Reactive iodine species in a semi-polluted environment

Transcription

1 GEOPHYSICAL RESEARCH LETTERS, VOL. 36, L16803, doi: /2009gl038018, 2009 Reactive iodine species in a semi-polluted environment Anoop S. Mahajan, 1 Hilke Oetjen, 1 Alfonso Saiz-Lopez, 2 James D. Lee, 3 Gordon B. McFiggans, 4 and John M. C. Plane 1 Received 4 March 2009; revised 28 May 2009; accepted 9 June 2009; published 22 August [1] Iodine chemistry in the marine boundary layer has attracted increasing attention over the last few years because iodine oxides cause ozone destruction, change the atmospheric oxidising capacity, and can form ultrafine particles. However, the chemistry of iodine in polluted environments is not well understood: its effects are assumed to be inhibited by reactions involving NO x (NO 2 & NO). This paper describes Differential Optical Absorption Spectroscopy (DOAS) observations of iodine species (I 2, OIO and IO) and the nitrate radical (NO 3 ) at a semi-polluted coastal site in Roscoff, France. Significant concentrations of IO and I 2 were measured during daytime, indicating efficient recycling of iodine from iodine nitrate (IONO 2 ). I 2, IO, OIO and NO 3 were observed at night. These observations are interpreted using a one dimensional model to demonstrate that iodine plays an important role even in polluted environments due to recycling mechanisms not considered previously. Citation: Mahajan, A. S., H. Oetjen, A. Saiz-Lopez, J. D. Lee, G. B. McFiggans, and J. M. C. Plane (2009), Reactive iodine species in a semi-polluted environment, Geophys. Res. Lett., 36, L16803, doi: /2009gl because the quantum yield of (R8) is 10% [Atkinson et al., 2007]. Cycle 1: IO + IO cycle ði þ O 3! IO þ O 2 Þ2 ðr1þ IO þ IO! OIO þ I OIO þ hv! I þ O 2 2O 3! 3O 2 Cycle 2: IO + HO 2 cycle I þ O 3! IO þ O 2 IO þ HO 2! HOI þ O 2 ðr2þ ðr3þ ðr4þ 1. Background [2] The current interest in molecular iodine (I 2 ), iodine oxide (IO) and iodine dioxide (OIO) was initiated by their first detection in the marine boundary layer (MBL) at Mace Head, Ireland and Cape Grimm, Tasmania [Alicke et al., 1999; Allan et al., 2001; Saiz-Lopez and Plane, 2004]. Reactive iodine species (RIS) affect the atmospheric oxidising capacity through depletion of O 3 [Davis et al., 1996], and changing the HO 2 /OH and NO 2 /NO ratios [Saiz-Lopez et al., 2008]. IO depletes O 3 through its self reaction (cycle 1, see below), and by reactions with HO 2 (cycle 2) and NO 2 (cycle 3) [McFiggans et al., 2000]. Cycles 1 and 2 are important in low NO x areas, whereas cycle 3 assumes importance in polluted environments. A very recent study shows that f R3 is close to 1 [Gómez Martín et al., 2009], which substantially increases the efficiency of cycle 1. In contrast, cycle 3 is not efficient HOI þ hv! I þ OH ðr5þ HO 2 þ O 3! OH þ 2O 2 Cycle 3: IO + NO 2 cycle I þ O 3! IO þ O 2 IO þ NO 2! IONO 2 ðr6þ IONO 2 þ hv! I þ NO 3 ðr7þ NO 3 þ hv! NO þ O 2 ðr8þ 1 School of Chemistry, University of Leeds, Leeds, UK. 2 NASA Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California, USA. 3 National Centre for Atmospheric Science, Department of Chemistry, University of York, York, UK. 4 Centre for Atmospheric Science, University of Manchester, Manchester, UK. Copyright 2009 by the American Geophysical Union /09/2009GL NO þ O 3! NO 2 þ O 2 ðr9þ 2O 3! 3O 2 Iodine is also implicated in ultrafine particle formation in coastal atmospheres, where new particle bursts were observed correlating with I 2 emissions from exposed macro-algae at low tide [O Dowd et al., 2002; McFiggans et al., 2004]. The mechanism for particle formation is still L of6

