Properties Of Atmospheric Aerosols Research Articles

Analysis of optical properties of atmospheric aerosols inferred from spectral AODs and Ångström wavelength exponent

Abstract Aerosol measurements over a tropical semi-arid site in Anantapur (14.62°N, 77.65°E, 331 m asl), India, provide the variabilities of aerosol characteristics for the period of January 2007–December 2008. The mean values of aerosol optical depth at 500 nm (AOD500 nm) over 2-year period are found to be 0.37 ± 0.11, 0.39 ± 0.13 and 0.30 ± 0.10 during winter, summer and monsoon seasons, respectively. A rather systematic variation is seen with AOD peaking in the month of March, falling off gradually to the lowest value in June and increasing slowly superposed with some modulations, which show a weak secondary peak in October. The mean and standard values of the Angstrom exponent, α (turbidity coefficient, β), are found to be 1.06 ± 0.33 (0.17 ± 0.05) during winter, 0.97 ± 0.35 (0.22 ± 0.04) during summer and 0.85 ± 0.26 (0.16 ± 0.06) during monsoon. The frequency distribution of α is relatively broad with two modes for dusty (α 1) situations. This frequency distribution of α reveals a great dispersion of a values in all seasons thus denoting variability in the aerosol-size distribution. In this study, spectral AOD and Angstrom exponent data are analyzed to obtain information about the adequacy of the simple use of the Angstrom exponent and its variation for characterizing optical properties of aerosols, and exploring possibilities for a more efficient characterization of them. Using the least-squares fit method, the Angstrom parameter, α, is calculated in the spectral interval 380–850 nm, along with coefficients a1 and a2 of the second-order polynomial fit to the plotted logarithm of AOD versus the logarithm of wavelength. The correlations between the coefficients a1 and a2 with months and α are discussed in the paper.

Characterization of chemical properties of atmospheric aerosols over Anmyeon (South Korea), a super site under Global Atmosphere Watch

This paper reports aerosol chemical properties for the first time over a Korean Global Atmosphere Watch (GAW) supersite, Anmyeon (36°32′N; 126° 19′E), during 2003–2004 period. Total suspended Particulates (TSP) showed significant seasonal variation with consistent higher mass concentrations during spring season (average of up to 230 ± 190 μg/m3). PM10 also followed similar trend with higher concentrations during spring (average of up to 170 ± 130 μg/m3) and showed reduced concentrations during summer. PM2.5 showed a significant increase during summer (average of up to 60 ± 25 μg/m3), which could be due to the influx of fine mode sea salt aerosols associated with the Changma front (summer monsoon). Chemical composition analysis showed enhanced presence of acidic fractions, majorly contributed by sulphates (SO42-) and nitrates (NO3-) in TSP, PM10 and PM2.5 during different seasons. Enhanced presence of Calcium (Ca2+) was observed during sand storm days during spring. The high correlation obtained on matrix analysis between crustal ions and acidic ions suggests that the ionic compositions over the site are mainly contributed by terrestrial sources of similar origin. The neutralization factors has been estimated to find the extend of neutralization of acidicity by main basic components, and found to have higher value for Ammonium (up to 1.1) in different seasons, indicating significant neutralization of acidic components over the region by NH4+. Back trajectory analysis has been performed during different seasons to constrain the possible sources of aerosol origin and the results are discussed in detail.

Evaluation of simulated aerosol properties with the aerosol-climate model ECHAM5-HAM using observations from the IMPACT field campaign

