3.4 East Pacific Bromoform Emission Experiment
3.4.3 Final bromoform emission fields
The final calculation of the bromoform emission fields is summed up in Figure 3.8, and the resulting emission fields for the two model experiments; the El Niño Exp and the
(A) (B)
FIGURE3.9: Monthly averaged total released bromoform (Oct-Dec), per 1◦×1◦grid, in the FLEXPART simulations of (A) the El Niño Exp and (B)
the ENSO Neutral Exp.
ENSO Neutral Exp, is shown in Figure 3.9. Negative emissions in the ENSO Neutral Exp is omitted in the following. There is no negative emissions in the El Niño Exp. The monthly mean 10 m wind speed from ERA-Interim was used to calculate the bromoform emission calculations for the EP bromoform emission experiments.
3.4.4 FLEXPART Setup
Two model runs were exhibited, one for the El Niño Exp and one for the ENSO Neutral Exp. The model setup was the same for both runs (Table 3.3), except for the runtime. The simulation of the El Niño experiment started 05.07.2015 and ended 31.03.2016. The simu-lation of the ENSO Neutral Exp started 12.09.2012 and ended 31.03.2013. The simusimu-lation time was longer for the El Niño Exp since the ASTRA-OMZ cruise was conducted two months earlier than the M91 cruise, and I wanted to have an estimate of the atmospheric bromoform VMR at the time of the two cruises. 5 particles were released from each 1◦×1◦ release box, at the surface. The total number of released particles were 797,180.
The total released bromoform mass for the two experiments was 9.2 million kg for AS-TRA and 0.9 million kg for M91.
TABLE3.3: FLEXPART model setup for the East Pacific bromoform emis-sion experiment.
Compund Input interval [hours]
Sync time [seconds]
Output interval [hours]
Averaging time [seconds]
Sampling rate [seconds]
CHBr3 3 1800 12 7200 1800
Chapter 4
Results and Discussions
The Results and Discussions Chapter is divided into three sections. Meteorological ob-servations and halocarbon measurements from the two cruises; ASTRA-OMZ (October 2015) and M91 (December 2012), are presented in Section 4.1. The first research question will be discussed in this first section: 1. To what extent is the tropical East Pacific (EP) a source for very short-lived halogenated substances (VSLS) to the atmosphere? Sec-tion 4.2 is mainly concerned with the second: 2. How much of these VSLS are trans-ported to the stratosphere? The last section (Section 4.3) further investigates the trans-port of bromoform to the stratosphere in a "seasonal" case study. The third research question is mainly answered in this last section: 3. How does El Niño affect the VSLS transport from the tropical EP to the stratosphere?
4.1 ASTRA-OMZ and M91 Data
Data from two cruises ASTRA-OMZ and M91 have been used as a base for this thesis. The ASTRA-OMZ cruise was conducted on the R/VSONNE(SO243; 5 to 22 October 2015) from Guayaquil in Ecuador to Antofagasta in Chile, and the M91 cruise was carried on R/VMETEOR (1 to 26 December 2012) starting and ending in Lima, Peru (Figure 4.1).
The ASTRA-OMZ cruise went farther north, crossing the Equator, and also further south than the M91 cruise. Both cruises alternated between open-ocean sections and sections close to the coast.
The overall goal of the ASTRA-OMZ cruise was to determine the impact of low oxy-gen conditions on trace element cycling and distributions, and how air-sea exchange of tracers is influenced by the pronounced productivity in the EP (Marandino, 2016).
Whereas, the main goal of M91 was to study on the upwelling region off Peru in order to investigate its importance for emission of climate-relevant atmospheric trace gases and for tropospheric chemistry (Bange, 2013). This thesis is concerned with the contribu-tion of the VSLS, emitted from the tropical EP, to the stratospheric halogen budget. The meteorological measurements and the halocarbon measurements of methyl iodide, bro-moform, and dibromomethane, taken during the ASTRA-OMZ cruise, are presented in this section, and compared with M91, previously analyzed by Fuhlbrügge et al. (2016a).
An overview of the ASTRA-OMZ DSHIP data is shown in Figure 4.2. Time periods of oceanic upwelling is shaded blue in the figure, chosen according to when the SST dropped to around 18◦C. Stramma et al. (2016) observed upwelling of warm, saline, and oxygen-rich water about 9◦S. Hence, it is likely that periods of upwelling, not shown in Figure 4.2, occurred at the more northern parts of the route. The same figure is shown for M91 (Figure 4.3), adapted from Fuhlbrügge et al. (2016a), for comparison. Larger SST drops occurred during M91, up to 5◦C, than during ASTRA-OMZ; with a max of 3.5◦C.
