Optical measurements of aerosol size distributions in Great Smoky Mountains National Park: dry aerosol characterization

Aerosol size distributions were measured during the summertime 1995 Southeastern Aerosol and Visibility Study (SEAVS) in Great Smoky Mountains National Park using an Active Scattering Aerosol Spectrometer (ASASP-X) optical particle counter. We present an overview of the experimental method, our data inversion technique, timelines of the size distribution parameters, and calculations of dry accumulation mode aerosol density and refractive index. Aerosol size distributions were recorded during daylight hours for aerosol in the size range 0.1 < Dp < 2.5 microns. The particle refractive index used for the data inversion was calculated with the partial molar refractive index approach using 12-hr measured aerosol chemical composition. Aerosol accumulation mode volume concentrations ranging from 1 to 26 micron 3 cm-3 were observed, with an average of 7 +/- 5 micron 3 cm-3. The study average dry accumulation mode geometric volume median diameter was 0.27 +/- 0.03 micron, and the mean geometric standard deviation was 1.45 +/- 0.06. Using an internally mixed aerosol model, and assuming chemical homogeneity across the measured particle distribution, an average accumulation mode dry sulfate ion mass scattering efficiency of 3.8 +/- 0.6 m2 g-1 was calculated.     

Aerosol properties,in-canopy gradients,turbulent fluxes and VOC concentrations at a pristine forest site in Amazonia

Aerosol physical and chemical properties were measured in a forest site in central Amazonia (Cuieiras reservation, 2.61S; 60.21W) during the dry season of 2004 (Aug–Oct). Aerosol light scattering and absorption, mass concentration, elemental composition and size distributions were measured at three tower levels (Ground: 2 m; Canopy: 28 m, and Top: 40 m). For the first time, simultaneous eddy covariance fluxes of fine mode particles and volatile organic compounds (VOC) were measured above the Amazonian forest canopy. Aerosol fluxes were measured by eddy covariance using a Condensation Particle Counter (CPC) and a sonic anemometer. VOC fluxes were measured by disjunct eddy covariance using a Proton Transfer Reaction Mass Spectrometer (PTR-MS). At nighttime, a strong vertical gradient of phosphorus and potassium in the aerosol coarse mode was observed, with higher concentrations at Ground level. This suggests a source of primary biogenic particles below the canopy. Equivalent black carbon measurements indicate the presence of light-absorbing aerosols from biogenic origin. Aerosol number size distributions typically consisted of superimposed Aitken (76 nm) and accumulation modes (144 nm), without clear events of new particle formation. Isoprene and monoterpene fluxes reached respectively 7.4 and 0.82 mg m^?2 s^?1 around noon. An average fine particle flux of 0.05 ± 0.10 10⁶ m^?2 s^?1 was calculated, denoting an equilibrium between emission and deposition fluxes of fine mode particles at daytime. No significant correlations were found between VOC and fine mode aerosol concentrations or fluxes.     

Aerosol physical,chemical and optical properties during the Rocky Mountain Airborne Nitrogen and Sulfur study

During the Rocky Mountain Airborne Nitrogen and Sulfur (RoMANS) study, conducted during the spring and summer of 2006, a suite of instruments located near the eastern boundary of Rocky Mountain National Park (RMNP) measured aerosol physical, chemical and optical properties. Three instruments, a differential mobility particle sizer (DMPS), an optical particle counter (OPC), and an aerodynamic particle sizer (APS), measured aerosol size distributions. Aerosols were sampled by an Interagency Monitoring of Protected Visual Environments (IMPROVE) sampler and a URG denuder/filter-pack system for compositional analysis. An Optec integrating nephelometer measured aerosol light scattering. The spring time period had lower aerosol concentrations, with an average volume concentration of 2.2 ± 2.6 μm³ cm^?3 compared to 6.5 ± 3.9 μm³ cm^?3 in the summer. During the spring, soil was the single largest constituent of PM_2.5 mass, accounting for 32%. During the summer, organic carbon accounted for 60% of the PM_2.5 mass. Sulfates and nitrates had higher fractional contributions in the spring than the summer. Variability in aerosol number and volume concentrations and in composition was greater in the spring than in the summer, reflecting differing meteorological conditions. Aerosol scattering coefficients (b_sp) measured by the nephelometer compared well with those calculated from Mie theory using size distributions, composition data and modeled RH dependent water contents.     

