1 Ozone formation induced by the impact of reactive bromine and

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Ozone formation induced by the impact of reactive bromine and iodine species on photochemistry in a polluted marine environment Moshe Shechner, and Eran Tas Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b02860 • Publication Date (Web): 07 Nov 2017 Downloaded from http://pubs.acs.org on November 12, 2017

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Environmental Science & Technology

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Ozone formation induced by the impact of reactive bromine and iodine species on

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photochemistry in a polluted marine environment

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M. Shechner1 and E. Tas*1

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Food and Environment, Hebrew University of Jerusalem, Rehovot, Israel

The Department of Soil and Water Sciences, The Robert H. Smith Faculty of Agriculture,

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* Correspondence to: Eran Tas, The Department of Soil and Water Sciences, The Robert

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H. Smith Faculty of Agriculture, Food and Environment, Hebrew University of Jerusalem,

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Rehovot, Israel, +972-525402510, Fax: +972-8-9475181 [email protected].

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ACS Paragon Plus Environment


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Abstract

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Reactive iodine and bromine species (RIS and RBS, respectively) are known for altering

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atmospheric chemistry, causing sharp tropospheric ozone (O3) depletion in polar regions

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and significant O3 reduction in the marine boundary layer (MBL). Here we use

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measurement-based modelling to show that, unexpectedly, both RIS and RBS can lead to

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enhanced O3 formation in a polluted marine environment under volatile organic compound

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(VOC)-limited conditions associated with high nitrogen oxide (NOX = [NO] + [NO2])

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concentrations. Under these conditions, the daily average O3 mixing ratio increased up to

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~44% and ~28% for BrO and IO mixing ratios of up to ~6.8 ppt and 4.7 ppt, respectively.

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The increase in O3 was partially induced by enhanced ClNO3 formation for higher Br2 and

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I2 emission flux. The O3 increase was associated with an increased mixing ratio of

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hydroperoxyl radical to hydroxyl radical ([HO2]/[OH]) and increased [NO2]/[NO] with

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higher RBS and/or RIS. NOX-rich conditions are typical to the polluted MBL, near

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coastlines and ship plumes. Considering that O3 is toxic to humans, plants and animals and

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is a greenhouse gas, our findings call for adequate updating of local and regional air-

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quality models with the effects of RBS and RIS activities on O3 mixing ratios in the

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polluted MBL.

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Table of Contents (TOC)

50 51 52 53

Remote marine boundary layer

[O3]

[O3] Br2 , I2

54 55

Polluted marine boundary layer

[HO2]/[OH]

Br2 , I2 [HO2]/[OH]

Br2 , I2

56

[NOX] / [VOC]

NOX-limited

Br2 , I2 VOC-limited

57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73

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74

Introduction

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Atmospheric reactive halogen species (RHS; containing Cl, Br or I and their oxides) are

76

highly reactive. They are released by inorganic and biological mechanisms from sea water

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and strongly affect the atmosphere's composition and oxidation capacity, as well as

78

climate components1. Since the 1980s, much evidence has accumulated demonstrating a

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key role for RHS in stratospheric ozone (O3) destruction2 and sharp tropospheric O3-

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depletion events in the polar regions3. In 1997, the first O3-depletion event outside of the

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polar regions was discovered in the boundary layer over the Dead Sea4. Since then, many

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studies have indicated the widespread occurrence of RHS over the ocean at mid-latitudes.

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Reactive iodine species (RIS) over the open ocean originate predominantly from reaction

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of O3 at the ocean surface which leads to I2 and hypoiodous acid (HOI) emission5; release

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of alkyl iodides (CH3I, C3H7I, CH2ClI, or CH2I2) from the sea surface probably makes a

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less important contribution1,

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important source in coastal areas7, 8, depending on macro-algal type and amounts7, 9, 10.