Abstract. In May 2008, the measurement campaign IMPACT for observation of atmospheric aerosol and cloud properties was conducted in Cabauw, The Netherlands. With a nudged version of the coupled aerosol-climate model ECHAM5-HAM we simulate the size distribution and chemical composition of the aerosol and the associated aerosol optical thickness (AOT) for the campaign period. Synoptic scale meteorology is represented realistically through nudging of the vorticity, the divergence, the temperature and the surface pressure. Simulated concentrations of aerosol sulfate and organics at the surface are generally within a factor of two from observed values. The monthly averaged AOT from the model is 0.33, about 20% larger than observed. For selected periods of the month with relatively dry and moist conditions discrepancies are approximately −30% and +15%, respectively. Discrepancies during the dry period are partly caused by inaccurate representation of boundary layer (BL) dynamics by the model affecting the simulated AOT. The model simulates too strong exchange between the BL and the free troposphere, resulting in weaker concentration gradients at the BL top than observed for aerosol and humidity, while upward mixing from the surface layers into the BL appears to be underestimated. The results indicate that beside aerosol sulfate and organics also aerosol ammonium and nitrate significantly contribute to aerosol water uptake. The simulated day-to-day variability of AOT follows synoptic scale advection of humidity rather than particle concentration. Even for relatively dry conditions AOT appears to be strongly influenced by the diurnal cycle of RH in the lower boundary layer, further enhanced by uptake and release of nitric acid and ammonia by aerosol water.

Aerosol chemistry over a high altitude station at northeastern Himalayas, India.

BackgroundThere is an urgent need for an improved understanding of the sources, distributions and properties of atmospheric aerosol in order to control the atmospheric pollution over northeastern Himalayas where rising anthropogenic interferences from rapid urbanization and development is becoming an increasing concern.Methodology/Principal FindingsAn extensive aerosol sampling program was conducted in Darjeeling (altitude ∼2200 meter above sea level (masl), latitude 27°01′N and longitude 88°15′E), a high altitude station in northeastern Himalayas, during January–December 2005. Samples were collected using a respirable dust sampler and a fine dust sampler simultaneously. Ion chromatograph was used to analyze the water soluble ionic species of aerosol. The average concentrations of fine and coarse mode aerosol were found to be 29.5±20.8 µg m−3 and 19.6±11.1 µg m−3 respectively. Fine mode aerosol dominated during dry seasons and coarse mode aerosol dominated during monsoon. Nitrate existed as NH4NO3 in fine mode aerosol during winter and as NaNO3 in coarse mode aerosol during monsoon. Gas phase photochemical oxidation of SO2 during premonsoon and aqueous phase oxidation during winter and postmonsoon were the major pathways for the formation of SO4 2− in the atmosphere. Long range transport of dust aerosol from arid regions of western India was observed during premonsoon. The acidity of fine mode aerosol was higher in dry seasons compared to monsoon whereas the coarse mode acidity was higher in monsoon compared to dry seasons. Biomass burning, vehicular emissions and dust particles were the major types of aerosol from local and continental regions whereas sea salt particles were the major types of aerosol from marine source regions.Conclusions/SignificanceThe year-long data presented in this paper provide substantial improvements to the heretofore poor knowledge regarding aerosol chemistry over northeastern Himalayas, and should be useful to policy makers in making control strategies.

Physical and optical properties of atmospheric aerosols by in-situ and radiometric measurements

Abstract. Physical and optical properties of atmospheric aerosols collected by using a high resolution (1.5 nm) spectroradiometer (spectral range 400–800 nm), a 13-stage Dekati Low Pressure Impactor (size range 30 nm–10 μm), and an AE31 Aethalometer (7 wavelenghts from 370 nm to 950 nm), have been examined in a semi-rural site in Southwest Italy (Tito Scalo, 40°35' N, 15°41' E, 750 m a.s.l.). In particular, daily averaged values of AOD and Ångström turbidity parameters from radiometric data together with mass-size distributions from impactor data and Black Carbon (BC) concentrations have been analyzed from May to October 2008. Furthermore, by inverting direct solar radiances, aerosol columnar number and volume size distributions have been obtained for the same period. The comparison of different observation methods, allowed to verify if, and in what conditions, changes in aerosol properties measured at ground are representative of columnar properties variations. Agreement between columnar and in-situ measurements has been obtained in case of anthropogenic aerosol loading, while in case of Saharan dust intrusions some discrepancies have been found when dust particles were located at high layers in the atmosphere (4–8 km) thus affecting columnar properties more than surface ones. For anthropogenic aerosols, a good correlation has been confirmed through the comparison of fine aerosol fraction contribution as measured by radiometer, impactor and aethalometer, suggesting that, in this case, the particles are more homogeneously distributed over the lower layers of atmosphere and columnar aerosol optical properties are dominated by surface measured component.

