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Deriving vertical profiles of free tropospheric trace gases from ground based measurements: Implications for oxidation of atmospheric mercury Sean Coburn 1,2, Siyuan Wang 1,2, Barbara Dix 2, Arnout terSchure 3, and Rainer Volkamer 1,2 1 Dept. of Chemistry, University of Colorado, Boulder, CO 2 Cooperative Institute for Research in Environmental Science (CIRES), Boulder, CO 3 Electric Power Research Institute (EPRI), Palo Alto, CA, ESA-SOLAS-EGU Conference 28-31 October 2014 Frascati,Italy
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TORERO Field Study Outline Background Halogens and mercury Instrumentation/Technique MAX-DOAS Measurements Results Derived vertical columns Implications for atmospheric oxidation Summary/Conclusions
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Why are halogen important? Modify oxidative capacity of atmosphere through reaction with O 3 Linked to atmospheric HO x and NO x cycles. Participate in new particle formation Involved in mercury oxidation reactions Pechtl et al., 2006 (ACP); Holmes et al., 2009 (AE) <2ppt BrO in MBL What about FT? Tropospheric Halogens
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BrO overview: observations and models Theys et al. [2011] Satellite: 1-3 x10 13 molec cm -2 (Chance et al., 1998; Wagner et al., 2001; Richter et al., 2002; Van Roozendael et al., 2002; Theys et al., 2011) Ground : 0.2-3 x10 13 molec cm -2 (Schofield et al., 2004, Hendrick et al., 2007; Theys et al., 2007; Coburn et al., 2011; Coburn et al., 2014, in prep.) Balloon: 0.2-0.3 x10 13 molec cm -2 (Pundt et al., 2002; Dorf et al., 2008) Models: 0.2-1.0 x10 13 molec cm -2 (~ 0.2-0.5 ppt) (Saiz Lopez et al., 2012; Parrella et al., 2012) – in the tropics
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Mercury Prevailing winds Atmosphere biggest source of mercury to terrestrial environment Where in the atmosphere is oxidation occurring?
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MAX-DOAS observations of the FT Greatest sensitivity at instrument altitude Ground based still contains information about layers aloft Aircraft can directly probe layers at different altitudes
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MAX-DOAS Platforms/Instrumentation spectrographs/detectors Telescope pylon motion stabilized Volkamer et al., SPIE 2009, Coburn et al., 2011, Baidar et al., AMT 2013
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MAX-DOAS Data overview # of meas. days~300 PeriodsJune – Oct. 2009; March – June 2010; Aug. – Oct. 2010; Feb. 2011 # of spectra>200,000 Trace gasesBrO, IO, CHOCHO, HCHO, NO 2, O 4 % Cloudy<40% FiltersDetection limit, absolute RMS, SZA Spectral proof for the detection of IO and BrO BrO = 2.1 ppt IO = 3.1 ppt MAX-DOAS Measurements Long term measurements Retrieve multiple species
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wavelength [nm] 3.4 ppt BrO @ 14.2km 2.2 ppt BrO @ 9.0km 0.3 ppt BrO @ 4.1km no BrO @ 0.9km 340359 BrO measured by CU AMAX-DOAS 18E-4 13E-4 5E-4 <1-2E-4 BrO is detectable over most of the tropospheric air column ~ 0.3 ppt BrO in lower FT (4.1km) ~ 3.4 ppt BrO above 14km DOAS detection limit: ~ 0.3 ppt BrO @ 1min data Spectral proof of BrO in the tropical FT
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CU AMAX/GMAX VCDs: BrO & IO In line with other free tropospheric observations 1-3x10 13 molec cm -2 global background VCD IO more variable -> background 2-4x10 12 molec cm -2 IO VCD? No BrO in the MBL!
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Vertical profiles: Non-linear Optimal Estimation Simulated O 4 SCDs Radiative transfer (1) Inversion Measured O 4 SCDs convergence? Weighing function Measured Trace gas SCDs Radiative transfer Trace gas profile Aerosol profile Volkamer et al., 2009, SPIE; Baidar et al., 2013, AMT; Rodgers (2000); http://rtm.iup.uni-heidelberg.de/McArtim Trace gases and aerosol extinction profiles
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CU GMAX/AMAX Profiles: BrO and IO Converge on 1 profile, independent of a priori Coburn et al., 2014, in prep; Volkamer Group, unpublished Now have vertical distribution of BrO in the troposphere -> Use to assess impact on GEM oxidation Comparison with previous profile measurements
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Comparison with model profiles Models are under predicting BrO in the free troposphere Effect on GEM oxidation? TORERO, GEOS-ChemThis study, WACCM Coburn et al., 2014, in prep; Volkamer Group, unpublished
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Relevance of FT-BrO for mercury Even with lower amounts from models oxidation by Br radicals is dominant pathway ModeledMeasured Coburn et al., 2014, in prep
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Relevance of FT-BrO for mercury Free tropospheric Hg brought to the BL through deep convection
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Conclusions Field observations of halogens: First simultaneous observations of BrO and IO in the tropical FT – First vertical profiles of BrO from aircraft in the tropics Vertical profiles and VCDs derived from ground-based MAX-DOAS measurements IO and BrO are widespread in the FT and detected in NH and SH – 0.1-0.5 ppt IO have been detected in the FT in NH and SH – 2 ppt BrO are measured at altitudes >12km in NH and SH – IO and BrO show different vertical profiles – Some similarities between SH and NH from AMAX and GMAX Relevance for mercury: Bromine dominates mercury oxidation rates (chlorine < 1%). GEM lifetime wrt bromine: ~1 d in upper FT; 8 to 80 d in lower FT
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Volkamer Group EPA staff NCAR/EOL/RAF the entire TORERO team Funding EPRI TI Program EPRI Air toxics program NSF-TORERO Acknowledgements
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