Hampden D. Kuhns

The contributions of snow, fog, and dry deposition to the summer flux of anions and cations at Summit, Greenland

Experiments were performed during the period May–July of 1993 at Summit, Greenland. Aerosol mass size distributions as well as daily average concentrations of several anionic and cationic species were measured. Dry deposition velocities for SO42− were estimated using surrogate surfaces (symmetric airfoils) as well as impactor data. Real-time concentrations of particles greater than 0.5 μm and greater than 0.01 μm were measured. Snow and fog samples from nearly all of the events occurring during the field season were collected. Filter sampler results indicate that SO42− is the dominant aerosol anion species, with Na+, NH4+, and Ca2+ being the dominant cations. Impactor results indicate that MSA and SO42− have similar mass size distributions. Furthermore, MSA and SO42− have mass in both the accumulation and coarse modes. A limited number of samples for NH4+ indicate that it exists in the accumulation mode. Na, K, Mg, and Ca exist primarily in the coarse mode. Dry deposition velocities estimated from impactor samples and a theory for dry deposition to snow range from 0.017 cm/s +/− 0.011 cm/s for NH4+ to 0.110 cm/s +/− 0.021 cm/s for Ca. SO42− dry deposition velocity estimates using airfoils are in the range 0.023 cm/s to 0.062 cm/s, as much as 60% greater than values calculated using the airborne size distribution data. The rough agreement between the airfoil and impactor-estimated dry deposition velocities suggests that the airfoils may be used to approximate the dry deposition to the snow surface. Laser particle counter (LPC) results show that particles > 0.5 μm in diameter efficiently serve as nuclei to form fog droplets. Condensation nuclei (CN) measurements indicate that particles < 0.5 μm are not as greatly affected by fog. Furthermore, impactor measurements suggest that from 50% to 80% of the aerosol SO42− serves as nuclei for fog droplets. Snow deposition is the dominant mechanism transporting chemicals to the ice sheet. For NO3−, a species that apparently exists primarily in the gas phase as HNO3(g), 93% of the seasonal inventory (mass of a deposited chemical species per unit area during the season) is due to snow deposition, which suggests efficient scavenging of HNO3(g) by snowflakes. The contribution of snow deposition to the seasonal inventories of aerosols ranges from 45% for MSA to 76% for NH4+. The contribution of fog to the seasonal inventories ranges from 13% for Na+ and Ca2+ to 26% and 32% for SO42− and MSA. The dry deposition contribution to the seasonal inventories of the aerosol species is as low as 5% for NH4+ and as high as 23% for MSA. The seasonal inventory estimations do not take into consideration the spatial variability caused by blowing and drifting snow. Overall, results indicate that snow deposition of chemical species is the dominant flux mechanism during the summer at Summit and that all three deposition processes should be considered when estimating atmospheric concentrations based on ice core chemical signals.

Development of a United States-Mexico emissions inventory for the big bend regional aerosol and visibility observational (BRAVO) study

Abstract The Big Bend Regional Aerosol and Visibility Observational (BRAVO) Study was commissioned to investigate the sources of haze at Big Bend National Park in southwest Texas. The modeling domain of the BRAVO Study includes most of the continental United States and Mexico. The BRAVO emissions inventory was constructed from the 1999 National Emission Inventory for the United States, modified to include finer-resolution data for Texas and 13 U.S. states in close proximity. The first regional-scale Mexican emissions inventory designed for air-quality modeling applications was developed for 10 northern Mexican states, the Tula Industrial Park in the state of Hidalgo, and the Popocatépetl volcano in the state of Puebla. Emissions data were compiled from numerous sources, including the U.S. Environmental Protection Agency (EPA), the Texas Natural Resources Conservation Commission (now Texas Commission on Environmental Quality), the Eastern Research Group, the Minerals Management Service, the Instituto Nacional de Ecología, and the Instituto Nacional de Estadistica Geografía y Informática. The inventory includes emissions for CO, nitrogen oxides, sulfur dioxide, volatile organic compounds (VOCs), ammonia, particulate matter (PM) <10 μm in aerodynamic diameter, and PM <2.5 μm in aerodynamic diameter. Wind-blown dust and biomass burning were not included in the inventory, although high concentrations of dust and organic PM attributed to biomass burning have been observed at Big Bend National Park. The SMOKE modeling system was used to generate gridded emissions fields for use with the Regional Modeling System for Aerosols and Deposition (REMSAD) and the Community Multiscale Air Quality model modified with the Model of Aerosol Dynamics, Reaction, Ionization and Dissolution (CMAQ-MADRID). The compilation of the inventory, supporting model input data, and issues encountered during the development of the inventory are documented. A comparison of the BRAVO emissions inventory for Mexico with other emerging Mexican emission inventories illustrates their uncertainty.

