Laser Optical Particle Counter Research Articles

Comparison of mesozooplankton production estimates from Saanich Inlet (British Columbia, Canada) using the chitobiase and biomass size spectra approaches

Zooplankton production estimates are necessary to understand the availability and transfer of energy to higher trophic levels in marine food webs. Methods have been developed to quantify zooplankton production; however, they are difficult to compare as they focus on single species, groups, stages, or size classes of zooplankton. We compared 2 methods for estimating crustacean production: the chitobiase method (based on a crustacean moulting enzyme), and 3 empirical growth rate models (Huntley-Lopez, Hirst-Lampitt, and Hirst-Bunker) applied to optically resolved mesozooplankton normalized biomass size spectra (NBSS). Mesozooplankton net samples were collected between March and August of 2010 and 2011 in Saanich Inlet (British Columbia, Canada) and analyzed in the laboratory using microscopy and a bench-top laser optical particle counter (lab-LOPC). Microscope and lab-LOPC estimates of abundance and biomass were in close agreement. Crustacean production estimates were highest using Huntley-Lopez (0.20-185.3 mg C m-3 d-1), followed by Hirst-Bunker (0 .01-18.3 mg C m-3 d-1), chitobiase (0.05-15.6 mg C m-3 d-1), and Hirst-Lampitt (0.03-14.3 mg C m-3 d-1). Hirst-Lampitt-, Hirst-Bunker-, and chitobiase-based estimates of crustacean production and trophic transfer efficiency (TTE) yielded similar patterns/magnitude, while the Huntley-Lopez model was more variable. Estimates showed stronger agreement in 2011 than in 2010, attributed to the shift from El Niño to La Niña conditions. We highlight similarities/differences associated with these techniques and suggest that Hirst-Bunker estimates of production and TTE are most consistent with chitobiase-based values.

Estimating zooplankton vertical distribution from combined LOPC and ZooScan observations on the Brazilian Coast

Although the technology for sampling zooplankton is rapidly evolving, it remains a challenge to determine species distributions with high spatial resolution. This study reports on a technique developed to estimate the mesozooplankton vertical distribution by combining the spatial resolution possible with laser optical particle counter (LOPC) profiles with the taxonomic resolution possible with vertically integrated zooplankton net samples. Data obtained from different coastal and oceanic ecosystems off Brazil were used for our analysis. The aggregate signals from LOPC data sets were removed, and the distributions of the residual Zo-particles, believed to describe zooplankton, were calculated. Significant correlations were then identified between the abundances of the Zo-particles in different size classes and the abundance and size distributions of zooplankton collected in a vertical integrated net sample and classified with a ZooScan system. A principal component analysis was carried out to identify and exclude outliers. After these procedures, multiple linear regression analyses were used to detect relationships among abundances of different zooplankton groups and multiple Zo-particle size classes. The resulting relationships were used to estimate the vertical distributions of the dominant zooplankton taxa—calanoids, poecilostomatoids, shrimp-like, and appendicularians—through the water column from the LOPC data. The results achieved the goal of estimating distributions of dominant zooplankton taxa with an improved spatial resolution. The analytical technique described here enabled the analysis of zooplankton vertical structure in a wide range of marine ecosystems off Brazil and could serve as a basis to revisit historical data collected with similar methods elsewhere.

Plankton and seston size spectra estimated by the LOPC and ZooScan in the Abrolhos Bank ecosystem (SE Atlantic)