Comparisons of aerosol properties measured by impactors and light scattering from individual particles: refractive index,number and volume concentrations,and size distributions

The southeastern aerosol and visibility study (SEAVS) was conducted in Great Smoky Mountains National Park in summer 1995 to investigate variations in ambient aerosol size distributions and their effect on visibility. In this paper, we compare dry aerosol size distribution parameters from a MOUDI impactor and two different optical particle counters (OPCs). Size distributions from the various instruments are expressed in a common measure of size, specifically, optical and aerodynamic diameters are converted to a dry, geometric diameter basis. Comparisons of the real part of particle refractive index obtained directly from light scattering measurements and inferred from aerosol composition measurements are also shown. Real refractive indices from direct measurements and those computed from measured fine aerosol chemical composition were generally within ±0.02. Maximum differences in estimated accumulation mode integrated volume concentrations from all instruments were within ∼22%. Accumulation mode integrated number concentrations and geometric standard deviations from the two OPCs agreed within ∼30% and ∼3%, respectively. Differences between MOUDI- and OPC-derived accumulation mode number concentrations and geometric standard deviations were ∼20% and ∼8%, respectively. The average geometric volume mean diameters derived from the three instruments agreed within 15% or less. The volume median diameters obtained by fitting the CSU number concentration data to a lognormal function were typically the smallest. We show that these discrepancies can be related to the differences and biases in the measurement and data analysis techniques.     

Aerosol size distributions and visibility estimates during the Big Bend regional aerosol and visibility observational (BRAVO) study

The Big Bend Regional Aerosol and Visibility Observational (BRAVO) study was conducted in Big Bend National Park in 1999. The park is located in a remote region of southwest Texas but has some of the poorest visibility of any Class 1 monitored area in the western US. The park is frequently influenced by air masses carrying emissions from Mexico and eastern Texas. Continuous physical, optical and chemical aerosol measurements were performed in an effort to understand the sources of and contributions to haze in the park. As part of this characterization, dry aerosol size distributions were measured over the size range of 0.05<D_p<20 μm. Three instruments with different measurement techniques were used to cover this range. Complete size distributions were obtained from all of the instruments in terms of a common measure of geometric size using a new technique. Size parameters for accumulation and coarse particle modes were computed and demonstrate periods when coarse mode volume concentrations were significant, especially during suspected Saharan dust episodes in July and August. Study average (and one standard deviation) geometric volume mean diameters for the accumulation and coarse particle modes were 0.26±0.04 and 3.4±0.8 μm, respectively. Dry light scattering coefficients (b_sp) were computed using measured size distributions and demonstrated periods when contributions to b_sp from coarse particles were significant. The study average computed b_sp was 0.026±0.016 km⁻¹. Computed dry b_sp values were highly correlated with measured values (r²=0.97). Real-time sulfate measurements were correlated with accumulation mode volume concentrations (r²=0.89) and computed dry light scattering coefficients (r²=0.86), suggesting sulfate aerosols were the dominant contributor to visibility degradation in the park.     

