3.1 Air Mass Origins
 NOAA Hysplit back trajectories were calculated for the time and location of each data point in order to establish the origin of the air parcel at each specific latitude, longitude, and altitude where samples were collected. The data were then categorized according to five distinct air mass origins (Figure 1). The majority of the air masses followed the expected northeasterly trade winds and traveled parallel to the African coastline, without any contribution from continental air. A few of the air masses arriving at high altitude originated from over Africa; moderate levels of CO (77–83 ppb) and O3 (23–25 ppb) in these air masses indicated that they were not strongly influenced by contaminated or biomass burning sources. VSLH levels in these air masses were not depleted and in some cases were enhanced relative to those in open ocean trajectories, suggesting either a terrestrial source or strong coastal input (Figure 2). High O3 (blue +, Figure 2e) was observed in open ocean trajectories over the Canaries which will have significant European influence (Figure 1). The presence of hurricane Fred south of flight B477 (09 September 2009), which traveled west from Cape Verde islands at 17°N, produced unusual, southerly trajectories which looped around and were possibly influenced by the Southern Hemisphere African continent. This is supported by the unusual CO and O3 concentrations for these trajectories (Figures 2e and 2f), which displayed relatively low O3 characteristic of Southern Hemisphere air masses but also enhanced CO which may be attributable to African biomass burning. O3 concentrations are noticeably higher over the upwelling region; this could be attributed to the emission of NOx from shipping close to the coast of Mauritania.
 Average VSLH mixing ratios measured during TROMPEX are shown in Table 1. CH2I2 was not detected in any of the canister samples, likely due to its rapid photolysis (<5 min [Montzka et al., 2011]) resulting in levels below the limit of detection at the altitudes sampled. The mean MBL mixing ratio of CH2ClI of 0.27 pptv agrees well with springtime measurements in the western north Pacific of 0.27 pptv [Kurihara et al., 2010] and previous measurements made in the Mauritanian upwelling region of 0.24 pptv in the summer of 2007 [Jones et al., 2010]. The mean mixing ratio, however, is skewed by the presence of the upwelling and by changes in altitude. Outside of the upwelling region, the average MBL background level was between 0.1–0.2 pptv (Figure 3), which is more consistent with fieldwork measurements of 0.11 pptv at Mace Head, Ireland [Carpenter et al., 1999; Carpenter et al., 2000], 0.1 pptv at Christmas Island [Varner et al., 2008], and with long-term measurements at Hateruma Island (east of Taiwan, 24°3′N 123°48′E) and Cape Ochiishi (Oshiishinishi, Nemino, Hokkaido 43°9′N 145°30′E) of 0.12 and 0.18 pptv, respectively [Yokouchi et al., 2011]. To our knowledge, these are the first airborne measurements of CH2ClI reported to date. Due to its short atmospheric photodissociation lifetime of ≈2 h [Rattigan et al., 1997], mixing ratios of CH2ClI at altitudes >MBL (≈500 m) were very low (<0.02 pptv). This suggests that the majority of CH2ClI is photolyzed below 500 m, releasing iodine atoms into the boundary layer.
Table 1. Overview of Measured VSLH Concentrations
| ||Mixing Ratio (pptv)|
| ||MBL||MBL Open Ocean||Upwellinga||Free Troposphere|
|CHCl3||17.8 (11.8–32.7)||15.1 (11.7–20.0)||23.3 (20.2–26.3)||14.2 (10.9–19.3)|
|C2H5Ib||0.1 (nd–0.7)||0.05 (nd–0.14)||0.2 (0.1–0.7)||0.1 (nd–0.3)|
|CH2ClI||0.3 (nd–0.7)||0.2 (nd–0.4)||0.6 (0.4–0.7)||0.1 (nd–0.1)|
|CH2Br2||1.1 (0.1–2.6)||0.9 (0.6–1.2)||2.1 (1.7–2.6)||0.8 (0.5–1.3)|
|CHBr3||1.2 (0.1–4.8)||0.5 (0.2–1.1)||4.0 (3.4–4.8)||0.3 (0.1–1.3)|
Figure 3. MBL concentrations of CH2ClI, C2H5I, CHBr3, and CH2Br2 scaled by size and color, overlaid onto a gray-scaled chlorophyll a image (MODIS) showing the Mauritanian upwelling region.