2 oxidation (R1). Reaction (R2) and the reaction between IO and NO 3 (R12) then produce OIO. I 2 þ hv! 2I ðr10þ I 2 þ NO 3! IONO 2 þ I ðr11þ I þ O 3! IO þ O 2 IO þ NO 3! OIO þ NO 2 ðr12þ IONO 2 þ I! I 2 þ NO 3 ðr11 0 Þ Figure 1. Daytime DOAS observations of (a) I 2 and (b) OIO on 19th September 2006 and (c) IO on 9th September The black circles (with 2s error bars) are the measured mixing ratios, and the rectangles show the instrument detection limit for each observation. Tidal height is shown as a dashed line. not well understood but it is suggested that IO and OIO recombine followed by oxidation with O 3 to form I 2 O 5 which then polymerizes [Saunders and Plane, 2005]. [3] The established sources of RIS in the MBL are the emissions of organohalogens and I 2 by exposed macroalgae, resulting in an anti-correlation between RIS concentrations and tidal height [Alicke et al., 1999; Peters et al., 2005; Saiz-Lopez and Plane, 2004]. Organohalogens, such as diiodomethane (CH 2 I 2 ), photolysis lifetime = 5 minutes, are emitted by macroalgae [Carpenter et al., 1999], while Saiz-Lopez and Plane [2004] reported the first observations of I 2, which was shown to be the dominant iodine source at Mace Head, and explained the daytime and night-time measurements of IO: photolysis during the day (R10) and reaction with NO 3 during the night (R11), followed by [4] Organohalogens have been reported from sites other than Mace Head [Carpenter et al., 2003], although positive detection of I 2 is reported at only one other location, i.e., California [Finley and Saltzman, 2008]. For instance, Peters et al. [2005] carried out a study in Lilia, on the north-west coast of France, where they observed IO (maximum 8 ppt) but could not detect any OIO or I 2 (detection limit 3 ppt and 10 ppt respectively). Another study in the Gulf of Maine reported high levels of IO and OIO in a polluted environment, leading Stutz et al. [2007] to propose that IONO 2, which acts as an iodine reservoir, reacts with O 3 to form OIO. However, Kaltsoyannis and Plane [2008], using quantum chemistry calculations combined with detailed balance of the measured rate constant of (R11) have shown that the reverse reaction (R11 0 ) between IONO 2 and I will recycle rapidly back to I 2. The subsequent photolysis of I 2 produces more I atoms that react with IONO 2 (R11 0 ), rather than O 3 (R1), if the ratio [IONO 2 ]/[O 3 ]is>0.01 [Kaltsoyannis and Plane, 2008]. This sequence represents an autocatalytic cycle limiting the build up of IONO 2, and indicates that iodine chemistry should still be active even in a relatively high NO x environment, as found by Stutz et al. [2007]. In this paper we present measurements of iodine species in a semi-polluted coastal environment, and use a one dimensional model, including this new chemistry, to explain the observations. 2. Experiment [5] Observations of I 2, OIO, IO, NO 2 and NO 3 were made at Roscoff, on the north-west coast of France (48.7 N 4.0 W) for five weeks during August and September 2006 as part of the Reactive Halogens in the Marine Boundary Layer (RHaMBLe) project. Roscoff was chosen as it has a large tidal range (the maximum 9.5 m occurred during RHaMBLe) and, due to a shallow coastal shelf, large beds of macroalgae are exposed during low tide. [6] The Long Path-DOAS technique [Plane and Saiz-Lopez, 2006] was used to measure the concentrations of I 2, OIO, IO and NO 3. The absorption path extended 3.35 km from the research institute Station Biologique, on the western edge of Roscoff to Ile de Batz, a small island north of Roscoff, where a retroreflector array was placed 2of6

3 to fold the optical path. The total optical path length was 6.7 km and the beam was 7 to 12 m above the mean sea level. The exposed macroalgae bed extended 1 km offshore (2 km of the DOAS light path). [7] Details about the DOAS instrument employed can be found elsewhere [Saiz-Lopez and Plane, 2004]. Briefly, spectra were recorded at a resolution of 0.25 nm before being converted into differential optical density spectra and the contributions of individual species determined by simultaneous fitting their absorption cross-sections using singular value decomposition [Plane and Saiz-Lopez, 2006]. Three different spectral regions were used: IO [Gómez Martín et al., 2005], NO 2 [Harder et al., 1997] and H 2 O[Rothman et al., 2003] were retrieved in the nm window; I 2 [Saiz-Lopez et al., 2004], OIO [Gómez Martín et al., 2005], NO 2 [Harder et al., 1997] and H 2 O[Rothman et al., 2003] in the nm window; and NO 3 [Yokelson et al., 1994] and H 2 O[Rothman et al., 2003] in the nm window. During the day, the instrument was run on either the OIO/I 2 or IO spectral regions, while NO 3 was measured at night. Simultaneous in situ measurements of O 3, using a standard UV absorption instrument and of NO 2, using a chemiluminesence NO instrument with photolytic NO 2 converter, were made at the research institute Station Biologique. IO was also measured by Light Induced Fluorescence (LIF) [Whalley et al., 2007] and cavity ring down spectroscopy [Wada et al., 2007]. 