Estimates of Particle Hygroscopicity during the Southeastern Aerosol and Visibility Study

ABSTRACT

Aerosol water content was determined from relative humidity controlled optical particle counter (ASASP-X) size distribution measurements made during the Southeastern Aerosol and Visibility Study (SEAVS) in the Great Smoky Mountains National Park during summer 1995. Since the scattering response function of the ASASP-X is sensitive to particle refractive index, a technique for calibrating the ASASP-X for any real refractive index was developed. A new iterative process was employed to calculate water mass concentration and wet refractive index as functions of relative humidity. Experimental water mass concentrations were compared to theoretically predicted values assuming only ammonium sulfate compounds were hygroscopic. These comparisons agreed within experimental uncertainty. Estimates of particle hygroscopicity using a rural aerosol model of refractive index as a function of relative humidity demonstrated no significant differences from those made with daily varying refractive index estimates. Although aerosol size parameters were affected by the assumed chemical composition, forming ratios of these parameters nearly canceled these effects.     

Link between aerosol optical,microphysical and chemical measurements in an underground railway station in Paris

Measurements carried out in Paris Magenta railway station in April–May 2006 underlined a repeatable diurnal cycle of aerosol concentrations and optical properties. The average daytime PM₁₀ and PM_2.5 concentrations in such a confined space were approximately 5–30 times higher than those measured in Paris streets. Particles are mainly constituted of dust, with high concentrations of iron and other metals, but are also composed of black and organic carbon. Aerosol levels are linked to the rate at which rain and people pass through the station. Concentrations are also influenced by ambient air from the nearby streets through tunnel ventilation. During daytime approximately 70% of aerosol mass concentrations are governed by coarse absorbing particles with a low Angström exponent (～0.8) and a low single-scattering albedo (～0.7). The corresponding aerosol density is about 2 g cm^?3 and their complex refractive index at 355 nm is close to 1.56–0.035 i. The high absorption properties are linked to the significant proportion of iron oxides together with black carbon in braking systems. During the night, particles are mostly submicronic, thus presenting a greater Angström exponent (～2). The aerosol density is lower (1.8 g cm^?3) and their complex refractive index presents a lower imaginary part (1.58–0.013 i), associated to a stronger single-scattering albedo (～0.85–0.90), mostly influenced by the ambient air. For the first time we have assessed the emission (deposition) rates in an underground station for PM₁₀, PM_2.5 and black carbon concentrations to be 3314 ± 781(?1164 ± 160), 1186 ± 358(?401 ± 66) and 167 ± 46(?25 ± 9) μg m^?2 h^?1, respectively.     

Particulate matter in the indoor and outdoor air of a gymnasium and a fronton

An indoor/outdoor monitoring programme of PM₁₀ was carried out in two sports venues (a fronton and a gymnasium). Levels always below 50 μg m^?3 were obtained in the fronton and outdoor air. Due to the climbing chalk and the constant process of resuspension, concentrations above 150 μg m^?3 were registered in the gymnasium. The chalk dust contributed to CO₃ ^2? concentrations of 32?±?9.4 μg m^?3 in this sports facility, which represented, on average, 18 % of the PM₁₀ mass. Here, the carbonate levels were 128 times higher than those registered outdoors. Much lower concentrations, around 1 μg m^?3, were measured in the fronton. The chalk dust is also responsible for the high Mg²⁺ concentrations in the gym (4.7?±?0.89 μg m^?3), unfolding a PM₁₀ mass fraction of 2.7 %. Total carbon accounted for almost 30 % of PM₁₀ in both indoor spaces. Aerosol size distributions were bimodal and revealed a clear dependence on physical activities and characteristics of the sports facilities. The use of climbing chalk in the gymnasium contributed significantly to the coarse mode. The average geometric mean diameter, geometric standard deviation and total number of coarse particles were 0.77 μm, 2.79 cm^?3 and 28 cm^?3, respectively.     

The Aerosol Research and Inhalation Epidemiology Study (ARIES): PM25 Mass and Aerosol Component Concentrations and Sampler Intercomparisons

ABSTRACT

The Aerosol Research and Inhalation Epidemiology Study (ARIES) was designed to provide high-quality measurements of PM₂₅, its components, and co-varying pollutants for an air pollution epidemiology study in Atlanta, GA.

