Recent progress in aerosol instrumentations enables the measurement of the size distribution and chemical components of atmospheric aerosol particles in real time. However, the secondary aerosol formation process, where nanometer-sized particles are generated via gas-particle conversion, has still not been well understood because of the difficulty in the measurement of nanometer-sized aerosol particles under ‘real’ atmospheric conditions such as low temperatures and/or reduced pressure: the higher time- and size-resolution measurement techniques are necessary to analyze particle formation rates (nucleation rates) under various conditions. In addition, appropriate attention is not paid to fundamental studies on carbonaceous particles, although they seem to have large effects on the environment. In this paper, recent developments of the instrumentation technologies for aerosol measurement in the nanometer-size range, and generation of aggregate carbonaceous particles with controlled size and density, which are available as standard particles, are reviewed. The applications of aerosol instruments to the study of new atmospheric particle formation, as well as to laboratory model experiments on the formation and growth of sulfuric acid aerosols by binary homogeneous nucleation and ion-induced nucleation, are introduced.Another application of the generation of aggregate carbonaceous particles for the study of effective density and refractive index is also introduced.
Secondary organic aerosols (SOAs) are produced through chemical reactions initiated by the oxidation of volatile organic compounds (VOCs). Understanding of SOA formation mechanisms is still insufficient because multi-step reactions from a variety of VOCs are involved. In order to clarify the chemical mechanisms for SOA formation, we are investigating gas- and aerosol-phase species produced from VOC oxidation by using mass spectrometry and laser spectroscopy techniques. Here, we describe the application of mass spectrometry to SOA component analyses and recent results, including those from our group. Chemical kinetics studies of the VOC oxidation reactions in relation to SOA formation are also mentioned.
This research developed international standard methods using virtual impactors and diluters for the in-stack measurement of (i) PM2.5 and PM10 mass concentrations and (ii) condensable particles within the flue gases of stationary emission sources (such as factories and power plants). Based on this re search, which reflects the importance of the standardization of sampling and measurement methods, international standards for both methods (ISO 13271 and ISO 25597) were published. For the purposes of the ISO publication, the performance of virtual impactors was compared with that of a commercial, conventional two-stage cascade impactor (Real impactor) that collects particles on its solid impaction surfaces. Virtual impactor measurements delivered consistent data in model aerosol testing systems, and in combustion flue gases at testing and commercial plants for coal and other fuels in Japan, Malaysia, and China. The emission behaviors of PM2.5 and condensable nanoparticles with organic and inorganic trace elements were characterized at each plant by using total surface area-based concentration, size distribution, and a variety of other measurement methods.
To evaluate aerosol impacts on human health and plants, introduce control technology to the emission sources, and formulate a reduction strategy, three subjects must be comprehensively studied: (i) characteristics of the emission sources, and physical and chemical behavior of the aerosols in the atmosphere; (ii) biological and medical influences; and (iii) integrated quantification of the source receptor relationship based on the achievements of the foregoing two studies. Additionally, measuring the characteristics of the source–receptor relationship (SRR) should not be limited to individual regions or countries but considered on an international scale to solve the problems of pollution at regional levels, as well as at transboundary levels, through a socio-economic lens. This is done because the anthropogenic sources are associated with global economic activities and globalization induces the economy–environment linkage, which creates a hidden emission structure. In the first part, we show a general mathematical framework of SRR and review the SRR studies of acid depositions, surface ozone and aerosol concentrations. A further research review is made of the intercontinental SRR of adverse health impacts, such as premature mortalities caused by aerosol or gaseous air pollutant exposure. We also introduce a SRR study of PM2.5 by sectors in source regions of east and south-east Asia. In the second part, we present socio-economic evaluation methods of anthropogenic sources of air pollutants, such as consumption-based accounting of their emissions and emissions embodied in bilateral trade.Applications of multiregional input-output (MRIO) analysis to analyze environmental implications of consumption in terms of CO2, aerosol and other air pollutants are shown in particular. Finally, a consumption-based impact analysis utilizing the combined models of SRR and MRIO is presented.