3. Observations [8] Figure 1 shows a selection of daytime observations: Figures 1a and 1b show simultaneous measurements of I 2 and OIO, and Figure 1c shows IO (on another day). The IO and I 2 concentrations correlate strongly with tide. In fact, the highest mixing ratio of IO (10.1 ± 0.7 ppt) was observed on 9th September 2006, when the (spring) tidal variation was 9.5 m. Figure 1 also shows that even during the daytime measurement of high levels of I 2 (25.9±3.1 ppt on 19th September 2006; see Figure S3 1 for an example of a spectral fit), OIO was not observed above the detection limit (4 ppt). NO 2 concentrations averaged 2 ppb with distinct peaks in the early morning and late evening. Figure 2 shows an example of night-time observations (on three different days). I 2 and IO again correlate with low tide and, in contrast to the daytime, OIO was also observed. The maximum night-time mixing ratios were: [I 2 ] = 52.3 ± 4.2 ppt, [IO] = 3.0 ± 0.9 ppt and [OIO] = 8.7 ± 2.3 ppt. IO was detected on two out of eight nights, whereas OIO and I 2 were detected on seven out of thirteen nights. NO 3 was measured on nine nights with concentrations ranging from <2 ppt to 72.1 ± 1.8 ppt (on 7th 8th September, Figure 2d). [9] The complete data time series can be found in the auxiliary material (Figure S1). 1 During daytime low tide conditions, peak concentrations for iodine species were variable, ranging from ppt for I 2 with an average of 28.7 ppt and ppt for IO with an average of 7.5 ppt. Over the entire time series, the daytime I 2 /IO ratio ranged from 2.4 to 5.8, with an average of 3.8. OIO was not observed within error above the detection limit on all 1 Auxiliary materials are available in the HTML. doi: / 2009GL days of daytime measurements. It should be noted that because three separate spectral windows were used, simultaneous measurements of OIO/I 2, IO and NO 3 were not possible. Days with similar meteorological conditions (easterly wind during the day, north-westerly wind at night) were therefore chosen for all the data shown in Figures 1 and Discussion and Modelling [10] Iodine species measured during this study correlate closely with low tide, indicating that exposed macroalgae are their major source. A similar correlation was found previously at Mace Head [Saiz-Lopez and Plane, 2004] and Lilia [Peters et al., 2005]. The I 2, IO and OIO mixing ratios at Roscoff are also comparable to clean MBL sites [Alicke et al., 1999; Allan et al., 2000; Saiz-Lopez and Plane, 2004]. Whalley et al. [2007] and Wada et al. [2007] have also reported higher in situ [IO] peaking 27.6 ± 3.2 pptv and 54 ± 18 pptv during the same campaign indicating the presence of RIS hotspots. The occurrence of elevated levels of RIS is surprising given that NO 2 should act as a sink through formation of IONO 2 (R6). One explanation for the observable IO is recycling of IONO 2 to I 2 by reaction (R11 0 ), as discussed in the Introduction. [11] In order to quantify these observations and examine the impact of RIS in the semi-polluted coastal MBL, we now employ the one dimensional photochemistry and transport Tropospheric Halogen Chemistry Model (THAMO), details of which are given by Saiz-Lopez et al. [2008]. A table of reactions used in the model is included in the auxiliary material. The 1-D model is used with a vertical resolution of 1 m and boundary layer height of 1 km. The eddy diffusion coefficient (K z ) was calculated as shown in the auxiliary material (equation (S1)) and ranged from cm 2 s 1 to cm 2 s 1 for the lowermost 200 m of the boundary layer (Figure S2). The sensitivity of the model to K z is discussed in the auxiliary material. The concentrations of all the iodine species, O 3 