Air pollution epidemiology studies have typically relied on available data on particle mass often collected using filter-based methods. Filter-based PM_2.5 sampling is susceptible to both positive and negative errors in the measurement of aerosol mass and particle-phase component concentrations in the undisturbed atmosphere. These biases are introduced by collection of gas-phase aerosol components on the filter media or by volatilization of particle phase components from collected particles. As part of the ARIES, we collected daily 24-hr PM_2.5 mass and speciation samples and continuous PM_2.5 data at a mixed residential-light industrial site in Atlanta. These data facilitate analysis of the effects of a wide variety of factors on sampler performance. We assess the relative importance of PM_2.5 components and consider associations and potential mechanistic linkages of PM_2.5 mass concentrations with several PM_2.5 components.

For the 12 months of validated data collected to date (August 1, 1998-July 31, 1999), the monthly average Federal Reference Method (FRM) PM_{2 5} mass always exceeded the proposed annual average standard (12-month average = 20.3 ± 9.5 ug/m³). The particulate SO₄ ^2- fraction (as (NH₄)₂SO₄) was largest in the summer and exceeded 50% of the FRM mass. The contribution of (NH₄)₂SO₄ to FRM PM_2.5 mass dropped to less than 30% in winter. Particu-late NO₃ ^- collected on a denuded nylon filter averaged 1.1 ± 0.9 ug/m³. Particle-phase organic compounds (as organic carbon × 1.4) measured on a denuded quartz filter sampler averaged 6.4 ± 3.1 ug/m³ (32% of FRM PM_{2 5} mass) with less seasonal variability than SO₄ ^2-.     

Estimates of particle hygroscopicity during the Southeastern Aerosol and Visibility Study

Aerosol water content was determined from relative humidity controlled optical particle counter (ASASP-X) size distribution measurements made during the Southeastern Aerosol and Visibility Study (SEAVS) in the Great Smoky Mountains National Park during summer 1995. Since the scattering response function of the ASASP-X is sensitive to particle refractive index, a technique for calibrating the ASASP-X for any real refractive index was developed. A new iterative process was employed to calculate water mass concentration and wet refractive index as functions of relative humidity. Experimental water mass concentrations were compared to theoretically predicted values assuming only ammonium sulfate compounds were hygroscopic. These comparisons agreed within experimental uncertainty. Estimates of particle hygroscopicity using a rural aerosol model of refractive index as a function of relative humidity demonstrated no significant differences from those made with daily varying refractive index estimates. Although aerosol size parameters were affected by the assumed chemical composition, forming ratios of these parameters nearly canceled these effects.     

The physical characteristics of sulfur aerosols

A review of the physical characteristics of sulfur-containing aerosols, with respect to size distribution of the physical distributions, sulfur distributions, distribution modal characteristics, nuclei formation rates, aerosol growth characteristics, and in situ measurement, has been made.Physical size distributions can be characterized well by a trimodal model consisting of three additive lognormal distributions.When atmospheric physical aerosol size distributions are characterized by the trimodal model, the following typical modal parameters are observed:1. Nuclei mode – geometric mean size by volume, DGV_n, from 0.015 to 0.04 μm. σ_gn=1.6, nucler mode volumes from 0.0005 over the remote oceans to 9 μm³ cm⁻³ on an urban freeway.2. Accumulation mode – geometric mean size by volume, DGV_a, from 0.15 to 0.5 μm, σ_ga=1.6–2.2 and mode volume concentrations from 1 for very clean marine or continental backgrounds to as high as 300 μm³ cm⁻³ under very polluted conditions in urban areas.3. Coarse particle mode – geometric mean size by volume, DGV_c, from 5 to 30 μm, σ_gn=2–3, and mode volume concentrations from 2 to 1000 μm³ cm⁻³.It has also been concluded that the fine particles (D_p<2 μm) are essentially independent in formation, transformation and removal from the coarse particles (D_p>2 μm).Modal characterization of impactor-measured sulfate size distributions from the literature shows that the sulfate is nearly all in the accumulation mode and has the same size distribution as the physical accumulation mode distribution.Average sulfate aerodynamic geometric mean dia. was found to be 0.48±0.1 μm (0.37±0.1 μm vol. dia.) and σ_g=2.00±0.29. Concentrations range from a low of about 0.04 μg m⁻³ over the remote oceans to over 8 μg m⁻³ under polluted conditions over the continents.Review of the data on nucleation in smog chambers and in the atmosphere suggests that when SO₂, is present, SO₂-to-aerosol conversion dominates the Aitken nuclei count and, indirectly, through coagulation and condensation, the accumulation mode size and concentration. There are indications that nucleation is ubiquitous in the atmosphere, ranging from values as low as 2 cm⁻³ h⁻¹ over the clean remote oceans to a high of 6×10⁶ cm⁻³ h⁻¹ in a power plant plume under sunny conditions.There is considerable theoretical and experimental evidence that even if most of the mass for the condensational growth of the accumulation mode comes from hydrocarbon conversion, sulfur conversion provides most of the nuclei.     