Volatile organic compounds, VOCs, are emitted from various sources into the atmosphere. Through the reactions of VOCs with atmospheric radicals (eg. daytime OH and nighttime NO3) and all-day O3, the formation of secondary organic aerosols, SOA, is common and therefore an important subject of study. To investigate the mechanisms of reactions in the atmosphere and to control SOAs effectively, it is essential to capture the behavior of emitted VOCs with consideration of their radical reactivity.Meanwhile, organic nitrates (ONs) like alkyl nitrates (ANs) and peroxyalkyl nitrates (PNs) have been focused upon as important intermediates for SOA. To investigate the mechanisms of SOA formation from VOCs, it is essential to capture the behavior of intermediates such as ONs. In this manuscript, tools for measuring (1) total radical reactivity of VOCs and (2) total particulate organic nitrates were developed as applications for present NO2 analyzers.
This study utilized an inertial filter (INF) sampler to evaluate gas adsorption and chemical components in ultrafine particles (Dp < 0.1 μm; PM0.1). They were measured at suburban roadside and background sites under low conditions of PM vaporization and gas adsorption and compared with the components of fine particles (Dp < 2.5 μm; PM2.5). Atmospheric samplings of PM0.1 and PM2.5 were performed in summer and winter with 11.5 h sampling durations to observe the variation of chemical components between daytime and nighttime and to clarify the emission sources and the growth behavior of PM0.1to PM2.5. It was shown that the main components in PM0.1 and PM2.5 were different. Carbonaceous components at both sites accounted for 70 % in PM0.1 and for 40 % in PM2.5 respectively, because ionic components were dominant in PM2.5. The carbonaceous and ionic components near the roadside in PM0.1 had a tendency to be transported to background sites as they expanded via heterogeneous nucleation, especially in winter. Ultrafine particles with a diameter of 20-30 nm and OC1 fraction in OC components were confirmed only in winter, and OC1 fraction indicated higher correlation with soot EC components at both sites. Therefore, it was thought that OC1 fraction was emitted from the diesel exhaust and played an important role in the growth of PM0.1 to PM2.5.
Heterogeneous/multiphase reactions can play an important role in the formation, chemical transforma tion, and loss/removal of organic aerosol components in the atmosphere. In this paper, we review recent laboratory experiments on the ozonolysis of condensed-phase organics and heterogeneous/multiphase reactions involving aldehydes. This review includes studies on the hygroscopicity of condensed-phase organics formed by heterogeneous/multiphase reactions. Further, we present our experimental methods to investigate these two types of reactions, using extracts from atmospheric aerosol samples collected on filters. We emphasize the importance of performing laboratory experiments using ambient aerosol com ponents or organic mixtures with complex compositions (e.g., secondary organic aerosols generated in a chamber) to better understand the significance of these reactions under actual atmospheric conditions, and to find reaction pathways that have been overlooked in previous studies. Field studies may also be useful to assess laboratory-based knowledge and to find new reaction pathways.
The observational studies of the distribution and movement of aerosols using a lidar network and surface observation networks are reviewed in this paper. This study was conducted for a project with the Grant in-Aid for Scientific Research on Innovative Areas entitled “Impacts of Aerosols in East Asia on Plants and Human Health.” Three-dimensional distribution and movement of aerosols were studied in various temporal and spatial scales using the lidar network, a surface monitoring network which includes mountainous areas, the existing air pollution monitoring network, and chemical transport models. The Raman lidar receivers for measuring the extinction coefficient profiles were added to the lidars in six primary stations of the network (AD-Net), and continuous observations were performed. In the intensive observation periods, aerosol sampling was carried out in the mountainous area, and analyses using transmission electron microscope (TEM) and energy dispersive X-ray (EDX) were performed to study the composition and mixing states of aerosols. A long-term data set of the lidar extinction coefficients for dust and air pollution aerosols was constructed and provided for the studies of the effects of aerosols on human health.
In this paper, research studies on the long-range transport of particulate air pollutants such as PM2.5 and Polycyclic Aromatic Hydrocarbons (PAH), which are of great concern with regard to human health, around Japan and the East Asian region are reviewed briefly, with an emphasis on the results of a research project which the authors have been conducting for four years. Year-round measurements of PM2.5 at upwind area, i.e., at remote islands and urban sites in the northern Kyushu area of Japan, since spring 2009, have shown that PM2.5 mass concentration and major chemical composition in the northern Kyushu area, even in such a large city as Fukuoka, are dominated by the influx of regional air pollution from the Asian continent throughout the year. Organic pollutants in the particulate state such as PAH and n-alkenes also appeared to be transported from the Asian continent in spring, fall, and winter. Single particle analysis employing Laser Ionization Single Particle Aerosol Mass Spectrometer (LISPA-MS) revealed the internal mixing state of Pb and other metallic elements such as Zn and Sn in aerosol col lected at Fukue Island in spring, suggesting the substantial contribution of coal and industrial waste burnings to the long-range transported fine aerosols. The knowledge obtained in the series of studies by the authors complemented the previously reported short-term measurements conducted in a number of locations in the middle and northern part of the East China sea, providing a larger picture of the aerosol pollution status in the area and suggesting a direction for the countermeasure.
