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Very Short-lived substances

Data assimilation

Stratospheric ozone

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Background

In recent years it has been realized that in addition to the long-lived ozone depleting substances (such as CFCs and halons) very short-lived substances (VSLS), mostly of natural origin, can also contribute significantly to the stratospheric halogen loading, and thus stratospheric ozone loss (e.g., WMO 2007). While the contribution of VSLS to stratospheric chlorine is small (several percent), short-lived organic bromine compounds significantly contribute to stratospheric bromine (approximately 25%), and presumably all stratospheric iodine is due to iodinated VSLS. Very little is currently known how the impact of VSLS on the stratospheric ozone layer will evolve in the next decades under the influence of a changing climate (WMO 2007, Chapter 2). Improving the knowledge on the atmospheric fate of the VSLS is an urgent task since in a changing climate their source strength, atmospheric transport and the efficiency of the halogens for stratospheric ozone loss is likely to change.

Recent Work

In recent studies (Aschmann et al., 2009) we used our 3D chemical transport model (CTM) [Link auf Modellseite] to simulate the transport of bromine VSLS into the stratosphere. The novelty of our approach is to use an isentropic model with a parameterization of deep convective transport. In contrast to the fast and localized convective mass flux we calculate the slow large-scale vertical motion from diabatic heating rates. With this tool we have studied the impact of convection on stratospheric bromine loading, investigated the relative importance of different source regions using several emission scenarios and assessed the impact of dehydration on soluble inorganic bromine. We give here an short overview of our study. For further details and references please refer to the paper[Link auf Publication-Seite]itself.

Transport of very short-lived substances into the stratosphere

It is generally assumed that the most important pathway of air entering the stratosphere is in the tropics through the tropical tropopause layer (TTL) via convection. Here, the level of zero clear sky radiative heating (at an altitude of about 15.5 km) marks the transition from large-scale subsidence to large-scale ascent, as diagnosed from diabatic heating rates and thus representing a natural border for air parcels (see Fig 1). In general, air masses have to be lifted by deep convection above this border to leave the troposphere, which is considered to be the common pathway into the stratosphere. Since air lifted by convective clouds is able to rise several kilometers within a few hours this transport mechanism is fast enough for VSLS whose life times are rather in the order of weeks.

Fig. 1: Schematic of vertical transport in the TTL. Beside the slow large-scale transport driven by diabatic heating rates wr there is a fast localized convective mass flux wc. The amount of air leaving the convective cloud is given by the convective detrainment rate dc. Fig. 2: Snapshot of idealized bromoform tracer (TT20) distribution at 380 K (about 17 km altitude) on December 4, 2005.


Due to their short lifetime the distribution of VSLS in the upper troposphere/lower stratosphere (UTLS) is generally localized in areas with significant convective activity as shown in Fig. 2, for example South America or the Maritime Continent. However, even for short-lived species like bromoform (CHBr3, average lifetime of 20 days) the horizontal transport can be very important as the far-stretching filaments in Fig. 2 clearly illustrate.

In order to evaluate our model for bromoform results we compare them with aircraft observations from the Pre-AVE and Cr-AVE campaign (see Fig. 3). Both campaigns were performed in January/February 2004 and 2006 for Pre-AVE and Cr-AVE, respectively. There is a general good agreement between the model calculations and the aircraft observations (Fig. 4).

Fig. 3: Map of Pre-AVE and Cr-AVE campaign Fig. 4: Comparison of measured and modeled bromoform. The horizontal bars denote one standard deviation.


Transport efficiency for different regions

Convective activity is not uniformly distributed but tends to be concentrated in distinct areas. To investigate the most efficient pathways of VSLS into the stratosphere we divided the globe into different areas with a single VSLS tracer each (Fig. 5). The contribution of a single region tracer to the total amount of this species gives information of the relative importance of this region regarding to the transport of VSLS. The results are given in Fig. 6. Clearly the West Pacific dominates all other source regions. At the top of the UTLS over 60% of the idealized bromoform tracer (TT20) is originated from this region.

Fig. 5: Different source regions for idealized bromoform tracer Fig. 6: Relative contribution of individual source regions to tropical TT20 abundance


Stratospheric bromine loading

The contribution of brominated VSLS to stratospheric bromine abundance is mainly defined by the emission strength, convective activity and loss of soluble inorganic bromine by dehydration. The two most abundant bromine VSLS, bromoform (CHBr3)and dibromomethane (CH2Br2), are mainly of natural origin, produced by marine microorganisms. However, the magnitude of the emission remains uncertain. For this study we only include bromoform with an average detrainment mixing ratio of 1 pptv, which is in general agreement with other estimations. The convective transport was discussed in the previous sections thus the remaining relevant process is the dehydration of the TTL. In order to reach the exceptional dry stratosphere tropospheric air masses have to be dehydrated during their ascent. Several studies indicate that this dehydration occurs mainly in exceptional cold areas in the TTL, typically residing over the Maritime Contintent or West Pacific, the so called “cold trap”. This dehydration affects the contribution of VSLS to stratospheric bromine loading since VSLS decay into inorganic bromine species (Bry) which are in part very soluble. Therefore some amount of solved bromine is lost due to falling ice.

To study the impact of this effect we defined two extremal cases as upper and lower boundary of VSLS contribution. For the upper boundary, we assumed that dehydration has no effect on Bry at all. In case of the lower boundary all Bry is removed instantaneously whenever dehydration occurs in a model grid box. The resulting profiles are shown in Fig. 7: The idealized bromoform tracer (TT20) has a detrainment mixing ratio of 1 pptv, i.e. air that leaves a convective cloud includes 1 pptv of bromoform. When one molecule of CHBr3 decays it produces three molecules of Bry. In case of the insoluble inorganic bromine tracer there are no other sinks present thus the mixing ratio approaches 3 pptv in the stratosphere. On the other hand the soluble Bry tracer is affected by dehydration; its stratospheric mixing ratio is about 1.6 pptv. Consequently even in the “worst case” scenario less than 50% of bromine from VSLS is affected by dehydration.

 
Fig. 7: Average tropical profiles of idealized bromoform tracer (TT20) and soluble and insoluble Bry tracer.