Aerosol scattering properties in northern China

The aerosol scattering properties were investigated at two continental sites in northern China in 2004. Aerosol light scattering coefficient (σ_sp) at 525 nm, PM₁₀, and aerosol mass scattering efficiencies (α) at Dunhuang had a mean value of 165.1±148.8 M m⁻¹, 157.6±270.0 μg m⁻³, and 2.30±3.41 m² g⁻¹, respectively, while these values at Dongsheng were, respectively, 180.2±151.9 M m⁻¹, 119.0±112.9 μg m⁻³, and 1.87±1.41 m² g⁻¹. There existed a seasonal variability of aerosol scattering properties. In spring, at Dunhuang PM₁₀, σ_sp, and α were 184.1±211.548 μg m⁻³, 126.3±89.6 M m⁻¹, and 1.05±0.97 m² g⁻¹, respectively, and these values at Dongsheng were 146.4±142.1 μg m⁻³, 183.4±81.7 M m⁻¹, and 1.98±1.52 m² g⁻¹, respectively. However, in winter at Dunhuang PM₁₀, σ_sp, and α were 158.1±261.4 μg m⁻³, 303.3±165.2 M m⁻¹, and 3.17±1.93 m² g⁻¹, respectively, and these values at Dongsheng were 155.7±170.1 μg m⁻³, 304.4±158.1 M m⁻¹, and 2.90±1.72 m² g⁻¹, respectively. σ_sp and α in winter were higher than that in spring at both the sites, which coincides with the characteristics of dust aerosol and pollution aerosol. Overall, the dominant aerosol types in spring and winter at both sites in northern China are dust aerosol and pollution aerosol, respectively.     

Aerosol radiative forcing over the Indo-Gangetic plains during major dust storms

Indo-Gangetic (IG) alluvial plains, one of the largest river basins in the world, suffers from the long range transport of mineral dust from the western arid and desert regions of Africa, Arabia and Rajasthan during the summer (pre-monsoon season, April–June). These dust storms influence the aerosol optical depth (AOD) across the IG plains. The Kanpur AERONET (Aerosol Robotic Network) station and Moderate Resolution Imaging Spectro-radiometer (MODIS) data show pronounced effect on the aerosol optical properties and aerosol size distribution during major dust storm events over the IG plains that have significant effect on the aerosol radiative forcing (ARF). The multi-band AOD, from AERONET and MODIS, show contrasting changes in wavelength dependency over dust affected regions. A time collocated (±30 min) validation of AERONET AOD with MODIS Terra (level 2 swath product) over Kanpur, at a common wavelength of 550 nm for the period 2001–2005 show moderate correlation (R²∼0.6) during the summer season. The average surface forcing is found to change by −23 W m⁻² during dust events and the top of the atmosphere (TOA) forcing change by −11 W m⁻² as compared to the non-dusty clear-sky days. A strong correlation is found between AOD at 500 nm and the ARF. At surface, the correlation coefficient between AOD and ARF is found to be high (R²=0.925) and is found to be moderate (R²=0.628) at the TOA. The slope of the regression line gives the aerosol forcing efficiency at 500 nm of about −46±2.6 W m⁻² and −17±2.5 W m⁻² at the surface and the TOA, respectively. The ARF is found to increase with the advance of the dry season in conjunction with the gradual rise in AOD (at 500 nm) from April (0.4–0.5) to June (0.6–0.7) over the IG plains.     