The biomass size spectrum provides valuable information about the functioning of plankton systems. We evaluated hydrographic and bathymetric influences on biomass size spectra and on vertical distributions of plankton and seston above the Abrolhos Bank and in adjacent oceanic areas off Eastern Brazil. We used both in situ Laser Optical Particle Counter (LOPC) and preserved plankton samples analyzed with a ZooScan system to determine seston and plankton abundances, size distributions, and biomasses. Shelf stations, including those on the Abrolhos Bank, had higher particle concentrations and mesozooplankton biomasses than the vertically stratified oceanic stations. The latter were influenced by cold, nutrient-rich South Atlantic Central Water (SACW) below the mixed layer, particularly toward the south of the study area. Small particles (<1mm) were more abundant above and within the pycnocline, whereas large particles (>1mm) had a more heterogeneous vertical distribution, but were more abundant above the pycnocline, especially at the oceanic stations. Calanoid copepods usually dominated the mesozooplankton biomass spectra, but were accompanied by cyclopoids, appendicularians, and ostracods, the latter being particularly abundant during nighttime stations on the Abrolhos Bank. Both LOPC and ZooScan data showed significant differences in NBSS slopes and intercepts between shelf and oceanic stations. The higher intercepts and steeper slopes over the shelf are characteristic of higher productivity. The shallower slopes and presence of more biomass in larger particles indicate a more important contribution of large organisms and higher energy transfer efficiencies at the open ocean stations. Our results highlight the importance of the Abrolhos Bank for pelagic production in an otherwise oligotrophic ocean.

Aggregates and their distributions determined from LOPC observations made using an autonomous profiling float

The vertical flux of particles in the ocean drives the movement of organic carbon to the deep ocean. We have been studying the distribution and flux of these particles using the SOLOPC, a profiling Lagrangian (SOLO) float with a Laser Optical Particle Counter (LOPC). We have been able to distinguish between aggregate-like and zooplankton-like particles with diameters >2mm but needed a way to separate the smaller particles into aggregates and zooplankton. Observations included a lognormal-shaped fraction in the normalized volume distribution similar to that observed in results for simulations of particles in the euphotic zone. By fitting a lognormal distribution to the volume spectrum of particles with diameters ≤2mm, we have been successful at making a separation of marine snow material from other, presumably living, particles. The particle volumes derived using the separations are positively correlated with fluorescence, particulate organic carbon, and the volume of larger particles classified as aggregate-like, which supports the conclusion that these particles are truly aggregates, in some cases derived from phytoplankton. The residual volumes (total less the above fit) are highly correlated with the volumes of large, zooplankton-like particles. Downward velocities of the aggregate fraction calculated from time series of particle profiles are consistent with previous estimates of particle settling rates (20–70md−1). We now have a tool to estimate aggregate distributions, properties, and vertical fluxes in the euphotic zone, including when and where they change.

Mesozooplankton and particulate matter responses to a deep-water frontal system in the southern California Current System

Author(s): Ohman, MD; Powell, JR; Picheral, M; Jensen, DW | Abstract: We analyzed the abundance of mesozooplankton and suspended particulate matter across the deep-water in the southern sector of the California Current System. We characterized the A-Front with two novel devices, a free-fall Moving Vessel Profiler (MVP) and an Underwater Vision Profiler 5 (UVP5), together with quantitative bongo samples analyzed by ZooScan. The MVP permitted real-time visualization of vertical density structure, chlorophyll a fluorescence and particle size structure (from a laser optical particle counter) across the front to a depth of 200 m with the research vessel moving at 6 m s-1. The UVP5 quantified in situ vertical distributions from digital images of planktonic organisms and particles in profiles to 300 m. Both the MVP and UVP5 indicated that organic aggregates increased several-fold at the A-Front. The A-Front was a region of elevated abundance of mainly particle-grazing mesozooplankton, including calanoid copepods, Oithona spp., appendicularians and euphausiids, as well as a site of elevated ratio of nauplii copepod-1. In contrast, poecilostomatoid copepods, ostracods, chaetognaths and radiolaria, most of which are more carnivorous or omnivorous, were all elevated in abundance to the south of the front. We provide evidence that submesoscale fronts can be regions of locally elevated plankton abundance and production, as well sites of faunal transitions. © 2012 The Author.