Three campaigns of aerial observations were carried out by using Goto-Fukue Airport as the base of observations in autumn, winter, and spring, which are the seasons of air pollutant transportation from East Asia. The aerial observation campaigns were made in October 2009 for autumn, in December2010 for winter, and in March 2012 for spring. In the autumn observations three characteristic types of different air mass were observed. One was an air mass affected by sea salts with high concentrations of Na＋ and Cl-. Second was an air mass that had passed above the Korean Peninsula (judged so based on calculated backward trajectories) and, thus, containing anthropogenic emission from Korea with high concentration of SO42- and NH4＋. Third was an air mass with Kosa (Asian Dust) with high concentration of Ca2＋ and metallic elements originating from soil, such as Al and Fe. In the winter observations more Kosa could be observed, again with high concentration of Ca2＋ and metallic elements of soil origin, such as Al and Fe. In this case, SO42- and NH4＋ were also high. In the spring observations, high concentrations of SO2 (higher than 20 ppb) were found, as well as other anthropogenic pollutants within dense haze, which strongly suggested the impact of emissions from East Asia.
Particulate matter (PM) can have adverse effects on human health. The mechanisms of PM formation and behavior in the atmosphere are very complicated. To reduce PM concentrations effectively and attain environmental standards, source–receptor relationships must be clearly understood. Stable isotope ratios can be used to detect and distinguish primary materials, chemical processes, and sources. It has recently become possible to determine stable isotope ratios from small sample volumes with high accuracy and precision by using an isotope ratio mass spectrometer (IRMS) coupled with an elemental analyzer (EA).In environmental science, and especially in research on aerosols, stable isotope ratios are expected to constitute a powerful tool for source identification.This review paper introduces a possible formation mechanism and source apportionment of ammonium and nitrate ions in suspended particulate matter (SPM) using δ15N of ammonium (δ15N-NH4+)， nitrate ions (δ15N-NO3-) of SPM and ammonia (δ15N-NH3), nitrogen dioxide (δ15N-NO2), and nitrate gas(δ15N-HNO3).
Aerial observations were conducted over the East China Sea in October 2009 (autumn), December 2010(winter) and March 2012 (spring). The base airport was Goto Fukue Airport in Fukue Island, Nagasaki, Japan. The flights were performed between Fukue Island and the southern offing of Jeju Island and the flight altitudes were 500, 1000, 2000 and 3000 metres. The concentration ratios of particulate nitrate(NO3-(p)) to inorganic total nitrate (T.NO3) were less than 0.5 in most of the flights except under high concentrations of dust particles (Kosa) or trans-boundary air pollutants. Most of the NO3-(p) would be comprised of NaNO3 formed by the reaction of gaseous nitric acid (HNO3(g)) with sea salt aerosols during the observations in autumn and winter, with the exceptions of October 17 and December 11, when high concentrations of Kosa were transported. In the spring observation, the fraction of NaNO3 in NO3-(p) was low and a large part of NO3-(p) would be originated from reactions of HNO3(g) with gas phase ammonia and soil dust particles.
Aerosol retrieval work from satellite data is divided into three parts: satellite data analysis, aerosol modeling, and multiple light scattering simulations in the atmosphere modeling what is known as the radiative transfer problem. It is well known that the aerosol model has been developed using the accumulated measurements during more than ten years provided with the world-wide aerosol monitoring network (AERONET). The radiative transfer simulations incorporate the Rayleigh scattering by molecules and Mie scattering by aerosols in the atmosphere. Thus the aerosol properties are estimated by comparing satellite measurements with the numerical values of radiation simulations in the Earth-atmosphere model. It is reasonable to consider that the precise simulation of multiple light scattering processes is necessary, and requires a long computational time, especially for an optically thick atmosphere. Therefore, efficient algorithms for radiative transfer problems are indispensable to retrieve aerosols from space. It is shown here that dense aerosol events can be well simulated by a semi infinite radiative-transfer model. For an example, the air pollution event observed by Aqua/MODIS in June of 2010 over Beijing is interpreted based on the proposed aerosol models and the semi-infinite radiative transfer simulation code known as MSOS (Method of Successive Order of Scattering).
