Atmospheric Environment (v.35, #30)
List of Forthcoming Papers (I-II).
Aerosol radiative forcing for Asian continental outflow by S Kinne; R Pueschel (5019-5028).
Asian aerosols in elevated layers over the Pacific Ocean were sampled with NASA wire-impactors and a FSSP optical particle spectrometer-probe aboard the NASA DC-8 aircraft in early March 1994. Strong variations in aerosol properties, primarily aerosol concentration, lead to derived mid-visible extinctions between 0.003 and 0.5/km. FSSP data usually identified two size-modes. The larger ‘coarse mode’ (radii of 1–3 μm) was assumed to be dust. The composition of the smaller ‘accumulation mode’ (radii of 0.1–0.3 μm) was based on the analysis of the wire-impactor samples, as significant amounts of soot reduce mid-visible single scattering albedos to the 0.87–0.92 range.Radiative forcing simulations investigated the impact of Asian outflow aerosol on atmospheric radiative fluxes and heating rates. Only events with larger optical depths were important. In those events the solar attenuation of the smaller size mode dominated the net-flux losses at the surface, with values similar those of urban-polluted and/or biomass burning aerosol types (as observed during the TARFOX and INDOEX field experiments). In contrast, changes to net-fluxes at the top of the atmosphere (ToA) for outflow cases are less negative—primarily due to the added greenhouse effect of the dust component. For the climate of the Earth-Atmosphere-System, ToA net-flux losses are considered a cooling, ToA net-flux gains are associated with warming. Weak cooling is determined for the Asian outflow cases under cloud-free conditions. The addition of a reported 50% cloud cover below the aerosol layer causes a switch to slight warming.
Keywords: Aerosol; Radiative forcing; Dust; Asia; Aircraft samples;
Abrupt change of the mid-summer climate in central east China by the influence of atmospheric pollution by Qun Xu (5029-5040).
Following the great flooding of summer 1998, the mid-lower Yangtze Basin further suffered from another large flooding in summer 1999. Successive droughts through 3 recent summers (1997–1999) appeared in north China in addition, leading to an abnormal summer climate pattern of “north drought with south flooding”. Such southward move of the summer monsoon rainy belt in east China started in the late 1970s–early 1980s. Its main cause may not be a purely natural climate change, but the acceleration of industrialization in east China could play a major role by emitting large volumes of SO2, especially from the rapidly growing rural factories of east China. The annual release of SO2 in China exceeded 20 Tg during 1992–1998, so dense sulfate aerosols covered the central east China which significantly reduced the sunlight. Although present estimates for the changes of clear sky global solar radiation may include some error, they show that the negative radiative forcing of sulfate aerosols in central east China by far exceeds the effect of greenhouse warming in summer. Hence the mid-summer monsoon rainy belt of east China has a trend moving southward in 21 recent years (1979–1999), showing the very sensitive characteristic of the summer monsoon system to the change in heat equilibrium of the land surface. The occurrence rate of summer climate pattern of “north drought with south flooding” in east China during 21 recent years is the largest since AD 950; such anomalous climate has brought large losses to China. The only possible way to reverse this southward trend of summer monsoon rainy belt is to significantly reduce air pollution by using more clean energy. Recently, the PRC has paid serious attention to this problem by adopting a series of countermeasures.
Keywords: Summer monsoon rainy belt; Southward retreat; Increasing amount of sulfate aerosol; Enterprises outside cities; Negative radiative forcing;
A case study of the impact of boundary layer aerosol size distribution on the surface UV irradiance by Ülle Kikas; Aivo Reinart; Mai Vaht; Uno Veismann (5041-5051).
