Dean B. Atkinson

The Reno Aerosol Optics Study: An Evaluation of Aerosol Absorption Measurement Methods

The Reno Aerosol Optics Study (RAOS) was designed and conducted to compare the performance of many existing and new instruments for the in situ measurement of aerosol optical properties with a focus on the determination of aerosol light absorption. For this study, simple test aerosols of black and white particles were generated and combined in external mixtures under low relative humidity conditions and delivered to each measurement system. The aerosol mixing and delivery system was constantly monitored using particle counters and nephelometers to ensure that the same aerosol number concentration and amount reached the different instruments. The aerosol light-scattering measurements of four different nephelometers were compared, while the measurements of seven light-absorption instruments (5 filter based, 2 photoacoustic) were evaluated. Four methods for determining the aerosol light-extinction coefficient (3 cavity ring-down instruments and 1 folded-path optical extinction cell) were also included in the comparisons. An emphasis was placed on determining the representativeness of the filter-based light absorption methods, since these are used widely and because major corrections to the raw attenuation measurements are known to be required. The extinction measurement from the optical extinction cell was compared with the scattering measurement from a high-sensitivity integrating nephelometer on fine, nonabsorbing ammonium sulfate aerosols, and the two were found to agree closely (within 1% for blue and green wavelengths and 2% for red). The wavelength dependence of light absorption for small kerosene and diesel soot particles was found to be very near λ− 1, the theoretical small-particle limit. Larger, irregularly shaped graphite particles showed widely variable wavelength dependencies over several graphite runs. The light-absorption efficiency at a wavelength of 530 nm for pure kerosene soot with a number size distribution peak near 0.3 μ m diameter was found to be 7.5 ± 1.2 m2 g− 1. The two most fundamental independent absorption methods used in this study were photoacoustic absorption and the difference between suspended-state light extinction and scattering, and these showed excellent agreement (typically within a few percent) on mixed black/white aerosols, with the photoacoustic measurement generally slightly lower. Excellent agreement was also observed between some filter-based light-absorption measurements and the RAOS reference absorption method. For atmospherically relevant levels of the aerosol light-absorption coefficient (< 25 Mm− 1), the particle soot absorption photometer (PSAP) absorption measurement at mid-visible wavelengths agreed with the reference absorption measurement to within ∼ 11% for experiment tests on externally mixed kerosene soot and ammonium sulfate. At higher absorption levels (characterized by lower single-scattering albedo aerosol tests), this agreement worsened considerably, most likely due to an inadequate filter loading correction used for the PSAP. The PSAP manufacturers filter loading correction appears to do an adequate job of correcting the PSAP absorption measurement at aerosol single-scattering albedos above 0.80–0.85, which represents most atmospheric aerosols, but it does a progressively worse job at lower single-scattering albedos. A new filter-based light-absorption photometer was also evaluated in RAOS, the multiangle absorption photometer (MAAP), which uses a two-stream radiative transfer model to determine the filter and aerosol scattering effects for a better calculation of the absorption coefficient. The MAAP absorption measurements agreed with the reference absorption measurements closely (linear regression slope of ∼ 0.99) for all experimental tests on externally mixed kerosene soot and ammonium sulfate.

A portable pulsed cavity ring-down transmissometer for measurement of the optical extinction of the atmospheric aerosol

A small portable system is described which is used to directly determine the optical extinction of the atmospheric aerosol. The requisite highly sensitive measurement of the optical extinction is accomplished simultaneously at two wavelengths in the near-infrared (1064 nm) and visible (532 nm), using the pulsed cavity ring-down (CRD) approach. The measurement at the two wavelengths can aid in separating the scattering and absorption components of the optical extinction. Rayleigh equivalent optical extinction of approximately 10 x 10(-6) m(-1) from particulate matter in the atmospherically important 0.1-2.5 pm diameter size range (fine particle accumulation mode) can be readily observed with short (<5 s) integration times. Optical extinction is inversely related to the visual range, and so the instrument provides a direct measurement of this particulate-related air quality indicator. The instrument can also provide particle size range-selected multiwavelength optical property measurements, which can be inverted to provide valuable information about the extant airborne particulate distribution.