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 Figure 3 shows that mixing ratios of CH2ClI in the MBL (defined as 500 m above mean sea level in this region shown by NOAA Hysplit analysis) were elevated above background over the open ocean as well as over the Mauritanian upwelling, a known source region [Jones et al., 2010]. Concentrations were higher in the morning than in the afternoon presumably due to rapid photolysis of CH2ClI; this was also apparent in the two flights between the Canaries and Cape Verde Islands (Figure 4, top). Back trajectory analyses (Figure 1) showed that the open ocean regions with elevated CH2ClI corresponded to trade wind- and hurricane-influenced air masses of marine origin. These air masses had not been influenced by any coastal or upwelling regions for at least 24 h, which is more than 10 times the photolytic lifetime of CH2ClI. CH2ClI can be produced directly from phytoplankton or indirectly from the photolysis of CH2I2 in surface seawater [Martino et al., 2005; Jones and Carpenter, 2005]. The observations suggest an open ocean, possibly phytoplankton-derived source of either CH2ClI, its precursor CH2I2, or both.
Figure 4. Top: CH2ClI concentrations measured during Trade Wind Ozone Photochemistry Experiment (TROMPEX). Black squares are flight B476 from Canaries to Cape Verde Islands (07:16–10:00 08 September 2009), and open circles are flight B478 returning to the Canary Islands 2 days later (15:16–17:30 10 September 2009). Red and blue lines are the sea surface temperatures (SST) measured during each flight, respectively. Bottom: Black squares denote CH2ClI fluxes measured during the RHaMBLe cruise [Jones et al., 2010]. Red line denotes measured SST.
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 During the open ocean flights between the Canaries (25°N) and Cape Verde Islands (17°N), a latitudinal dependence of CH2ClI and C2H5I was observed with their concentrations doubling and quadrupling, respectively. The mixing ratio increased with decreasing latitude between these latitudes and broadly corresponded to changes in sea surface temperature (Figure 4). CH2ClI fluxes measured during the RHaMBLe (Reactive Halogens in the Marine Boundary Layer) campaign conducted in the same geographical region between the Canaries and Cape Verde Islands in summer 2007 (Figure 4, bottom, Jones et al. ) showed a similar pattern; thus we suggest that the latitudinal dependence of MBL-iodinated VSLH may be driven by temperature or photochemically driven processes in the surface ocean.
 CH3I was measured in all of the samples, but due to problems with the calibration at the time of analysis, the concentrations could not be quantified. The trend in the CH3I data was very similar to CH2Br2 and CHBr3 (section 3.3) with concentrations increasing over the upwelling region. CH3I concentrations also increased with decreasing latitude, similarly to CH2ClI, although these did not show elevated amounts over the open ocean.
 The mean C2H5I mixing ratio was 0.13 pptv, in agreement with measurements from Hateruma Island (0.15 pptv) and Cape Ochiishi (0.08 pptv, Yokouchi et al. ), Asian seas (0.09 pptv, [Yokouchi et al., 1997]), and at Mace Head (0.06 pptv, [Carpenter et al., 1999]).
 CHBr3 and CH2Br2 both showed distinct upwelling influence (Figures 2a and 2b) with CHBr3 concentrations increasing to 3–5 pptv, more than threefold higher than the mean background MBL CHBr3 concentration in that area. Bromocarbon levels in the southerly air masses were similar to, although at the low end, of those in Northern Hemisphere open ocean air. CH2Br2 concentrations were strongly correlated to those of CHBr3 (R2 = 0.89) throughout the flights (Figure 2h (inset)). The injection of fresh emissions from the upwelling into the free troposphere is indicated in Figure 2h, which shows CH2Br2/CHBr3 ratios grouped by air mass origin. CH2Br2 has an atmospheric lifetime of 124 days and is primarily removed via reaction with OH [Montzka et al., 2011]. CHBr3 is photolyzed within 24 days; therefore, an increasing CH2Br2/CHBr3 ratio suggests aging of the air mass, if the emission ratio remains constant. The southerly and African air masses, plus some trade wind trajectories from Europe, contain relatively aged emissions.
 The mean mixing ratios of CH2Br2 and CHBr3 of 1.01 and 1.08 pptv are within the range of previous, ship-based measurements in the area. Higher mean mixing ratios of 2.4 pptv for CH2Br2 and 6.2 pptv for CHBr3 [Quack et al., 2007] were observed in spring 2005 and lower mean mixing ratios of 0.4 pptv for CH2Br2 and 1.1 pptv for CHBr3 [Carpenter et al., 2009] during summer 2007. Over the open ocean, CHBr3 surface concentrations are generally higher than those of CH2Br2 [Butler et al., 2007; Kurihara et al., 2010]. However, throughout the flights, background levels of CH2Br2 were generally higher than CHBr3. This can be explained by considering the altitude at which the TROMPEX samples were taken (500–3000 m) and from the shorter lifetime of CHBr3 compared to that of CH2Br2, indicating an aged background air mass.
 Unlike the iodocarbons, the brominated species did not show a clear latitudinal dependence.