Indoor-Outdoor Atmospheric Particulate Matter Relationships in Naturally Ventilated Offices

Atmospheric particle size distribution and number concentrations were investigated in three naturally ventilated offices and outdoors. The offices were located in a University suburban campus in newly constructed, one-storey buildings. Measurements were conducted with a laser optical particle counter. Synoptic meteorological data, for the duration of the experiment, were obtained from the NOAA satellite data archive. The indoor to outdoor averaged number concentration ratios in the offices during working hours were in the range of 0.35-14.68. Construction activities in the vicinity of one office had a pronounced effect on the indoor number concentrations of particulate matter (PM). Indoor sources were traced to the movement of people, computer operation and environmental tobacco smoke. Re-suspension, appears to contribute to the number concentrations of PM, over all size ranges. Meteorological conditions with respect to office orientation also had a significant impact on indoor PM number concentrations.

Atmospheric Aerosol Deposition on Snow Surfaces and Its Effect on Albedo

This paper investigated how atmospheric aerosol deposition into snowpack affects snow albedo reduction using radiation budget observation, atmospheric aerosol monitoring, and snow pit work during the winter of 2003/2004 in Sapporo, Japan. The mass concentration of snow impurities was less than 10 parts per million by weight (ppmw) in the core accumulation season and exceeded 100 ppmw in the melting season due to a heavy dust event on March 11 to 12, 2004. The relationship between the visible albedo and snow impurities suggested the considerable effect of snow impurities in reducing the visible albedo even during the accumulation season in Sapporo. After the dust event, high impurity concentrations of approximately 600 ppmw in the top snow layer were maintained, but the visible albedo still decreased because the increasing snow grain size continued to reduce the visible albedo. The size distribution of snow impurities measured from a snow sample with a Coulter counter during the dust event was compared with the size distributions of snow impurities calculated as wet and dry depositions from a laser optical particle counter for atmospheric aerosols. The result confirmed that the contribution from wet depositions was important except for giant particles with a radius larger than 2.5 µm.

Possible Effect of Anthropogenic Aerosol Deposition on Snow Albedo Reduction at Shinjo, Japan

We observed broadband snow albedos in the visible and the near infrared spectral regions with snow pit works of several-day intervals, during the winters of 2001/2002 and 2002/2003 at Shinjo, Japan. We examined the dependence of albedos on snow grain size and on concentration of snow impurities, comparing observations and theoretical calculations using a radiative transfer model for atmosphere-snow system. The comparisons revealed that the snow was contaminated by strong absorptive impurities such as soot additional to moderate absorptive impurities such as mineral dust. Snow albedo reduction after snowfalls (snow aging effect on albedo) observed in both spectral regions corresponded to the growth tendency of snow grains and the increasing concentration of snow impurities with elapsed time after snowfalls. Measurement of the atmospheric aerosols above the snow surface using a laser optical particle counter suggested that wet deposition of atmospheric aerosols caused snow impurities of more than 1 ppmw in mass concentration.

In situ measurements of background aerosol and subvisible cirrus in the tropical tropopause region

