OTRAS REVELACIONES

The long-range transport event emerging from South Africa in July 2008 that was ana-lyzed in Subsection 7.1.2 can also be found in MACC-II reanalysis data. The individual observations of this event as well as accompanying meteorological conditions are shown in Figure 8.4.

The trajectory of the event in MACC-II reanalysis data shows significant deviations from the trajectory in satellite data. It is also apparent that tropospheric NO2 vertical column densities in MACC-II reanalysis data are significantly lower than estimated from GOME-2 / MetOp-A data, showing little more than 10 % of the estimated NO2 content in the plume. This is symptomatic of the Southern Hemisphere, as will be shown in Subsection 8.2.1. While the plume is hardly visible in visual inspection (Figure 8.4), it is still detected in the algorithm – due to low noise in the model data over the remote Indian Ocean.

Figure 8.2: Illustration of the backtrajectory of the penultimate observation of this long-range transport event (18 December 2007) in MACC-II reanalysis data. Data as in Figure 8.1, but purple circles indicate the locations of the backtrajecto-ries of the plume from 19 December 2007 at the respective dates. Only parts of the plume follow the backtrajectory to the estimated source region in the Chicago area. Many of the detected cells stray off the plume backtrajec-tory, showing implications of the coarse horizontal and vertical resolution of the plume detection algorithm. Also, MACC-II reanalysis model data and HYSPLIT_v4 backtrajectories are based on different meteorological data sets.

Figure 8.3: Altitudes of the backtrajectories for the long-range transport event over the North Atlantic on 16 to 19 December 2007, based on MACC-II reanalysis data. As in Figure 7.2. The observations on the first day show a prominent uplift about 24 hours after emission, while later observations do no longer indicate this so strongly.

Figure 8.4: As for Figure 7.5, but showing the long-range transport event as found in MACC-II reanalysis data. Only four observations of the plume are made. It appears to leave a trail of NO2 behind, which might indicate replenishing of the plume from the Highveld plateau until 09 July 2008. The trajectory of the plume is significantly further south than in GOME-2 / MetOp-A data.

8.1 Case studies

The plume can already be observed on 08 July 2008 – one day earlier than in GOME-2 / MetOp-A observations, where regular outflow leads to a diminished sensitivity of the algorithm near the coast of South Africa. It moves towards the east-southeast, steadily increasing in size.

The plume is last detected on 11 July 2008. On this date, the plume was located around 20^◦ or about 1, 000 km west of the Australian west coast – heading east – in GOME-2 / MetOp-A data, whereas MACC-II reanalysis data show the plume signifi-cantly displaced to the south and show no indications of the plume impacting Australian air quality. This might be an effect of different emission times – bearing the chaotic nature of meteorology in mind – or of the meteorological data set used for MACC-II reanalysis. On 12 July 2008, the plume is so strongly diluted that it is no longer detected in MACC-II reanalysis data by the algorithm, while it can still be seen in GOME-2 / MetOp-A data.

The plume in this long-range transport event does not show a distinct arc-like structure or a compact shape as seen in Figure 7.5. It appears that this plume might not have been emitted due to a violent event such as a passing cold front, but rather through much more subtle processes that suffice for long-range transport in this exposed emission region.

Instead, this plume consists of a compact bulk head (slowly increasing in horizontal extent) and an elongated tail, which only slowly moves away from the emission region.

It appears likely that this plume’s NO2 content was still replenished from the emission region on the first two observations, which leads to an increasing NO2 content. This also hints at a less violent emission mechanism.

As with the analsys in GOME-2 / MetOp-A data, age estimation of this plume is not consistent over consecutive days. This may be an effect of the more subtle weather conditions, the unusual shape of the plume and the elevated geography of the Highveld plateau. A joint analysis of all the estimated ages yields 43 hours as the best fitting age at the first time of observations. This is reasonable as the slow traveling speeds indicate a slow separation process from the source region.

It is difficult to look at the evolution of the NO2 content in the plume. As the plume is continuously replenished, its NO2 content increases. This leaves only the last two observations to determine the actual decay time of the NO2. This analysis leads an initial mass m0 = 0.71 Gg N with a NO2 lifetime of τ = 67 h, which – surprisingly – is consistent with the previous case study.

Looking at the altitudes of the backtrajectories for all four observations of the plume shows strong inconsistencies. In particular, the initial height of the bulk of the plume steadily increases with every observation. This may be caused by the selection algorithm that selects the backtrajectory with the most plume cells in the continental boundary layer. If the dispersion rate of plume pixels is roughly constant at all altitudes, this will lead to a preference of high altitudes for the estimation of the plume altitude at observation, where wind speeds are higher and the remote continent is reached faster.