Redox and electrophilic properties of vapor- and particle-phase components of ambient aerosols.

Particulate matter (PM) has been the primary focus of studies aiming to understand the relationship between the chemical properties of ambient aerosols and adverse health effects. Size and chemical composition of PM have been linked to their oxidative capacity which has been postulated to promote or exacerbate pulmonary and cardiovascular diseases. But in the last few years, new studies have suggested that volatile and semi-volatile components may also contribute to many adverse health effects. The objectives of this study were: (i) assess for the first time the redox and electrophilic potential of vapor-phase components of ambient aerosols and (ii) evaluate the relative contributions of particle- and vapor-fractions to the hazard of a given aerosol. To achieve these objectives vapor- and particle-phase samples collected in Riverside (CA) were subjected to three chemical assays to determine their redox and electrophilic capacities. The results indicate that redox active components are mainly associated with the particle-phase, while electrophilic compounds are found primarily in the vapor-phase. Vapor-phase organic extracts were also capable of inducing the stress responding protein, heme-oxygenase-1 (HO-1), in RAW264.7 murine macrophages. These results demonstrate the importance of volatile components in the overall oxidative and electrophilic capacity of aerosols, and point out the need for inclusion of vapors in future health and risk assessment studies.

Comparison of in situ and columnar aerosol spectral measurements during TexAQS-GoMACCS 2006: testing parameterizations for estimating aerosol fine mode properties

Abstract. During the 2006 Texas Air Quality Study and Gulf of Mexico Atmospheric Composition and Climate Study (TexAQS-GoMACCS 2006), the optical, chemical and microphysical properties of atmospheric aerosols were measured on multiple mobile platforms and at ground based stations. In situ measurements of the aerosol light extinction coefficient (σep) were performed by two multi-wavelength cavity ring-down (CRD) instruments, one located on board the NOAA R/V Ronald H. Brown (RHB) and the other located at the University of Houston, Moody Tower (UHMT). An AERONET sunphotometer was also located at the UHMT to measure the columnar aerosol optical depth (AOD). The σep data were used to extract the extinction Ångström exponent (åep), a measure of the wavelength dependence of σep. There was general agreement between the åep (and to a lesser degree σep) measurements by the two spatially separated CRD instruments during multi-day periods, suggesting a regional scale consistency of the sampled aerosols. Two spectral models are applied to the σep and AOD data to extract the fine mode fraction of extinction (η) and the fine mode effective radius (Reff,f). These two parameters are robust measures of the fine mode contribution to total extinction and the fine mode size distribution, respectively. The results of the analysis are compared to Reff,f values extracted using AERONET V2 retrievals and calculated from in situ particle size measurements on the RHB and at UHMT. During a time period when fine mode aerosols dominated the extinction over a large area extending from Houston/Galveston Bay and out into the Gulf of Mexico, the various methods for obtaining Reff,f agree qualitatively (showing the same temporal trend) and quantitatively (pooled standard deviation = 28 nm).

Aerosol Remote Sensing From Space: Determination of Atmospheric Aerosol Properties Using Satellite Measurements; Bad Honnef, Germany, 16–19 August 2009

Aerosol optical depth (AOD), a measure of how much light is attenuated by aerosol particles, provides scientists information about the amount and type of aerosols in the atmosphere. Recent developments in aerosol remote sensing was the theme of a workshop held in Germany. The workshop was sponsored by the Wilhelm and Else Heraeus Foundation and attracted 67 participants from 12 countries.The workshop focused on the determination (retrieval) of AOD and its spectral dependence using measurements of changes to the solar radiation backscattered to space. The midvisible AOD is usually applied to define aerosol amount, while the size of aerosol particles is indicated by the AOD spectral dependence and is commonly expressed by the Angstrom parameter. Identical properties retrieved by different sensors, however, display significant diversity, especially over continents. A major reason for this is that the derivation of AOD requires more accurate determination of nonaerosol contributions to the sensed satellite signal than is usually available. In particular, surface reflectance data as a function of the viewing geometry and robust cloud‐clearing methods are essential retrieval elements. In addition, the often needed assumptions about aerosol properties in terms of absorption and size are more reasons for the discrepancy between different AOD measurements.

