Marc Valdez - Academia.edu (original) (raw)

Papers by Marc Valdez

Journal of Geophysical Research, 1987

A diffusion/reaction model of SO 2 uptake by snow containing liquid water is used to examine the ... more A diffusion/reaction model of SO 2 uptake by snow containing liquid water is used to examine the snowpack processes controlling accumulation of dissolved S(IV) and S(VI). Surface deposition velocity v d (defined as overall accumulation rate divided by surface SO2 concentration) depends on the amount of liquid water in the snow, the time scale considered, the rate of S(IV) to S(VI) conversion in the aqueous phase, and the SO 2 concentration. In the absence of any oxidation, v d for dense snow (specific gravity 0.4) with a moderate liquid water mass fraction (X m = 0.01) and SO 2 concentration of 20 ppbv (parts per billion by volume) is calculated to be 0.026 cm s-! after 6 hours. Deposition velocity increases by a factor of 3.2 for each tenfold increase in X m and by a factor of 2.5-3.2 for each tenfold decrease in time. SO 2 penetration into the snowpack is about 5 cm for the 6-hour case. In the presence of air containing 30 ppbv ozone and otherwise identical conditions, 80% of the accumulated sulfur is as S(VI), and the calculated v d is 0.036 cm s-!. A further tenfold increase in ozone concentration gives only a 70% increase in vd. A similar dependence is calculated for oxygen (catalyzed by Fe(III)) as oxidant, but that for hydrogen peroxide is much stronger and almost linear. A tenfold increase in S02 concentration (with ozone at 30 ppb) results in a 2.3-fold decrease in vd. Wet snow with a liquid water mass fraction of about 0.1 gives a deposition velocity a•t 20 ppb SO 2 of 0.12 cm s-, with a penetration of only 2 cm. Calculated and observed uptakes of S02 by snow are in good agreement. 1 ß Introduction Dry deposition of acid trace gases to snow surfaces is in part responsible for the winter accumulation of acidity in mountain snowpacks. The subsequent melting and runoff results in an "acid shock" in the affected watersheds. Dry deposition is a multistep process involving transport of reactants (e.g., oxides of sulfur, nitrogen, and other trace gases) to the snow surface, adsorption or absorption at the snow-air interface, and chemical reaction to produce the oxidized products. Any of these steps could limit the rate of the overall process. The purpose of this paper is to present a model for deposition of SO 2 onto snow near 0øC

One primary objective of ACRP Project 02-34, “Quantifying Aircraft Lead Emissions at Airports,” w... more One primary objective of ACRP Project 02-34, “Quantifying Aircraft Lead Emissions at Airports,” was to review and improve upon existing methodologies to quantify and characterize aircraft-related lead (Pb) emissions at airports with significant populations of aircraft that use leaded aviation gasoline. The study involved the five major phases: 1) A review of existing methodologies for quantifying aircraft-related Pb emissions; 2) Development of a refined methodology for estimating aircraft-related Pb emissions inventories that addresses shortcomings with existing methodologies identified during the critical review; 3) Conducting month-long field studies at each of three selected airports to gather site-specific data regarding aircraft activity, the lead content of aviation gasoline used at the airport, and data regarding ambient Pb concentrations, Pb particle size distributions, and Pb isotope ratios; 4) Application of the refined methodology to develop Pb emission inventories for three selected airports using both readily available activity data as well as the site-specific data; and 5) Validation of the refined methodology through comparison of dispersion modeling results based on the inventory computed using site-specific data with ambient Pb measurements made during the field study.

Journal of Geophysical Research, 1989

An experimental study has been made of the incorporation of SO2 into ice depositing from the vapo... more An experimental study has been made of the incorporation of SO2 into ice depositing from the vapor at −15°C. Surprisingly, SO2 was captured in deposited ice at concentrations comparable to those given by SO2/S(IV) aqueous equilibrium at 0°C. A consequence of this result is that, in the remote troposphere, unrimed snow scavenging ratios for SO2 may be comparable to those for sulfate. In addition, ozone and HCHO appeared to inhibit, rather than enhance, SO2 uptake. An aqueous‐film model is developed to account for SO2 capture. If SO2 dissolves in a liquidlike layer on growing ice surfaces, the concentration of S(IV) species may become enhanced within the layer as a result of retarded diffusional transport away from the advancing ice/layer interface. Such a concentration increase can produce significant solute incorporation into the bulk ice, despite effective solute rejection from the ice.

