S. Pechtl et al.: Modeling the role of iodine oxides in new particle formation 507

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Fig. 1. Scheme of gas and liquid phase iodine chemistry as implemented in MISTRA.

question marks in Fig. 1 denote uncertain reactions in thegas phase. Sensitivity studies, especially with respect to ourkey species OIO were performed in order to address some ofthese uncertainties (see Sect. 3). In contrast to earlier stud-ies (Vogt et al., 1999; von Glasow et al., 2002a) we did notinclude I2O2 in our reaction scheme since recent thermody-namic calculations indicate that the asymmetric dimer IOIOthat forms to up to 60% from the self-reaction of IO (Atkin-son et al., 2004) breaks down in less than a second (Saun-ders and Plane, 2005). There might also be some contribu-tion of the symmetric dimer IOOI (less than 20%), which de-cays quickly to 2I+O2. Hence, we assume IO+IO−→OIO+I(>80%) and IO+IO−→2I+O2 (<20%). Gas phase reactionsand respective rate constants involving OIO as used in thepresent study are provided in Table 1. The accommodationcoefficient for uptake of OIO on aerosol is assumed to beunity. The entire reaction mechanism including the completeset of rate constants and references can be found as electronicattachement to this paper (http://www.atmos-chem-phys.org/acp/6/505/acp-6-505-sp.pdf).

The nucleation module developed for MISTRA consists ofa two-step parameterization: In the first step, the “real” nu-

cleation rate of thermodynamic stable clusters is calculated,while in the second step, the growth of these nuclei into themodel’s lowest particle size bin (diameter 10 nm) is com-puted. As the particles can be included into the model onlyafter the second step of nucleation, we denote the nucleationrate in the model’s lowest size bin the “apparent” nucleationrate. Note that this quantity is very useful for field mea-surements, too, because only an “apparent” nucleation rateof particles exceeding a certain size can be observed.

Regarding the “real” nucleation rate, we implementedtwo options: (1) homogeneous homomolecular nucleation ofOIO, and (2) ternary H2SO4-NH3-H2O nucleation relying onthe parameterization by Napari et al. (2002). No parameteri-zation has been available so far describing the nucleation ofOIO. We developed a parameterization using the approachof Burkholder et al. (2004): They combined laboratory ex-periments with a coupled chemical – aerosol model, wherethe experimental results were used to derive necessary modelparameters. The nucleation steps of OIO clusters are treatedfully kinetically in this model, the model bins increment bysingle OIO molecules. The temporal evolution of gas phaseOIO and OIO clusters are described by differential equations

www.atmos-chem-phys.org/acp/6/505/ Atmos. Chem. Phys., 6, 505–523, 2006

3.1. Differential Optical Absorption Spectroscopy 27

Figure 3.3: Differential absorption cross sections σ�(λ) of selected atmospheric

trace gases measurable by the DOAS technique. The indicated detection limit as-

sumes a minimal detectable optical density of 10−3 on the given light path.

36 3. DOAS

Figure 3.6: The MAX-DOAS principle: Spectra of scattered sunlight are taken

under different elevation angles. Especially for small elevation angles, a high sen-

sitivity towards tropospheric absorbers is achieved. Adapted from [Sinreich 2008]

event is assumed for each photon. In reality multi scattering can also occur,

which makes the determination of the light path more difficult. The green

shaded areas indicate layers of stratospheric and tropospheric absorbers. As

can be seen, lower elevation angles result in a longer light path and, thus, a

better detection limit.

Passive DOAS measurements yield slant column densities (SCDs) of the ab-

sorbing trace gases present in the probed atmosphere. The SCD is defined

as integral of the trace gas concentration along the light path s in the atmo-

sphere:

S =

�c(s) ds (3.23)

The SCD depends on the length of the light path, which is influenced by

observation geometry and meteorological conditions. It can be converted to

the vertical column density (VCD), V , which is the concentration integrated

Identifying the mesoscale horizontal and vertical distribution of reactive halogen

oxides in polar regions

NSERC CREATE Summer School in Arctic Atmospheric Science, July 2012

MotivationThe Earths atmosphere can be severely influenced by halogen compounds, e.g. the familiar stratospheric ozone hole. During the last two decades, halogen species have also been identified as playing an important role in the chemistry of the troposphere. In particular, chlorine, bromine and iodine released from sea salt particles or produced by the photolysis of halocarbons and I2 emitted by the ocean, contribute considerably to the destruction of ozone and have the potential to significantly alter the balance of the tropospheric chemical system.One of the most striking effects of halogens on tropospheric chemistry occurs each spring in the Arctic and Antarctic lower troposphere, when strong and sudden increases in reactive bromine over several million square kilometres are being observed. These events are frequently coincident with the complete destruction of ozone in the polar boundary layer. The reactive bromine is released via autocatalytic processes on sea salt surfaces. Under certain conditions, these heterogeneous processes can lead to an exponential increase of reactive bromine in the gas phase and therefore have become known as the ``bromine explosion´´.Reactive iodine compounds are expected to have an important impact on the ozone budget as well, even if they are present at very low mixing ratios of only a few parts per trillion (ppt). Inorganic iodine is believed to originate from the photolytic destruction of organic iodine compounds and I2. Their short lifetimes, ranging between minutes and days, initiates an inorganic iodine reactions, in particular involving the iodine radicals I, IO and OIO, that lead to the destruction of ozone in catalytic cycles and particle formation.Despite intensive scientific efforts and major field campaigns during the last few years, substantial uncertainties in the amount and distribution of halogen radicals in the troposphere, their sources and sinks, as well as in some of the key processes of tropospheric halogen chemistry still exist to date.