and NO x were allowed to vary. The model was initialised with [NO 2 ]=2 ppb and aerosol surface area = cm 2 cm 3 (typical of the measurements during the RHaMBLe campaign). The midday values for HO 2 and OH were set to 6 and 0.1 ppt, respectively. [12] At the typical observed wind speed of 5 m s 1,anair parcel would have taken at least 200 s to travel over the exposed macroalgae area 1 km wide at low tide. Hence, in the model we prescribe an I 2 injection pulse lasting for 200 s to simulate the release of I 2 by macroalgae. Furthermore, assuming that I 2 is released mostly by the exposed macroalgae, the iodine species would be confined in the first 1 km of the 3.35 km path between the DOAS and the retroreflectors. Therefore the measured concentrations should be multiplied by a factor of 3.4, yielding daytime peak values of 23 ppt IO and 78 ppt I 2 for the most realistic comparison with the model. [13] We consider two scenarios for daytime chemistry: scenario 1 contains all the chemistry, including the reaction between I and IONO 2 (R11 0 ) set at the recommended rate constant of cm 3 molecule 1 s 1 at 290K; scenario 2 is similar to scenario 1 except the rate constant for (R11 0 ) is set to the lower limit of cm 3 molecule 1 s 1 [Kaltsoyannis and Plane, 2008]. The I 2 3of6

4 Figure 2. Night-time measurements of (a) I 2 and (b) OIO on the 4th 5th of September, (c) IO on the 15th 16th of September, and (d) NO 3 (circles) and NO 2 (solid line) on the 7th and 8th of September. The tidal height (dashed line) is shown in Figures 2a 2c. The 2s error and detection limit for each measurement are indicated as in Figure 1. Night-time periods are indicated by grey shading. injection rate required to simulate the measured levels of [IO] is then molecules cm 2 s 1. Figure 3 shows the time evolution of iodine compounds following the pulse of I 2. In scenario 1, the peak levels predicted at the DOAS measurement height (8 12 m) is 23 ppt IO and 80 ppt I 2. Importantly, this I 2 concentration is in excellent agreement with the adjusted observed concentration of 78 ppt (see above). In fact, a key test for the model is replicating the observed I 2 /IO ratio of 3.7, since this does not depend on the pathlength correction. [14] In scenario 2, most of the iodine in the gas phase forms IONO 2 through (R6). As discussed by Kaltsoyannis and Plane [2008], the other known mechanisms for recycling IONO 2, photolysis to I + NO 3 using either of the literature cross sections [Joseph et al., 2007; Mössinger et al., 2002], thermal decomposition (to IO + NO 2 ), or uptake into aerosol and emission as IBr and ICl, are not fast enough to compensate for the removal by (R6). IO would then be significantly below the observed concentrations (Figure 3), and I 2 would be below the DOAS detection limit. If the I 2 injection rate is increased to molecules cm 2 s 1, then [IO] increases to the observed mixing ratio of 23 ppt, but I 2 is only 5 ppt, a factor of 16 below the observed daytime level. To match the observed [I 2 ], the I 2 injection rate needs to be increased even more to molecules cm 2 s 1, but then [IO] increases to 71 ppt, nearly 3 times the observed level. It is therefore not possible to reproduce the observed mixing ratios of IO and I 2 in scenario 2. [15] We therefore conclude that the rate constant for (R11 0 ) must be at least cm 3 molecule 1 s 1,in accord with the theoretical estimate [Kaltsoyannis and Plane, 2008]. Note that the speculative reaction between O 3 and IONO 2 [Stutz et al., 2007] is therefore not required to account for the iodine recycling, and would in any case lead to daytime OIO above the DOAS detection limit of 4 ppt, which was not observed. The absence of measurable OIO during daytime was also reported in previous campaigns at Mace Head [Saiz-Lopez and Plane, 2004] and Cape Grim [Allan et al., 2001], in contrast to the recent study in the Gulf of Maine [Stutz et al., 2007]. [16] One interesting point in scenario 1 is that the model predicts 10 ppt of NO 3 during the day at the height of measurements. Although the predicted [NO 3 ] depends on the rate of uptake of IONO 2, even at g IONO2 = 1 the model predicts [NO 3 ] > 8 ppt. This indicates that high levels of iodine in the semi-polluted boundary layer could be a source for daytime NO 3. Unfortunately, daytime measurements of NO 3 during low tide were not performed during this campaign. [17] Figure 3 (bottom) shows depletion of O 3 for both scenarios, and the predicted depletion at the height of the DOAS beam for scenario 1 is 1.3 ppb and 0.8 ppb for scenario 2, following the pulse of I 2. However, this depletion is quickly diluted by vertical mixing of O 3 from aloft, as can be seen in Figure 3. Consequently, although (R11 0 ) increases the effect of iodine on ozone depletion, significant depletion at low tide should not be observed (as can be seen in Figure S1). It should be noted that (R11 0 ) causes a 60% increase in O 3 depletion and thus, even in polluted environments, the efficient recycling of IONO 2 sustains the O 3 - depleting impact of iodine. [18] Night-time IO can be explained by reaction of I 2 with NO 3 (R11), as was previously observed at Mace Head [Saiz-Lopez and Plane, 2004]. Although the levels of NO 3 seen during some nights at Roscoff are much higher than at Mace Head, the peak night-time IO levels are comparable (Figure S1), possibly because of removal by reaction with 4of6

5 be between the density of H 2 O and solid I 2 O 5, as particles will be hydrated in the MBL. This shows that significant ultra-fine particle formation should occur even in the polluted MBL. Indeed, ultrafine particles (diameter = 3 10 nm) were measured at concentrations up to particles cm 3 during the RHaMBLe study [Whitehead et al., 2009]. 5. Summary [20] Elevated levels of IO and I 2 during the day and OIO, IO, I 2 and NO 3 during the night were observed in a relatively high NO x marine environment. A model was used to study the evolution of iodine species and their effect on the oxidising capacity of the boundary layer. The observations and modelling suggest that the reaction between I and IONO 2 plays an important role in recycling iodine species, so that ozone destruction and particle formation can occur even in polluted environments. [21] Acknowledgments. We thank Philippe Potin, Catherine Leblanc, and Station Biologique, CNRS, Roscoff for logistical support and acknowledge the UK NERC Surface Ocean Lower Atmosphere Study for financial support (RHaMBLe project NE/D006554/1). ASM thanks the School of Chemistry, University of Leeds, for a PhD studentship. Figure 3. One-dimensional model results for daytime conditions, where I 2 is injected 2 minutes after model initialisation. For scenario 1, k 6 = cm 3 molecule 1 s 1 ; in scenario 2, k 6 is set to the lower limit of cm 3 molecule 1 s 1. NO 3 (R12) and by recombination with the elevated levels of NO 2. The variability of IO and OIO during low tide at night is consistent with the variability of NO 3 produced from local NO x sources. Furthermore, during the night, the wind speed was seen to drop on an average to 2 ms 1, and this was used to calculate K z values ranging between cm 2 s 1 and cm 2 s 1 in the lowermost 200 m (Figure S2). Using an I 2 flux similar to the daytime ( molecules cm 2 s 1 for 200 s), we produce 215 ppt of I 2, 4 ppt of IO and 15 ppt of OIO in the presence of 40 ppt of NO 3 and 0.5 ppb of NO 2, which on average was lower during the night (Figure S1). These predicted values are in good accord with the average night-time observations (adjusted for path length) of 204 ppt I 2, 6.8 ppt IO and 20 ppt OIO. Reaction (R11 0 ) does not have a strong effect during the night as the levels of I atoms should be low (<1 ppt). [19] A final point is the formation of iodine oxide particles. Recycling of IONO 2 by (R11 0 ) leads to higher [IO] and [OIO], which are assumed to form I 2 O 5 [Saunders and Plane, 2005]. After 200 s, the model predicts molecules cm 3 of I 2 O 5 at 10 m, which corresponds to particles cm 3 of diameter 7 nm using a particle density of 2 g cm 3. This density is chosen to References Alicke, B., et al. (1999), Iodine oxide in the marine boundary layer, Nature, 397, Allan, B. J., G. 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