Solving chemical problems of environmental importance using cavity ring-down spectroscopy

Cavity ring-down (CRD) is a sensitive variant of traditional absorption spectroscopy that has found increasing use in a number of chemical measurement applications. This review focuses on applications of cavity ring-down spectroscopy that will be of interest to environmental chemists and analytical chemists working on environmental problems. The applications are classified into direct monitoring approaches, indirect analysis methods and ancillary studies and a differentiation is made between field-tested instruments and proof of principle studies.

Discrete Sums for the Rapid Determination of Exponential Decay Constants

Several computational methods are presented for the rapid extraction of decay time constants from discrete exponential data. Two methods are found to be comparably fast and highly accurate. They are corrected successive integration and a method involving the Fourier transform (FT) of the data and the application of an expression that does not assume continuous data. FT methods in the literature are found to introduce significant systematic error owing to the assumption that data are continuous. Corrected successive integration methods in the literature are correct, but we offer a more direct way of applying them which we call linear regression of the sum. We recommend the use of the latter over FT-based methods, as the FT methods are more affected by noise in the original data.

Laboratory Validation of Aerosol Extinction Coefficient Measurements by a Field-Deployable Pulsed Cavity Ring-Down Transmissometer

A pulsed cavity ring-down transmissometer is shown here to be capable of sensitively measuring the aerosol extinction coefficient at two wavelengths (λ = 532, 1064 nm) simultaneously. This instrument can be coupled with a nephelometer (yielding a measurement of the 530 nm scattering coefficient of the same aerosol) and a particle counter to allow the in situ measurement of extensive and intensive optical properties and particle number concentrations of aerosols. From the scattering and extinction coefficient measurements, the aerosol absorption coefficient can be calculated and the intensive properties single scattering albedo and extinction Ångström exponent can be determined. In this report, the pulsed cavity-ring down transmissometer (CRDT) is validated through a series of laboratory experiments. Agreement between the extinction coefficients from the cavity ring-down transmissometer and the scattering coefficient from the nephelometer is demonstrated for purely scattering sub-micron particles. Then agreement between measured extinction and Mie theory is demonstrated when using size-selected particles of measured number concentration. The agreement with Mie theory ranges from excellent (less than 1% deviation) to marginal (12%) depending on the particle size and composition. Similar deviations from Mie theory have been observed (Baynard et al. 2007) and we suggest that they could be due to the influence of multiply charged particles in the size-selection (DMA) process. The 95% confidence level limits of detection and quantitation for the extinction measurement by the CRD transmissometer are estimated to be bext = 4.0 Mm−1 and 13.4 Mm−1, respectively (Skoog et al. 2004).

Evolution of multispectral aerosol optical properties in a biogenically-influenced urban environment during the CARES campaign

Ground-based aerosol measurements made in June 2010 within Sacramento urban area (site T0) and at a 40-km downwind location (site T1) in the forested Sierra Nevada foothills area are used to investigate the evolution of multispectral optical properties as the urban aerosols aged and interacted with biogenic emissions. Along with black carbon and non-refractory aerosol mass and composition observations, spectral absorptio (β abs ), scattering (β sca ), and extinction (β ext ) coefficients for wavelengths ranging from 355 to 1064 nm were measured at both sites using photoacoustic (PA) instruments with integrating nephelometers and using cavity ring-down (CRD) instruments. The daytime average Angstrom exponent of absorption (AEA) was ~1.6 for the wavelength pair 405 and 870 nm at T0, while it was ~1.8 for the wavelength pair 355 and 870 nm at T1, indicating a modest wavelength-dependent enhancement of absorption at both sites throughout the study. The measured and Mie theory calculations of multispectral β sca showed good correlation ( R 2 =0.85–0.94). The average contribution of supermicron aerosol (mainly composed of sea salt particles advected in from the Pacific Ocean) to the total scattering coefficient ranged from less than 20% at 405 nm to greater than 80% at 1064 nm. From 22 to 28 June, secondary organic aerosol mass increased significantly at both sites due to increased biogenic emissions coupled with intense photochemical activity and air mass recirculation in the area. During this period, the short wavelength scattering coefficients at both sites gradually increased due to increase in the size of submicron aerosols. At the same time, BC mass-normalized absorption cross-section (MAC) values for ultraviolet wavelengths at T1 increased by ~60% compared to the relatively less aged urban emissions at the T0 site. In contrast, the average MAC values for 870 nm wavelength were identical at both sites. These results suggest formation of moderately brown secondary organic aerosols formed in biogenically-influenced urban air.