1Institute of Environmental Physics, University of Heidelberg, Im Neuenheimer Feld 229, 69120 Heidelberg, GermanyJohannes Zielcke1, Udo Friess1, Denis Pöhler1, Ulrich Platt1

References[1] Pöhler, D. et al., Observation of halogen species in the Amundsen Gulf, Arctic, by active long-path differential optical absorption spectroscopy. PNAS April 13, 2010 vol. 107 no. 15 6582-6587, (2010).[2] Pechtl, S. et al., Modeling the possible role of iodine oxides in atmospheric new particle formation. Atmos. Chem. Phys., 6, 505-523, (2006).[3] Friess, U. et al., Spectroscopic measurements of tropospheric iodine oxide at Neumayer Station, Antarctica. Geophys. Res. Lett., 28(10), 1941–1944, (2001).[4] Friess, U. et al., Iodine monoxide in the Antarctic snowpack. Atmos. Chem. Phys., 10, 2439–2456, (2010).[5] Haussmann, M. and Platt, U., Spectroscopic measurement of bromine oxide and ozone in the high Arctic during Polar Sunrise Experiment 1992, J. Geophys. Res., 99(D12), 25,399–25,413, (1994).

Corresponding author: Johannes Zielcke, jzielcke@iup.uni-heidelberg.de, Tel.: +49 6221 546527

Measurement instrumentation

Chemistry

Bromine explosion:BrO + HO2 HOBr + O2HOBrgas + Br-

aq + H+aq Br2, gas + H2O

Br2 + hν 2 Br2 Br + 2 O3 2 BrO + 2 O2

Future work• Field work in Antarctica in southern hemispheric spring 2012• Setting up automated near real-time vertical profile calculations for the instrument in Alert• Evaluation of the data from Barrow and Alert, including creation of profiles and maps• How about chlorine chemistry and its interaction with other halogen species?

Differential Optical Absorption Spectroscopy (DOAS)DOAS utilizes narrowband optical absorption structures, which are characteristic for individual molecule species. The diagram on the right shows absorption structures of some of the species of interest, which can be measured contactless with DOAS.

Longpath DOASThis technique uses an active light source such as LEDs or xenon arc lamps and a reflector array to send the light back to the telescope. The concentration of the species of interest averaged over the light path can be readily acquired contiuously during day and night.

Cavity-enhanced DOASSimilarly to Longpath DOAS, an active light source is used, however, the light path of several kilometers is folded using an optical resonator with ca. 2m separation. This allows point measurements of trace gases like IO or NO2 and thus identification of sources.

MeasurementsArcticAlert, Nunavut, Canada:Since 2008, a MAX-DOAS instrument in the ultraviolet wavelength range is running at the Global Atmosphere Watch (GAW) labora tory. Due to technical problems, spring 2012 was the first season with a continuous set of data showing regular ozone depletion events (ODEs).

AntarcticRoss Island:In Antarctic spring 2012 an intensive campaign is planned on Ross Island around polar sunrise with the instruments explained on the left. A Longpath DOAS will be set up, several MAX-DOAS instruments will be deployed, both stationary and mobile, and a newly developed mobile Cavity-enhanced instrument will be used to investigate the distribution of bromine and iodine monoxide but also other halogen compounds, while supplementary ozone measurements and snow sampling will be undertaken as well. The goal is to get a better understanding of the sources and pathways of reactive halogens and iodine in particular, which is practically absent in the Arctic, but present in coastal Antarctica.

Alert

Barrow

Ross Island

MAX-DOAS in AlertLongpath DOAS

Cavity-enhanced DOAS

MAX-DOAS

ALAR in Barrow

Barrow, Alaska, USA:Airborne measurements were conducted in spring 2012 with an imaging downward and a forward-looking instrument on board the Airborne Laboratory for Atmospheric Research (ALAR, Atmospheric chemistry group of Prof. Paul Shepson, Purdue University). Sea ice, open leads and the arctic tundra were mapped

Multi Axis (MAX)-DOASutilizes scattered sun light and scans several elevation angles to get slant column densities (SCDs), which can be used to compute vertical profiles of aerosols and absorbers up to a few kilometers.

highly reflective mirrors

absorber

Iodine chemistry in the marine boundary layer(adapted from [2])

Objectives• What is the vertical distribution of reactive halogens in polar regions?• What is the source of reactive bromine in polar spring?• To what extent is BrO released from blowing snow and the snowpack?• How do huge amounts of iodine radicals accumulate in the Antarctic snowpack, how can these be sustained, and why is reactive iodine found in Antarctica, but not in the Arctic?

Iodine monoxide in the Antarctic troposphere was first discovered by Friess et al[3]. Later, huge amounts of iodine monoxide were found to be in the snowpack[4], the primary sources of which are thought to be either biogenic processes in the ocean, or algae at the bottom of the sea ice.

Bromine monoxide in the Arctic troposphere was first measured by Haussmann and Platt (1994)[5]. Although the general reaction scheme is thought to be understood, some major aspects are still not known, e.g. the actual release mechanism and the meteorological conditions necessary for this process to happen.