Susanne V. Hering

Origins of aerosol sulfur size distributions in the Los Angeles basin

Low pressure impactor measurements show two distinct types of fine particle sulfur size distributions in Los Angeles, California. These two types of aerosol sulfur have mass median diameters of 0.54 ± 0.07μm and 0.20 ± 0.02 μm, respectively. Factors which may account for the two distribution types, including effects of relative humidity, coagulation, fogs and formation mechanisms, are discussed. Calculations show the 0.5 μm sulfur is consistent with chemical reactions in the aerosol phase, whereas the 0.2 μm sulfur results from homogeneous gas phase SO2 oxidation. While the growth of the total aerosol volume distribution is not too sensitive to the mechanism, the chemical species distribution strongly depends on the growth law, and can be used to establish its form.

Greased and oiled substrates as bounce-free impaction surfaces

Abstract Oiled membrane filters and greased surfaces were evaluated for suitability as collection surfaces in cascade impactors. Collection efficiencies of dry, solid particles were measured as a function of substrate loading. Efficiencies of greased surfaces decreased dramatically with particle loading. An oil coated 10 μm Teflon membrane filter gave high collection efficiencies, which did not decrease with substrate loading. A minimum of 2 μl of oil applied to the center of the filter is required. The impactor cutpoints were unchanged by the collection surface. Other oiled membrane filters tested behaved less ideally.

The nitric acid shootout: field comparison of measurement methods

Eighteen instruments for measuring atmospheric concentrations of nitric acid were compared in an eight day field study at Pomona College, situated in the eastern portion of the Los Angeles Basin, in September 1985. The study design included collocated and separated duplicate samplers, and the analysis by each laboratory of a set of quality assurance filters, so that the experimental variability could be distinguished from differences due to measurement methods. nFor all sampling periods, the values for nitric acid concentrations reported by the different instruments vary by as much as a factor of four. The differences among measurement techniques increase with nitric acid loading, corresponding to a coefficient of variation of 40%. In contrast, samplers of the same design operated by the same group show variability of 11–27 %. nOverall, the highest reported concentrations are observed with the filter packs and lower concentrations are observed by the annular denuders and tunable diode laser absorption spectrometers. When the nitric acid concentrations are high enough to be detected by the FTIR, the FTIR values are close to those obtained by the denuder difference method and to the mean value from the other sampler groups. nIn the absence of a reference standard for the entire study, measurement methods are compared to the average of four denuder difference method samplers (DDM). Filter pack samplers are higher than the DDM for both daytime and night-time sampling. Two different filter packs using Teflon® prefilters are higher than the DDM by factors of 1.25 and 1.4. The results from the three annular denuders do not agree; the ratios of means to the DDM value are 1.0,0.8 and 0.6. For the transition flow reactor method and for two dichotomous samplers operated as denuder difference samplers, the ratio of means to the DDM are 1.09 and 0.93, respectively. The tunable diode laser absorption spectrometers gave lower daytime and higher night-time readings compared to the DDM, especially during the last three days of the study. Averaged over the entire measurement period, the daytime ratio of TDLAS to DDM is 0.8 and the night-time ratio is 1.7.

Hypersonic impaction of ultrafine particles

The performance of a highly supersonic aerosol impactor as a size-discriminating instrument is explored experimentally with ultrafine particles having diameters as small as 51 A. A hypersonic jet is formed by expansion of a gas-particle mixture through an orifice of diameter dn, from a source region maintained at a pressure po into an evacuated region kept at a background pressure p1 several hundred times smaller than po. Facing the jet perpendicularly, at a variable distance, L, from the nozzle exit, is a bounce-free flat target plate which collects a fraction, E, of the incoming particles. The collection efficiency, E(dp, L), is determined on-line by using monodisperse charged particles of diameter dp and measuring the electrical current they transport to the conducting target plate, which is grounded through an electrometer. The impactor shows sharp separation efficiency curves E(dp, L) only when L is smaller than some critical distance L∗. For L > L∗, the curves E(L) become non-monotonic as a result of some gas dynamic reasons not fully understood yet. Provisionally, over the limited range of pressure ratios explored, it appears that L∗dn = 0.13 √(pop1). The size discrimination behaviour deteriorates also when Ldn becomes smaller than one, though this lower limit seems to be somewhat more flexible and geometry dependent than the upper one. Within the region 0.8 < Ldn < 0.13 √(pop1), E(dp, L) depends in a step-like manner on both these variables. The relatively sharp transition between E = 0 and E = 1 occurs roughly when the Stokes number in the impact region, S ∼ 32SoLdn, is approximately one, where So is a standard Stokes number based on the nozzle exit diameter, dn, and the gas sound speed, co, at source conditions. For particles sufficiently small to be in the free-molecule limit, So = 0.1983 ρpdpco2(dnpo), where ρp is their density. The fact that the fundamental variable S governing the impaction process is proportional to the product Ldp, makes these two factors conjugates of each other, and allows determining the diameter of an unknown monodisperse aerosol by measuring the value of L at which the capture efficiency E undergoes a step. This feature makes it possible to operate this impactor as an aerodynamic size spectrometer for ultrafine particles. With a pumping capacity of 500 l/min and an electrical noise level of 10−16 A, we have been able to ‘measure’ NaCl particles with a diameter as small as 51 A, and to discriminate between particle diameters only 3–5 A apart.