This suggest that stronger upwelling occured during M91 than ASTRA-OMZ. This is in accordance with a deep pycnocline and thermocline observed in October 2015, indicating reduced equatorial upwelling (Stramma et al., 2016). As can be seen from both figures,
FIGURE4.1: Cruise tracks for the ASTRA-OMZ (red) and the M91 (blue).
Equator is pictured as a white line. The ASTRA cruise took place in Octo-ber 2015 and the M91 cruise in DecemOcto-ber 2012.
14
Oct 06 2015 Oct 08 2015 Oct 10 2015 Oct 12 2015 Oct 14 2015 Oct 16 2015 Oct 18 2015 Oct 20 2015 Oct 22 2015 date [UTC]
FIGURE4.2: Meteorological data collected by the ASTRA-OMZ ship cam-paign (Oct 2015). In the three first panels a 30 min average of (a) SAT (red) and SST (blue), (b) RH (dark green) and AH (light green), and (c) wind di-rection (dark orange) and wind speed (light orange) is shown. In the three next panels measurements of (a) oceanic concentration, (b) atmospheric volume mixing ratio, and (c) emissions of; methyl iodide (pink), bromo-form (light blue) and dibromomethane (purple) is shown. The shaded blue areas are time periods when oceanic upwelling occurred, chosen as where SST drop≤18◦C. It was a stop near the harbour of Lima becasue of
prob-lems with nitrogen Oct 14-15.
15
Dec 02 2012 Dec 05 2012 Dec 08 2012 Dec 11 2012 Dec 14 2012 Dec 17 2012 Dec 20 2012 Dec 23 2012 Dec 26 2012 date [UTC]
FIGURE4.3: Meteorological data collected by the M91 ship campaign (Dec 2012). See Figure 4.2. Data were adapted from Fuhlbrügge et al. (2016a).
there is an increase in the RH and the atmospheric volume mixing ratio (VMR) of the halocarbons when oceanic upwelling occurred. This can be explained by the fact that when cold water is transported to the surface, it cools the air above, leading to a a sta-ble atmospheric surface layer with suppressed vertical mixing and higher atmospheric mixing ratios (Fuhlbrügge et al., 2016a). It is also visible from the Figures 4.2 and 4.3 that the measured oceanic concentration of methyl iodide is low, and that the measured bromocarbon concentration is high for ASTRA-OMZ, while the opposite for M91. The atmospheric VMR of the halocarbons are similar for the two cruises, resulting in low methyl iodide emissions and high bromocarbon emissions for ASTRA-OMZ, and again the opposite for M91.
(A) (B)
(C) (D)
FIGURE4.4: The vector display wind speed and direction for (A) ASTRA-OMZ and (B) M91, and the scatter plots are estimated methyl iodide sea-air
flux for (C) ASTRA-OMZ and (D) M91.
The Figures 4.4 and 4.5 provide information about the spatial distribution of the air-sea fluxes of halocarbons for the two cruises ASTRA-OMZ and M91, together with mea-sured wind data. Looking first at the winds (Figure 4.4a and 4.4b) we see that they are
(A) (B)
(C) (D)
FIGURE4.5: Estimated bromoform sea-air flux for (A) ASTRA-OMZ and (B) M91, and estimated dibromomethan sea-air flux for (C) ASTRA-OMZ
and (D) M91.
generally stronger further away from the coast. The measured wind speed and wind direction, in the region between 10◦S and 16◦S (for comparison), is on average a south-easterly fresh breeze (8.3 m/s, 155◦) for ASTRA-OMZ, and for M91 a southeasterly mod-erate breeze (6.3 m/s and 163◦). The winds have the typical direction of the southeasterly trades.
In Figure 4.4c and 4.4d methyl iodide emissions for ASTRA-OMZ anf M91 are dis-played. The emissions are calculated as explained in Chapter 3. Low emissions of methyl iodide occurred north of 12◦S for ASTRA-OMZ. The methyl iodide sea-air flux is gener-ally higher for M91 than ASTRA-OMZ, because the surface sea water concentration of methyl iodide is much higher for M91 than for ASTRA-OMZ.