Biomass burning signatures in the atmosphere of central Greenland

Daily atmospheric concentrations of participate oxalate measured at the Summit of the Greenland Ice Sheet are presented for the summers 1992–1995. We believe that four episodes of elevated concentrations are due to biomass burning plumes passing over the site. In at least two cases the source regions of the fires are located in northern Canada. Further characteristics of the aerosol are examined during one of these events. A large increase of particle number concentrations in the accumulation mode can be observed, while the increase is much more limited for total particle number. The suite of chemical species enriched in the aerosol includes typical biomass burning tracers like fine K, large concentrations of ammonium, particulate formate and acetate, as well as other organic species like glycolate. The size distributions of K, oxalate, and glycolate are skewed toward the accumulation mode and exhibit the very same shape as sulfate, suggesting internal mixing of these species in the same particles. Molar ratios S/K indicate incorporation of S during transport, most probably by production of sulfate. Concentrations of these species were measured in fog samples for radiative events that occurred during the plume passage. There is a good agreement in the relative variation of concentrations between the aerosol and fog for oxalate and glycolate, while the gas phase probably dominates incorporation in the fog droplets for acetate, formate, chloride, nitrate, and sulfate (incorporated as SO2, which is further oxidized). The complexity of the transfer of the organic acids from the atmosphere to fog is underlined.

Deposition and Removal of Fugitive Dust in the Arid Southwestern United States: Measurements and Model Results

Abstract This work was motivated by the need to better reconcile emission factors for fugitive dust with the amount of geologic material found on ambient filter samples. The deposition of particulate matter with aerodynamic diameter less than or equal to 10 μm (PM10), generated by travel over an unpaved road, over the first 100 m of transport downwind of the road was examined at Ft. Bliss, near El Paso, TX. The field conditions, typical for warm days in the arid southwestern United States, represented sparsely vegetated terrain under neutral to unstable atmospheric conditions. Emission fluxes of PM10 dust were obtained from towers downwind of the unpaved road at 7, 50, and 100 m. The horizontal flux measurements at the 7 m and 100 m towers indicated that PM10 deposition to the vegetation and ground was too small to measure. The data indicated, with 95% confidence, that the loss of PM10 between the source of emission at the unpaved road, represented by the 7 m tower, and a point 100 m downwind was less than 9.5%. A Gaussian model was used to simulate the plume. Values of the vertical standard deviation σ z and the deposition velocity V d were similar to the U.S. Environmental Protection Agency (EPA) ISC3 model. For the field conditions, the model predicted that removal of PM10 unpaved road dust by deposition over the distance between the point of emission and 100 m downwind would be less than 5%. However, the model results also indicated that particles larger than 10 μm (aerodynamic diameter) would deposit more appreciably. The model was consistent with changes observed in size distributions between 7 m and 100 m downwind, which were measured with optical particle counters. The Gaussian model predictions were also compared with another study conducted over rough terrain and stable atmospheric conditions. Under such conditions, measured PM10 removal rates over 95 m of downwind transport were reported to be between 86% and 89%, whereas the Gaussian model predicted only a 30% removal. One explanation for the large discrepancy between measurements and model results was the possibility that under the conditions of the study, the dust plume was comparable in vertical extent to the roughness elements, thereby violating one of the model assumptions. Results of the field study reported here and the previous work over rough terrain bound the extent of particle deposition expected to occur under most unpaved road emission scenarios.

