Development of a Real-Time Method for the Measurement of Sulfur(iv) in Cloud Water with a Counter-Flow Virtual Impactor.

The in-cloud oxidation of dissolved SO _2 is believed to be an important contributor to sulfate production in the atmosphere. The first step of this process is the dissolution of SO_2 into cloud droplets. Field measurements of dissolved SO_2 and its ionic forms should help determine what governs the dissolution and subsequent oxidation processes. A novel method, using a counter-flow virtual impactor (CVI) to measure S(IV) concentrations in cloud water, is described. The CVI collects and evaporates cloud droplets, converting aqueous S(IV) into gaseous SO_2 for analysis with a pulsed-fluorescence detector. Based on calculations and laboratory experiments with acidic droplets, S(IV), except that complexed as hydroxymethanesulfonate (HMS), is expected to be released to the gas phase during droplet evaporation. Evidence for the production of HMS in aerosol particles following droplet evaporation also was obtained from measurements of complexes (probably as HMS) S(IV) in ambient aerosol samples. Field measurements were performed with a CVI mounted on a research aircraft during the Frontal Boundary Study in Ohio to evaluate the CVI for measuring S(IV) in cloud water. From the signal of the SO_2 analyzer, the aqueous S(IV) concentration (in units of nmol per m^3 of air) was determined. Measurements in clouds showed similar temporal variation of S(IV) (aq) with other cloud parameters following required data treatment. A detection limit of 0.1 nmol per m^3 of air was achieved over one minute intervals (based on the response time of the analyzer) and was limited mainly by the noise of the SO_2 analyzer. Determination of molar S(IV) (aq) concentrations (in units mu M) requires simultaneous measurements of the water vapor released by the evaporated droplets collected by the CVI which was not made. With the inclusion of measurements of the liquid water content collected by the CVI, a detection limit for S(IV) (aq) of about 0.4 μm should be achievable. The fast response, good detection limit, ability to collect droplets of different sizes, and separation of S(IV) (aq) from oxidants and HMS are advantages of this method over bulk collection and wet chemical analysis of cloud water.