Laser radar characterization of atmospheric aerosols in the troposphere and stratosphere using range dependent lidar ratio

Laser radar (lidar) provides an excellent tool for characterizing the physical properties of atmospheric aerosols which play a very important role in modifying the radiative budget of the Earth's atmosphere. One of the important issues in lidar research is to derive accurate backscattering or extinction coefficient profiles required for understanding the basic mechanisms in the formation of aerosols and identifying their sources and sinks. Most of the inversion methods used for deriving the aerosol coefficients assume a range independent value for the extinction-to- backscattering ratio [lidar ratio, (LR)]. However, it is known that in a realistic atmosphere the value of LR is range dependent and varies with the physical and chemical properties of the aerosols. In this paper, we use a variant of widely applied Klett's method to obtain the range dependent LR values and derive the aerosol extinction profiles with good accuracy. We present the lidar derived aerosol extinction profiles in the upper troposphere and lower stratosphere corresponding to different seasons of the year of two distinctly different stations in the Indian subcontinent namely Trivandrum (8.33° N, 77° E), Kerala, India, a coastal station and Gadanki (13.5° N, 79.2° E), Tirupati, India an inland station. The range dependent LR is derived corresponding to different seasons of the year at the two stations. The lidar ratio, aerosol extinction coefficient (AEC), aerosol scattering ratio and aerosol optical depth show strong to medium seasonal variation at both the stations. The lidar ratio values at Trivandum vary in the range of 11-38 sr whereas the values range from 20-34 sr at Gadanki. AEC values at the Trivandum station vary from 7.9x10-6 to 6.9x10-5 m-1 and at Gadanki station the variation is from 1.27x10-5 to 6.9x10-5 m-1. It is proposed to use back-trajectory analysis to understand the sources of aerosol at the two stations.

Spatial and vertical heterogeneities in aerosol properties over oceanic regions around India: Implications for radiative forcing

AbstractThe influence of atmospheric aerosols on Earth's radiation budget and hence climate, though well recognized and extensively investigated in recent years, remains largely uncertain mainly because of the large spatio‐temporal heterogeneity and the lack of data with adequate resolution. To characterize this diversity, a major multi‐platform field campaign ICARB (Integrated Campaign for Aerosols, gases and Radiation Budget) was carried out during the pre‐monsoon period of 2006 over the Indian landmass and surrounding oceans, which was the biggest such campaign ever conducted over this region. Based on the extensive and concurrent measurements of the optical and physical properties of atmospheric aerosols during ICARB, the spatial distribution of aerosol radiative forcing was estimated over the entire Bay of Bengal (BoB), northern Indian Ocean and Arabian Sea (AS) as well as large spatial variations within these regions. Besides being considerably lower than the mean values reported earlier for this region, our studies have revealed large differences in the forcing components between the BoB and the AS. While the regionally averaged aerosol‐induced atmospheric forcing efficiency was 31 ± 6 W m−2 τ−1 for the BoB, it was only ∼18 ± 7 W m−2 τ−1 for the AS. Airborne measurements revealed the presence of strong, elevated aerosol layers even over the oceans, leading to vertical structures in the atmospheric forcing, resulting in significant warming in the lower troposphere. These observations suggest serious climate implications and raise issues ranging from the impact of aerosols on vertical thermal structure of the atmospheric and hence cloud formation processes to monsoon circulation. Copyright © 2009 Royal Meteorological Society

Hygroscopic growth of urban aerosol particles in Beijing (China) during wintertime: a comparison of three experimental methods

