Thomas G. Ellestad

A multilayer model for inferring dry deposition using standard meteorological measurements

In this paper, we describe the latest version of the dry deposition inferential model, which is used to estimate the deposition velocities (Vd) for SO2, O3, HNO3, and particles with diameters less than 2 μm. The dry deposition networks operated by the National Oceanic and Atmospheric Administration (NOAA) and the Environmental Protection Agency (EPA) use this model to estimate dry deposition on a weekly basis. This model uses a multilayer approach, discretizing the vegetated canopy into 20 layers. The use of canopy radiative transfer and simple wind profile models allows for estimates of stomatal (rs) and leaf boundary layer (rb) resistances to be determined at each layer in the plant canopy for both sunlit and shaded leaves. The effect of temperature, water stress, and vapor pressure deficits on the stomatal resistance (rs) have been included. Comparisons of modeled deposition velocities are made with extensive direct measurements performed at three different locations with different crops. The field experiment is discussed in some detail. Overall, modeled O3 deposition velocities are in good agreement with measured values with the average mean bias for all surfaces of the order of 0.01 cm/s or less. For SO2, mean biases range from −0.05 for corn to 0.15 cm/s for soybeans, while for HNO3, they range from 0.09 for corn to 0.47 cm/s for pasture.

Ozone and sulfur dioxide dry deposition to forests: Observations and model evaluation

Fluxes and deposition velocities of O3 and SO2 were measured over both a deciduous and a mixed coniferous-deciduous forest for full growing seasons. Fluxes and deposition velocities of O3 were measured over a coniferous forest for a month. Mean deposition velocities of 0.35 to 0.48 cm/s for O3 and 0.6 to 0.72 cm/s for SO2 were observed during the growing seasons of 1997 and 1998. Weekly averages of O3 deposition velocity ranged from 0.25 cm/s at the beginning and end of the season to 1.25 cm/s in late June. SO2 had a smaller seasonal variation, from 0.75 to 1.5 cm/s between the beginning and peak of the season. Because O3 concentrations are higher, the flux of O3 to forests is considerably greater than the flux of SO2. Daytime deposition velocities are very similar at each site, from 0.75 to 0.79 cm/s for O3, and from 1.01 to 1.04 cm/s for SO2. Diurnal cycles for both gases are discussed, as are the impact of some weather events. The peak time for O3 deposition velocity is in midmorning, while it is near midday for SO2. Surface wetness is usually associated with a small increase in deposition velocity, but for some rain events a major increase was noted. Minimum deposition velocities usually occur at night and increase slowly in the predawn hours before light. Comparisons are made between observations of deposition velocity and predictions made with the Meyers multilayer deposition velocity model. While the model is, on average, unbiased for O3, it tends to underpredict the higher deposition velocity values. The model is slightly biased low (underpredicts) for SO2 deposition velocity. The strengths of the model are noted, as are opportunities for improvement.

General Motors Sulfate Dispersion Experiment: Summary of EPA Measurements

In October 1975, General Motors sponsored a study of sulfate exposures utilizing a fleet of catalyst equipped motor vehicles in controlled, simulated, highway driving conditions. This paper reports some EPA sponsored measurements. Sulfuric acid aerosol, in the Aitken nuclei mode, geometric mean diameter (GMD) of about 0.02 µm, is emitted in the exhaust of catalyst equipped vehicles. Measurement of sulfuric acid 20 m downwind of the roadway indicated a lack of complete neutralization by ammonia. When the wind was perpendicular to the roadway there was little coagulation of sulfuric acid into the accumulation mode, GMD of about 0.24 µm From measurement of the mass flow rate of aerosol sulfur from the simulated freeway, the aerosol sulfur emission rate per car was determined to be 3.5 ± 0.8 µg/m (5.6 ±1.3 mg/mile) corresponding to a 12 ± 3% conversion of fuel sulfur into emitted aerosol sulfur.

Nitric acid-nitrate aerosol measurements by a diffusion denuder: A performance evaluation∗

A nitric acid diffusion denudcr made of nylon was operated in Riverside, CA, Houston, TX and Claremont, CA. The pre-exponential and diffusion coefficients for the first term of the Gormley-Kennedy equation were estimated by regressing the log (nitrate mass deposited) against the axial distance from the entrance of the denuder. The field study average values of the two coefficients were compared by the two-tail t-test to the average values for the HNO3-air system determined in the laboratory; the hypothesis that the means of the diffusion coefficients are equal may be rejected at the < 5% level of significance. The nitrate ion mass patterns in the denuder for the field cases do not adhere to the Gormley-Kennedy model applied to the simple HNO3-air system. The source of excess nitrate deposited in the nylon tube could not be identified by this analysis. Candidate sources are deposition of gaseous N-compounds (analyzed as nitrate) other than nitric acid and release of nitric acid vapor from particles during transit in the denuder tube.

A technique for estimating dry deposition velocities based on similarity with latent heat flux

Abstract Field measurements of chemical dry deposition are needed to assess impacts and trends of airborne contaminants on the exposure of crops and unmanaged ecosystems as well as for the development and evaluation of air quality models. However, accurate measurements of dry deposition velocities require expensive eddy correlation measurements and can only be practically made for a few chemical species such as O3 and CO2. On the other hand, operational dry deposition measurements such as those used in large area networks involve relatively inexpensive standard meteorological and chemical measurements but rely on less accurate deposition velocity models. This paper describes an intermediate technique which can give accurate estimates of dry deposition velocity for chemical species which are dominated by stomatal uptake such as O3 and SO2. This method can give results that are nearly the quality of eddy correlation measurements of trace gas fluxes at much lower cost. The concept is that bulk stomatal conductance can be accurately estimated from measurements of latent heat flux combined with standard meteorological measurements of humidity, temperature, and wind speed. The technique is tested using data from a field experiment where high quality eddy correlation measurements were made over soybeans. Over a four month period, which covered the entire growth cycle, this technique showed very good agreement with eddy correlation measurements for O3 deposition velocity.

Comparison of Measured and Modeled Surface Fluxes of Heat, Moisture, and Chemical Dry Deposition

Realistic air quality modeling requires accurate simulation of both meteorological and chemical processes within the planetary boundary layer (PBL). Surface energy and moisture fluxes control the temperature and humidity profiles. Similarly, chemical fluxes (dry deposition) have an important influence on PBL concentrations of trace chemical species. Therefore, accurate and consistent methods for simulation of both meteorological and chemical surface exchange processes are critical for realistic modeling of boundary layer atmospheric chemistry.

The sampling of reactive atmospheric species by transition-flow reactor: Application to nitrogen species

Concentrations of nitric acid vapor and fine particulate nitrate were measured during the 1985 Nitrogen Species Methods Comparison Study at Claremont, California, with a transition-flow reactor. This system separates atmospheric gases and particles by differential diffusion in a transition-flow stream and thereby minimizes sampling artifacts for these reactive species. Error analysis showed that [HNO3] was determined with typical uncertainties (1σ) of 5–8% and fine particulate [NO3−] at typical uncertainties of 5–11%. Both species showed a strong diurnal pattern with daytime maxima. Daytime [HNO3] ranged from 25 to 495 nmole m−3, while night-time values ranged from less than 1 to 107 nmole m −3. The time-weighted average concentrations of 12- and 10-h samples were equivalent to the corresponding 22-h sample for each day. Simple filtration with Teflon® and nylon filters in series over-estimated [HNO3] by 30–50 % for 10- and 12-h samples, and by 70% for 22-h samples. Analysis of ionic balance suggests that the cause of the overestimate was dissociation of NH4NO3 from equilibrium changes, and not displacement by strong acids.