Atmospheric polycyclic aromatic hydrocarbons (PAHs), accounting for most (35–82%) of the total mutagenic activity of ambient aerosols, originate mostly from the incomplete combustion of carbon based fuels. Hence, reduction of PAHs emissions into the atmosphere is essential for effective air quality control, which requires reliable source apportionment. In certain situations molecular fingerprinting and/or stable carbon isotope ratios (δ13C) of individual PAHs can provide insight about the contributions from specific sources. Unfortunately, the broad utility of both approaches is limited to source apportionment between combustion sources. It is because deviations in source end-member values, arising both from carbon source variation and from differences in the conditions of PAHs formation, sometimes exceed differences between source end-member values. The 5730 year half-life of 14C makes it an ideal tracer for identifying combustion products derived from fossil fuels (14C-free; Δ14C＝－1000‰) versus those from modern biomass (contemporary 14C; Δ14C＞0‰). The recently developed preparative capillary gas chromatograph (PCGC) system and micro-scale 14C analysis by accelerator mass spectrometry (AMS) made compound specific radiocarbon analysis (CSRA) applicable to organic compounds existing in environmental samples. This review introduces recent progress in development of ultra micro-scale14C AMS analyses and compiles results from PAHs source apportionment studies using CSRA and/or compound class-specific radiocarbon analysis (CCSRA).
To clarify the effect of aerosols as trans-boundary air pollutants on the forest trees of East Asia, we developed methods of visualization of submicron-sized aerosols that were artificially deposited onto leaves or needles to show their localizations and distributions. Firstly, the leaves were artificially exposed to gold (Au) particles as model particles at submicron range. After exposure (deposition of aerosols), we visualized gold particles on the surface of the leaves using field-emission scanning electron microscopy (FE-SEM) and energy dispersive X-ray spectrum (EDX). Secondly, depositions of black carbon aerosol particles onto the adaxial and abaxial surfaces of the leaves or needles of four species of woody plants were examined after exposure to black carbon particles at submicron range. The localization and deposition patterns of black carbon particles differed depending on the type of exposure apparatus and the surface structure of the leaves or needles. Therefore, we concluded that these methods are useful for understanding the mechanisms of the effects of submicron-sized aerosols on the growth and physiological response of trees.
Since the 1990s, trans-boundary air pollution induced by aerosols such as black carbon particles has been one of several serious environmental problems in East Asia. However, there is very limited information on the effects of aerosols on forest tree species. In this review paper, we survey the chemical and physical effects of aerosols such as dust on plants based on results from previous field surveys and experimental studies, and give an outline of our experimental study on the effects of submicron-sized black carbon particles on the growth, foliar gas exchange rates, and amounts of particle deposition on the foliar surface of four Japanese forest tree species and its species difference.
Recent research on aerosol dry deposition in forests in East Asia, focusing particularly on field measurements, estimations of deposition rates, and developments of measurement technique, was reviewed to contribute assessments of the impacts of aerosols on vegetation. Some measurements of aerosol deposition by the gradient method indicated that the deposition velocities were in agreement with other experimental results in North America and Europe, and were higher than those calculated by theoretical models. Higher deposition velocities were associated with larger friction velocities and unstable conditions. The deposition velocities were possibly influenced by humidity, due to the growth in size of hygroscopic aerosols and the enhancement of surface collection efficiency. Some estimations of aerosol deposition rates indicated i) high ratios of sulfate to SO2 in deposition rates in remote areas of Japan, ii) high dry deposition of elemental carbon on a tropical forest, and iii) about 10~20% year to-year variations in aerosol deposition, with a higher percentage variation in tropical areas. Regarding aerosol deposition on forest leaves, there was larger deposition in higher parts than in lower parts, and approximately 70% of total aerosol deposition onto forests probably deposit on forest leaves during the leafy season. It is indicated that the relaxed eddy accumulation method could obtain more accurate data on the fluxes of aerosol components than data obtained with the gradient method.