On-road measurement of automotive particle emissions by ultraviolet Lidar and transmissometer: theory

An on-road vehicle emissions remote sensing system (VERSS) utilizing an ultraviolet laser (operating at a wavelength of 266 nm) has been developed to quantify the particulate mass in vehicle exhaust. The system simultaneously measures the backscatter and the transmission of the laser light. Obtaining the particulate matter mass concentration from the measurements depends upon a prior laboratory calibration as well as knowledge of the physical, chemical and optical characteristics of the exhaust particles.

The Treasure Valley secondary aerosol study I: measurements and equilibrium modeling of inorganic secondary aerosols and precursors for southwestern Idaho

Abstract The SCAPE2 aerosol equilibrium model was applied to measured concentrations of PM2.5 aerosol and precursor gases. Ambient measurements in the Treasure Valley, Idaho were collected during stagnation episodes between December 1999 and March 2000 when conditions were favorable for the formation of secondary inorganic aerosol. SCAPE2 results agreed well with measurements for the episodes tested; Monte Carlo simulation using the uncertainties of the model input variables indicated that discrepancies between measured and modeled results were within the range of analytical precision. Air pollution control strategies were evaluated by perturbing the model input concentrations of total sulfate, nitrate, and ammonium. The largest reductions in secondary aerosol concentrations occurred when total nitrate was reduced, indicating that the formation of ammonium nitrate was limited by the availability of nitrate.

A lumped parameter model for the atmosphere‐to‐snow transfer function for hydrogen peroxide

Of the main atmospheric oxidants, only hydrogen peroxide (H 2 O 2 ) is preserved in polar ice cores. To make use of the peroxide record, however, requires a quantitative understanding of the transfer function or relation between atmospheric concentrations of H 2 O 2 and those preserved in the ice core. Snow-pit H 2 O 2 profiles adjacent to three automatic snow-depth gages from Summit, Greenland were used to estimate parameters and evaluate the performance of a lumped parameter model to relate concentrations in the atmosphere with those in surface snow and shallow firn. Three of the model parameters define an equilibrium partitioning coefficient between snow and atmosphere as a nonlinear function of depositional temperature. Model parameters yielded a function that closely matched previous laboratory estimates [Conklin et al., 1993]. A fourth parameter reflects the disequilibrium that may be preserved during periods of rapid accumulation. The final model parameter describes the exchange of H 2 O 2 between near-surface snow and the atmosphere, allowing already buried snow to either take up or release H 2 O 2 as conditions in and above the snowpack change. We simulated snow pit profiles by combining this transfer function model with a finite-difference model of gas-phase diffusion in the snowpack. Two applications for this transfer function are (1) to estimate the local seasonal or annual atmospheric H 2 O 2 concentration in the past from snow-pit and ice-core records and (2) to invert snow-pit and ice-core H 2 O 2 profiles to obtain estimates of the seasonal or annual accumulation time series. In the first case, an independent estimate of snow accumulation is needed, and in the second application, an independent estimate of the annual H 2 O 2 atmospheric cycle is needed.

Particulate emission factors for mobile fossil fuel and biomass combustion sources

