G. A. Dawson

Wet removal of highly soluble gases

Partition, not kinetics, ultimately determines the concentration of highly soluble gases in cloud droplets. Partition equations are formulated and applied to idealized air-mass thunderclouds and precipitating stratus. Contribution to aqueous concentrations from sub-cloud scavenging of highly soluble gases is estimated at between 10 and 20% under relatively unpolluted conditions. Data indicate that evaporation can produce enhancements in concentration of as much as a factor of 3. The calculations give large-scale mean coefficients of wet removal of highly soluble gases of about 2.8×10-6 s-1 (4-day residence time) for air-mass thunderclouds and precipitating stratus. Removal is so effective that the mean scale heights of these gases should be decreased to 2 km or less. The criterion of high solubility in this paper is that KH (Henrys Law coefficient) > 105 mol l-1 atm-1. Gases that are effectively highly soluble include HCl, HNO3, H2SO4, H2O2, NH3 in acid droplets, SO2 in oxidizing droplets (and probably some light amines and sulfonic acids), but not SO2 in the absence of oxidants, nor HCHO. A variation of removal coefficient and scale height with solubility is presented. A comparison of atmospheric NH3 concentrations deduced from rain NH4+ and measured directly gives reasonable agreement.

A Preliminary Study of the Carbon-Isotopic Content of Ambient Formic Acid and Two Selected Sources: Automobile Exhaust and Formicine Ants

Relatively large quantities (≈1 mg) of formic acid have been collected from the atmosphere and subjected to carbon-isotopic analysis, as a means of source discrimination. Ambient formic acid was captured on Ca(OH)2-treated filters using a high-volume sampler. The collection method was not only efficient (>96%), but also appears to have low artifact production.Most of the samples (36 out of 52) were collected over a two-year period at the summit of Mount Lemmon, Arizona, where a strong seasonality in HCOOH mixing ratio was observed (≈0.2 ppb during winter months to 1.5 ppb in the summer). Other collection sites included the Oregon coast, Colorado Rockies, urban Tucson, and the North Dakota prairie. The carbon-13 content of atmospheric HCOOH was found to be have little variation (−18 to −25‰), regardless of location or season. This is consistent with a single dominant source of formic acid. The carbon-14 measurements of 6 Mount Lemmon samples showed high levels of modern carbon (93–113% modern).The emissions from formicine ants and automobile combustion were selected as two other potential sources for isotopic analysis. The HCOOH collected from auto exhaust was much more depleted in13C than the atmospheric samples, with a δ13C of −28.0 and −48.6‰ from a leaded and unleaded automobile, respectively. Formicine ants, on the other hand, ranged from −17.2 to −20.6‰.

Oxidation of sulfur dioxide by ozone in highly dispersed water droplets

The oxidation of S(IV) by ozone in aqueous solution has been studied in highly dispersed droplets in a laboratory could chamber at 20°C. Urban air was used. Ozone concentrations ranged from 15 to 120 ppb, orders of magnitude lower than most previous works. Rate constants were derived from differential product determination, rather than from rates of decay of bulk reactants. Comparison of the results obtained here with those from previous work indicates that the reaction rate is indeed first order with respect to ozone. The second-order reaction rate constants found in this work lay between, or close to, other recent results, and could be represented by:k=1.23×104 [H+]−0.51 M−1 s−1.

Selective oxidation of formaldehyde to carbon dioxide from high ionic strength solution for carbon-13 analysis by mass spectrometry

A method for determining the carbon isotopic composition of sub-milligram to milligram amounts of formaldehyde in high ionic strength aqueous solution has been developed. The procedure involves a highly selective two-step oxidation to CO2, followed by purging, drying and cryogenic trapping of the CO2. The 13C content of processed samples is determined by mass spectrometry. Calibration of the method with an isotopic standard gave yields in the range 85–92%, while isotopic reproducibility (compared with quantitative combustion) was excellent. It is anticipated that the oxidation procedure will prove useful for the analysis of atmospheric formaldehyde collected by sulphito-based substrates.