Abstract. The hygroscopic properties of atmospheric aerosols are highly relevant for the quantification of radiative effects in the atmosphere, but also of interest for the assessment of particle health effects upon inhalation. This article reports measurements of aerosol particle hygroscopicity in the highly polluted urban atmosphere of Beijing, China in January 2005. The meteorological conditions corresponded to a relatively cold and dry atmosphere. Three different methods were used: 1) A combination of Humidifying Differential Mobility Particle Sizer (H-DMPS) and Twin Differential Mobility Particle Sizer (TDMPS) measurements, 2) A Hygroscopic Tandem Differential Mobility Analyzer (H-TDMA), and 3) A simplistic solubility model fed by chemical particle composition determined from Micro Orifice Uniform Deposit Impactor (MOUDI) samples. From the H-DMPS and TDMPS particle number size distributions, a size-resolved descriptive hygroscopic growth factor (DHGF) was determined for the relative humidities (RH) 55%, 77% and 90%, and particle diameters between 30 and 400 nm. In Beijing, the highest DHGFs were observed for accumulation mode particles, 1.40 (±0.03) at 90% RH. DHGF decreased significantly with particle size, reaching 1.04 (±0.15) at 30 nm. H-TDMA data also suggest a decrease in growth factor towards the biggest particles investigated (350 nm), associated with an increasing fraction of nearly hydrophobic particles. The agreement between the H-DMPS/TDMPS and H-TDMA methods was satisfactory in the accumulation mode size range (100–400 nm). In the Aitken mode range (<100 nm), the H-DMPS/TDMPS method yielded growth factors lower by up to 0.1 at 90% RH. The application of the solubility model based on measured chemical composition clearly reproduced the size-dependent trend in hygroscopic particle growth observed by the other methods. In the case of aerosol dominated by inorganic ions, the composition-derived growth factors tended to agree (± 0.05) or underestimate (up to 0.1) the values measured by the other two methods. In the case of aerosol dominated by organics, the reverse was true, with an overestimation of up to 0.2. The results shed light on the experimental and methodological uncertainties that are still connected with the determination of hygroscopic growth factors.

Optical and Physical Properties of Atmospheric Aerosols over the Bay of Bengal during ICARB

Abstract Simultaneous and collocated measurements of total and hemispherical backscattering coefficients (σ and β, respectively) at three wavelengths, mass size distributions, and columnar spectral aerosol optical depth (AOD) were made onboard an extensive cruise experiment covering, for the first time, the entire Bay of Bengal (BoB) and northern Indian Ocean. The results are synthesized to understand the optical properties of aerosols in the marine atmospheric boundary layer and their dependence on the size distribution. The observations revealed distinct spatial and spectral variations of all the aerosol parameters over the BoB and the presence of strong latitudinal gradients. The size distributions varied spatially, with the majority of accumulation modes decreasing from north to south. The scattering coefficient decreased from very high values (resembling those reported for continental/urban locations) in the northern BoB to very low values seen over near-pristine environments in the southeastern BoB. The average mass scattering efficiency of BoB aerosols was found to be 2.66 ± 0.1 m2 g−1 at 550 nm. The spectral dependence of columnar AOD deviated significantly from that of the scattering coefficients in the northern BoB, implying vertical heterogeneity in the aerosol type in that region. However, a more homogeneous scenario was observed in the southern BoB. Simultaneous lidar and in situ measurements onboard an aircraft over the ocean revealed the presence of elevated aerosol layers of enhanced extinction at altitudes of 1 to 3 km with an offshore extent of a few hundred kilometers. Back-trajectory analyses showed these layers to be associated with advection from west Asia and western India. The large spatial variations and vertical heterogeneity in aerosol properties, revealed by the present study, need to be included in the regional radiative forcing over the Bay of Bengal.

Source characterization from ambient measurements of aerosol optical properties

A general method to identify air masses impacted by distinct primary aerosol sources from in situ observations of optical properties of atmospheric aerosols is developed theoretically and tested against measurements made during the second New England Air Quality Study (NEAQS 2004) on board the NOAA R/V Ronald H. Brown. Distinct events are identified based on the observation of a temporal coherence in the relationship between a measured intensive optical property (i.e., single scatter albedo) and the extensive property of total light absorption of the sub‐micron aerosol. We show that this relationship can be used to differentiate between and quantitatively determine properties of the background aerosol and recent inputs of primary aerosol. Additionally, we show that the fraction of aerosol extinction (or aerosol mass) from primary emissions can be estimated from the albedo‐absorption relationship. During the NEAQS 2004 study, the primary aerosol was usually found to be highly absorbing, and thus likely to be derived from anthropogenic combustion sources. Overall primary aerosol contributed a relatively small amount (ca. 11%) to the total sub‐micron aerosol burden sampled onboard R/V Ron Brown.