This paper gives overviews of a sophisticated one-dimensional land surface model (SOLVEG) and a new particle dry deposition scheme for chemical transport models (CTMs) developed by the authors for assessment of the effects of aerosols on vegetation. The current major particle dry deposition schemes for CTMs are reviewed to examine the issue of less prediction accuracy in deposition velocity of sub-micron particles. The authors’ new particle dry deposition scheme overcomes the issue by including state-of-the-art knowledge based on SOLVEG simulations and providing better estimations of deposition velocity in this size range. As an example of regional scale analysis using this new scheme, simulation results of atmospheric black carbon (BC) onto forest areas in Eastern Asia are demonstrated.Our studies presented in this paper show that the land surface model developed by the authors is effective to evaluate the impacts of aerosols on the terrestrial biosphere.
To clarify the effects of aerosols on trees in cool-temperate forests in Hokkaido, the concentrations of atmospheric aerosols were measured using the filter pack method from spring to autumn in eastern Hokkaido. The components of fog water collected by an active string fog collector and the diameters of fog droplets measured via fog monitor were analyzed in the same area. The concentrations of SO42- and NH4＋ in aerosols were highest during the growing period of forest trees in eastern Hokkaido. These ions were also the main components of fog water in eastern Hokkaido. The mean diameter of fog droplets was larger than that of other areas. This suggests that fog deposition velocity would increase in this area.In addition, the shape and components of aerosols deposited on the surfaces of leaves of white birch trees were observed by SEM-EDX. Soil particles and particles including S, and NaCl particles were found. The results of washing the leaf surfaces of white birch leaves showed that it is possible that many of the particles deposited on leaf surfaces can be washed off with raindrops.
In this article, we give an overview of East Asian epidemiological studies, including part P12 of Grant in-Aid for Scientific Research in Innovative Areas: “Impacts of Aerosols in East Asia on Plants and Human Health.” In particular, we focus on “health effects of East and Southeast Asian air pollution.” The body of research surveyed can be divided into studies on short-term effects and long-term effects.The short-term effect studies can further be divided into studies on local effects and trans-boundary effects. We reviewed research on Japanese cedar pollen diseases and particulate matter for short-term local effects, and Asian dust and particulate matter for trans-boundary effects. For long-term effects, we reviewed critically some research that utilized existing cohort studies.
This paper reviews the effects of chemicals such as polycyclic aromatic hydrocarbons (PAHs) and their derivatives, which are major chemical components in particulate matter (PM), on immune cells. First, we introduce some biomarkers for the health effects of PM. We also show that cytotoxicity and/or inflammatory responses were induced by some PAH derivatives containing hydroxyl, carbonyl, nitro, or amino groups, indicating that the toxicological and inflammatory effects of chemicals may be related to their structures, such as functional group and isomer.
This paper reports on the absorption of environmental pollutants into various airborne pollen grains, aggravation of allergenicity, damage to the pollen cell walls, and the airborne behavior of fine allergenic particles, especially those released from Cryptomeria japonica (Japanese cedar) pollen, as abundantly determined on sunny days following rainfall. From the result of rainfall sampling and analysis, it was indicated that a great number of pollens were trapped in initial precipitation. At the same time, many burst pollen grains were also observed in the rainwater. Thus, it was possible that fine allergenic particles such as fractions of cell wall and contents of pollen were released from the bursting of pollen grains. On the other hand, elution of allergenic contents was increased when contacted with a weakly basic solution containing Ca2＋ ions. Therefore, we think that one important factor contributing to the small-sized allergenic particles was induced by contact with rainfall after long-range transportation events of Asian dust. We succeeded for the first time in detecting fine allergenic contents containing 3-nitrotyrosine in urban atmosphere and clarified that 3-nitrotyrosine induces HeLa cell apoptosis. We summarized the various studies to elucidate the scattering behavior of various pollens, focusing on Cryptomeria japonica pollen and its allergenic particles in the urban atmosphere. This study has the potential for future developments in palynology.
