Deriving vertical profiles of free tropospheric trace gases from ground based measurements:

Implications for oxidation of atmospheric mercury

Sean Coburn1,2, Siyuan Wang 1,2, Barbara Dix2, Arnout terSchure3, and Rainer Volkamer1,2

1Dept. of Chemistry, University of Colorado, Boulder, CO2Cooperative Institute for Research in Environmental Science (CIRES), Boulder,

CO3Electric Power Research Institute (EPRI), Palo Alto, CA,

ESA-SOLAS-EGU Conference28-31 October 2014

Frascati ,Italy

TORERO Field Study

Outline

Background• Halogens and mercuryInstrumentation/Technique• MAX-DOAS • MeasurementsResults• Derived vertical columns• Implications for

atmospheric oxidationSummary/Conclusions

Why are halogen important?• Modify oxidative capacity of atmosphere through reaction with O3

• Linked to atmospheric HOx and NOx 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 x1013 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 x1013 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 x1013 molec cm-2

(Pundt et al., 2002; Dorf et al., 2008)

Models: 0.2-1.0 x1013 molec cm-2

(~ 0.2-0.5 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

motionstabilized

Volkamer et al., SPIE 2009, Coburn et al., 2011, Baidar et al., AMT 2013

MAX-DOAS Data overview

# of meas. days ~300

Periods June – Oct. 2009; March – June 2010; Aug. – Oct. 2010; Feb. 2011

# of spectra >200,000

Trace gases BrO, IO, CHOCHO, HCHO, NO2, O4

% Cloudy <40%

Filters Detection 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

optic

al d

ensi

ty [a

.u.]

wavelength [nm]

3.4 ppt BrO @ 14.2km

2.2 ppt BrO @ 9.0km

0.3 ppt BrO @ 4.1km

no BrO @ 0.9km

340 359

BrO measured by CU AMAX-DOAS

18E-4

13E-4

5E-4

<1-2E-4

optic

al d

ensi

ty [a

.u.]

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

CU AMAX/GMAX VCDs: BrO & IO

In line with other free tropospheric observations 1-3x1013 molec cm-2 global background VCD

IO more variable -> background 2-4x1012 molec cm-2 IO VCD?

No BrO in the MBL!

Vertical profiles: Non-linear Optimal Estimation

Simulated O4 SCDs

Radiative transfer (1)

Inversion

Measured O4 SCDs

convergence?

Weighing function

Measured Trace gas

SCDs

Radiative transfer

Trace gasprofile

Aerosolprofile

Volkamer et al., 2009, SPIE; Baidar et al., 2013, AMT; Rodgers (2000); http://rtm.iup.uni-heidelberg.de/McArtim

Þ 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-Chem This 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

Modeled Measured

Coburn et al., 2014, in prep

Relevance of FT-BrO for mercury

Free tropospheric Hg brought to the BL through deep convection

ConclusionsField 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

Volkamer GroupEPA staffNCAR/EOL/RAFthe entire TORERO team

FundingEPRI TI ProgramEPRI Air toxics programNSF-TORERO

Acknowledgements