Aerosol mass-size distributions at a tropical coastal environment: response to mesoscale and synoptic processes

Regular measurements of total mass concentration and mass-size distribution of near-surface aerosols, made using a ten-channel Quartz Crystal Microbalance (qcm) Impactor for the period October 1998–December 1999 at the tropical coastal station Trivandrum (8.5°N, 77°E), are used to study the response of aerosol characteristics to regional mesoscale and synoptic processes. Results reveal that aerosol mass concentrations are generally higher under land breeze conditions. The sea breeze generally has a cleansing effect, depleting the aerosol loading. The continental air (LB regime) is richer in accumulation mode (submicron) aerosols than the marine air. On a synoptic scale, aerosol mass concentration in the submicron mode decreased from an average high value of ∼86 μg m⁻³ during the dry months (January–March) to ∼11 μg m⁻³ during the monsoon season (June–September). On the contrary mass concentration in the supermicron mode increased from a low value of ∼15 μg m⁻³ during the dry months to reach a comparatively high value of ∼35 μg m⁻³ during April, May. Correspondingly, the effective radius (R_eff) increased from a low value of 0.15–0.17 μm to ∼0.3 μm indicating a seasonal change in the size distribution. The mass-size distribution shows mainly three modes, a fine mode (∼0.1 μm); a large mode (∼0.5 μm) and a coarse mode (∼3 μm). The fine mode dominates in winter. In summer the large mode becomes more conspicuous and the coarse mode builds up. The fine mode is highly reduced in monsoon and the large and coarse modes continue to remain high (replenished) so that their relative dominance increases. The size distribution tends to revert to the winter pattern in the post-monsoon season. Accumulation (submicron) aerosols account for ∼98% of the total surface area and ∼70% of the total volume of aerosols during winter. During monsoon, even though they still account for ∼90% of the area, their contribution to the volume is reduced to ∼50%; the coarse aerosols account for the rest.     

Aerosol formation yields from the reaction of catechol with ozone

The formation of secondary organic aerosol from the gas-phase reaction of catechol (1,2-dihydroxybenzene) with ozone has been studied in two smog chambers. Aerosol production was monitored using a scanning mobility particle sizer and loss of the precursor was determined by gas chromatography and infrared spectroscopy, whilst ozone concentrations were measured using a UV photometric analyzer. The overall organic aerosol yield (Y) was determined as the ratio of the suspended aerosol mass corrected for wall losses (M_o) to the total reacted catechol concentrations, assuming a particle density of 1.4 g cm^?3. Analysis of the data clearly shows that Y is a strong function of M_o and that secondary organic aerosol formation can be expressed by a one-product gas–particle partitioning absorption model. The aerosol formation is affected by the initial catechol concentration, which leads to aerosol yields ranging from 17% to 86%. The results of this work are compared to similar studies reported in the literature.     

