Marc Pitchford

Revised Algorithm for Estimating Light Extinction from IMPROVE Particle Speciation Data

Abstract The Interagency Monitoring of Protected Visual Environments (IMPROVE) particle monitoring network consists of approximately 160 sites at which fine particulate matter (PM2.5) mass and major species concentrations and course particulate matter (PM10) mass concentrations are determined by analysis of 24-hr duration sampling conducted on a 1-day-in-3 schedule. A simple algorithm to estimate light extinction from the measured species concentrations was incorporated in the 1999 Regional Haze Rule as the basis for the haze metric used to track haze trends. A revised algorithm was developed that is more consistent with the recent atmospheric aerosol literature and reduces bias for high and low light extinction extremes. The revised algorithm differs from the original algorithm in having a term for estimating sea salt light scattering from Cl - ion data, using 1.8 instead of 1.4 for the mean ratio of organic mass to measured organic carbon, using site-specific Rayleigh scattering based on site elevation and mean temperature, employing a split component extinction efficiency associated with large and small size mode sulfate, nitrate and organic mass species, and adding a term for nitrogen dioxide (NO2) absorption for sites with NO2 concentration information. Light scattering estimates using the original and the revised algorithms are compared with nephelometer measurements at 21 IMPROVE monitoring sites. The revised algorithm reduces the underprediction of high haze periods and the overprediction of low haze periods compared with the performance of the original algorithm. This is most apparent at the hazier monitoring sites in the eastern United States. For each site, the PM10 composition for days selected as the best 20% and the worst 20% haze condition days are nearly identical regardless of whether the basis of selection was light scattering from the original or revised algorithms, or from nephelometer-measured light scattering.

Uncertainties in PM2.5 Gravimetric and Speciation Measurements and What We Can Learn from Them

ABSTRACT The U.S. Environmental Protection Agency (EPA) and the federal land management community (National Park Service, United States Fish and Wildlife Service, United States Forest Service, and Bureau of Land Management) operate extensive particle speciation monitoring networks that are similar in design but are operated for different objectives. Compliance (mass only) monitoring is also carried out using federal reference method (FRM) criteria at approximately 1000 sites. The Chemical Speciation Network (CSN) consists of approximately 50 long-term-trend sites, with about another 250 sites that have been or are currently operated by state and local agencies. The sites are located in urban or suburban settings. The Interagency Monitoring of Protected Visual Environments (IMPROVE) monitoring network consists of about 181 sites, approximately 170 of which are in nonurban areas. Each monitoring approach has its own inherent monitoring limitations and biases. Determination of gravimetric mass has both negative and positive artifacts. Ammonium nitrate and other semivolatiles are lost during sampling, whereas, on the other hand, measured mass includes particle-bound water. Furthermore, some species may react with atmospheric gases, further increasing the positive mass artifact. Estimating aerosol species concentrations requires assumptions concerning the chemical form of various molecular compounds, such as nitrates and sulfates, and organic material and soil composition. Comparing data collected in the various monitoring networks allows for assessing uncertainties and biases associated with both negative and positive artifacts of gravimetric mass determinations, assumptions of chemical composition, and biases between different sampler technologies. All these biases are shown to have systematic seasonal characteristics. Unaccounted-for particle-bound water tends to be higher in the summer, as does nitrate volatilization. The ratio of particle organic mass divided by organic carbon mass (Roc) is higher during summer and lower during the winter seasons in both CSN and IMPROVE networks, and Roc is lower in urban than nonurban environments. IMPLICATIONS Particulate matter less than 2.5 microns in size (PM2.5) National Ambient Air Quality Standards (NAAQS) are based on gravimetric analysis of particulate matter collected on a Teflon substrate, using federal reference methodologies, whereas compliance under the Regional Haze Rule (RHR) is based on atmospheric extinction, derived from measurements of individual aerosol species. Gravimetric mass can be over- or underestimated because of volatilization issues and water retention by inorganic species, whereas species-specific estimates of mass are dependent on assumptions concerning their detailed chemical composition. Over- or underestimation of aerosol species or gravimetric mass could result in violation of standards or failure to meet visibility goals established under the RHR.

