Chemical sources and sinks of Hg(II) in the remote atmospheric marine boundary layer

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1 Cruise May 22 Atlantic Cruise August 23 April 24 Chemical sources and sinks of Hg(II) in the remote atmospheric marine boundary layer Christopher D. Holmes Daniel J. Jacob Department of Earth & Planetary Sciences Harvard University Robert P. Mason Department of Marine Sciences University of Connecticut Dan Jaffe Interdisciplinary Arts and Sciences Department University of Washington

2 Hg-halogen chemistry Simultaneous O 3 and Hg depletion event Hg lifetime for April days Spitzbergen Sprovieri et al., 25 Pressure, hpa o S 3 o S 3 o N 6 o N Gas-phase reaction mechanism Br Br, OH,... k1 k3 Hg HgBr HgBrX k2 Global lifetime due to Hg-Br reaction: τ Hg = 2 d (range: d) Holmes et al., 26 Goodsite et al., 24

3 Mid-latitude marine boundary layer Rapid MBL cycling? Max. daily RGM, pg m RGM-O3 Anticorrelation Atlantic Subtropics Subtropics Midlatitudes Mason and Sheu, Ozone, ppbv following Laurier and Mason, 27

4 RGM diurnal cycles - regular features Cruise Atlantic Cruise RGM (pg m-3) Observations and Hourly medians Atlantic Subtropics Subtropics Midlatitudes Major features - Fast morning rise - Peak 11:-13: LT - Fast decline in afternoon - No significant change 2:-4: LT - Amplitudes 1-3X mean Local Hour Detection Limits Cruises Data from Laurier et al., 24 Jaffe et al., 25 Laurier and Mason, 27

5 Free troposphere Sources and sinks of Hg(II) in the marine boundary layer Marine boundary layer Entrainment Hg O3, OH, Br, Cl aq. photoreduction Hg(II) Dry & Wet Deposition Sea salt aerosol Hg Hg(II) MeHg Hg(p) d[rgm] MBL dt = P L + F entrain F dep F entrain = v entrain ([RGM] FT [RGM] MBL ) F dep = [RGM] MBL (v dry (u) + H F ss (u))

6 5 OH Free troposphere Sources and sinks of Hg(II) in the marine boundary layer Marine boundary layer Hg Hg O3, OH, Br, Cl aq. photoreduction Dry & Wet Deposition Entrainment Hg(II) Hg(II) Sea salt aerosol MeHg Hg(p) 16 molecules cm-3 16 molecules cm Midlatitudes Atlantic Subtropics Subtropics Atomic Br Atlantic Subtropics Subtropics Midlatitudes Atomic Chlorine 1 ppt BrO for 1 ppb O3 1 ppt BrO for 3 ppb O3 d[rgm] MBL dt = P L + F entrain F dep F entrain = v entrain ([RGM] FT [RGM] MBL ) F dep = [RGM] MBL (v dry (u) + H F ss (u)) 14 molecules cm-3.5 Hour

7 Site-specific simulations RGM, relative Midlatitudes τ Hg(II) = 6.5 h..5 τ Hg(II) = 8.1 h. RGM, relative Subtropics τ Hg(II) = 2.6 h Hour Atlantic Subtropics τ Hg(II) =.7 h Hour Observations Hg+Br sim Hg+OH sim 24

8 RGM Budget Flux, pg m-3 d Sources Entrainment Hg+Cl Hg+O3 Hg+Br Sinks Entrainment Dry deposition Sea salt aerosols midlatitudes subtropics τ Hg(II) = 8.1 h τ Hg, chem. = 1 d τ Hg(II) = 6.5 h τ Hg, chem. = 127 d τ Hg(II) = 2.6 h τ Hg, chem. = 86 d τ air, entrain = 28 h

9 Conclusions and Implications Loss of RGM at numerous MBL sites is controlled by wind-driven sea salt aerosols Hg+Br supplies 2-4% of MBL RGM in the midlatitudes and subtropics. Hg+Cl contributes less than 1% of RGM. Entrainment supplies 3-5% of MBL RGM. ~1% of marine Hg emissions are immediately oxidized in the MBL vs. 4% suggested by Mason and Sheu (22) Hg(II) deposition should be fastest in high-latitude storm tracks. RGM lifetime by sea salt aerosols 23 mean hours Funding Source: EPA STAR