PM emission factors (EFs) for gasoline- and diesel-fueled vehicles and biomass combustion were measured in several recent studies. In the Gas/Diesel Split Study (GD-Split), PM(2.5) EFs for heavy-duty diesel vehicles (HDDV) ranged from 0.2 to ~2 g/mile and increased with vehicle age. EFs for HDDV estimated with the U.S. EPA MOBILE 6.2 and California Air Resources Board (ARB) EMFAC2007 models correlated well with measured values. PM(2.5) EFs measured for gasoline vehicles were ~two orders of magnitude lower than those for HDDV and did not correlate with model estimates. In the Kansas City Study, PM(2.5) EFs for gasoline-powered vehicles (e.g., passenger cars and light trucks) were generally <0.03 g/mile and were higher in winter than summer. EMFAC2007 reported higher PM(2.5) EFs than MOBILE 6.2 during winter, but not during summer, and neither model captured the variability of the measured EFs. Total PM EFs for heavy-duty diesel military vehicles ranged from 0.18±0.03 and 1.20±0.12 g/kg fuel, corresponding to 0.3 and 2 g/mile, respectively. These values are comparable to those of on-road HDDV. EFs for biomass burning measured during the Fire Laboratory at Missoula Experiment (FLAME) were compared with EFs from the ARB Emission Estimation System (EES) model. The highest PM(2.5) EFs (76.8±37.5 g/kg) were measured for wet (>50% moisture content) Ponderosa Pine needles. EFs were generally <20 g/kg when moisture content was <20%. The EES model agreed with measured EFs for fuels with low moisture content but underestimated measured EFs for fuel with moisture content >40%. Average EFs for dry chamise, rice straw, and dry grass were within a factor of three of values adopted by ARB in Californias San Joaquin Valley (SJV). Discrepancies between measured and modeled emission factors suggest that there may be important uncertainties in current PM(2.5) emission inventories.

Application of the Tracer-Aerosol Gradient Interpretive Technique to Sulfur Attribution for the Big Bend Regional Aerosol and Visibility Observational Study

Abstract A simple data analysis method called the Tracer-Aerosol Gradient Interpretive Technique (TAGIT) is used to attribute particulate S and SO2 at Big Bend National Park in Texas and nearby areas to local and regional sources. Particulate S at Big Bend is of concern because of its effects on atmospheric visibility. The analysis used particulate S, SO2 , and perfluorocarbon tracer data from six 6-hr sampling sites in and near Big Bend National Park. The data were collected in support of the Big Bend Regional Aerosol and Visibility Observational (BRAVO) Study; the field portion was conducted from July through October 1999. Perfluorocarbon tracer was released continuously from a tower at Eagle Pass, TX, approximately 25 km northeast of two large coal-fired power plants (Carbon I and II) in Coahuila, Mexico, and approximately 270 km east-southeast of Big Bend National Park. The perfluorocarbon tracer did not properly represent the location of the emissions from the Carbon power plants for individual 6-hr sampling periods and attributed only 3% of the particulate S and 27% of the SO2 at the 6-hr sites in and near Big Bend to sources represented by the tracer. An alternative approach using SO2 to tag “local” sources such as the Carbon plants attributed 10% of the particulate S and 75% of the SO2 at the 6-hr sites to local sources. Based on these two approaches, most of the regional (65–86%) and a small fraction (19–31%) of the local SO2 was converted to particulate S. The analysis implies that substantial reductions in particulate S at Big Bend National Park cannot be achieved by only reducing emissions from the Carbon power plants; reduction of emissions from many sources over a regional area would be necessary.

The Treasure Valley secondary aerosol study II: modeling of the formation of inorganic secondary aerosols and precursors for southwestern Idaho

Abstract Many locations with very high mixing ratios of ammonia, nitrogen oxides and relatively low sulfate mixing ratios are found in urban areas of the western United States during the wintertime. These urban areas may also experience episodes with high humidity with high levels of secondary inorganic ammonium nitrate particles. Photochemical simulations of the formation of secondary sulfate and ammonium nitrate aerosol were made to investigate possible secondary particle control strategies for Treasure Valley, Idaho. The simulation conditions were based on the field study of Kuhns et al. (Atmos. Environ. 2002, this issue). It was found that under these conditions that almost all of the sulfate and approximately 95% of the nitric acid would be found in the particulate phase. Reductions in the emission rates of volatile organic compounds were found to be most effective in reducing secondary inorganic aerosol concentrations while reductions in nitrogen oxide emission rates would be expected to increase aerosol concentrations. This response of aerosol formation rates is due to the effects of the nitrogen oxide and volatile organic compound emission rates on the concentration of hydroxyl radical mixing ratios.