C. F. Rogers

Photoacoustic and filter measurements related to aerosol light absorption during the Northern Front Range Air Quality Study (Colorado 1996/1997)

A new photoacoustic instrument for the measurement of aerosol light absorption was collocated with conventional aerosol instrumentation during the 1996-1997 winter intensive monitoring period of the Northern Front Range Air Quality Study. Measurements of the light absorption efficiency for black carbon were 5 m 2 /g at 685 nm and 10 m 2 /g at 532 nm, and for elemental carbon, they were 3.6 m 2 /g at 685 nm. We show that these values together with previous photoacoustic measurements of aerosol light absorption shed some light on the wavelength dependence of absorption efficiency for carbonaceous aerosol in the visible and near-visible region. Integrating plate type filter measurements of aerosol light absorption result in far larger values than those measured with the photoacoustic instrument. We demonstrate that a recently published correction technique [Horvath. 1997] can yield improved agreement.

Exhaust Particle Size Distribution Measurements at the Tuscarora Mountain Tunnel

On-road particle size distributions were measured at the Tuscarora Mountain tunnel on the Pennsylvania Turnpike in May 1999. The data were obtained using a scanning mobility particle sizer. The nucleation modes of the size distributions contained most of the particles on a number concentration basis and exhibited peak diameters ranging from 11 to 17 nm. This observation is consistent with previous calculations and measurements, indicating that significant numbers of ultrafine aerosol particles can be expected in close proximity to busy motorways. The experiment provided 4 case studies for which the tunnel inlet data could be used to correct data obtained at the outlet, allowing for estimates of particle production within the tunnel. Exhaust particle production rates per vehicle kilometer were estimated; the results are presented with the caveat that the measurements were affected by ambient dilution. The 4 case study nucleation mode sizes varied inversely with ambient temperature. The light-duty vehicle contributions to the ultrafine particle distributions were apparently dominated by the heavy-duty vehicle contributions.

Methods for Real-Time, In Situ Measurement of Aerosol Light Absorption

Light extinction by aerosols is due to scattering and absorption. The anthropogenic contribution is generally dominated by light scattering by sulfate particles and light absorption by elemental carbon. While real-time, in situ instrumentation for the measurement of ambient light scattering exists and is widely used (i.e., nephelometers), no such instrumentation is currently in use for the sensitive measurement of ambient light absorption by aerosols. Instrumentation for this purpose has been developed in the past, mostly for the measurement of gaseous light absorption, but it has also been applied to the measurement of aerosol light absorption. This instrumentation is based on measuring the absorbed energy, as opposed to measuring light extinction, which is complicated by the scattering component and is also less sensitive. For aerosols, the absorbed energy heats the gas, leading to its thermal expansion. The two most sensitive techniques to detect this expansion are photoacoustic detection, in which the light source is modulated and the periodic expansion of the gas results in a sound wave at the modulation frequency, which may be detected with a microphone; and optical homodyne interferometry, in which the changed gas density is detected with a Mach-Zehnder type interferometer via the directly related change in refractive index. This article reviews the current state of both photoacoustic and interferometric detection methods. In addition, new ideas are discussed that are currently implemented by our group and should lead to substantial improvements. Size and reliability are being improved by utilizing modern, compact solid state lasers. New designs both for the photoacoustic cell and the interferometer promise to be less susceptible to acoustic background noise. In the case of the photoacoustic cell, the new design also virtually eliminates the previously dominant noise source, coherent window noise. Furthermore, an acoustic amplifier, based on the thermoacoustic effect, is being integrated into the photoacoustic cell to further improve its sensitivity.

Middle- and neighborhood-scale variations of PM10 source contributions in Las Vegas, Nevada

The Las Vegas Valley PM10 Study was conducted during 1995 to determine the contributions to PM10 aerosol from fugitive dust, motor vehicle exhaust, residential wood combustion, and secondary aerosol sources. Twenty-four-hr PM10 samples were collected at two neighborhood-scale sites every sixth day for 13 months. Five week-long intensive studies were conducted over a middle-scale sub-region at 29 locations that contained many construction projects emitting fugitive dust. The study found that the zone of influence around individual emitters was less than 1 km. Most of the sampling sites in residential and commercial areas yielded equivalent PM10 concentrations in the neighborhood region, even though they were more distant from each other than they were from the nearby construction sources. Based on chemical mass balance (CMB) receptor modeling, fugitive dust accounted for 80-90% of the PM10, and motor vehicle exhaust accounted for 3-9% of the PM10 in the Las Vegas Valley.

Design and testing of a new size classifying isokinetic sequential aerosol sampler

Two versions of a size-classifying isokinetic sequential aerosol sampler (SCISAS) have been designed, built, tested, and deployed in a field program in the southwestern US. The SCISAS units can operate at unattended sites, exposing four or more filter types simultaneously, in two size ranges, for six sampling time intervals. Design considerations included theoretical estimates of aerosol particle losses in the 0-15 {mu}m size range. SCISAS prototypes have been tested to evaluate their sampling efficiency as a function of flow rate, the sensitivity of the sampling efficiency to isokinetic matching within the SCISAS sampling stack, the equivalency of their sampling ports, and their passive deposition characteristics. The prototypes were also compared to several other types of aerosol filtration samplers already in common use. These tests show that particle loss mechanisms within the SCISAS usually cause no more than 5% losses, and that the SCISAS units agree, within one to two measurement uncertainty intervals, with other types of aerosol samplers.