In situ aerosol measurements were performed in the Indian Ocean Intertropical Convergence Zone (ITCZ) region during the Airborne Polar Experiment‐Third European Stratospheric Experiment on Ozone (APE‐THESEO) field campaign based in Mahé, Seychelles between 24 February and 6 March 1999. These are measurements of particle size distributions with a laser optical particle counter of the Forward Scattering Spectrometer Probe (FSSP)‐300 type operated on the Russian M‐55 high‐altitude research aircraft Geophysica in the tropical upper troposphere and lower stratosphere up to altitudes of 21 km. On 24 and 27 February 1999, ultrathin layers of cirrus clouds were penetrated by Geophysica directly beneath the tropical tropopause at 17 km pressure altitude and temperatures below 190 K. These layers also were concurrently observed by the Ozone Lidar Experiment (OLEX) lidar operating on the lower‐flying German DLR Falcon research aircraft. The encountered ultrathin subvisual cloud layers can be characterized as (1) horizontally extending over several hundred kilometers, (2) persisting for at least 3 hours (but most likely much longer), and (3) having geometrical thicknesses of 100–400 m. These cloud layers belong to the geometrically and optically thinnest ever observed. In situ particle size distributions covering diameters between 0.4 and 23 μm obtained from these layers are juxtaposed with those obtained inside cloud veils around cumulonimbus (Cb) anvils and also with background aerosol measurements in the vicinity of the clouds. A significant number of particles with size diameters around 10 μm were detected inside these ultrathin subvisible cloud layers. The cloud particle size distribution closely resembles a background aerosol onto which a modal peak between 2 and 17 μm is superimposed. Measurements of particles with sizes above 23 μm could not be obtained since no suitable instrument was available on Geophysica. During the flight of 6 March 1999, upper tropospheric and lower stratospheric background aerosol was measured in the latitude band between 4°S and 19°S latitude. The resulting particle number densities along the 56th meridian exhibit very little latitudinal variation. The concentrations for particles with sizes above 0.5 μm encountered under these background conditions varied between 0.1 and 0.3 particles/cm3 of air in altitudes between 17 and 21 km.

Intercomparison of number concentration measurements by various aerosol particle counters

Total aerosol particle number concentrations, as measured by means of 16 different measurement systems, have been quantitatively compared during an international workshop at the Institute for Experimental Physics of the University of Vienna, Austria, which was coordinated within the Committee on Nucleation and Atmospheric Aerosols (ICCP-IUGG). The range of measuring instruments includes Pollak counters (PCO) in use already for several decades, presently available commercial particle counters, as well as laboratory prototypes. The operation of the instruments considered was based on different measurement principles: (1) adiabatic expansion condensation particle counter, (2) flow diffusion condensation particle counter, (3) turbulent mixing condensation particle counter, (4) laser optical particle counter, and (5) electrostatic particle measurement system. Well-defined test aerosols with various chemical compositions were considered: DEHS, sodium chloride, silver, hydrocarbons, and tungsten oxide. The test aerosols were nearly monodispersed with mean particle diameters between 4 and 520 nm, the particle number concentrations were varied over a range from about 4×10 1 to 7×10 6 cm −3. A few measurements were performed with two-component aerosol mixtures. For simultaneous concentration measurements, the various instruments considered were operated under steady state conditions in a linear flow system. A series of at least 10 single concentration measurements was performed by each individual instrument at each set of test aerosol parameters. The average of the concentration data measured by the various instruments was defined as a common reference. The number concentrations obtained from the various instruments typically agreed within a factor of about two over the entire concentration range considered. The agreement of the measured concentrations is notable considering the various different measurement principles applied in this study, and particularly in view of the broad range of measurement instruments used. Significant deviations and nonlinear response were observed only in a few cases and are possibly related to calibration errors. For certain conditions, a dependence of aerosol counter response on particle composition has been found. The scatter of the number concentrations obtained from each individual instrument during measurements with constant test aerosol typically did not exceed 20% to 25%. At concentrations below 10 3 cm −3, however, several of the instruments, including electrostatic particle measurement systems, tend to show increased experimental scatter.

Stratospheric aerosol measurements in the Arctic winter of 1996/1997 with the M-55 Geophysika high-altitude research aircraft

In-situ aerosol measurements were performed in the northern hemispheric stratosphere up to altitudes of 21 km between 13 November 1996 and 14 January 1997, inside and outside of the polar vortex during the Airborne Polar Experiment (APE) field campaign. These are measurements of particle size distributions with a laser optical particle counter of the FSSP-300 type operated during 9 flights on the Russian M-55 high-altitude research aircraft Geophysika. For specific flights, the FSSP-300 measurements are compared with balloon-borne data (launched from Kiruna, Sweden). It was found that the stratospheric aerosol content reached levels well below the background concentrations measured by the NASA operated ER-2 in 1988/89 in the northern hemisphere. During the APE campaign, no PSC particle formation was observed at flight altitudes although the temperatures were below the NAT condensation point during one flight. The measured correlations between ozone and aerosol give an indication of the subsidence inside the 1996/97 polar vortex. Despite the lower aerosol content in the winter 1996/97 compared to the 1989 background, the heterogeneous reactivity of the aerosol (as calculated from the measured data with additional model input) is comparable. This is due to the dependency of the reactive uptake coefficients on the atmospheric water vapor content. Under the described assumptions the reaction rates on the background aerosol are significantly smaller than for competing gas phase chlorine activation, as can be expected for stratospheric background conditions especially inside the polar vortex.

