Jonathan O. Allen

The chemical composition of atmospheric ultrafine particles

Atmospheric ultrafine particles (with diameter less than 0.1 μm) may be responsible for some of the adverse health effects observed due to air–pollutant exposure. To date, little is known about the chemical composition of ultrafine particles in the atmosphere of cities. Ultrafine particle samples collected by inertial separation on the lower stages of cascade impactors can be analysed to determine a material balance on the chemical composition of such samples. Measurements of ultrafine particle mass concentration made in seven Southern California cities show that ultrafine particle concentrations in the size range 0.056–0.1 μm aerodynamic diameter average 0.55–1.16 μg m−3. The chemical composition of these ultrafine particle samples averages 50% organic compounds, 14% trace metal oxides, 8.7% elemental carbon, 8.2% sulphate, 6.8% nitrate, 3.7% ammonium ion (excluding one outlier), 0.6% sodium and 0.5% chloride. The most abundant catalytic metals measured in the ultrafine particles are Fe, Ti, Cr, Zn, with Ce also present. A source emissions inventory constructed for the South Coast Air Basin that surrounds Los Angeles shows a primary ultrafine particle emissions rate of 13 tonnes per day. Those ultrafine particle primary emissions arise principally from mobile and stationary fuel combustion sources and are estimated to consist of 65% organic compounds, 7% elemental carbon, 7% sulphate, 4% trace elements, with very small quantities of sodium, chloride and nitrate. This information should assist the community of inhalation toxicologists in the design of realistic exposure studies involving ultrafine particles.

Source apportionment of lead-containing aerosol particles in Shanghai using single particle mass spectrometry.

Lead (Pb) in individual aerosol particles was measured using single particle aerosol mass spectrometer (ATOFMS) in the summer of 2007 in Shanghai, China. Pb was found in 3% of particles with diameters in the range 0.1-2.0 microm. Single particle data were analyzed focusing on the particles with high Pb content which were mostly submicron. Using the ART-2a neural network algorithm, these fine Pb-rich particles were classified into eight main classes by their mass spectral patterns. Based on the size distribution, temporal variation of number density, chemical composition and the correlation between different chemical species for each class, three major emission sources were identified. About 45% of the Pb-rich particles contained organic or elemental carbon and were attributed to the emission from coal combustion; particles with good correlation between Cl and Pb content were mostly attributed to waste incineration. One unique class of particles was identified by strong phosphate and Pb signals, which were assigned to emissions from phosphate industry. Other Pb-rich particles included aged sea salt and particles from metallurgical processes.

Atmospheric deposition of carbon and nutrients across an arid metropolitan area.

Urbanization is increasing rapidly in semi-arid environments and is predicted to alter atmospheric deposition of nutrients and pollutants to cities as well as to ecosystems downwind. We examined patterns of wet and coarse dry deposition chemistry over a five-year period at 7 sites across the Central Arizona-Phoenix (CAP) study area, one of two urban sites within the National Science Foundations Long-Term Ecological Research (LTER) program. Wet and dry deposition of organic carbon (oC) were significantly elevated in the urban core; in contrast, mean annual wet and dry fluxes of nitrogen (N) were low (<6 kg ha(-1) yr(-1)) compared to previous estimates and did not differ significantly among sites. Wet deposition of sulfate (SO(4)2-) was high across CAP (mean 1.39 kg ha(-1) yr(-1) as S) and represented the dominant anion in rainfall. Dry deposition rates did not show strong seasonal trends with the exception of oC, which was 3-fold higher in winter than in summer; ammonium (NH4+) deposition was high but more variable. Dry deposition of NO3- and oC was strongly correlated with particulate base cations and dust-derived soluble reactive phosphorus (SRP), suggesting that urban-derived dust is scrubbing the atmosphere of acidic gases and entrained particles and increasing local deposition. Differences between measured and predicted rates of dry N deposition to the urban core may be explained by incomplete collection of gas phase N on surrogate deposition surfaces in this hot and arid environment. The extent of urban enhancement of cations and oC inputs to desert ecosystems appears to be restricted to the urbanized metropolitan area rather than extending far downwind, although a low number of sites make it difficult to resolve this spatial pattern. Nevertheless, wet and dry inputs may be important for biogeochemical cycles in nutrient and carbon-poor desert ecosystems within and near arid cities.