Relationship between measured water vapor growth and chemistry of atmospheric aerosol for Grand Canyon, Arizona, in winter 1990

Size-resolved aerosol growth measurements (growth ≡ moist particle diameter/dry particle diameter) and chemical composition monitoring were conducted during a 3 month period in the winter of 1990 at the South Rim of Grand Canyon National Park, AZ as part of the Navajo Generating Station Visibility Study. Particle growth data are from a Tandem Differential Mobility Analyzer (TDMA). Typically for relative humidities above 75%, the TDMA-measured moist particle distribution is distinctly bimodal, indicating two aerosol fractions based on growth and providing direct evidence of an external mixture of soluble and insoluble constituents. In this study both particle fractions grew in size, thus the terms “more hygroscopic” and “less hygroscopic” were used to distinguish them. Micro-Orifice Uniform Deposit Impactors (MOUDI) collected size-segregated 24 h duration samples for subsequent analysis by XRF, ion chromatography, and by thermo-optical analysis (i.e. for carbon). A model that synthesizes growth and compositional information was developed to partition the overall volume fraction of the soluble material as determined from the MOUDI composition data, e, to the two growth fractions obtained from the TDMA data (i.e. e is partitioned between em and e1). The model calculates em and e1 for each TDMA measurement as well as growth capacity of the soluble material at the measurement relative humidity, Gs, which is assumed to be the same for both growth fractions. Model results indicate that on average, the more hygroscopic particles are composed of equal volumes of soluble and insoluble materials, while the less hygroscopic fraction is dominated by insoluble material (about 85%).

Development and applications of a standard visual index

Abstract A standard visual index appropriate for characterizing visibility through uniform hazes, is defined in terms of either of the traditional metrics: visual range or extinction coefficient. This index was designed to be linear with respect to perceived visual changes over its entire range in a way that is analogous to the decibel scale for sound. Neither visual range nor extinction coefficient have this useful property, which has resulted in their misuse for some applications. By assuming the availability of sensitive scenic targets at every distance, it can be demonstrated that any specific fractional change in extinction coefficient (or visual range) is equally perceptible regardless of baseline visibility conditions. The new index is defined so that its scale, which is expressed in deciview (dv), is linear with respect to fractional changes in extinction coefficient, bexv as 10 In(bext/0.01 km−1 ), where extinction coefficient is expressed in km−1. A I dv change is about a 10% change in extinction coefficient, which is a small but perceptible scenic change under many circumstances. Since the deciview scale is near zero for a pristine atmosphere (dv=0 for Rayleigh conditions at about 1.8 km elevation) and increases as visibility is degraded, it measures perceived haziness. Use of the deciview scale in visibility perception and benefits research will promote improved study design and more appropriate use of results. It should also allow the visibility consequences of alternative air pollutionrelated decisions to be more easily presented and evaluated.

Characterization of particles in the arid west

Abstract This paper describes spatial and temporal variations of airborne paniculate matter in the eight western states included in the western fine particle network (WFPN). The samples were generated by a 40-site monitoring network of remotely sited dichotomous samplers of the stacked filter and virtual design, with a coarse fraction between 15 μm and 2.5 μm dia. and a fine fraction below 2.5 μm dia. The units operate for two 72 h period each week, delivering samples analyzed gravimetrically for mass and by particle induced X-ray emission (PIXE) for elements sodium and heavier. Results are presented for the period October 1979 to May 1980, showing regional patterns of particles especially in the fine modes. A sulfur episode which occurred in the southwest is examined via trajectory analysis, while factor analysis is applied to the entire data set to generate information on particulate sources.

Size and critical supersaturation for condensation of jet engine exhaust particles

In situ measurements of jet engine exhaust from a Sabreliner were made by instruments on board the NCAR Electra during a brief period of coordinated flying. Particle size distribution and critical supersaturation spectra were monitored before, during, and after the encounter with the jet exhaust plume by a condensation nucleus counter, an active scattering aerosol spectrometer probe (ASASP), and a cloud condensation nuclei (CCN) spectrometer. The relationships between particle size and corresponding critical supersaturation in the background air and within the jet exhaust plume are developed. Results indicate that background particles are much more active as CCN than exhaust particles of the same size, of which less than 1:100 are active at 0.8% supersaturation. An estimate of the engine sooting efficiency lies between 3×10−5 and 10−6.

