Choosing surface atmospheric concentrations

Three different atmospheric bromoform VMR fields were considered for calculation of the ocean-atm bromoform flux. The first option was to use a constant atmospheric bro-moform VMR of 1 ppt. This option does not reflect the increased atmospheric concentra-tion of bromoform close to the coast, but it is relevant for the open ocean, and close to the global average of 1.2 ppt (Carpenter et al., 2014b, Table 1-7). The second option was

FIGURE 3.3: 10 days backward trajectories for ASTRA-OMZ and M91 (Steffen Fuhlbrügge, personal communication, Nov. 2017).

(A) (B)

FIGURE3.4: Surface concentrations of bromoform Stemmler et al. (2015) which are scaled with the (A) ASTRA-OMZ and (B) M91 cruise ments. This was done by comparing each surface concentration measure-ment of bromoform with the corresponding regional and temporal value in the Stemmlers field, using the average difference as the scaling factor,

which is 2.28 for ASTRA-OMZ and 0.61 for M91.

to use an average bromoform VMR measured during the two cruises, using the ASTRA-OMZ average of 3.2 ppt for the El Niño Exp, and the M91 average of 2.9 ppt for the ENSO Neutral Exp. A problem with using these numbers is that it overestimates the VMR over the open ocean, because both cruises were close to the Peruvian coast, where the con-centrations are generally higher. The third option was to use the Ziska updated fields (Figure 3.5), were the original Ziska et al., 2013 field has been updated with the M91 measurements among others, but does not include the ASTRA-OMZ data yet. Thus, it includes the difference in VMR close to land and the open ocean, although not the ENSO variations.

FIGURE3.5: The Ziska updated bromoform VMR at the surface (Ziska et al., 2013) .

FIGURE3.6: Calculated bromoform emission fields using oceanic surface concentrations by Stemmler et al. (2015) and three optional atmospheric bromoform volume mixing ratios; 1 ppt (a and c), 3.2 ppt (b and d), and the field by Ziska et al. (2013) (c and f). The bromoform emission estimations from the ASTRA-OMZ (a, b, and c) and the M91 (d, e, and f) cruises are

plotted on top.

(A)

(B)

FIGURE3.7: Bromoform emission comparison between the ship emission estimations from a) ASTRA-OMZ and b) M91, and three different esti-mated emission fields. The emission field are estiesti-mated using the Stemm-ler et al. (2015) surface water bromoform concentration field and a marine bromoform VMR field; either a constant field value of 1 ppt (blue), the av-eraged measured volume mixing ratio; a 3.2 ppt field for ASTRA-OMZ and a 2.9 ppt field for M91 (red), or the updated Ziska et al., 2013 field (yellow).

In Figure 3.6 the resulting bromoform emission fields for the three options, stated above, with the estimated ASTRA-OMZ and M91 emissions plotted on top is presented.

Negative values are set to zero in this plots. The three options give quite different re-sulting emission fields. The Peruvian coast is a region with oceanic upwelling, but the upwelling is not continuous (Stemmler et al., 2015), thus the three fields shows emissions according to an upwelling average. I therefore found it the best to compare the average field values with the average cruise values, and an overview of the averages are given in Table 3.2 below:

TABLE3.2: Mean oceanic bromoform emissions for two experiments; the El Niño Exp and the ENSO Neutral Exp. In the first column the mean of the corresponding cruise emission estimated are shown (ASTRA-OMZ for the El Niño Exp and M91 for the ENSO Neutral Exp). For the other columns a mean over the stated field is shown. The field averages were taken over a big enough lat-lon box to include the respective cruise track. The values in

the brackets includes negative emissions.

BROMOFORM EMISSIONS [p mol m⁻²h⁻¹]

Experiment Cruise 1 ppt 2.9 ppt 3.2 ppt Ziska update field El Niño Exp 1639 (1588) 1225 (1225) – 821 (818) 1079 (1079) ENSO Neutral Exp 232 (117) 227 (227) 15 (-97) – 36 (-32)

FIGURE 3.8: Schematic of method for calculating emission fields for ASTRA-OMZ and M91.

The average oceanic bromoform emissions of the fields were calculated for a large area for the East Pacific (EP) including the respective cruise track. By taking the mean over this area, open ocean values are included. However, open ocean emissions are gen-erally lower than the coastal emissions (Quack and Wallace, 2003). As the two cruises followed the Peruvian coast, measuring mostly coastal emissions, hence higher mean emissions from the cruises are expected than from the generated EP. It is apparent that cruise mean for both cruises is closest to the mean when using a constant 1 ppt VMR for the overlaying atmosphere (Table 3.2). To check further in detail the in situ cruise emis-sions are compared with the three optional field emisemis-sions at that same location (Figure 3.7). It can be seen that the ASTRA-OMZ emissions corresponded best with the emission field using the 1 pp for the atmospheric VMR, with a correlation coefficientR² = 0.11.

Thus, using this atmospheric field for the El Niño Exp seemed the best. It is also no-ticeable that all emissions for the "1 ppt" emission field of the El Niño Exp, are positive.

However, this is not the case for the ENSO Neutral emission fields. The best field cor-relation with M91 data (Figure 3.7b) is R² = 0.05 for the "2.9 ppt" emission field, but including quite a lot of negative emissions. The next best correlation is with the "1 ppt"

emission field where R² = 0.04. Since this field included far less negative emissions, and the overall calculated averages corresponded best with this field (3.2), the "1 ppt"

emission field is used for both cruise experiments.