ωo, b, ΔF/δ, and Ångström exponent (å)] and the amount of aerosol present (measured by σsp) that are qualitatively similar but quantitatively different among the four stations. These types of systematic relationships and the regional and temporal variations in aerosol optical properties should be considered when using “climatological” averages.

A North American Arctic Aerosol Climatology using Ground-based Sunphotometry

The Arctic is known as a key area for the detection of climate changes and atmospheric pollution on a global scale. In this paper we describe a new Canadian sunphotometer network called AEROCAN, whose primary mandate is to establish a climatology of atmospheric aerosols. This network is part of AERONET, the worldwide federated sunphotometer network managed by the NASA Goddard Space Flight Center. The potential of sunphotometer data from the AERONET/AEROCAN network for monitoring of Arctic aerosols is illustrated, using examples of the multiyear variation of aerosol optical properties and atmospheric precipitable water vapour content at some stations, and in particular at Bonanza Creek, Alaska since 1994. Despite its sparse spatial density, the network represents an important tool for monitoring the spatio-temporal variation of Arctic aerosols. It also represents an important source of independent aerosol data, which we feel should be further developed in northern areas to improve our understanding of how atmospheric aerosols influence global climate.

Four years of continuous surface aerosol measurements from the Department of Energy's Atmospheric Radiation Measurement Program Southern Great Plains Cloud and Radiation Testbed site

Continuous measurements of the optical and microphysical properties of aerosol particles have been made at the Department of Energy's Atmospheric Radiation Measurement Program Southern Great Plains Cloud and Radiation Testbed (CART) site covering the 4‐year period from July 1996 through June 2000. Hourly, daily, and monthly statistics have been calculated that illustrate aerosol variability over a range of timescales. A pronounced peak in total particle number, centered on the midafternoon hours (local time), is evident in the hourly statistics. A broad early morning peak in the concentration of particles >0.1‐μm aerodynamic diameter corresponds with a similar peak in aerosol light‐scattering coefficient, σsp. No strong cycles were observed in the daily statistics, suggesting that day of the week has only a minor influence on the observed aerosol variability. The σsp at a wavelength of 550 nm for the 4‐year period showed a median value of 33 Mm−1 and was highest in February and August. The median fraction of aerosol light scattering at 550 nm due to particles <1‐μm aerodynamic diameter was 0.85 over the entire record. The median aerosol light absorption coefficient, σap, for the 4‐year period was ∼1.5 Mm−1 and was observed to be highest in late summer and autumn. The σap showed an increasing trend of nearly 0.5 Mm−1 yr, possibly due to increased agricultural field burning in the area. The occurrence of an autumn decrease in single‐scattering albedo, ω0, was observed and may be caused by regional‐scale agricultural or transportation activities or seasonal changes in atmospheric flow patterns. The median value for ω0 over the 4‐year period was 0.95, but this value has decreased ∼1–2% yr−1 presumably due to increased agricultural burning. Numerous field fires during the second half of 1999 influenced the surface aerosol at the CART site causing substantial variability of aerosol optical properties. The aerosol hygroscopic growth factor (f(RH)), corresponding to a relative humidity increase of 40–85%, showed a median value of 1.83 for 1999, although much lower values were observed during periods that were probably influenced by locally generated smoke and dust aerosols (median f(RH) = 1.55 and 1.59, respectively).

Observations of the vertical and regional variability of aerosol optical properties over central and eastern North America

Aerosol optical properties were measured in situ from a research aircraft during three recent field experiments in the central and eastern parts of North America as well as over areas of the western Atlantic Ocean. Regional and vertical variability of the aerosol properties for boundary layer and free tropospheric air were determined. In general, the differences between distributions of aerosol properties measured at low (planetary boundary layer) and high (free troposphere) altitudes were small but statistically significant at the 3% level or better. However, most of the aerosol optical thickness of the layers studied (∼5 km down to ∼100 m) was encountered in the lowest 1 km of each layer. As a result, the near‐surface measurements of aerosol optical properties adequately represented the portion of the lower column that dominates the radiative effects. The estimated error encountered by using near‐surface aerosol measurements to calculate the layer forcing was typically <10%.

Determining marine aerosol scattering characteristics at ambient humidity from size‐resolved chemical composition

Although the variation of aerosol optical properties with humidity depends critically on particle size and chemical composition, existing models usually employ empirical growth factors for water uptake. For some size ranges and many types of particles these empirical factors are not well known. We describe here a model that applies a thermodynamic analysis to arbitrary aerosol chemical size distributions to compute water uptake, refractive index, number distribution, and optical extinction. By starting with the fundamental chemistry of the aerosols the model is able to compute the derivative of extinction with sulfate, sulfate plus ammonium, and other scenarios that may be derived from global change and pollution control strategies. It can also supply information on the variation of aerosol size and extinction with humidity and with details of the aerosol size distribution. We demonstrate this model using five detailed chemical size distributions measured with a cascade impactor in the marine boundary layer. The results are in good agreement with published extinction coefficients and with nephelometer data collected in concert with one of our samples. Calculated non‐sea‐salt sulfate mass‐scattering efficiencies of our samples ranged from 0.6 to 2.6 g m−2 for dry particles and 5.3 to 13 g m−2 at 80% relative humidity.

Variability of aerosol optical properties over Hefei during the period from September 1993 to September 1994

Variability of aerosol optical properties over Hefei during the period from September 1993 to September 1994

Arctic aerosols: Model estimates of interactive influences upon the surface-atmosphere clearsky radiation budget

It is known that a significant fraction of the Arctic's tropospheric aerosol is of anthropogenic origin. But in order to interpret the possible impact of this anthropogenic component upon the Arctic climate, it is necessary to understand how the aerosol alters the radiation budget of the Arctic's surface-troposphere system. The present study addresses several aspects of this possible radiative perturbation, such as the importance of the aerosol's vertical distribution, the importance of vertical variations of aerosol optical properties as induced by vertical variations of relative humidity, the estimation of diurnally-averaged seasonal perturbations of the surface-troposphere system and how these are influenced by interactions between the anthropogenic and natural components of the aerosol and a suggestion as to how to interpret observed aerosol-induced changes in the surface radiation budget.