The relationship between scattering characteristics of surface aerosol and surface UV irradiance was examined on the basis of the measurements carried out in June–August 1999 in Pärnu, Estonia on the Eastern coast of the Baltic Sea (58°22′27″N, 24°30′43″E). The UV radiation spectra (300–340 nm) were measured with the Ocean Optics Inc. UV spectrometer PC1000, the aerosol size distributions (3–10 000 nm) were measured with the electric aerosol spectrometer EAS.A case study was conducted for six sequential cloudless days, when the decrease of the surface UV irradiance was seemingly influenced by atmospheric aerosol. Aerosol radiative properties were calculated from the measured size distributions that represented the maritime polar (North Atlantic) and mixed maritime–continental air. The aerosol optical depths at 500 nm for the North Atlantic air were estimated to be from 0.08 to 0.13. The spectral aerosol optical depth agreed well with the Ångström law, the Ångström exponent α varied from day to day between values of 0.52–0.90. Aerosol asymmetry factor at 300 nm changed between values of 0.76 and 0.80, and was highly correlated with the mean radius of aerosol number distribution. The total aerosol UV scattering was mostly influenced by changes in aerosol with a diameter of 100–560 nm. The aerosol scattering coefficients were positively correlated with the relative humidity of air.The ground aerosol properties were used for calculating the surface UV irradiance from the radiative transfer model of Bird and Riordan (J. Climate Appl. Meteorol. 25 (1986)). The calculated UV irradiances correlated quite well with the measured ones, showing that the use of ground aerosol data for radiative transfer calculations turned out reasonable results. However, ignoring the changes in the aerosol vertical distribution resulted in overestimation of aerosol optical depth on hazy days.
Keywords: Surface UV irradiance; Aerosol scattering; Radiative properties of aerosol;
On the relation between the size and chemical composition of aerosol particles and their optical properties by A Molnár; E Mészáros (5053-5058).
The light scattering and absorption coefficients of fine atmospheric aerosol particles were recorded in Hungary under rural conditions in 1998–1999 by an integrating nephelometer and particle soot absorption photometer, respectively. In some cases optical properties in the fine size range were compared to those in the coarse particles. Results obtained indicate, as expected, that fine particles control the scattering and absorption caused by the aerosol. In 1999 the size distribution of aerosol particles was also monitored by means of an electric low pressure impactor (ELPI). This makes it possible the study of the relationship between the number, surface and mass concentration in the size range of 0.1–1.0 μm and the optical characteristics by also considering the chemical composition of the particles.
Processes of long-term relaxation of stratospheric aerosol layer in Northern Hemisphere midlatitudes after a powerful volcanic eruption by V.V Zuev; V.D Burlakov; A.V El’nikov; A.P Ivanov; A.P Chaikovskii; V.N Shcherbakov (5059-5066).
Multiyear lidar measurements of characteristics of stratospheric aerosol layer, made at midlatitude observatories in Tomsk (56.5°N, 85.0°E) and Minsk (53.9°N, 27.5°E), are analyzed and used to study the processes of long-term relaxation of the aerosol-perturbed stratosphere after powerful volcanic eruptions to background state. The absence of significant seasonal variations of vertical stratification of stratospheric aerosol and exponential altitudinal decrease of aerosol backscattering coefficient are proposed as criteria of background state of stratospheric aerosol layer for Northern Hemisphere midlatitudes.
Keywords: Stratosphere; Aerosol backscattering coefficient; Lidar; Scattering ratio;
Evolution of stratospheric aerosols in the post-Pinatubo period measured by solar occultation by Didier Fussen; Filip Vanhellemont; Christine Bingen (5067-5078).
This paper presents particle size distributions of stratospheric aerosols derived from solar occultation data measured by the instrument ORA (an acronym for Occultation RAdiometer) during the period August 1992–May 1993. Starting from the UV-visible wavelength dependence of extinction coefficient profiles, an algorithm is developed that allows to retrieve the three parameters of an equivalent log-normal distribution and that makes use of vertical regularization. Comparison of retrieved mode radius and particle number density with existing data is found to be satisfactory. The evolution of the stratospheric aerosols is clearly influenced by sedimentation and coagulation as expected but also by vertical circulation. In a simple 1-D model, we derive the vertical wind profile and we interpret the temporal evolution of the particle mode radius.