Springer eBooks, 1987

... Auteur(s) / Author(s). BALES RC (1) ; VALDEZ MP ; DAWSON GA ; STANLEY DA ; ... Mots-clés espa... more ... Auteur(s) / Author(s). BALES RC (1) ; VALDEZ MP ; DAWSON GA ; STANLEY DA ; ... Mots-clés espagnols / Spanish Keywords. Nieve ; Dióxido sulfúrico ; Fase gaseosa ; Depósito químico ; Agua ; Cinética ; Penetración ; Laboratorio ; Localisation / Location. ...

Journal of Geophysical Research, 1987

A series of 48 laboratory experiments has determined depth profiles of S(IV) and S(VI) accumulati... more A series of 48 laboratory experiments has determined depth profiles of S(IV) and S(VI) accumulation in snow exposed to 20-140 ppb SO 2 for 6-12 hours. Surface deposition velocity, calculated from the amount of sulfur taken up, divided by the gas-phase concentration, averaged 0.06 cm s-1. Snow held near-2øC had an average deposition velocity of 0.04 cm s-1, with new snow having a value about double that for older, metamorphosed snow (0.05 versus 0.02 cm s-l). Snow held near 0øC had an average deposition velocity of 0.07 cm s-1, and snow held below zero, but with surface melting due to sunlight, had a value of 0.06 cm s-1. When sunlight and temperature allowed draining of meltwater, dep?sition velocities were much higher (0.14 cm s). Deposition velocities for snow at-2øC were as much as 60% higher for measurements over 6 hours versus 10-12 hours. Penetration of sulfur into the snow was generally about 7 cm for colder experiments and only 4 cm for snow with more liquid water present. Uptake of SO 2 by snow is apparently determined largely by the liquid-water-to-air ratio in the snowpack. SO 2 and ozone concentrations had only small effects on deposition velocity. Most sulfur in the snow was found as S(VI), even in the absence of ozone, indicating that another oxidant is readily available, especially in new snow. Sunlight had no effect on uptake, other than increasing the liquid-water-to-air ratio by surface melting of the snow. Four measurements of NO 2 deposition to snow were also made; uptake was small, with deposition velocities averaging 0.005 cm s-1 in the dark and 0,012 cm s-1 in the sunlight.

Depth profiles of S(IV) and S(VI) in snow exposed to 20-140 ppbv SO₂ for 6 to 12 hours have been ... more Depth profiles of S(IV) and S(VI) in snow exposed to 20-140 ppbv SO₂ for 6 to 12 hours have been determined in 48 laboratory experiments. Surface deposition velocity (v(d)) averaged 0.06 cm s⁻¹. Well-metamorphosed snow, longer run times, higher SO₂ concentrations and colder snow were associated with lower values of v(d), and vice versa. Melting followed by draining increased v(d) greatly (0.14 cm s⁻¹. Any effect of ozone on SO₂ v(d) was undetectable. Most sulfur in the snow was as S(VI), even without added ozone, indicating the presence of other oxidants, especially in new snow. Four NO₂ deposition experiments (average v(d) = 0.007 cm s⁻¹), and one combined SO₂-NO₂ deposition experiment were conducted. Ozone, sunlight and SO₂ did not enhance NO₂ deposition; NO₂ and sunlight did not enhance SO₂ deposition. The deposition of SO₂ into a snowpack is modelled as an aqueous system, where the liquid water is considered to be present on snow grain surfaces. Gas transport into the snow, air-water partitioning, and aqueous-phase reactions are explicitly considered. Three oxidants (Fe- or Mn-catalyzed O₂, O₂, and H₂O₂) act to convert S(IV) to S(VI), acidify the film, and inhibit further S(IV) uptake. Model calculations illustrate the primary importance of liquid-water mass fraction (X(m)) and the secondary importance of oxidative reactions on SO₂ v(d) to snow. Model and experimental results are similar for assumed X(m) on the order of one percent. Experiments were also conducted on the incorporation of SO₂ into ice depositing from the vapor at -7 and -15°C. Remarkably, SO₂ is captured in deposited ice at concentrations comparable to Henry's Law equilibrium with water at 0°C. Ozone and HCHO appear to inhibit, not enhance, SO₂ capture. An aqueous-film model accounting for the capture of SO₂ by depositing ice was developed. S(IV) concentrations may be enhanced in the liquid-like layer on growing ice surfaces due to solute exclusion from the bulk ice and greatly-retarded diffusional transport from the ice/film interface, leading to significant incorporation into the ice despite low distribution coefficients. SO₂ snow scavenging ratios may be comparable to sulfate scavenging ratios in the remote troposphere