1. Discerning mercury-halogen chemistry from diurnal cycles of RGM Christopher Holmes, Daniel Jacob, Noelle Eckley Selin Harvard University Dan Jaffe, Phil Swartzendruber University of Washington Rob Mason University of Connecticut

2. The Mercury Cycle Rapid oxidation and deposition Hg 0 Hg(II)

3. Observations of RGM in the Marine Boundary Layer Major features of RGM diurnal cycles - Fast morning rise - Peak 11:00-13:00 LT - Fast decline in afternoon - No significant change 20:00-4:00 LT - Amplitudes 1-3X mean Jaffe et al. JGR 2005 Laurier et al. JGR 2003 Laurier & Mason JGR 2007

4. All RGM Observations Smoothed Diurnal Cycle Obs GEOS- Chem Selin et al. JGR 2007 Okinawa diurnal cycle Δ Phase due to halogens? Hg 0 RGM OH, O 3 Deposition (~1d) Sea salt (7h) hv Reduction (~1d) Standard GC Mechanism Inconsistent with nighttime RGM > DL Okinawa DL Cruise DL

5. Okinawa Hg 0 RGM O3O3 Deposition (~1d) Sea salt (7h) hv Reduction (~1d) “Excess” Oxidation OH, Br, Cl… Simple Model Calculating [Br] from RGM observations Other oxidants, e.g. Cl, OH, also contribute Pacific Cruise [Br] < 1 x 10 7 cm -3 in MOCCA and MISTRA (Dickerson et al. 1999; von Glasow et al. 2002)

6. Hg 0 RGM Br, Cl, O 3, (OH?) Deposition (~1d) Sea salt (slower) hv Reduction (<1d?) Revised MBL Mechanism Implications Major features of RGM diurnal cycles - Fast morning rise oxidation of Hg 0 by Br and Cl - Peak 11:00-13:00 LToxidation of Hg 0 is primarily in the morning - Fast decline in afternoonrapid sink could be photochemical reduction - No significant change 20:00-4:00 LTdirect deposition (1 st order) is a minor RGM sink - Amplitudes 1-3X meandifferent RGM sink rates Smoothed Diurnal Cycle