Size and Composition Distribution of Fine Particulate Matter Emitted from Wood Burning, Meat Charbroiling, and Cigarettes

A dilution source sampling system is augmented to measure the size-distributed chemical composition of fine particle emissions from air pollution sources. Measurements are made using a laser optical particle counter (OPC), a differential mobility analyzer/condensation nucleus counter (DMA/CNC) combination, and a pair of microorifice uniform deposit impactors (MOUDIs). The sources tested with this system include wood smoke (pine, oak, eucalyptus), meat charbroiling, and cigarettes. The particle mass distributions from all wood smoke sources have a single mode that peaks at approximately 0.1−0.2 μm particle diameter. The smoke from meat charbroiling shows a major peak in the particle mass distribution at 0.1−0.2 μm particle diameter, with some material present at larger particle sizes. Particle mass distributions from cigarettes peak between 0.3 and 0.4 μm particle diameter. Chemical composition analysis reveals that particles emitted from the sources tested here are largely composed of organic compounds. Noticeable concentrations of elemental carbon are found in the particles emitted from wood burning. The size distributions of the trace species emissions from these sources also are presented, including data for Na, K, Ti, Fe, Br, Ru, Cl, Al, Zn, Ba, Sr, V, Mn, Sb, La, Ce, as well as sulfate, nitrate, and ammonium ion when present in statistically significant amounts. These data are intended for use with air quality models that seek to predict the size distribution of the chemical composition of atmospheric fine particles.

Short-term particulate matter mass and aerosol-size distribution measurements: transient pollution episodes and bimodal aerosol-mass distributions

Aerosol-size distribution of fine aerosols were measured in Pocatello, Idaho along with hourly average PM10 mass concentrations. Aerosol size distribution measurements are made with a laser optical particle counter while short-term mass concentrations are measured with a Tapered Element Oscillating Microbalance. The short-term mass concentration measurements show wide variations in mass concentration over 24h periods. The accumulation mode of the aerosol-size distribution is bimodal in a few cases, primarily when elevated mass concentrations exist.

Comparison of modeled backscatter using measured aerosol microphysics with focused CW lidar data over Pacific

During NASA's GLObal Backscatter Experiment (GLOBE) II flight mission over the Pacific Ocean in May‐June 1990, extensive aerosol backscatter data sets from two continuous wave, focused CO2 Doppler lidars and an aerosol microphysics data set from a laser optical particle counter (LOPC) were obtained. Changes in aerosol loading in various air masses with associated changes in chemical composition, from sulfuric acid and sulfates to dustlike crustal material, significantly affected aerosol backscatter, causing variation of about 3 to 4 orders of magnitude. Some of the significant backscatter features encountered in different air masses were the low backscatter in subtropical air with even lower values in the tropics near the Intertropical Convergence Zone (ITCZ), highly variable backscatter in the ITCZ, mid‐tropospheric aerosol backscatter background mode, and high backscatter in an Asian dust plume off the Japanese coast. Differences in aerosol composition and backscatter for northern and southern hemisphere also were observed. Using the LOPC measurements of physical and chemical aerosol properties, we determined the complex refractive index from three different aerosol mixture models to calculate backscatter. These values provided a well‐defined envelope of modeled backscatter for various atmospheric conditions, giving good agreement with the lidar data over a horizontal sampling of ∼18,000 km in the mid‐troposphere.

