Discerning mercury-halogen chemistry from diurnal cycles of RGM

Christopher Holmes, Daniel Jacob, Noelle Eckley SelinHarvard University

Dan Jaffe, Phil SwartzendruberUniversity of Washington

Rob MasonUniversity of Connecticut

The Mercury Cycle

Rapid oxidation and deposition

Hg0Hg(II)

Observations of RGM in the Marine Boundary Layer

Major features of RGMdiurnal 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

All RGM Observations

Smoothed Diurnal Cycle

Obs GEOS-Chem

Selin et al. JGR 2007

Okinawa diurnal cycle

Δ Phase due to halogens?

Hg0 RGMOH, O3

Deposition (~1d)

Sea salt (7h)

hv Reduction (~1d)

Standard GC Mechanism

Inconsistent with nighttime RGM > DL

Okinawa DLCruise DL

Okinawa

Hg0 RGMO3

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 107 cm-3 in MOCCA and MISTRA(Dickerson et al. 1999; von Glasow et al. 2002)

Hg0 RGMBr, Cl, O3, (OH?)

Deposition (~1d)

Sea salt (slower)

hv Reduction (<1d?)

Revised MBL Mechanism

Implications

Major features of RGM diurnal cycles

- Fast morning rise oxidation of Hg0 by Br and Cl

- Peak 11:00-13:00 LT oxidation of Hg0 is primarily in the morning

- Fast decline in afternoon rapid sink could be photochemical reduction

- No significant change 20:00-4:00 LT direct deposition (1st order) is a minor RGM sink

- Amplitudes 1-3X mean different RGM sink rates

Smoothed Diurnal Cycle