Keywords: Aerosols; Stratosphere; Pinatubo;
Aerosol maps from GOME data by Rodolfo Guzzi; Giovanni Ballista; Walter Di Nicolantonio; Elisa Carboni (5079-5091).
In this paper, we present a methodology to calibrate the surface reflectance seen by satellite and validate the aerosol optical properties retrieved by the GOME instrument. Data are also visualized in maps by a tool properly developed, named GOMEView. The validation procedure is based on ground measurements obtained by sunphotometers. Results show that calibration of the surface reflectance is crucial to obtain the best results, i.e. in agreement with the ground measurements. Aerosol data have also been classified on the basis of their optical properties evidencing for instance, the presence of desert aerosol over the sea along the west coast of Sahara. Cloud retrievals were also analyzed in terms of their occurrence and amount.
Keywords: Aerosol optical depth; Atmospheric radiative transfer; Earth observation; Remote-sensing calibration; Sea reflectance;
A preliminary study on the correlation between TOMS aerosol index and ground-based measured aerosol optical depth by Francesco Esposito; Giulia Pavese; Carmine Serio (5093-5098).
The aim of this work is to study the correlation between ground-based measured aerosol optical depth (AOD) and TOMS Aerosol Index. For this reason, two AOD data-sets have been analysed. The first set of measurements has been obtained in a desert plateau in Namibia during July 1998, while the second one has been collected in Tito Scalo (Italy), a very small industrial zone surrounded by a large rural area, in June–July 2000. The AOD has been computed in the spectral range 400–870 nm with a resolution of 3 nm by measuring the direct solar irradiance. The used spectroradiometer is an Optical Spectrum Analyser, equipped with a continuously rotating diffraction grating. Successively, a correlation between the Earth Probe TOMS Aerosol Index, whose definition uses backscattered radiances at 331 and 360 nm, and the AOD in the visible range was searched for. A satisfying correlation was found, whose Pearson correlation coefficient R 2 values range from 0.64 to 0.91.
Keywords: Spectrophotoradiometer; Desert aerosol; Anthropogenic aerosol;
Retrieval of the aerosol size distribution in the complex anomalous diffraction approximation by Ghislain R Franssens (5099-5104).
This contribution reports some recently achieved results in aerosol size distribution retrieval in the complex anomalous diffraction approximation (ADA) to MIE scattering theory. This approximation is valid for spherical particles that are large compared to the wavelength and have a refractive index close to 1.The ADA kernel is compared with the exact MIE kernel. Despite being a simple approximation, the ADA seems to have some practical value for the retrieval of the larger modes of tropospheric and lower stratospheric aerosols.The ADA has the advantage over MIE theory that an analytic inversion of the associated Fredholm integral equation becomes possible. In addition, spectral inversion in the ADA can be formulated as a well-posed problem. In this way, a new inverse formula was obtained, which allows the direct computation of the size distribution as an integral over the spectral extinction function. This formula is valid for particles that both scatter and absorb light and it also takes the spectral dispersion of the refractive index into account.Some details of the numerical implementation of the inverse formula are illustrated using a modified gamma test distribution. Special attention is given to the integration of spectrally truncated discrete extinction data with errors.
Keywords: Aerosols; Size distribution; Spectral inversion; Anomalous diffraction approximation;
On applicability of model aerosol distributions for urban region of Bratislava city by Miroslav Kocifaj; Igor Kohút; Pavol Zaujec (5105-5115).