The underlying risk factors for stroke, such as hypertension, diabetes, dyslipidemia, and atrial fibrillation, have been established. However, the factors that trigger stroke in patients at high risk are still uncertain. Recent epidemiological studies have documented the possibility that Asian dust may be related to diseases and health conditions. Recently, utilizing data from the Fukuoka Stroke Registry(FSR), we have shown that Asian dust exposure is associated with the incidence of atherothrombotic brain infarction, a subtype of ischemic stroke. Several studies have reported small but statistically significant associations between short-term particulate matter (PM) exposure and incidence of ischemic stroke. Several biological pathways responsible for PM-associated cardiovascular diseases have been explored, including endothelial dysfunction, production of oxygen species, hypercoagulability, induction of arrhythmia, and systemic inflammation. Because the concentrations of PM were higher in the periods with Asian dust than those in the periods without Asian dust, PM may contribute, at least in part, to the occurrence of atherothrombotic infarction in response to Asian dust exposure. Further studies are needed to elucidate the precise mechanisms.
The frequency and scale of Asian dust events have increased rapidly in East Asia since 2000. In connection with this, the effects of Asian dust (kosa) on human health, especially on allergic diseases, are major concern in Japan. We herein discuss the effects of kosa on allergic diseases, including asthma, chronic cough and Japanese cedar pollinosis. Epidemiological studies, as well as experimental studies, have demonstrated the association between kosa and the exacerbation of asthma and allergic diseases.The kosa particles increase airway inflammation as one of the major sources of atmospheric particulate matter. Furthermore the kosa particles absorb various atmospheric gases, including air pollution. Such environmental pollution enhances the response to allergens, including Japanese cedar pollen. Recently, some epidemiological studies used the kosa data obtained by the light detection and ranging (LIDAR) system, which distinguish between mineral dust and other spherical particles, by identifying differences in the shape of the particles. Further studies using the LIDAR system will help to identify the kosa aerosol components that have adverse health effects, leading to provide new strategies to prevent environmentally induced allergic diseases.
A non-destructive simultaneous analytical method for multi-elements in aerosols using the latest type of energy dispersive X-ray fluorescence spectrometry (EDXRF) has been developed recently. In this paper, a couple of previous studies regarding aerosol analysis achieved using the latest EDXRF are reviewed. Generally, EDXRF with a fundamental parameter (FP) quantification technique gave results similar to those obtained by acid digestion/ICP-MS analysis. Moreover, the EDXRF-FP quantification was found to be independent of its collection site, particle size, and filter type. This paper also reports the preliminary results of the indoor/outdoor ratios for the concentrations of aerosols and their chemical composition as obtained by EDXRF. No significant differences between indoor and outdoor PM2.5concentrations were observed. It was also found that the I/O ratio for each element was often different from that for aerosol concentration. EDXRF would be suitable for application to epidemiological studies since it could analyze enormous number of samples to obtain chemical compositions of aerosols.
There is increasing concern for possible adverse health effects of Asian dust. In this paper, we reviewed the epidemiological evidence of potential effects of Asian dust events on mortality, from six studies retrieved from PubMed. In addition, one study was identified through a manual search of conference proceedings and reports. Three studies were conducted in Seoul, two in Taipei, and the remaining two in Japan. In Seoul, statistically significant effects of Asian dust on all-cause and circulatory mortality among people 65 years of age or over and respiratory mortality among general population were reported.In Taipei, one study reported a significant effect on cardiovascular mortality and another study reported no significant effect on all-cause and cause-specific mortality. A study of 47 cities in western Japan reported a significant increase in circulatory and respiratory mortality with the increase in Asian dust particles, while no evidence of effects on mortality was reported in the study conducted in Nagasaki.
Exposure assessment is an essential part of epidemiological studies evaluating the health effects of atmospheric environments. Asian dust exposure has been assessed by using 1) particulate matter concentration, 2) visibility, and 3) extinction coefficients for non-spherical particles measured by light detection and ranging (LIDAR). We evaluated the association between Asian dust exposure and emergency ambulance dispatches using a LIDAR-derived extinction coefficient for nonspherical particles in Nagasaki. In this study, we examined how the altitude and the cut-off values of extinction coefficient to determine that the Asian dust exposure influenced the magnitude of health effects estimated to result from Asian dust exposure. We found an increasing risk of emergency ambulance dispatches for Asian dust exposure at and above the cut-off level of 0.07/km dust extinction coefficient.The effect estimates were greater when dust extinction coefficients at a lower altitude were used.