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 October 2014 Frascati,Italy

TORERO Field Study Outline Background Halogens and mercury Instrumentation/Technique MAX-DOAS Measurements Results Derived vertical columns Implications for atmospheric oxidation Summary/Conclusions

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

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 : 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: x10 13 molec cm -2 (Pundt et al., 2002; Dorf et al., 2008) Models: x10 13 molec cm -2 (~ ppt) (Saiz Lopez et al., 2012; Parrella et al., 2012) – in the tropics

Mercury Prevailing winds Atmosphere biggest source of mercury to terrestrial environment Where in the atmosphere is oxidation occurring?

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

MAX-DOAS Platforms/Instrumentation spectrographs/detectors Telescope pylon motion stabilized Volkamer et al., SPIE 2009, Coburn et al., 2011, Baidar et al., AMT 2013

MAX-DOAS Data overview # of meas. days~300 PeriodsJune – Oct. 2009; March – June 2010; Aug. – Oct. 2010; Feb # 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

wavelength [nm] 3.4 ppt 14.2km 2.2 ppt 9.0km 0.3 ppt 4.1km no 0.9km 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 1min data Spectral proof of BrO in the tropical FT

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!

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);  Trace gases and aerosol extinction profiles

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

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

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

Relevance of FT-BrO for mercury Free tropospheric Hg brought to the BL through deep convection

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 – 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

Volkamer Group EPA staff NCAR/EOL/RAF the entire TORERO team Funding EPRI TI Program EPRI Air toxics program NSF-TORERO Acknowledgements