Figure 4.5 presents the bromocarbon emissions. Negative fluxes means that the mass flux is directed from the air to the ocean. This happens when the air close to the ocean
surface is over-saturated with the specific trace gas of interest. Many negative fluxes oc-cur especially for bromoform and during M91. Positive bromoform emissions are only found at around 14◦S. Both cruises display negative bromoform fluxes relatively near to the coast at around 10◦S. In contrast, at about 16◦S there is positive and strong bro-moform emissions for ASTRA-OMZ and negative brobro-moform emissions for M91. What is striking in Figure 4.5c is the high dibromomethane emissions in the equatorial open ocean, although lower emissions of bromoform and methyl iodide appear in the same area. Overall, the halocarbon emissions vary a lot during the transect of both cruise campaigns. Higher emissions are mostly foundnear the coast, except for the strong di-bromomethane emissions around the equator, at about 85◦W, for ASTRA-OMZ.
TABLE4.1: Measured oceanic concentration, atmospheric VMR and calcu-lated emission for CH3I (methyl iodide), CHBr3(bromoform) and CH2Br2 (dibromomethane). The mean ±one standard deviation and the and the maximum values are listed. Data from M91 is adapted from (Fuhlbrügge
et al., 2016a).
October 2015 CHBr3 20.1 ± 15.5
[
0.1−
102.6]
December 2012 CHBr3 6.6 ± 5.5
[
0.2−
21.5]
The surface ocean concentration, atmospheric VMR, and emission of methyl iodide (CH3I), bromoform (CHBr3) and dibromomethane (CH2Br2), for the ASTRA and the M91 cruise campaigns, are given in Table 4.1. The mean concentration of methyl iodide in the surface water is approximately three times as high for M91 than for ASTRA-OMZ, the calculated mean emission is more than double, and the mean atmospheric VMR is 30% larger for for M91 than for ASTRA-OMZ. For the bromocarbons we have the oppo-site behavior. The mean oceanic bromocarbon concentration is higher for ASTRA-OMZ
than for M91. The mean atmospheric bromocarbon VMR is again not that different, but still greater during ASTRA-OMZ than M91, and the mean emission of the bromocarbons was higher for M91 than ASTRA-OMZ. The difference between the two cruises are high-est for bromoform. The mean emission of bromoform is approximately 13 times larger for ASTRA-OMZ than for M91. The different distribution of halocarbon sources and emissions during the varying ENSO states is currently under further investigation (Birgit Quack and Kirstin Krüger, personal communication, Nov 2017).
TABLE4.2: Mean emission (pmol m−2hr−1) of methyl iodide (CH3I), bro-moform (CHBr3) and dibromomethane (CH2Br2) from several cruises and model-based studies. EP = East Pacific, WP = West Pacific, IO = Indian Ocean, and OLS = ordinary least square method. Adapted from Fiehn et
al. (2017).
Study Cruise, region CH3I CHBr3 CH2Br2
This study ASTRA-OMZ, tropical EP 350 1590 620
Fuhlbrügge et al. (2016a) M91, tropical EP 860 120 240
Fuhlbrügge et al. (2016b) SHIVA, tropical WP 433 1486 405 Tegtmeier et al. (2012, 2013) TransBrom, WP 320 608 164
Fiehn et al. (2017) OASIS, West IO 460 910 930
Hepach et al. (2015) MSM 18/3, equatorial Atlantic 425 644 187 Hepach et al. (2014) DRIVE, tropical Atlantic 254 787 341 Chuck et al. (2005) ANT XVIII/1, tropical Atlantic 625 125 –
Butler et al. (2007) Tropics 541 379 108
Ordóñez et al. (2012) Tropics – 956 –
Warwick et al., 2006 Tropics – 580 –
Liang et al. (2010) Tropics, open ocean – 854 81
Carpenter et al. (2009) Atlantic open ocean – 367 158
Yokouchi et al. (2008) Global open ocean – – 119
Quack and Wallace (2003) Global open ocean – 625 –
Bell et al. (2002) Global ocean 670 – –
Ziska et al. (2013) Equatorial EP (OLM) ≈200 ≈500 ≈400
Stemmler et al. (2015) Equatorial EP – ≈500 –
Stemmler et al. (2013) Tropical Atlantic ≈500 – –
Average emissions = 469 689 321
Table 4.2 provides an overview of VSLS mean emissions form several cruises and model-based studies. An analysis of the tropical EP as a source region for atmospheric VSLS is given now. The methyl iodide emission from the tropical EP appear to be impor-tant, especially for M91 where the emission was 83% larger than the overall study average emission of 469 pmol m−2hr−1. The tropical EP is a hot spot region for bromoform, since the highest mean emission of 1590 pmol m−2 hr−1 are found for ASTRA-OMZ. How-ever, since M91 has the lowest mean emission of 120 pmol m−2 hr−1 it reveals a large variability during different El Niño–Southern Oscillation (ENSO) phases but also during different seasons and years. The tropical EP seem to be a moderate to strong source re-gion for atmospheric dibromomethane. In summary, these results show that the tropical EP is an important source region for VSLS to the atmosphere with a large differences between ENSO phases.