Continuous measurements of optical properties of atmospheric aerosols in Mukteshwar, northern India

Particulate pollution of mainly anthropogenic origin is a widely spread phenomenon in southern Asia, influencing climate and causing adverse health effects on humans. In this paper, we analyze continuous measurements of aerosol scattering and absorption properties that were conducted in Mukteshwar, a remote site in the Himalayan Mountains in northern India, during September 2005–September 2007. While aerosol concentrations were somewhat lower than those measured closer to urban areas in India, some optical parameters had a noticeable resemblance to those measured elsewhere in India. The average value of the measured single‐scattering albedo was 0.81 at 525 nm, being indicative of a substantial amount of absorbing material. Annually, there were two periods when aerosol scattering and absorption coefficients were relatively low. These were the rainy season during July and August and the winter months December and January when particulate pollution seemed to reside partly below the altitude of the measurement station. During December and January, local cooking and warming of houses by biomass burning, normally not visible in the data, became evident yet weak sources of absorbing material.

Improvements for ground-based remote sensing of atmospheric aerosol properties by additional polarimetric measurements

We discuss the improvements in the aerosol properties characterization resulting from the additional multi-wavelength polarization measurements measured by a new CIMEL polarized sun/sky-photometer, CE318-DP. In order to process direct-sun, sky and polarization measurements in a wide spectral range (340–1640 nm), we developed new calibration methods and strategies, e.g. using the Langley plot method to calibrate both direct-sun irradiance and sky radiance, as well as combining laboratory facilities with a vicarious method to calibrate the polarized sky measurements. For studying the impact of new polarimetric measurements on the retrievals of aerosol properties, we have processed an extensive record of field measurements using an updated Dubovik and King retrieval algorithm [Dubovik O, Sinyuk A, Lapyonok T, Holben BN, Mishchenko MI, et al. Application of spheroid models to account for aerosol particle nonsphericity in remote sensing of desert dust. J Geophys Res 2006;111:D11208.]. A preliminary analysis shows that adding polarization in the inversion can reduce possible errors (notably for about 30% of our field cases) in the fine mode size distribution, real part of refractive index and particle shape parameter retrievals, especially for small particles.

Determination of Atmospheric Aerosol Properties Over Land Using Satellite Measurements

Mostly, aerosol properties are poorly understood because the aerosol properties are very sparse. The first workshop on the determination of atmospheric aerosol properties over land using satellite measurements is convened in Bremen, Germany. In this workshop, the topics of discussions included a variety of satellite aerosol optical thickness retrieval methods, the description of modern satellite instrumentation, and the information content of satellite measurements with respect to the aerosol properties retrieval from spaceborne observations. The presentations are mostly concerned with the aerosol optical thickness retrieval over land.

Optical properties of atmospheric aerosols obtained by in situ and remote measurements during 2006 Campaign of Air Quality Research in Beijing (CAREBeijing‐2006)

In order to investigate the optical properties of atmospheric aerosol in the urban area of Beijing, in situ and remote measurements of particulate pollutants were conducted at an urban site (39.98°N, 116.35°E, ∼20 m above ground level) in Beijing during the Campaign of Air Quality Research in Beijing (CAREBeijing) campaign from 16 August to 10 September 2006. During the intensive measurement period, average PM2.5 mass concentration and light extinction coefficient (bext) were measured to be 102.1 ± 55.0 μg m−3 and 880.7 ± 685.9 Mm−1, respectively. Average mass scattering efficiency of PM10 and PM2.5 particle were found to be 2.5 ± 1.1 and 3.4 ± 1.2 m2 g−1, respectively. Average single scattering albedo (SSA) of dry PM10 was characterized to be 0.82 ± 0.09. It was found that SSA decreased to lower values of ∼0.75 during the relatively clean condition while it increased up to ∼0.86 during polluted condition mainly owing to the increases of ammonium sulfate, ammonium nitrate, and organic mass by carbon's contributions to PM mass concentrations. Pollution episodes in Beijing were strongly related to wind speed and wind direction. Stagnant weather conditions with southerly wind and low wind speed accelerated the accumulation of the pollutants in Beijing, which led to severe haze. It has been found that high PM2.5/AOT (aerosol optical thickness) ratio of 228.2 μg m−3 was observed when air mass was transported from western or northern China while significantly lower PM2.5/AOT of 107.6 μg m−3 was observed when it was affected mostly by local air pollutants in Beijing.