Long-term measurements of spectral atmospheric transparency are analysed to describe the aerosol size distribution as well as the aerosol optical thickness in the urban region of Bratislava city, capital of Slovak Republic. Aerosol characteristics are related to the most frequent air masses, especially to the continental polar (cP—with a 54% occurrence) and maritime polar (mP—with 34% occurrence), to the wind direction and speed, as well as to the relative humidity. Including both random and systematic errors of the observations into the calculation procedures, the aerosol optical thickness is obtained with approximately 4% error at all wavelengths. Averaged values of the aerosol optical thickness τ a(λ) at reference wavelength λ=520 nm vary over a wide range, from 0.1 to 0.7. Besides, the aerosol optical thickness of the continental polar air mass is obviously higher than corresponding values in the maritime polar air mass. It is shown that the transformation inside the air mass reflects the changes of the optical characteristics of aerosols, especially during decay of air mass. The function τ a(λ) seems to be monomodal in the majority of cases, with the mode position about λ≈400 nm for cP, and λ≈500 nm for mP. A value of power parameter δ of the function τ a(λ)≈λ −δ is about 0.8–1.6 for maritime polar and about 0.3–1.2 for continental polar. Two simple model functions (Junge and gamma) are examined to find a best fit of real distribution retrieved from the aerosol optical thickness data using the inverse techniques based on Mellin transform. The gamma function much better than Junge's function supply the real aerosol component of all studied air masses (mainly for cP and mP). The average modal radius of gamma distribution practically does not exceed the value of 0.06 μm. Real distributions retrieved using a Mellin transform give an averaged morning value of particle modal radius r m about 0.084 μm, and averaged daily value r m about 0.054 μm.
Keywords: Aerosol size distribution; Solar radiation; Inverse problems; Aerosol optical thickness; Sky radiance;
A quantitative comparison of α-Å turbidity parameter retrieved in different spectral ranges based on spectroradiometer solar radiation measurements by V.E. Cachorro; R. Vergaz; A.M. de Frutos (5117-5124).
Spectroradiometric direct irradiance measurements in the 300–1100 nm wavelength range with a spectral resolution of 6.2 nm have been used in a study of the variation in the Ångström turbidity parameter α and its dependence on the spectral range used in its determination. The measurements have been carried out under clear sky conditions at two different climate stations in Spain. Least-square fits of the experimental spectral aerosol optical depth (AOD) to the Ångström formula in different spectral ranges, selected for convenience depending on the objective or application (e.g., UV–VIS (350–400 nm), VIS (400–670 nm), VIS–NIR (370–870 nm), etc.), result in different sets for the α parameter. Due to this dependence on the spectral range, where the α-values are determined, a quantitative comparative analysis is carried out in order to assess the differences for a given data-base covering very different atmospheric conditions. The study reveals the necessity of a ‘standard spectral range’ to achieve confident data comparisons. We show some applications that are relevant for aerosol studies, from UV absorption by aerosols to satellite remote sensing.
Keywords: Aerosols; Optical depth; Ångström parameters; Wavelength dependence;
Multilayer four-flux model of scattering, emitting and absorbing media by Claude Rozé; Thierry Girasole; Anne-Gaelle Tafforin (5125-5130).
Four-flux model allows to compute diffuse and collimated flux through a slab containing absorbing and scattering particles in an absorbing medium. An extension of this model is proposed so that the slab can be composed of an arbitrary number of layers. Moreover, emission of the particles and of the surrounding matrix is taken into account.
Keywords: Multiple scattering; Thermal radiation; Emission; Absorption;
Waves in air impurities and their influence on atmospheric optical properties by O.G. Khoutorova; D.N. Douryagin; A.A. Vasilyev; G.E. Korchagin (5131-5134).
A three years time series of data of gas concentrations and aerosol measurements have been obtained by five automatic stations separated by distances between 1 and 6 km. The data were analyzed statistically with respect to periodicity and wave structure of the pollutants/gases. The gas and aerosol concentrations as well as the meteorological parameters have the same spectra in the bottom troposphere. The periods of waves vary from 5 min up to 6 h and their spatial sizes vary from 1 up to 50 km. The influence of wave disturbances on the electromagnetic waves propagation is discussed.