3, 6

. I2 emission by macro-algae at low tide can also be an

88 89

The major source of reactive bromine species (RBS) in the mid-latitudinal marine

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boundary layer (MBL) and the free troposphere seems to be photochemical decomposition

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of CHBr3 and other organic Br-containing species11, 12. The well-established autocatalytic

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liberation of Br2 and BrCl from sea salt, induced by the uptake of hypobromous acid

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(HOBr) by acidified sea salt aerosols6, 13 seems to be less important in the mid-latitudinal

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MBL11,

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(VOCs), they are likely to contribute more to VOC oxidation, and thus to the oxidation

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capacity of the atmosphere, than to O3 depletion14, 15. However, the apparently widespread

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occurrence of BrO and IO over the mid-latitudinal ocean at mixing ratios of up to a few

12

. Due to the high reactivity of Cl radicals with volatile organic compounds

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parts per trillion indicates a strong factor in O3 depletion that is relevant on regional to

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global scales3.

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The impact of RHS on O3 level and oxidation capacity in the polluted MBL, in either ship

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plumes or coastal areas, is of special interest. Osthoff et al.16 showed that night-time

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interactions between sea salt chlorine and N2O5 can result in significant increases in

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daytime O3 mixing ratios, via ClNO2 formation and its subsequent photolysis to yield

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reactive Cl, which accelerates VOC oxidation. More recent findings suggest that the

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effects of Cl-mediated O3 formation on air quality are not limited to the coastline area, but

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spread much further inland1, 17.

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The interaction of RBS and RIS with photochemistry is completely different from that

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with reactive chlorine species (RCS). It is commonly accepted that photochemical O3

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formation by halogens is incomparably lower than O3 destruction induced by RBS or RIS,

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even at their typical levels over the open ocean3, associated with BrO and IO mixing ratios

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ranging from below 1 ppt up to ~4 ppt3, 18. Even more effective RBS- and RIS-induced O3

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destruction is expected in the polluted coastal MBL. It has been shown that RBS

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concentration can be enhanced under high nitrogen oxide concentrations (NOX = [NO] +

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[NO2]) and sulphate aerosols18-20, whereas high BrO concentrations (>10 ppt) have been

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observed near the eastern North-Atlantic coastline21. In coastal areas, IO has been

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measured at mixing ratios of up to ~ 50 ppt22, 23, induced by macro-algae under oxidative

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stress at low tide1, 3, 7, 24, 25. The basic reason for the different behaviour of RBS and RIS

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compared to RCS lies in the much more efficient reaction of Cl with VOCs compared to

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that of Br and I26. This leads to less efficient recycling of RCS throughout O3 destruction

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cycles, because reaction of RHS with VOCs acts as a terminator of the catalytic O3

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destruction cycles. In addition, due to its high reactivity with VOCs, Cl acts similar to

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other photolytic oxidants (OH, O3 and NO3) in facilitating O3 formation, mainly during the

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morning hours16, 17, 27.

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Here we study the impact of RBS and RIS on photochemistry under photochemically

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imbalanced atmospheric conditions, accounting for VOC-limited and NOX-limited

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conditions in terms of O3 formation28. The simulations are based on previous

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measurements performed at the Dead Sea as described in the following.

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2. Methods

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2.1. Model description

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The model simulations are based on previous measurements taken at the Dead Sea (Sect.

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S3), as described briefly in the following. The model used in this analysis is the

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comprehensive heterogeneous CAABA/MECCA box model29,

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includes an explicit kinetic heterogeneous chemical mechanism, accounting for gas and

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aqueous phase reactions and heterogeneous reactions for two aerosol modes.

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Heterogeneous recycling of HOX, XNO2 and XNO3 via MBL aerosols has been shown to

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play an important role in recycling and increasing RBS concentrations18 and in the

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recycling and gas-aerosol partitioning of RIS31. In the gas phase, species are subjected to

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photochemical decomposition and dry deposition; aerosol processes include both

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scavenging and new particle formation. Gas–aerosol partitioning is performed based on

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Henry’s law and kinetic limitations for coarse soluble and accumulation soluble aerosol

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modes. CAABA/MECCA was run with these two aerosol modes—coarse soluble and

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accumulation soluble—accounting for the O–H–C–N–S–Cl–Br–I chemical mechanism.