4.2 Cruise VSLS Emissions Experiment
The aim of the Cruise VSLS Emission Experiment is to investigate the path of the esti-mated oceanic halocarbon emitted to the air, during the two cruises ASTRA-OMZ and M91, and to diagnose the amount of the emitted mass that reaches the stratosphere. The experimental setup is described in Section 4.2. Figure 4.6 present a 10 day forward trans-port pathway for the particles that reached the 17 km height, for the two cruises ASTRA-OMZ (A) and M91 (B). In can be seen that the wind pattern during the two cruises are quite different. During the ASTRA-OMZ cruise almost every trajectory travel north-wards, and to the 17 km height at around 10◦N, while the VSLS released during the M91 cruise follow several paths. Some travel over land, south-east, and convect to 17 km height around 30◦S. Others move northwards, as during the ASTRA-OMZ cruise.
But most trajectories reaching the entrainment height (17 km) are lifted above the An-des Mountain range, between 5◦S and 15◦S. This is consistent with apparent enhanced convection at about 10◦N in October 2015, and also the enhanced convection above the Andes Mountain range in December 2012 (Figure 2.8, Chapter 2.3).
The released mass, entrained mass, and the transport efficiency of methyl iodide, bromoform, and dibromomethane, for the two cruises, are shown in Figure 4.7. The entrained mass is here defined as the amount of mass that reaches the 17 km altitude.
Hence, the entrained mass is a measure of the amount of released mass that reached the stratosphere. There are a few points to be noticed in Figure 4.7. It is apparent in Figure 4.7a and 4.7c that the relative entrainment of methyl iodide and bromoform respectively increases equatorwards for the ASTRA-OMZ cruise. The latitudinal dependency is most clear for methyl iodide, and it is not noticeable for dibromomethane (Figure 4.7e). This diversity in latitudinal dependency is due to the different lifetimes of the three VSLS.
Methyl iodide is prescribed with a very short e-folding lifetime of 3.5 days in the atmo-spheric column, and the amount that is entrained is therefore more reliant on where it is released. We know from Figure 4.6 that the main entrainment location for the ASTRA-OMZ experiment is north for equator. Hence, methyl iodide released far south on the cruise track has short time to reach the main entrainment center before most of the mass is depleted. The mean lifetime of bromoform in the model run is longer (17 days), and mass released further south may still have time to reach the main entrainment center be-fore much of the mass has decayed. 150 days is the average lifetime of dibromomethane in the atmosphere. Hence, it is no surprise that we do not see the latitudinal trend for that compound. See Chapter 3.3.1 for information about the prescribed lifetimes of the VSLS used in this experiment. For M91, the VSLS was only released between 10◦S and 16◦S and the transport pathways to the stratosphere varied a lot more, thus the latitudinal dependency is not visible.
It can be seen that the relative entrainment during M91 fluctuates more than during ASTRA-OMZ, over the same latitudes. This agrees with that during ASTRA-OMZ there was one main transport pathway for entrainment, contrary to M91 were three main trans-port pathways were pointed out. A comprehensive comparison of these results with sim-ilar studies from other tropical ocean campaign, also applying FLEXPART ERA-Interim trajectory calculations for the VSLS transport to the stratosphere are summarized in Table 4.3, 4.4 and 4.5 for methyl iodide, bromoform and dibromomethane respectively. In the next three paragraphs a discussion of each of the compounds is given.