Comparison of sampling methods for carbonaceous aerosols in ambient air

Measurement methods for fine particle carbon were compared under field sampling conditions. Sampling methods included filtration, impaction, and adsorption-corrected filtration. Systems were operated side-by-side for nine consecutive days in the Los Angeles Basin. Analytical methods were compared separately. For organic carbon and total carbon, ambient measurements showed greater variability than could be accounted for by differences in analytical methods, and these differences are attributed to sampling methodology. The highest reported concentrations were obtained by quartz filter sampling; the lowest were from the sum of the impactor stages (excluding the quartz afterfilters). Positive artifact from the adsorption of gaseous vapors on quartz fiber filters was significant. However, correction for vapor adsorption by subtraction of the carbon value on the second of two filters in series did not completely eliminate the dependence of the apparent total aerosol carbon concentration on filter face velocity ...

Regional haze case studies in the southwestern U.S—I. Aerosol chemical composition

Abstract Aerosol chemical composition as a function of particle size was determined in the southwestern U.S.A. during four weeks of sampling in June, July and December, 1979 as a part of project VISITA. Samples were collected at two ground stations about 80 km apart near Page (AZ) and in two aircraft flying throughout the region. Several different size separating aerosol samplers and chemical analysis procedures were intercompared and were used in determining the size distribution and elemental composition of the aerosol. Sulfur was shown to be in the form of water soluable sulfate, highly correlated with ammonium ion, and with an average [NH + 4 ]/[SO 2− 4 ] molar ratio of 1.65. During the summer sampling period, three distinct regimes were observed, each with a different aerosol composition. The first, 24 h sampling ending 30 June, was characterized by a higher than average value of light scattering due to particles (b sp ) of 24 × 10 −6 m −1 and a fine particulate mass ( M f ) of 8.5 μg m −1 . The fine particle aerosol was dominated by sulfate and carbon. Aircraft measurements showed the aerosol was homogeneous throughout the region at that time. The second regime, 5 July, had the highest average b sp of 51 × 10 −6 m −1 during the sampling period with M f of 3.2 μgm −3 . The fine particle aerosol had nearly equal concentrations of carbon and ammonium sulfate. For all three regimes, enrichment factor analysis indicated fine and coarse particle Cu, Zn, Cl, Br, and Pb and fine particle K were enriched above crustal concentrations relative to Fe, indicating that these elements were present in the aerosol from sources other than wind blown dust. Particle extinction budgets calculated for the three regimes indicated that fine particles contributed most significantly, with carbon and (NH 4 ) 2 SO 4 making the largest contributions. Fine particle crustal elements including Si did not contribute significantly to the extinction budget during this study. The December sampling was characterized by very light fine particle loading with two regimes identified. One regime had higher fine mass and sulfate concentrations while the other had low values for all species measured.

Calibration of the QCM Impactor for Stratospheric Sampling

The performance of the QCM impactor (Model 3000, California Measurements, Sierra Madre, CA) has been evaluated at inlet pressures of 59 and 88 Torr, corresponding to the conditions of stratospheric sampling aboard the U-2 aircraft. Impactor mass flow rates and stage operating pressures were measured as a function of inlet pressure. The collection efficiencies of monodisperse liquid oleic acid aerosols were measured for individual impactor stages by counting particle penetration with an optical particle counter. Modification of the optical counter for this low pressure application is described. Response of the assembled QCM to near-mono-disperse Apiezon grease aerosols was measured using the signal from the quartz crystal impaction substrates. Aerodynamic cutoff diameters as a function of inlet pressure agree with impaction theory for most stages. Good comparison is found between the response of the QCM microbalance impaction plates and the optical particle counter data. For solid particles the QCM collect...

Characterization of the regional haze in the southwestern United States

Abstract As part of project VISTTA, aircraft sampling flights over non-urban portions of Arizona and Utah were conducted on 6 days in June and July 1979. The spatial distribution of “regional haze” is mapped in terms of the particle light scattering coefficient. Fine particle elemental composition was obtained from impactor and filter samples integrated over each flight path. One sampling day, 6/29/79, is characterized by reduced visibility throughout the sampling region. Possible sources are discussed.

Desorptivity Versus Chemical Reactivity of Polycyclic Aromatic Hydrocarbons (PAHs) in Atmospheric Aerosols Collected on Quartz Fiber Filters

Nine polycyclic aromatic hydrocarbons (PAHs) contained in air samples collected on quartz fiber filters inside an urban tunnel and in a nearby mixed commercial residential area in the city of Rio de Janeiro, Brazil, were exposed to scrubbed air (to measure desorption loss) and to particle-free ambient air (to measure chemical reaction losses in the absence of desorption). The exposures were conducted for 5.5 to 9 hour periods at ambient temperature (22-26 degrees C) at face velocities typical of high volume sampling. Under prevailing atmospheric conditions all nine PAHs experienced filter losses which (for most of them) followed first order kinetics. For the ambient samples, in a 6 hour exposure period, the following five PAHs showed filter losses (% in parantheses) attributed exclusively to chemical reaction: benzo(b)fluoranthene (43), benzo(k)fluoranthene (39), benzo(a)pyrene (70), benzo(ghi)perylene (44), and indeno (1,2,3-cd)pyrene (41). The other four showed the following unassigned losses: pyrene (100), fluoranthene (65), crysene (72), and benzo(a)anthracene (71). The results are discussed in the light of possible filter artifacts in PAH sampling and the use of PAH profile signatures for source identification of atmospheric particulate matter in receptor modeling.

A direct FTIR method for identifying functional groups, in size segregated atmospheric aerosols

Abstract A direct method for characterizing organic material in atmospheric aerosols has been devised. Size segregated particulates, collected on ZnSe disks, were analyzed using Fourier Transform Infrared (FTIR) spectroscopy. The FTIR spectra were used to identify compound classes present in the aerosols; the distribution of functional groups varied with particle size and depended on whether the aerosol was collected at a primary or secondary receptor site.