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The chemical mechanism included 186 gas-phase reactions capturing 58 photolysis

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30

. CAABA/MECCA

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reactions, 266 aqueous reactions and 154 heterogeneous reactions based on the default

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CAABA/MECCA mechanism (http://www.mecca.messy-interface.org/)30. The model

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includes a new condensed degradation scheme for isoprene which is a reduction of the

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corresponding detailed mechanism in the Master Chemical Mechanism (MCM v3.2;

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http://mcm.leeds.ac.uk/MCM/) (MIM)32.

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Newly formed sea salt aerosol composition was set based on the Dead Sea water

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composition, accounting for the Br- (8.3 g/l), Cl- (306 g/l) and I- (0.16 mg/l)

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concentrations reported by Tas et al.33. Based on reported halide contents in ocean water34,

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the corresponding enrichment factors for Cl-, Br-, I- and Br-/Cl- in the Dead Sea water are

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~12, ~ 128, ~3 and ~7 compared to normal ocean water, respectively.

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Boundary layer height, temperature, relative humidity and pressure, as well as aerosol

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number concentrations were set based on Tas et al.35-37 and Obrist et al.37. An average

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sulphate surface area of 55 µm2/cm3) was determined based on in situ measurements (Sect.

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2.2 in Tas et al.36), while a value of ~6 µm2/cm3 was used for the surface area of the sea

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salt. Photolysis rate coefficients were calculated by CAABA/MECCA using the method

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described in Landgraf and Crutzen38.

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Photochemistry representation by the model was independent of RHS activity. To achieve

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this, preliminary simulations were performed without inclusion of RHS chemistry to

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reproduce the basic photochemistry, using measurement days for which there was no

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evidence of RHS activity over the Dead Sea site33,

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VOCs, NO and NO2 flux were used to reproduce the NOX, VOC and O3 mixing ratios as

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boundary conditions for the simulations. VOC mixing ratio was partially available from

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35, 39, 40


. Fixed magnitude of some


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previous measurements41 and partially based on reported mixing ratios for semi-polluted

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regions28. Two scenarios were selected based on these preliminary simulations,

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representing VOC-limited and NOX-limited photochemical conditions. The photo-

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stationary state used for the VOC-limited and NOX-limited conditions was defined based

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on [VOC]/[NOX] (see Table 1). The photo-stationary state was validated based on the

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response of O3 to deviations in the initial concentrations of VOC and NOX. Both scenarios

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were run under the same meteorological conditions, representing the Dead Sea

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meteorological conditions in summer, with average temperature, relative humidity and

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pressure of ~307 K, ~37% and ~1066 hPa, respectively. For each scenario, several

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simulations were performed with varying fluxes of either Br2 or I2, to represent different

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source magnitudes, as summarised in Table 1. Note that the applied Br2 and I2 fluxes do

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not represent any specific source mechanism. The higher range of the used fluxes resulted

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in RBS and RIS loadings that are typical of the Dead Sea and coastal MBL, while the

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lower range also corresponds with BrO and IO concentrations over the open ocean (see

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Fig. 1). Note that in the case of Br-enriched simulations, the heterogeneous hydrolysis of

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BrNO3 occurs, while the formed HOBr42 can then activate the so-called "Bromine

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Explosion" mechanism6, 13. These two processes have been shown efficient at releasing

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Br2 from aerosols into the atmosphere, particularly under high sulphate aerosol and NOX

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concentrations18, 20, 35. In the case of I-enriched simulations, the applied I2 emission flux

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may be considered the only source of RIS, considering that the uptake of HOI, INO2 and

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INO3 does not result in higher RIS31.