Optical Properties of Atmospheric Aerosol over the Black Sea from May 2006 to September 2007

Measurements of the spectral aerosol optical depth (AOD), moisture content and Angstrom parameter (α) were carried out in Sevastopol under the AERONET program. The graphs and histograms of the Angstrom parameter (α) versus the moisture content and AOD were analyzed, and the spectral dependency of AOD was plotted. All measurements are presented in the plots and tables. Daily and monthly average characteristics of general parameters are described. Results allow one to reveal regional features of optical properties of atmospheric aerosol and their temporal variability over the Black Sea in the Sevastopol region.

Size‐resolved chemical composition of aerosol particles during a monsoonal transition period over the Indian Ocean

An aerosol time‐of‐flight mass spectrometer (ATOFMS) was used to measure the size‐resolved mixing state of particles over the northern Indian Ocean in October and November 2004. This period was chosen to observe the impact of the monsoonal transition on the size, chemistry, sources, and radiative properties of atmospheric aerosols in the region. Overall, elemental carbon with sulfate (EC‐sulfate), biomass/biofuel burning, fresh sea salt (SS), aged sea salt, fly ash, and EC mixed with sea salt were the dominant supermicron particle types, whereas EC‐sulfate, biomass/biofuel burning, and fly ash were the dominant submicron particle types. Interestingly, particles composed mostly of aged organic carbon and nitrate were virtually absent during the campaign. This is possibly from low ozone formation in the region or selective scavenging during transport. Notably, during long‐range transport periods when an aethalometer measured the highest black carbon concentrations, 77% of submicron particles between 0.5 and 2.5 μm and 71% of EC/soot particles contained an intense 39K+ ion (a known tracer for biomass/biofuel combustion). These observations suggest when the air mass originated from India, biofuel combustion represented a significant source of the regional atmospheric brown cloud. The majority (∼80%) of EC and biomass/biofuel burning particles were mixed with significant amounts of sulfate due to extensive secondary processing of these particles during transport. EC mixed with sea salt was also observed suggesting the particles had undergone cloud processing and become internally mixed during transport. These measurements support the use of an internal mixture of sulfate with EC/soot and biomass/biofuel burning in models to accurately calculate radiative forcing by aerosols in this region.

Relationship of refractive index to mass density and self-consistency of mixing rules for multicomponent mixtures like ambient aerosols

This paper focuses on two important yet poorly addressed aspects of ambient aerosols: relationship of refractive index to mass density (index–density relationship) and consistency of the mixing rules used to calculate these two quantities of a multicomponent mixture like ambient aerosols with the index–density relationship. Combined empirical and theoretical analyses show that a denser material generally tends to have a larger refraction index because the applied electric field induces a greater number of electric dipoles, and that the index–density relationship can be described reasonably well by the Lorentz–Lorenz relation. It is shown that the commonly used volume–mean mixing rule for calculating the effective mass density, the Lorentz–Lorenz mixing rule and the molar refraction mixing rule for calculating effective refractive index form a set of mixing rules that are consistent with the Lorentz–Lorenz relation. The molar fraction mixing rule and the Lorentz–Lorenz mixing rule are shown to be equivalent for the Lorentz–Lorenz mixture while the linear volume-mixing rule is an approximation of the Lorentz–Lorenz mixing rule for quasi-homogeneous mixtures wherein the refractive indices of the constituents do not differ much. The results highlight the need for consistency of the mixing rules for calculating the effective refractive index and mass density with the index–density relationship, which not only provides a theoretical guide for judiciously choosing the mixing rules to calculate effective properties of ambient aerosols but also poses new challenges to develop an effective medium theory that applies to more than one quantity. An empirical power-law expression is obtained from the published data that relates the effective specific refractive index to the effective mass density of aerosol particles.