Change of the ambient particle size distribution in East Germany between 1993 and 1999

Size distribution of particle number concentrations in the geometric equivalent diameter range 0.01–2.5 μm were determined in three communities, Zerbst, Bitterfeld and Hettstedt of the state of Sachsen-Anhalt in Eastern Germany, in the first half of 1993 and 1999. A Mobile Aerosol Spectrometer (MAS) consisting of a differential mobility particle spectrometer (DMPS) and a laser aerosol spectrometer (LAS-X) were used for size-selective particle number concentration measurements from which mass concentrations were derived based on an apparent mean density of the ambient aerosol of the closely situated city of Erfurt.The total number concentration was governed by ultra-fine particles (<0.1 μm) (81% in 1993 and 90% in 1999) and 0.1–0.5 μm size fraction dominates total mass concentration (approximately 80%). While the mass concentration of fine particles (PM2.5) decreased from 39 to 19 μg m⁻³, the geometric means of total number concentration showed constant concentration (13.3×10³ cm⁻³ in 1993 and 13.3×10³ cm⁻³ in 1999, p=0.975) and the geometric means of number concentration of ultra-fine particles (UP) between 10 and 30 nm increased from 5.9×10³ to 8.2×10³ cm⁻³ from 1993 to 1999 (p=0.016). The temporal changes of number and mass concentrations in the three communities are similar. The clear shift to smaller particle sizes within this six years period was caused by changes of the most prominent sources, traffic and domestic heating, since formerly dominating industries in Bitterfeld and Hettstedt had vanished grossly.     

Chemical composition and complex refractive index of Saharan Mineral Dust at Izaña,Tenerife (Spain) derived by electron microscopy

Samples from two strong homogeneous dust plumes from the Saharan desert reaching Izaña (Tenerife, Spain) in July and August 2005 were taken with a miniature impactor system and filter samplers. Size, aspect ratio and chemical composition of more than 22,000 individual particles were studied by scanning electron microscopy. The mineralogical phase composition of about 200 particles was investigated by transmission electron microscopy. In addition, the aerosol size distribution was measured with an optical particle spectrometer. In all samples, the aerosol was dominated by mineral dust with an average composition (by volume) of 64% silicates, 6% quartz, 5% calcium-rich particles, 14% sulfates, 1% hematite, 1% soot and 9% other carbonaceous material. Sulfate was found predominantly as coating on other particles with an average thickness of approximately 60 nm. The aerosol calcium content is correlated with the calcite concentrations of soils in the source region, highest values were observed for northern and central Algeria and Morocco. The average aspect ratio of the particles was 1.64. The distributions of the aspect ratios are parameterized by log-normal functions for modeling purpose. Single-scattering albedo (0.95) and asymmetry factor (0.74–0.81) was measured by polar aerosol photometry on filter samples using a light source resembling the solar spectrum. The apparent soot content of the sample (1 vol%) was determined by the same technique. From the mineralogical data, an average complex refractive index of 1.59–9×10⁻³i for visible light was derived. The imaginary part of the complex refractive index decreases with increasing particle size from −2.5×10⁻²i to <−10⁻³i, reflecting the decreasing hematite and soot contents. The imaginary part derived from optical measurements was −7×10⁻³i.     

Aerosol light-scattering in The Netherlands

The relation between the (midday) aerosol light-scattering and the concentrations of nitrate and sulfate has been assessed at a site near the coast of the North Sea in The Netherlands. Midday was selected for the measurements because this is the time at which the aerosol is most effective in the scattering of solar radiation. Automated thermodenuders were used for the hourly measurement of the concentration of nitrate and sulfate with a lower detection limit of 0.1 μ m⁻³. The site is operational since October 1993. The first-year average dry aerosol light-scattering (measured with an integrating nephelometer at a wavelength of 525 nm) was 0.71 × 10⁻⁴ m^1&#x0304;. In arctic marine air the aerosol light-scattering was a factor of 10 lower than the average value, in polluted continental air it was up to a factor of 10 higher. The ratio of the total aerosol light-scattering to the concentration of sulfate was 20 m² g⁻¹. The contribution of nitrate to the aerosol light-scattering was higher than that of sulfate in the winter and of about equal magnitude in the summer period. In November and December of 1993, the humidity dependence of the aerosol light-scattering was investigated. Two types of (continental) aerosol were found with respect to the humidity behavior. One type showed a significant increase in light-scattering at the deliquescence points of ammonium nitrate and ammonium sulfate, with that of ammonium nitrate the most pronounced. The second type of continental aerosol did not show deliquescence, but followed the typical humidity dependence of aerosol in a supersaturated droplet state. In this latter aerosol type, nitrate dominated over sulfate. It was concluded from the study that the aerosol light-scattering in The Netherlands, in particular its humidity dependence, is governed by (ammonium) nitrate.     