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2.2. Model simulations

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We performed the model simulations under two contrasting conditions: VOC-limited and

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NOX-limited in terms of the photochemical O3-production regime. The respective

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simulations were termed FULL-VOC-limited and FULL-NOX-limited, respectively. The

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two scenarios differed primarily in NOX mixing ratios, which resulted in lower O3 mixing

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ratios and somewhat lower VOC mixing ratios for the VOC-limited scenario (see Table

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1). Each scenario was run under a range of both I2 and Br2 emission fluxes. The model

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simulations indicated that gaseous RCS mixing ratios also increase with increases in both

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I2 and Br2 release. Hence, we performed additional simulations without including Cl

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chemistry, to evaluate the potential net contribution of RBS and RIS to O3 formation. This

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set of simulations, termed No-Cl, was run using a VOC-limited scenario similar to the

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FULL VOC-limited simulations (see Table 1), with no Cl chemistry. Note, however, that

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the exclusion of Cl chemistry from the chemical mechanism results in higher initial VOC

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mixing ratios (Table 1), following a model spin-up, and therefore these simulations cannot

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be used for accurate evaluation of RCS's contribution to RBS- and RIS-induced O3

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formation. For these latter simulations, O3 flux was added for a short time just after

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sunset. This O3 flux was required to enable a more effective night-time O3 recovery, as

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occurs in reality at the Dead Sea, as well as in other areas. This phenomenon is further

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discussed in Tas et al.20, 35. Table 1 summarises the key properties used by the different

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model simulations.

215 216 217 218 219 220 221 222

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Table 1. Key to different simulation types Simulation type Property NO, NO2b (ppb) VOCb (ppb)

No-Cla (VOC-limited)

FULL-VOClimiteda

FULL-NOXlimiteda

2.97 (NO) 19.5 (NO2)

0.0178 (NO) 0.31 (NO2)

2.4 (NO) 13.5 (NO2)

10.8

13.1

14.6

0.12 (OH) 1.7 (HO2)

0.11 (OH) 20 (HO2)

10.12 (OH) 1.7 (HO2)

OH, HO2b (ppt) O3 b (ppb) I2 flux (molecule cm-2 s-1) Br2 flux (molecule cm-2 s-1) O3 fluxc (molecule cm-2 s-1)

66

78

66

5.0E7-2.0E10

5.0E7-2.0E10

5.0E7-1.0E11

1.0E8-7.5E10

1.0E8-7.5E10

1.0E8-1.0E11

-

-

2.3E13

Halogen chemistry

I–Br–Cl

I–Br–Cl

I–Br

224 225 226 227 228 229 230 231 232

a

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3. Results and discussion

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The results obtained under the two contrasting scenarios, FULL-VOC-limited and FULL-

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NOX-limited, were fundamentally different. Figure 1 shows that increasing the emission

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flux of either I2 or Br2 under a VOC-limited regime led to an increase in daily average

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(DA)-O3 mixing ratios of up to 44% and 28% for the Br-enriched and I-enriched

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simulations, respectively, corresponding to DA mixing ratios of 6.8 ppt and 4.7 ppt for

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BrO and IO, respectively. The corresponding maximum daily mixing ratios of BrO and IO

FULL-VOC-limited and FULL-NOX-limited scenarios are described in Methods. No-Cl refers to a set of simulations that use the same model configuration as for the VOC-limited scenarios, but exclude Cl chemistry and apply different I2 and Br2 fluxes. b The values refer to the daily average mixing ratios of the background simulations (i.e., with no I2 or Br2 flux). c The flux was added for 0.5 and 1 h after sunset to account for recovery of O3 in the case of enriched RIS and RBS activity, respectively (see Methods).

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were approximately 9 ppt and 7 ppt, respectively. Higher IO and BrO corresponded with

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lower O3.

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Figure 1. The response of ozone (O3) mixing ratios to increasing reactive halogen species activity. The simulated daily average mixing ratios of O3 and XO (X = I, Br) are presented. (a, e) FULL-NOX-limited scenarios; (c, g) FULL-VOC-limited scenarios, versus the corresponding X2 (X = Br, I) emission fluxes. The daily average reaction rate (R) of O3 with the corresponding X is presented. (b, f) FULL-NOX-limited; (d, h) FULLVOC-limited. Dashed lines indicate maximum Br2 or I2 flux for which an increase in daily average O3 mixing ratios occurred.