Keywords: Wave processes; Atmospheric aerosol;
Black carbon (BC) in alpine aerosols and cloud water—concentrations and scavenging efficiencies by R. Hitzenberger; A. Berner; H. Giebl; K. Drobesch; A. Kasper-Giebl; M. Loeflund; H. Urban; H. Puxbaum (5135-5141).
During April 1999 and March 2000, intensive field campaigns were performed on a mid-level mountain (Rax, 1644 m a.s.l.) in Central Europe both under out-of-cloud and in-cloud conditions. The black carbon (BC) content of both aerosol and cloud water as well as BC scavenging efficiencies of Rax clouds were measured. As a tracer for the non-carbonaceous aerosol, sulfate was used. Although BC concentrations on Rax were low (April 1999 out-of-cloud average: 0.43 μg/m3, March 2000: 0.72 μg/m3), the BC mass fraction of the aerosol was fairly high (1999: 3.5%, 2000: 6.4%). Average BC concentrations in cloud water were 1.09 μg/ml (1999) and 1.4 μg/ml (2000). These values are far higher than literature values, but comparable to those found in an earlier study (J. Geophys. Res. 105 (D20) (2000) 24637) at a high-level mountain (Sonnblick, 3106 m a.s.l.) some 200 km distant from Rax. The average BC scavenging efficiency of the Rax clouds in March 2000 was 0.54. The increase of scavenging efficiency with increasing liquid water content of the clouds found earlier on Sonnblick for sulfate and aerosol carbon (J. Atmos. Chem 35 (2000) 33), organic carbon (J. Geophys. Res. 105 (2000) 19857), and BC (J. Geophys. Res. 105 (D20) (2000) 24637) was also confirmed on Rax.
Keywords: In-cloud black carbon (BC); Scavenging efficiency; Aerosol cloud interaction;
Distant contrast measurements through fog and thick haze by M. Gazzi; T. Georgiadis; V. Vicentini (5143-5149).
Koschmieder's contrast reduction law has been verified in medium visibility conditions by means of distant contrast measurements of two or more black targets placed at different distances from a telephotometer, as well as by comparing the contrast with transparency measurements. On the contrary, in foggy conditions the measurement results do not obey the law and markedly disagree with transmissometer readings. By reexamining Koschmieder's equations in the light of Foitzik's suggestion about the dependence of a target apparent luminance in fog on its geometrical dimensions, the following results are obtained: (i) contrast reduction law does not hold in fog because the target apparent luminance depends on the droplet size distribution, and (ii) this fact is entirely due to the truncated integration over the scattering angle of the air luminance gradient in the view direction because of the target's finite dimensions. These statements are confirmed by field experiments’ results.
Keywords: Apparent luminance; Visibility; Telephotometer; Stray light; Drop size distribution;
Apparent anomalous extinction in fog by M Gazzi; T Georgiadis; V Vicentini (5151-5156).
Experimental observations of anomalous extinction in rural fogs by means of trichromatic distant contrasts are explained in terms of Foitzik's suggestion about the loss of forward scattered daylight intercepted by the target.
Keywords: Luminance contrast; Telephotometer; Light scattering; Mist; Drop size distribution;
Visibility impairment during Yellow Sand periods in the urban atmosphere of Kwangju, Korea by Kyung W Kim; Young J Kim; Seung J Oh (5157-5167).