From Table 4.3 it is apparent that the methyl iodide emissions are quite different for the two cruises in this study, with a mean of 186 nmol for ATSRA-OMZ, and more than double for M91. The mean methyl iodide emission during M91, December 2012, is also high in comparison to the other campaigns listed in the table. This is in agreement with the findings of large amounts of idocarbons in the surface ocean during M91 (Hepach
(A)
(B)
FIGURE4.6: Flexpart 10 days forward trajectories for (a) the ASTRA-OMZ (Oct 2015) setup, and for (b) the M91 (Dec 2012) setup. The figure shows the general airmass transport pathways of those trajectories reaching the 17 km altitude. To get a good overview, only every (a) 60 and (b) 30 trajectory that reached the 17 km altitude is plotted here. The trajectories starts from
where they were released along the respective cruise tracks.
0.0
FIGURE4.7: Released mass, entrained mass and relative entrainment of methyl iodide (top), bromoform (center), and dibromomethane (bottom).
The mass is released along the cruise track of ASTRA-OMZ (left) and M91 (right) according to when VSLS measurement were taken. The entrained mass is defined as the amount of mass reaching the stratosphere at 17 m altitude (Chapter 2.3), and the relative entrainment is defined as the ratio
of entrained to released mass.
et al., 2016). However, three notices need to be taken when reading this table. Firstly, Tegtmeier et al. (2013) used the 20 m wind observation directly for the sea-air flux calcu-lation although sea-air flux calcucalcu-lations are set up for 10 m wind calcucalcu-lations, and has to be corrected to that level. The wind at 20 m over the ocean is generally higher than at 10 m, which was estimated and used in this study and Fiehn et al. (2017). Stronger wind leads to stronger mixing of the VSLS in the marine atmospheric boundary layer (MABL), and therefore increasing the ocean-atm halocarbon concentration gradient close to the surface, directly impacting the emission. Hence, calculated emissions are higher for Tegt-meier et al. (2013). Secondly, the lifetime of methyl iodide used in TegtTegt-meier et al. (2013), ranged between 2-3 days according to height. With a shorter lifetime, a lower transport
TABLE4.3: Emission of methyl iodide and entrainment at 17 km altitude.
The mean ±one standard deviation, the minimum value and the maximum value are listed. Negative emissions are excluded.
METHYL IODIDE
efficiency can be assumed, considering that the particles then has less time to reach the stratosphere to entrain the same amount of mass, compared to if they had longer lifetimes (less rapid mass decay). Lastly, Tegtmeier et al. (2013) (and Tegtmeier et al. (2012)) used the earlier FLEXPART version 8.0, where the stratospheric entrainment has been found to be systematically stronger for FLEXPART version 8 than 9 (Kirsten Krüger, personal communication, Nov 2017). This study and Fiehn et al. (2017) used the same lifetime of 3.5 days for Methyl iodide, and the same FLEXPART version 9.2. Having this in mind, we see that the transport efficiency of methyl iodide in the Coastal East Pacific region (this study) was one magnitude smaller than the other ocean regions, except for the East Atlantic. The methyl iodide transport efficiency for ASTRA-OMZ and M91 were very similar, with a mean of 0.1 % for both cruises. The highest transport efficiency, 4.0%, was found in the Coastal West Pacific (Tegtmeier et al., 2013). The West Pacific is, as described in the Chapter 2.1.2, a major convective region due to the Walker Circulation. At the time of the SHIVA campaign in the West Pacific, a moderate La Niña was active (CDB, Nov 2011), possibly contributing to the very high transport efficiency.
Turning now to bromoform and Table 4.4, we notice that the mean bromoform emis-sion during the ASTRA-OMZ cruise is almost four times larger than the mean emisemis-sions during the M91 cruise. The mean entrainment is nearly five times larger during ASTRA-OMZ cruise. Hence, the mean transport efficiency was larger too, with a mean of 1.5 % for ASTRA-OMZ and 1.2 % for M91. What is striking in this table is the formidable bro-moform emission estimated for ASTRA-OMZ. No other emission values in the table are even close to those measures. The strong transport efficiency in the coastal West Pacific is again striking.
By inspecting the results for dibromomethane (Table 4.5), it can be seen that the mean dibromomethane emission is more than the double during ASTRA-OMZ than M91.
TABLE4.4: Emission of bromoform and entrainment at 17 km altitude. The
TABLE4.4: Emission of bromoform and entrainment at 17 km altitude. The