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Figure 1 also indicates that within the increasing O3 range (IOR), the increase in X2 (X =

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Br, I) emission fluxes resulted in a corresponding increase in the reaction of O3 with the

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compatible X, and in the resultant XO. Under the FULL-NOX-limited conditions, a

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continuous decrease in O3 occurred in response to increases in Br2 and I2 emission fluxes.

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The mechanism via which addition of RBS and RIS induces O3 formation is similar to the

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well-established pure photochemical mechanism via which O3 increases in response to

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reduction in NOX, under NOX-rich conditions, as is typically discussed for urban areas28.

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The increase in O3 in that case results from alteration of the HOX-NOX balance and

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reduced O3 titration by NO, as reflected by an increase in the Leighton ratio43

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([NO2]/[NO])28. The present study demonstrates that a moderate increase in RBS and RIS

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mixing ratio can also lead to significant reduction in NOX under NOX-rich conditions,

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resulting in an increase in O3. As will be shown later in this section, the fundamental

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reason for the increase in O3 within the IOR is associated with a RIS- and RBS-induced

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increase in the photochemical O3-formation potential, due to reduction in NOX by

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formation of halo-nitrogenised reservoirs. Within the IOR, under NOX-rich conditions,

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this indirect increase in O3 by a moderate increase in RBS and RIS impacts O3 mixing

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ratios more than the direct O3 destruction by RBS and RIS.

269 270

Figure 2 demonstrates the sharp increase in the modified Leighton ratio (L") within the

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IOR. L'' is defined by the following equation (R = alkyl group):

272 L' ' 

[ NO2 ] [ NO]

273



[O3 ]  K NOO  [ HO2 ]  K HO  NO   [ RO2 ]  K RO  NO   [ XO]  K XO NO 3 2 2 J NO 2

(1),

274

to indicate that the increase in [NO2]/[NO] is partially induced by the reaction of XO with

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NO (see Fig. S3 and Sect. S1), resulting in conversion of XO to X and O3 removal by X.

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This conversion of XO to X is the reason for the general conception that the RHS-induced

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increase in [NO2]/[NO] is not expected to result in net O3 formation15.

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282 283 284 285 286 287 288 289 290 291 292 293 294 295 296

Figure 2. Response of Leighton (L) ratio and [HO2]/[OH] to increasing reactive halogen species. The simulated daily averages of the modified Leighton ratio (L'' ; [NO2]/[NO]]) and [HO2]/[HO] are presented versus X2 (X = Br, I) emission fluxes, for the FULL-NOX-limited (a, c) and FULL-VOC-limited (b, d) scenarios. Dashed lines indicate the maximum Br2 or I2 flux for which an increase in daily average O3 mixing ratio occurred.

297

However, Fig. 3 demonstrates that within the IOR, there was a significant increase in the

298

overall reaction rate of non-halogen radicals with NO (R([HO2+NO],[RO2+NO]; see

299

Figs. 3f, 3l and S4), leading to a non-halogenic increase in L'', and enhancing the O3-

300

formation potential within the IOR. The increase in R([HO2+NO],[RO2+NO]) within the

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IOR resulted from a sharp decrease in DA-NOX for increasing X2, which led to an increase

302

in DA-OH, DA-HO2 and DA-RO2 concentrations (e.g., Figs. 3e and 3k) that was more

303

significant than the associated reduction in DA-NO. The increase in OH with X2 in the

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IOR induced O3 formation via enhanced VOC oxidation rate. The reduction in DA-NOX

305

within the IOR resulted from enhanced formation rate of halo-nitrogenised reservoirs and

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HNO3 with increasing X2 emission flux. The conversion of NOX to HNO3 is demonstrated

307

in Figs. 3d and 3j, while the overall conversion of NOX into halo-nitrogenised reservoirs

308

and HNO3 is demonstrated in Fig. 4. The non-halogenic daytime increase in L'', due to the

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overall reaction rate of NO with HO2, CH3O2 and C2H5O2 within the IOR, accounted for

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about 45–77% and 47–78% of the total increase in L'', for increasing I2 and Br2,

311

respectively.