For continuous monitoring of atmospheric visibility in the city of Kwangju, Korea, a transmissometer system consisting of a transmitter and a receiver was installed at a distance of 1.91 km across the downtown Kwangju, Korea. At the transmitter site a nephelometer and an aethalometer were also installed to measure the scattering and absorption coefficients of the atmosphere, respectively. Unusually high number of Yellow Sand events had occurred in the Northeast Asia during the spring of 2000. Visibility in Kwangju under such conditions was greatly impaired over large area for a few days. In order to investigate the effects of Yellow Sand on visibility impairment, chemical and elemental analyses of aerosol samples were performed along with the optical measurement of visibility. Hourly averaged visual range decreased from 61.7 to 1.9 km when hourly averaged concentration of PM10 varied from 32.9 to 601.8 μg/m3 during Yellow Sand periods. Fine particulate (<2.5 μm) concentrations were relatively lower than coarse particulate matters. Results of the data analyses show that mineral dusts originated from continental sources were simultaneously transported along with anthropogenic sulfate aerosols and marine aerosols. Total light extinction coefficient, b ext, proposed by the IMPROVE network showed poor correlation with b ext measured by transmissometer. Coarse mass scattering efficiency was classified into three categories; E NHSOc, E mineral, and E sea-salt, which were determined as ammonium sulfate combined with nss-sulfate of 1.0, sea-salt of 0.4, and mineral of 0.77 m2/g, respectively. Mass fraction of NHSOc, sea-salt, and mineral dust was 0.20, 0.14, and 0.66, respectively.
Keywords: Yellow Sand; Visibility impairment; Light extinction equation; Mineral dust; Nss-sulfate aerosol; IMPROVE program;
Aerosol light scattering measurements as a function of relative humidity: a comparison between measurements made at three different sites by Derek E Day; William C Malm (5169-5176).
The water uptake by fine aerosol particles in the atmosphere has been investigated at three rural National Parks in the United States (Great Smoky Mountains, Grand Canyon and Big Bend National Parks). The relative humidity (RH) of sample aerosols was varied from less than 20% to greater than 90% using Perma Pure drying tubes as the scattering coefficient of the aerosol was measured with a Radiance Research M903 nephelometer. Data from these studies show that growth curves at all the three sites are similar in shape but the magnitude of growth can vary considerably from day to day. The growth curves from Great Smoky Mountains show smooth continuous growth over the entire range of RH, while the growth curves from the Grand Canyon and Big Bend show smooth and continuous growth on some days and deliquescence on other days. Comparing 12-h filter samples of chemical composition data with the aerosol growth curves, we find that higher fractions of soluble inorganic compounds (sulfate and nitrate) produce growth curves of greater magnitude than do higher concentrations of either organic carbon or soil material.
Keywords: Hygroscopic aerosol; Scattering coefficient; Deliquescence;
Diurnal and seasonal patterns in light scattering, extinction, and relative humidity by Kristi A. Gebhart; Scott Copeland; William C. Malm (5177-5191).
Since 1988, several federal and state governmental agencies in the US have coordinated efforts to operate the interagency monitoring of protected visual environments (IMPROVE) network at sites in remote areas. Most IMPROVE sites are equipped with either a transmissometer to measure light extinction (B ext) or a nephelometer to measure particle scattering (B sp). Optical, temperature, and relative humidity (RH) measurements are made hourly at these sites. The diurnal and seasonal patterns in these data are examined and discussed here. At many IMPROVE sites the diurnal patterns in RH and therefore B ext or B sp are as expected based on average temperature. On average, RH is higher at night and during the winter than during warmer times of the day and year. Also as expected, based on RH alone, at many sites hourly mean B ext or B sp values are either in phase with RH or weakly dependent on time of day. Usually, the diurnal differences are not as large as the seasonal differences. Another group of IMPROVE sites have mean RH patterns similar to those described above but have a different diurnal pattern in measured scattering or extinction. At these sites, the highest mean B sp or B ext occurs during mid-day rather than at night. At several of these sites, especially those on ridge tops, it is hypothesized that this is because the diurnal shifts in mixing height only allow the surface layer of the atmosphere to reach the monitor during mid-day. Several other sites have unique diurnal or seasonal patterns in average B sp or B ext that can usually be linked to emissions in nearby source regions or local meteorology and terrain.
Keywords: IMPROVE network; Diurnal patterns; Light extinction; Scattering; Seasonal trends;
Chemical species’ contributions to the upper extremes of aerosol fine mass by Rodger B Ames; William C Malm (5193-5204).
