Gas-phase HO•-Initiated reactions of elemental mercury:: Kinetics, product studies, and atmospheric implications

Mercury is an environmentally volatile toxic fluid metal that is assumed to have a long atmospheric residence time and hence is subject to long-range transport. The speciation and chemical transformation of mercury in the atmosphere strongly influences its bioaccumulation potential in the human food chain as well as its global cycling. To investigate the oxidation of Hg-0 by HO. the dominant daytime atmospheric oxidant,we performed kinetic and product studies over the temperature range 283353 K under near atmospheric pressure (100 +/- 0.13 kPa) in air and N-2 diluents. Experiments were carried out by the relative rate method using five reference molecules and monitored by gas chromatography with mass spectroscopic detection (GC-MS). The HO. were generated using UV photolysis of isopropyl nitrite at 300 less than or equal to lambda less than or equal to 400 nm in the presence of NO. The room-temperature rate constant was found to be (9.0 +/- 1.3) x 10(-14) cm(3) molecule(-1) s(-1). The temperature dependence of the reaction can be expressed as a simple Arrhenius expression (in unit of 10(-14) cm(3) molecule(-1) s(-1)) using ethane as the reference molecule: k(Hg + HO) = 3.55 x 10(-14) exp{(294 +/- 16)/T}. The major reaction product, HgO, was identified in the gaseous form, as aerosols and as deposits on the container walls, using chemical ionization mass spectrometry (CI-MS), electron impact mass spectrometry (EI-MS), GC-MS, and cold vapor atomic fluorescence spectrometry (CVAFS). Experimental results reveal that ca. 6% of the reaction products were collected on a 0.2 mum filter as suspended aerosol, ca. 10% were in the gaseous form, and about 80% were deposited on the reaction vessel wall. The potential implications of our results in the understanding of tropospheric mercury transformation are herein discussed.