312

313 314 315 316 317 318 319 320

Figure 3. The role of nitrogen oxides (NOX) in the impact of reactive halogen species on ozone (O3) formation. Daily average values are plotted versus Br2 and I2 flux, including NOX and HNO3 (a, d, g, j), OH and HO2 (b, e, h, k), and the total reaction rate of HO2, CH3O2 and C2H5O2 with NO (R([HO2+NO],[RO2+NO]) (c, f, i, l). Each panel indicates whether the simulation was performed under FULL-VOC-limited or FULLNOX-limited conditions. Dashed lines indicate the maximum Br2 or I2 flux for which an increase in daily average O3 mixing ratio occurred.

321

To investigate the possibility that O3 formation is related to an increase in RCS in

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response to the increase in I2 and Br2 emission flux, the FULL-VOC-limited scenario was

323

also run without RCS (Table 1). No-Cl showed an increase in DA-O3 in response to higher

324

X2 emission flux (Fig. S1), with a simultaneous increase in the reaction rate of O3 with Br

325

and with I. This indicates that within the No-Cl IOR, direct O3 destruction by RBS and

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RIS occurred, while the indirect RBS- and RIS-induced O3 formation was higher.

327

However the increase of DA-O3 within the IOR was higher for FULL-VOC-limited by

328

factors of ~1.3 and 3.0 for I-enriched and Br-enriched simulations, compared to the

329

corresponding No-Cl simulations (see Figs. 1 and S1). This indicates that the effect RBS-

330

and RIS-induced O3 formation shown in Fig. 1 is at least partially influenced by the

331

presence of RCS activity. The increase in RCS in these simulations results from enhanced

332

ICl and BrCl release from the aerosols for higher Br2 and I2 emission flux (see Sect. S2.1).

333

Previous studies have indicated the significant contribution of RCS to O3 formation via

334

formation of nitryl chloride (NO2Cl) in the polluted MBL16, 17, 27, 44, 45. However, within

335

the IOR, DA-ClNO2 decreased in response to decrease in NOX (see Figs. 3 and 4), and

336

therefore formation of ClNO2 cannot explain the RBS- and RIS-induced O3 formation

337

within the IOR. The decrease in DA-ClNO2 within the FULL-IOR was accompanied by

338

an increase in DA-ClNO3. Both of these trends were facilitated by an increase in DA-

339

[ClO]/[Cl] in response to the increase in DA-O320, 46. The increase in ClNO3 within the

340

IOR facilitates O3 formation by reducing NOX, in the same way that other halo-

341

nitrogenised reservoirs do. The increase in DA-ClNO3 within the IOR indicates the

342

dependence of O3 formation on its own initial concentrations. The IOR concentrations of

343

ClNO3 were higher in the Br-enriched simulations compared with I-enriched simulations,

344

and therefore the contribution of RCS to O3 formation within the IOR was higher in the

345

former case.

346 347

The No-Cl scenarios were also associated with lower XO for the same X2 emission flux

348

compared to FULL, resulting from the lower decrease in NOX within the IOR in the

349

absence of RCS chemistry, and leading to significantly higher XNO2 and XNO3 reservoir

350

formation at the expense of XOX (X = Br, I). To some extent, this is also the result of the

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more moderate increase in O3 within the No-Cl IOR, which leads to lower [XO]/[X] (Sect.

352

S2.3).

353 354

The indirect RBS- and RIS-induced photochemical O3 formation via the non-halogenic

355

increase in L'' can be stronger than direct O3 destruction by RBS and RIS only for VOC-

356

limited conditions and low enough XOX, as is demonstrated for the IOR. Under such

357

conditions, the impact of RBS and RIS on both [HO2]/[OH] and L'' is different than

358

previously shown for relatively low NOX conditions. Modification of [HO2]/[OH] by RHS

359

activity has been previously discussed in relation to the oxidation capacity of the

360

atmosphere47, 48. For both I-enriched and Br-enriched FULL-VOC-limited simulations, the

361

[HO2]/[OH] ratio increased within the IOR (Fig. 2). This is in contrast to previous

362

studies47-49 indicating that an increase in RHS and a decrease in NOX tend to decrease

363

[HO2]/[OH] by HO2 reaction with XO and NO, respectively, as follows 48, 49:

364

NO  HO2  NO2  OH

(2)

365

XO  HO2  HOX  O2

(3)

366

In the case of FULL-VOC-limited simulations, [HO2]/[OH] increases within the IOR due

367

to a significant decrease in NOX, in response to the higher I2 and Br2 emission fluxes,

368

resulting in a decrease in reaction (2), which is more significant than the increase in

369

reaction (3).