Foam droplets generated from natural and artificial seawaters

[1]xa0Submicrometer sea salt aerosol (SSA) particles are routinely observed in the remote marine boundary layer (MBL); these aerosols include cloud condensation nuclei and so affect the earths radiative balance. Here foams designed to mimic oceanic whitecaps were generated in the laboratory using a range of bubbling flow rates and aqueous media: unfiltered seawater, filtered seawater, artificial seawater, and mixtures of filtered and artificial seawater. The number and sizes of dried foam droplets in the particle diameter, Dp, range 15–673 nm were measured. Particle size distributions for natural and artificial seawaters were unimodal with a dN/d logDp mode at Dp ≈ 100 nm (≈200 nm at 80% RH). The foam droplet mode falls within the range of reported mode diameters (Dp = 40–200 nm) for submicrometer SSA particles observed in the remote MBL. The present laboratory results were scaled up to estimate submicrometer SSA particle fluxes; this extrapolation supports the hypothesis that foam droplets are the most important source of SSA particles by number. The foam droplet flux from the oceans was estimated to be 980 cm−2 s−1 for a fractional white cap coverage, W, of 0.2%. These results compared well with foam droplet fluxes reported elsewhere. The origins of variability in foam droplet fluxes were also evaluated. Natural organic matter affected foam droplet flux by a factor of 1.5; this was less than (1) the effect of bubbling flow rate on foam droplet flux (factor of 5) and (2) the uncertainty in W (factor of 3–7).

Hygroscopic Behavior and Liquid-Layer Composition of Aerosol Particles Generated from Natural and Artificial Seawater

[1] Sea-salt aerosol (SSA) particles affect the Earths radiative balance and moderate heterogeneous chemistry in the marine boundary layer. Using conventional and environmental transmission electron microscopes (ETEM), we investigated the hygroscopic growth and liquid-layer compositions of particles generated from three types of aqueous salt solutions: sodium chloride, laboratory-synthesized seawater (S-SSA particles), and natural seawater (N-SSA particles). Three levels of morphological change were observed with the ETEM as the laboratory-generated particles were exposed to increasing relative humidity (RH). The first level, onset of observable morphological changes, occurred on average at 70, 48, and 35% RH for the NaCl, S-SSA, and N-SSA particles, respectively. The second level, rounding, occurred at 74, 66, and 57% RH for NaCl, S-SSA, and N-SSA particles, respectively. The third level, complete deliquescence, occurred at 75% RH for all particles. Collected ambient SSA particles were also examined. With the exception of deliquescence, they did not exhibit the same hygroscopic characteristics as the NaCl particles. The ambient particles, however, behaved most similarly to the synthesized and natural SSA particles, although the onset of morphological change was slightly higher for the S-SSA particles. We used energy-dispersive X-ray spectrometry to study the composition of the liquid layer formed on the S-SSA and N-SSA particles. The layer was enriched in Mg, S, and O relative to the solid particle core. An important implication of these results is that MgSO 4 -enriched solutions on the surface of SSA particles may be the solvents of many heterogeneous reactions.

Synthetic musk emissions from wastewater aeration basins.

Wastewater aeration basins at publicly owned treatment works (POTWs) can be emission sources for gaseous or aerosolized sewage material. In the present study, particle and gas phase emissions of synthetic musks from covered and uncovered aeration basins were measured. Galaxolide (HHCB), tonalide (AHTN), and celestolide (ADBI) were the most abundant, ranging from 6704 to 344,306 ng m(-3), 45-3816 ng m(-3), and 2-148 ng m(-3) in the gas phase with particle phase concentrations 3 orders of magnitude lower. The musk species were not significantly removed from the exhaust air by an odor control system, yielding substantial daily emission fluxes (∼ 200 g d(-1) for HHCB) into the atmosphere. However, simple dispersion modeling showed that the treatment plants are unlikely to be a major contributor to ambient air concentrations of these species. Emission of synthetic musk species during wastewater treatment is a substantial fate process; more than 14% of the influent HHCB is emitted to the atmosphere in a POTW as opposed to the <1% predicted by an octanol-water partition coefficient and fugacity-based US EPA fate model. The substantial atmospheric emission of these compounds is most likely due to active stripping that occurs in the aeration basins by bubbling air through the sludge.

Quantitative extraction of organic tracer compounds from ambient particulate matter collected on polymer substrates.

Organic compounds in ambient particulate matter (PM) samples are used as tracers for PM source apportionment. These PM samples are collected using high volume samplers; one such sampler is an impactor in which polyurethane foam (PUF) and polypropylene foam (PPF) are used as the substrates. The polymer substrates have the advantage of limiting particle bounce artifacts during sampling; however these substrates may contain background organic additives. A protocol of two extractions with isopropanol followed by three extractions with dichloromethane (DCM) was developed for both substrate precleaning and analyte extraction. Some residual organic contaminants were present after precleaning; expressed as concentrations in a 24-h ambient PM sample, the residual amounts were 1 microg m(-3) for plasticizers and antioxidants, and 10 ng m(-3) for n-alkanes with carbon number lower than 26. The quantification limit for all other organic tracer compounds was approximately 0.1 ng m(-3) in a 24-h ambient PM sample. Recovery experiments were done using NIST Standard Reference Material (SRM) Urban Dust (1649a); the average recoveries for polycyclic aromatic hydrocarbons (PAHs) from PPF and PUF substrates were 117+/-8% and 107+/-11%, respectively. Replicate extractions were also done using the ambient samples collected in Nogales, Arizona. The relative differences between repeat analyses were less than 10% for 47 organic tracer compounds quantified. After the first extraction of ambient samples, less than 7% of organic tracer compounds remained in the extracted substrates. This method can be used to quantify a suite of semi- and non-polar organic tracer compounds suitable for source apportionment studies in 24-h ambient PM samples.