Airborne studies of emissions from savanna fires in southern Africa: 1. Aerosol emissions measured with a laser optical particle counter

During the SAFARI‐92 experiment (Southern Africa Fire Atmosphere Research Initiative, September–October 1992), we flew an instrumented DC‐3 aircraft through plumes from fires in various southern African savanna ecosystems. Some fires had been managed purposely for scientific study (e.g., those in Kruger National Park, South Africa), while the others were “fires of opportunity” which are abundant during the burning season in southern Africa. We obtained the aerosol (0.1–3.0 μm diameter) number and mass emission ratios relative to carbon monoxide and carbon dioxide from 21 individual fires. The average particle number emission ratio ΔN/ΔCO (Δ: concentrations in plume minus background concentrations) varied between 14 ± 2 cm−3 ppb−1 for grasslands and 23 ± 7 cm−3 ppb−1 for savannas. An exceptionally high value of 43 ± 4 cm−3 ppb−1 was measured for a sugarcane fire. Similarly, the mass emission ratio ΔM/ΔCO varied from 36 ± 6 ng m−3 ppb−1 to 83 ± 45 ng m−3 ppb−1, respectively, with again an exceptionally high value of 124 ± 14 ng m−3 ppb−1 for the sugarcane fire. The number and mass emission ratios relative to CO depended strongly upon the fire intensity. Whereas the emission ratios varied greatly from one fire to the other, the aerosol number and volume distributions as a function of particle size were very consistent. The average background aerosol size distribution was characterized by three mass modes (0.2–0.4 μm, ≈1.0 μm, and ≈2.0 μm diameter). On the other hand, the aerosol size distribution in the smoke plumes showed only two mass modes, one centered in the interval 0.2–0.3 μm and the other above 2 μm diameter. From our mean emission factor (4 ± 1 g kg−1 dm) we estimate that savanna fires release some 11–18 Tg aerosol particles in the size range 0.1–3.0 μm annually, a somewhat lower amount than emitted from tropical forest fires. Worldwide, savanna fires emit some 3–8 × 1027 particles (in the same size range) annually, which is expected to make a substantial contribution to the cloud condensation nuclei population in the tropics.

Multiwavelength comparison of modeled and measured remote tropospheric aerosol backscatter over Pacific Ocean

Aerosol concentrations and size distributions in the middle and upper troposphere over the remote Pacific Ocean were measured with a forward scattering spectrometer probe (FSSP) on the NASA DC‐8 aircraft during NASA's Global Backscatter Experiment (GLOBE) in May–June 1990. The FSSP size channels were recalibrated based on refractive index estimates from flight‐level aerosol volatility measurements with a collocated laser optical particle counter (LOPC). The recalibrated FSSP size distributions were averaged over 100‐s intervals, fitted with lognormal distributions and used to calculate aerosol backscatter coefficients at selected wavelengths. The FSSP‐derived backscatter estimates were averaged over 300‐s intervals to reduce large random fluctuations. The smoothed FSSP aerosol backscatter coefficients were then compared with LOPC‐derived backscatter values and with backscatter measured at or near flight level from four lidar systems operating at 0.53, 1.06, 9.11, 9.25, and 10.59 μm. Agreement between FSSP‐derived and lidar‐measured backscatter was generally best at flight level in homogeneous aerosol fields and at high backscatter values. FSSP data often underestimated low backscatter values especially at the longer wavelengths due to poor counting statistics for larger particles (&gt;0.8 μm diameter) that usually dominate aerosol backscatter at these wavelengths. FSSP data also underestimated backscatter at shorter wavelengths when particles smaller than the FSSP lower cutoff diameter (0.35 μm) made significant contributions to the total backscatter.