370 371

Hence, the RBS- and RIS-induced increase in [HO2]/[OH] for FULL-VOC-limited

372

conditions resulted from the NOX-rich conditions within the IOR. For higher RBS and

373

RIS, NOX decreased less significantly (Fig. 3j) and even increased (Fig. 3d), resulting in a

374

decrease in [HO2]/[OH] primarily via an increase in the overall rate of reactions (2) and

375

(3) (see Sect. S2.3). Under the NOX-limited regime, NOX decreased much more slowly for

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higher RBS and RIS compared to the IOR (Fig. 3), and therefore [HO2]/[OH] tended to

377

decrease for increasing X2 (Figs. 2a and 2c). Exceptional was the lower RBS increasing

378

range, for which the decrease in NOX was relatively sharp (Fig. 3a), resulting in an

379

increase in [HO2]/[OH].

380 381

L'' is strongly related to [HO2]/[OH] via Eq. (2) and by reaction of NO2 with OH, which

382

forms HNO3. Hence DA-L'' correlated with DA-[HO2]/[OH] for both the FULL-VOC-

383

limited and FULL-NOX-limited simulations (Fig. 2), whereas its increase within the IOR

384

was further induced by an increase in the titration rate of O3 by NO. Exceptional was the

385

low Br2 range of the FULL-NOX-limited simulations for which an anti-correlation

386

between L'' and [HO2]/[OH] was shown, due to the relatively sharp decrease in DA-HOX

387

and DA-NOX (Fig. 3), which reduced the rate of reaction (2) and thus also L''. The general

388

strong decrease in DA-L'' for increasing X2 under NOX-limited conditions, in contrast to

389

previous studies (e.g., Davis et al.50), from relatively high values of ~12, resulted from the

390

rapid DA-HO2 and DA-NOX decrease which reduced the rate of reaction (2). The rapid

391

reduction in HO2 was due to its reaction with XOX and in the case of Br-enriched

392

simulations, also due to the decrease in VOC by reaction with Br (Fig. S6). Hence, we

393

attribute the impact of RHS on L'', which differs from that found in other studies focusing

394

on NOX-limited conditions (e.g., Davis et al.50), to the high [VOC]/[NOX] used in the

395

present study. Similarly, the strong increase in [HO2]/[OH] within the IOR results from

396

the high [NOX]/[VOC]. This reinforces the important role of [NOX]/[VOC] in the way in

397

which RBS and RIS activities impact photochemical processes. The increase in

398

[HO2]/[OH] is associated with an increase in L'', which facilitates O3 formation. By

399

comparing Figs. 2 and S2, it is clear that RCS also contribute significantly to the observed

17



Page 18 of 24

400

increase in daily average [HO2]/[OH] within the IOR and beyond, thereby reflecting the

401

contribution of RCS to O3 formation (see Sect. S2.1). 402 403

425 426 427 428 429 430 431 432 433

404 405 406 407 408 409 410 411 412 413 414 415 416 417 418 419 420 421 422 423 424 Figure 4. Modification of halo-nitrogenised reservoir and HNO3 mixing ratios in response to increasing reactive halogen species. The simulated daily averages of halo-nitrogenised reservoirs and HNO3 are presented versus Br2 and I2 emission fluxes for NOX-limited scenarios (a, b) and VOC-limited scenarios (c, d). The four upper panel parts present the total simulated daily average mixing ratios of halo-nitrogenised reservoirs ([X-N]) and the sum of the daily average of halo-nitrogenised reservoirs and HNO3 ([X-N]+[HNO3]). Dashed lines indicate the maximum Br2 or I2 flux for which an increase in daily average O3 concentration occurred.