Optical injection unlocking for cavity ringdown spectroscopy

Continuous wave cavity ringdown spectroscopy requires a rapid termination of the injection of light into the cavity to initiate the decay (i.e., ringdown) event. We demonstrate a technique that accomplishes this through pulsed optical injection of a second laser into the main laser, resulting in 20-100 MHz frequency shifts in the otherwise cavity-locked main laser sufficient to create ringdown events at 3.5 kHz. Data on the frequency shift as a function of both main laser current and relative wavelength are presented, as well as a demonstration that single exponential decays are maintained in the process.

An atmospheric pressure static reactor – ion trap mass spectrometer for studying gas-phase reactions

The design and operation of an atmospheric pressure static reactor coupled to an ion trap mass spectrometer is described. The reactor is designed for studying gas-phase reactions that are important in atmospheric chemistry. The sys- tem provides a simple and robust method for identifying the products of gas-phase reactions. Results for the reaction of O3 with 2,3-dimethyl-2-butene (tetramethylethylene, TME) are demonstrated as a proof of the principle for the performance of the static reactor. All of the previously reported major primary products of the reaction were observed, and the yields of two compounds (acetone and hydroxyacetone) were quantified, in excellent agreement with previous work. Several mi- nor species were also observed, demonstrating the potential for this method to investigate the product channels for less well-studied atmospherically relevant reactions.

Evaluation of the reactivity of exhaust from various biodiesel blends as a measure of possible oxidative effects: A concern for human exposure

Diesel exhaust particles (DEP) are a major constituent of ambient air pollution and are associated with various adverse health effects, posing a major safety and public health concern in ambient and occupational environments. The effects of DEP from various biodiesel blends on biological systems was investigated using glutathione (GSH) as a marker of possible oxidative effects, based on the decrease in the concentration of GSH at physiological pH. The fluorophoric agent 2,3-naphthalenedicarboxaldehyde (NDA) was used as a selective probe of GSH in the presence of any likely interferents via fluorescence detection. Three different polar solvents (acetonitrile, methanol and water) were used to extract DEP generated during the combustion of different biodiesel blends (5%-99%). Oxidation of GSH to the disulfide (GSSG) was confirmed using electrospray ionization mass spectrometry. A decrease in the concentration of GSH was observed in the presence of DEP extracts from all of the biodiesel blends studied, with reaction rates that depend on the biodiesel blend. Interestingly the reactivity peaked at 50% biodiesel (B50) rather than decreasing monotonically with increased biodiesel content, as was expected. Organic solvent DEP extracts showed wider variations in reactivity with GSH, with methanol extracts giving the largest decrease in GSH concentrations. This may imply a more organic nature of the oxidants in the biodiesel exhaust. It is therefore important to consider ways of reducing concentrations of organic components in biodiesel exhaust that can cause different toxic activity before any blend is offered as a preferred alternative to petroleum diesel fuel.

The discrete Fourier transform algorithm for determining decay constants—Implementation using a field programmable gate array

Cavity ringdown spectroscopy (CRDS) uses the exponential decay constant of light exiting a high-finesse resonance cavity to determine analyte concentration, typically via absorption. We present a high-throughput data acquisition system that determines the decay constant in near real time using the discrete Fourier transform algorithm on a field programmable gate array (FPGA). A commercially available, high-speed, high-resolution, analog-to-digital converter evaluation board system is used as the platform for the system, after minor hardware and software modifications. The system outputs decay constants at maximum rate of 4.4 kHz using an 8192-point fast Fourier transform by processing the intensity decay signal between ringdown events. We present the details of the system, including the modifications required to adapt the evaluation board to accurately process the exponential waveform. We also demonstrate the performance of the system, both stand-alone and incorporated into our existing CRDS system. Details of FPGA, microcontroller, and circuitry modifications are provided in the Appendix and computer code is available upon request from the authors.