Instrument intercomparison study on cloud droplet size distribution measurements: Holography vs. laser optical particle counter

During the EUROTRAC Ground Based Cloud Experiment (GCE) 1990, a newly developed HODAR (Holographic Droplet and Aerosol Recording) was operated for the first time to measure cloud droplet size distributions by recording Fraunhofer in-line holograms of small cloud sample volumes in situ and analyzing the holographic images in the laboratory.

Aircraft studies of size‐dependent aerosol sampling through inlets

Accurate measurement of atmospheric aerosol from aircraft platforms is of importance to a variety of research interests. Representative measurement of aerosol from aircraft‐aspirated systems requires special efforts in order to (1) maintain near isokinetic sampling conditions, (2) estimate aerosol losses in the sample system, and (3) obtain a measurement of sufficient duration to be statistically significant for all sizes of interest. This last point is especially critical for aircraft measurements which typically require fast response times while sampling in clean remote regions. Here we present size‐resolved tests, intercomparisons, and analysis of aerosol inlet performance as determined by a custom laser optical particle counter. Measurements discussed here took place during the Global Backscatter Experiment (1988–1989) and the Central Pacific Atmospheric Chemistry Experiment (1988). System configurations are discussed including (1) nozzle design and performance, (2) system transmission efficiency, (3) nonadiabatic effects in the sample line and its effect on sample line relative humidity, and (4) the use and calibration of a virtual impactor.

Aerosol size distribution, composition, and CO2 backscatter at Mauna Loa Observatory

Continuous measurements of aerosol size distributions were obtained during January–March and November–December periods of 1988 at Mauna Loa Observatory, Hawaii. These periods were chosen in order to characterize aerosol physicochemistry during periods representative of “low‐dust” atmospheric conditions and periods associated with appreciable Asian dust transport to that site. Size distributions for particles with diameters between 0.15 and 7.6 μm were accumulated in 256 size bins of a laser optical particle counter for 3‐hour intervals during most of the period. The aerosol sample stream was heated to selected temperatures in order to provide size‐discriminated measurements of aerosol volatility. Resulting data were used to assess the variability in aerosol concentrations and properties related to aerosol backscatter values at a wavelength of 10.6 μm, βCO2 in the mid‐troposphere. Low aerosol concentrations, considered representative of mid‐tropospheric air, occurred in downslope flow between midnight and sunrise. Our measurements for these time periods suggest that βCO2 varied from a low of about 5×10−12m−1sr−1 to a high of 5×10−8m−1Sr−1. Coarse particles with diameters between 1.0 and 5.0 μm account for most of the derived values of βCO2 at all but the highest and lowest aerosol mass concentrations. Volatile aerosol appears to dominate aerosol mass during the cleanest periods but was a small fraction of the total during dust events. During low‐dust periods the aerosol volatile at 320°C often appears as coatings on refractory (e.g., dust) aerosol and is expected to influence measured βCO2 values. This low dust condition is also expected to be most representative of aerosol at higher altitudes in the free troposphere. We estimate that minimum values for βCO2 at about 8 km should usually fall in the range of 1–3×10−12m−1sr−1 and be dominated by a sulfate aerosol.

Performance of Disposable Respirators

AbstractAn experimental study of the filtration performance of several commercially available, NIOSH‐approved, disposable respirators was conducted using monodisperse DOP and NaCl particles with diameters in the 0.035 to 4 μm range.The aerosol penetration through the respirators were measured at flow rates of 16, 28 and 48 L/m using a condensation nucleus counter, an aerodynamic particle sizer, and a laser optical particle counter for purposes of comparison. The results obtained by these instruments were found to be in good agreement with each other in the overlapping region of particle size of the instruments.The peak penetration through the respirator filters was found to range from approximately 1.2% to 30% at 16 L/m, 3.5% to 37% at 28 L/m, and 6% to 45% at 48 L/m. The most penetrating particle size was found to lie in the 0.1 to 0.4 μm diameter range. Further, no significant difference in penetration for NaCl and DOP particles was found, suggesting that the particlebounce effect was not important in the present study.