434

Overall, we show here that under high NOX and VOC-limited conditions, the impact of

435

RBS and RIS on [HO2]/[OH], L'' and O3 within the IOR occurs indirectly, via alteration of

436

the NOX-HOX balance and reduced O3 titration by NO. For higher RBS and RIS than used

437

within the IOR, direct O3 destruction by RBS and RIS dominates their indirectly induced

438

O3 formation. Further increase in RBS and RIS leads to modification in the trends of

439

[HO2]/[OH] and L'', which begin to decrease. These changes in the trends of DA-O3, DA-

18


Page 19 of 24


440

[HO2]/[OH] and DA-L'' reveal different interactions of RBS as compared to RIS with O3,

441

HO2 and NOX. These are further discussed in Sect. S2.3.

442 443

As was demonstrated above, conversion of NOX to halo-nitrogenised reservoirs and HNO3

444

plays a key role in the way that RBS and RIS impact photochemistry. This conversion has

445

been reported previously, where it was largely attributed to reaction of BrNO3 in and on

446

the surface area of sulphate aerosols18, 35. INO3 can undergo similar uptake and react in

447

and on the surface of aerosols, but the fate of such processes is currently less certain3, 51, 52.

448

Hence, this research emphasizes the need to improve our knowledge about the fate of

449

halo-nitrogenised reservoirs, and in particular XNO3.

450 451

It is commonly accepted that photochemical O3 formation by halogens is incomparably

452

lower than O3 destruction induced by RBS or RIS, even at their typical levels over the

453

open ocean3, associated with BrO and IO concentrations ranging from below 1 ppt up to

454

~4 ppt3, 18. In contrast to most previous studies, the present study focused on NOX-rich

455

conditions within the MBL. It is shown that under the photochemically imbalanced

456

atmospheric conditions associated with limited VOCs and high NOX, RBS and RIS at

457

mixing ratios that are typical or even higher than the prevailing MBL mixing ratios can

458

induce significant photochemical O3 production. Under such conditions, conversion of

459

NOX into HNO3 and halo-nitrogenised reservoirs induced by increasing RBS and RIS is

460

associated with a non-halogenic increase in the Leighton ratio, which impacts O3 mixing

461

ratios more than the direct O3 destruction by RBS and RIS. NOX-rich conditions are more

462

typical to the polluted MBL near coastlines and ship plumes. In coastal areas, the mixing

463

ratios of IO and BrO also tend to be higher than over the open ocean: IO has been

464

measured at mixing ratios of up to ~50 ppt22, 23, induced by macro-algae under oxidative

19



465

stress at low tide1,

466

conditions19,

467

bromocarbons (e.g., CHBr3) from macro-algae and micro-algae54. This suggests that under

468

polluted conditions, RBS and RIS in the MBL can lead to either O3 formation or O3

469

destruction, based on both the photochemical characteristics and the RBS and RIS

470

concentrations. The fact that the polluted MBL covers extensive inhabited areas of the

471

earth's surface calls for an improved understanding of the interaction of RHS with

472

photochemical air pollution, to better characterise the implications of this interaction on

473

O3 concentration, and oxidation capacity of the atmosphere.

20, 53

3, 7, 24, 25

; BrO tends to reach higher concentrations under polluted

and near the coastline

21, 53

, at least partially induced by emission of

474 475

Supporting Information. Additional discussions, tables, and figures regarding the role of

476

RCS, RBS and RIS on O3 formation, L'' and [HO2]/[OH], additional information about the

477

field measurements.

478 479 480 481 482

Acknowledgements This study was supported by United States–Israel Binational Science Foundation, Grant No. 2012287. E.T. holds the Joseph H. and Belle R. Braun Senior Lectureship in Agriculture.

483 484

References

485 486 487 488 489 490 491 492 493 494 495

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1 Ozone formation induced by the impact of reactive bromine and

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