Instrument Busy Time and Mass Measurement using Aerosol Time-of-Flight Mass Spectrometry

Aerosol Time-of-Flight Mass Spectrometry (ATOFMS) instruments have been used widely to measure the size and composition of single ambient aerosol particles. ATOFMS data do not directly and quantitatively represent aerosol composition because the instruments exhibit non-linear response to particle concentration, size, and composition. Our approach is to analyze separately the components of non-linear ATOFMS response using field sampling data in order to understand ATOFMS response to ambient aerosols so that ATOFMS data can be scaled to more closely represent ambient aerosols. In this work we examine the effect of instrument busy time, mainly the time to process and save data, on ATOFMS response to ambient aerosols sampled during the 1999 Bakersfield Instrument Intercomparison Study (BIIS). During this study an ATOFMS instrument was operated alternately in normal and fast scatter data acquisition modes. In fast scatter mode, the instrument does not record mass spectra, minimizing instrument busy time; these data were used to determine particle arrival rates. Busy time in normal mode was found by a comparison of the number of particles detected to that expected for a Poisson process modified to include busy time. During the BIIS experiment, the ATOFMS instrument was busy between 5 and 95% of the nominal sampling time; thus busy time cannot be ignored for accurate quantitative analysis of ATOFMS data. ATOFMS data were scaled for on-line time and transmission efficiency, found by comparison with reference aerosol measurements, in order to estimate fine particle mass concentrations. Fine aerosol mass concentrations from scaled ATOFMS data demonstate semi-quantitative agreement with independent measurements using Beta Attenuation Monitors. We recommend that ATOFMS instruments be modified to measure busy time directly. Present Address: National Oceanic and Atmospheric Administration, Atmospheric Sciences Modeling Division, Mail Stop 876006, Research Triangle Park, North Carolina, USA.

Absorption of semi-volatile compounds in oiled impaction substrates : Measurement of pyrene absorption

Oiled impaction substrates have been used to prevent particle bounce during the collection of size-segregated aerosol samples, which have been analyzed for trace-level airborne organic compounds, including polycyclic aromatic hydrocarbons (PAHs). The use of the oiled impaction substrates, however, may introduce another sampling artifact-the absorption of semivolatile species from the gas phase which could artificially increase the amount of PAHs attributed to the aerosol. In this article, laboratory measurements of the absorption of a particular PAH, pyrene, from the gas phase to impaction substrates of polytetrafluoroethylene membranes impregnated with dibutyl phthalate are reported. Overall mass transfer coefficients are determined from the data. These results are used to calculate the absorption of gas phase PAHs during sampling of size-segregated atmospheric particles. Criteria are developed to determine if the absorption artifact is negligible. The first criterion requires that the analyte be negligi...

Impact of Asphalt Rubber Friction Course Overlays on Tire Wear Emissions and Air Quality Models for Phoenix, Arizona, Airshed

Tire wear contributes to atmospheric particulate matter (PM) and is regulated by the U.S. Environmental Protection Agency because PM has been shown to affect human health. Vehicle emissions are a significant source of both PM2.5 and PM10. Vehicle fleet emissions per mile traveled have been reduced significantly in the past 30 years as a result of improved engine operation and tailpipe controls. However, “zero emission” vehicles will continue to generate PM from tire wear, road wear, brake wear, and resuspended road dust. In this study, aerosol measurement techniques at Arizona State University were applied to evaluate tire wear emissions from the vehicle fleet by using the Deck Park Tunnel in Phoenix, Arizona. The Deck Park Tunnel highway surface was portland cement concrete (PCC) and was resurfaced with an asphalt rubber friction course (ARFC) layer as part of the Arizona Department of Transportation Quiet Pavements Program. This study took advantage of a rare opportunity to sample tire wear emissions at the tunnel before and after the ARFC overlay. The hypothesis was that an ARFC surface results in less tire wear than the existing PCC road surface. This paper reports on the measured PM emissions from the on-road vehicle traffic during typical highway driving conditions for the two different roadway surfaces. It presents the analysis of representative tire tread samples for tire wear marker compounds and a comparison of roughness and frictional surface characteristics as measured before and after the ARFC overlay. The study found that emission rates of tire wear per kilometer driven on PCC road surfaces were 1.4 to 2 times higher than emission rates of tire wear on ARFC road surfaces.