Comparison of calculated sulfate scattering efficiencies as estimated from size-resolved particle measurements at three national locations

Size distributions and resulting optical properties of sulfur aerosols were investigated at three national parks by a Davis Rotating-drum Universal-size-cut Monitoring (DRUM) impactor. Sulfur size distribution measurements for 88, 177, and 315 consecutive time periods were made at Grand Canyon National Park during January and February 1988, Meadview, AZ during July, August, and September 1992, and at Shenandoah National Park during summer, 1990, respectively. The DRUM impactor is designed to collect aerosols with an aerodynamic diameter between 0.07 and 15.0 μm in eight size ranges. Focused beam particle-induced X-ray emission (PIXE) analysis of the aerosol deposits produces a time history of size-resolved elemental composition of varied temporal resolution. As part of the quality assurance protocol, an interagency monitoring of protected visual environments (IMPROVE) channel A sampler collecting 0–2.5 μm diameter particles was operated simultaneously alongside the DRUM sampler. During these sampling periods, the average sulfur mass, interpreted as ammonium sulfate, is 0.49, 2.30, and 10.36 μg m−3 at Grand Canyon, Meadview, and Shenandoah, respectively. The five drum stages were “inverted” using the Twomey (1975) scheme to give 486 size distributions, each made up of 72 discreet pairs of dC/dlog(D) and diameter (D). From these distributions mass mean diameters (Dg), geometric standard deviations (σg), and mass scattering efficiencies (em)) were calculated. The geometric mass mean diameters in ascending order were 0.21 μm at Meadview, 0.32 μm at Grand Canyon, and 0.42 μm at Shenandoah corresponding σg were 2.1, 2.3, and 1.9. Mie theory mass scattering efficiencies calculated from dC/dlog(D) distributions for the three locations were 2.05, 2.59, and 3.81 m2 g−1, respectively. At Shenandoah, mass scattering efficiencies approached five but only when the mass median diameters were approximately 0.4 μm and σg were about 1.5. σg near 1.5 were frequently measured at Shenandoah, rarely at Grand Canyon, and never during the summer at Meadview.

Regional analysis of factors affecting visual air quality

The U.S. Environmental Protection Agency, National Park Service, Visibility Research Center, and University of California at Davis are currently operating a monitoring program in national parks and monuments throughout much of the western United States. Project VIEW, the Visibility Investigative Experiment in the West, includes measurement of visibility parameters using manual telephotometers, and measurement of particle concentrations averaged over 72 h. Variation of these parameters occurs in both space and time. To better understand these variations, several techniques including principal component analysis and data comparisons among sites are applied to Fall, 1979 data for much of the network. Then the Grand Canyon is chosen for additional analysis. Best and worst case visibility days are determined and compared with particle concentrations. Finally, hypothetical causes for visibility reduction are further verified by computing wind trajectories back in time for these special case days. Highlights of this preliminary investigation include evidence that fine sulfur and fine particles are responsible for visibility variation at the VIEW sites; that fine particle copper may be suitable as a tracer for copper smelter impact and that at the Grand Canyon, the majority of trajectories for days of visibility greater than 310km come from the north and west, over Utah and Nevada.

Characterization of the Winter Midwestern Particulate Nitrate Bulge

Abstract A previously unobserved multi-state region of elevated particulate nitrate concentration was detected as a result of the expansion of the Interagency Monitoring of Protected Visual Environments (IMPROVE) network of remote-area particulate matter (PM) speciation monitoring sites into the midwestern United States that began in 2002. Mean winter ammonium nitrate concentrations exceed 4 μg/m3 in a region centered in Iowa, which makes it responsible for as much as half of the particle light extinction. Before these observations, particulate nitrate in the United States was only observed to be a dominant component of the fine PM (PM2.5) in parts of California and some urban areas. Comparisons of the spatial patterns of particulate nitrate with spatial patterns of ammonia and nitrogen oxide emissions suggest that the nitrate bulge is the result of the high emissions of ammonia associated with animal agriculture in the Midwest. Nitrate episodes at several locations in the eastern United States are shown to be associated with transport pathways over the Midwest, suggesting long-range transport of either ammonia or ammonium nitrate. Thermodynamic equilibrium modeling conducted by others on data from the Midwest shows the relative importance of atmospheric ammonia and nitric acid in the production of PM2.5. This is a particular concern as the sulfur dioxide emissions in the United States are reduced, which increases the amount of ammonia available for ammonium nitrate production.

Silicon in submicron particles in the southwest

Silicon in the fine particle size range (less than about 2 μm) was investigated. Elemental and microscopical analysis of size segregated, ambient particulate and suspended soil samples from the rural southwest indicate that the particles are earth crust material. The composition of the soil related materials changes with changing particle size. Smaller particles tend to vary more in their elemental makeup than corresponding large particles. Numerous thin mineral plates with diameters of several micrometers are aerodynamically sized in the smallest sample size range (< 0.5 μm). Since silicon accounts for one-quarter to one-half the total mass in the fine particle size range, it may be a significant causal factor in regional visibility degradation.