Frequency distributions of the major chemical components of aerosol fine mass are shown to illustrate the respective species’ contributions to the range of observed fine particle mass concentration. The magnitude of a species’ contribution to the upper extremes of aerosol fine mass is relevant to control scenarios that seek to improve worst day fine particle conditions, or in many cases worst day visibility. We summarize the relative contributions of fine particle sulfate, nitrate, carbon, and soil plus sea salt to the upper extremes of aerosol fine mass based on Interagency Monitoring of PROtected Visual Environments (IMPROVE) data collected at monitoring locations across the United States during 1995 through 1999. The data show that the spatial pattern of a given chemical species’ contribution to the upper extremes of aerosol fine mass is often quite different than at lower fine mass concentrations. In some cases, the monitoring data suggest a casual relationship between specific aerosol source regions and the magnitude in which a species’ contribution to the upper extremes of fine mass is elevated above the contribution to median fine mass concentrations.
Keywords: IMPROVE network; Visibility; Class I area; Fine particles; Frequency distribution;
Haze trends over the United States, 1980–1995 by Bret A Schichtel; Rudolf B Husar; Stefan R Falke; William E Wilson (5205-5210).
The patterns and trends of haze over the United States for the period of 1980–1995 are presented. Haze measurements are based on human visual range observations at 298 synoptic meteorological stations operated by the United States Weather Service. There was a significant (∼10%) decline in haziness over the 15-yr period. The reductions were evident throughout the eastern United States as well as over the hazy air basins of California. During the same period, in the eastern United States sulfur emissions also declined by about 10%. However, a causality for the reductions has not been established. This report is an update of an earlier survey of haze patterns and trends from 1950 to 1980.
Keywords: Visibility; Light extinction; Trends; Eastern United States; SO2 emissions;
Linear trend analysis: a comparison of methods by Ann Hess; Hari Iyer; William Malm (5211-5222).
In this paper, we present an overview of statistical approaches available for detecting and estimating linear trends in environmental data. We evaluate seven methods of trend detection and make recommendations based on a simulation study. We also illustrate the methods using real data.
Assessment of metal concentrations in atmospheric particles from Burnaby Lake, British Columbia, Canada by R Brewer; W Belzer (5223-5233).
Trace metals were assessed in atmospheric particulates at Burnaby Lake, in the greater Vancouver area of British Columbia to assess concentrations, particle size distributions and deposition rates to an urban watershed. Week-long samples were collected over a period of 18 weeks in 1995 using a 13 stage low pressure impactor (LPI). Samples were analysed using inductively coupled plasma atomic emission spectroscopy (ICP). Aluminum, boron, calcium, iron, magnesium, manganese, sodium and strontium had a similar time series pattern and particle size distribution. For these metals, maximum concentrations occurred during weeks of low precipitation and exhibited a large peak in mid June. Their particle size distribution was mostly dominated by a large peak between 1.7–18.4 μm with a secondary peak at <0.08 μm. Metal concentrations were generally one to three orders of magnitude higher than those measured in a rural location 100 km away from Burnaby Lake but similar to those measured in urban Taipei, Taiwan. Concentrations of the highly toxic metals, arsenic, cadmium and lead were within current air quality guidelines, however boron exceeded the Ontario Ministry of Environment ambient air quality standard in two of the 16 samples. Deposition velocities ranged between 0.22 and 13 cm s−1 with the largest values corresponding to the coarse particle mode. Mean deposition rates ranged between 4.0 μg m−2 d−1 and 650 mg m−2 d−1. Depending on the metal, yearly loadings to the watershed ranged from 90 kg to several thousand tonnes. Calcium, aluminum, boron and magnesium had the highest metal loadings to the watershed. Manganese also had relatively high loadings, a reflection of the high traffic density in the area. The relatively high metal deposition rates indicate that metal contribution from atmospheric sources may represent a significant portion of the total metal load to the Burnaby Lake watershed.
Keywords: Metals; Deposition rates; Deposition velocities; Particle impactor; Size fractions;