Relationship between Sulfate Aerosol and Visibility

A PM₁₀ monitoring network was established throughout the South Coast Air Basin (SOCAB) in the greater Los Angeles region during the calendar year 1986. Annual average PM₁₀ mass concentrations within the Los Angeles metropolitan area ranged from 47.0 µg m^-3 along the coast to 87.4 µg m^-3 at Rubldoux, the furthest inland monitoring station. Measurements made at San Nicolas Island suggest that regional background aerosol contributes between 28 to 44 percent of the PM₁₀ aerosol at monitoring sites In the SOCAB over the long term average. Five major aerosol components (carbonaceous material, NO^- ₃, SO⁼ ₄, NH⁺ ₄, and soil-related material) account for greater than 80 percent of the annual average PM₁₀ mass at all on-land monitoring stations. Peak 24-h average mass concentrations of nearly 300 µg m^-3 were observed at inland locations, with lower peak values (?130–150 µg m^-3) measured along the coast. Peak-day aerosol composition was characterized by increased NO^- ₃ Ion and associated ammonium ion levels, as compared to the annual average. There appears to be only a weak dependence of PM₁₀ mass concentration on season of the year. This lack of a pronounced seasonal dependence results from the complex and contradictory seasonal variations in the major chemical components (carbonaceous material, nitrate, sulfate, ammonium ion and crustal material). At most sites within the Los Angeles metropolitan area, PM₁₀ mass concentrations exceeded both the annual and 24-h average federal and state of California PM₁₀ regulatory standards.     

Concentration and Size Distribution of Ultrafine Particles Near a Major Highway

Abstract

Motor vehicle emissions usually constitute the most significant source of ultrafine particles (diameter <0.1 μm) in an urban environment, yet little is known about the concentration and size distribution of ultrafine particles in the vicinity of major highways. In the present study, particle number concentration and size distribution in the size range from 6 to 220 nm were measured by a condensation particle counter (CPC) and a scanning mobility particle sizer (SMPS), respectively. Measurements were taken 30, 60, 90, 150, and 300 m downwind, and 300 m upwind, from Interstate 405 at the Los Angeles National Cemetery. At each sampling location, concentrations of CO, black carbon (BC), and particle mass were also measured by a Dasibi CO monitor, an aethalometer, and a DataRam, respectively. The range of average concentration of CO, BC, total particle number, and mass concentration at 30 m was 1.7?2.2 ppm, 3.4?10.0 μg/m³, 1.3?2.0 × 10⁵/cm³, and 30.2?64.6 μ/m³, respectively.

For the conditions of these measurements, relative concentrations of CO, BC, and particle number tracked each other well as distance from the freeway increased. Particle number concentration (6–220 nm) decreased exponentially with downwind distance from the freeway. Data showed that both atmospheric dispersion and coagulation contributed to the rapid decrease in particle number concentration and change in particle size distribution with increasing distance from the freeway. Average traffic flow during the sampling periods was 13,900 vehicles/hr. Ninety-three percent of vehicles were gasoline-powered cars or light trucks. The measured number concentration tracked traffic flow well. Thirty meters downwind from the freeway, three distinct ultrafine modes were observed with geometric mean diameters of 13, 27, and 65 nm. The smallest mode, with a peak concentration of 1.6 × 10⁵/cm³, disappeared at distances greater than 90 m from the freeway. Ultrafine particle number concentration measured 300 m downwind from the freeway was indistinguishable from upwind background concentration. These data may be used to estimate exposure to ultrafine particles in the vicinity of major highways.