Trends of stratospheric chlorine and fluorine reservoir species

In the present thesis, ground-based measurements with Fourier transform infrared (FTIR) spectrometers are evaluated and compared with calculations by atmospheric models of different architecture. The species of primary interest are the chlorine and fluorine reservoir gases hydrogen chloride (HCl), chlorine nitrate (ClONO2), and hydrogen fluoride (HF). Their source gases are mainly anthropogenic chlorofluorocarbons (CFCs), hydrochlorofluorocarbons (HCFCs), and halons, which are so-called ozone-depleting substances. Because of this property, their emission was restricted in the Montreal Protocol in 1987 and its amendments and adjustments, leading to a subsequent phase-out. As a result of these reductions in the source gas emissions, the stratospheric inorganic chlorine content decreases with time since the late 1990s. This decline is confirmed by the FTIR measurements of HCl and ClONO2 total column abundances reported in the present thesis, at nearly all of the 17 measurement sites taking part in this study and belonging to the Network for the Detection of Atmospheric Composition Change (NDACC). The calculations of five atmospheric chemistry models (i.e., the Bremen 2-D model, KASIMA, SLIMCAT, EMAC, and SOCOL) reproduce this decrease. The main reason for including HF in this comparison study of models and measurements is that this substance is an indicator of anthropogenic influences. Besides, it is often used as a tracer due to its high stability and a resulting long stratospheric lifetime. Although the emission of fluorine is not explicitly prohibited because it practically does not deplete stratospheric ozone, HF is expected to be also influenced by the Montreal protocol and its amendments because some of its source gases are restricted. However, CFC substitutes (e.g., hydrofluorocarbons) are allowed to contain fluorine so that the HF total column abundance is expected to have reached its peak value later than the two chlorine species. This can also be confirmed with the here-presented FTIR measurements and reproduced by the atmospheric models. In addition to the overall temporal evolution and absolute total column abundances of the three species, also the mean seasonal cycle of models and measurements at the 17 NDACC sites is compared. Trends from the above-described model and measurement time series are calculated by fitting a linear function combined with a (thirdor first-order) Fourier series accounting for the seasonal cycle of the species to the available data between 2000 and 2009. The error bars for the trends are determined with a bootstrap method. The influence of the type of fitting function, of time series length, the time period used, and sampling on the resulting trend and its confidence intervals is investigated. Furthermore, by multiple linear regression, the effect on the trend of including time series representing the 11-year solar variation and the quasi-biennial oscillation in the fitting function is studied. The trends of HCl and ClONO2 from models and measurements for the period 2000–2009 amount to roughly about -1 and -2%/yr, respectively, while HF increases by about 1%/yr during this period. However, there are differences between the measured trends and those simulated by the models. In the HCl and ClONO2 measurements, the calculated trends exhibit a dependence on hemisphere, with a stronger decrease in the northern than in the southern hemisphere. Analogously, the measured HF increase is stronger in the southern than in the northern hemisphere. These differences can be attributed to the time required for transport from the emission sources of the organic chlorine and fluorine species, which are mostly located in the northern hemisphere, to the southern hemisphere. This results in later peaks of stratospheric inorganic chlorine and fluorine in the southern than in the northern hemisphere. The models obviously do not include this effect. Furthermore, the ClONO2 trends from the FTIR measurements depend on latitude, with the strongest decreases at high and low latitudes, which is reproduced only by some of the models. Trends for the time range 2004–2009 show no significant trend or even a decrease of HF at many northern hemisphere sites. This confirms the stabilisation of the HF total column abundance in the northern hemisphere during the last few years. The second part of the present thesis investigates the difference between the trends of HCl and ClONO2 that was found before for example in the Kiruna data by Mikuteit (2008) and Kohlhepp et al. (2011). Also in the analyses described above, the ClONO2 total column abundance decreases stronger than the HCl abundance in the FTIR total column measurements at most of the included NDACC sites, and also in the model results. In the reactions forming HCl and ClONO2, methane (CH4) and nitrous oxide (N2O) are involved, the latter only indirectly via NO2. This is why sensitivity studies with the EMAC (ECHAM/MESSy Atmospheric Chemistry) model are conducted by analysing trends between 2000 and 2050 from a long-term simulation where CH4 and N2O increase according to the A1B scenario from IPCC (2007), and from two other simulations where once CH4 and once N2O were kept fixed on their value of the year 2000. These studies reveal a strong influence of the considerable CH4 increase during 2000–2050 in the reference simulation on chlorine partitioning into HCl and ClONO2. Although the stratospheric temperature decrease with time present in all three simulations is expected to result in an enhancement of ClONO2 production, HCl is strongly favoured if CH4 increases at the rate projected by the A1B scenario. Examination of FTIR trends of HCl and ClONO2 during a period of constant atmospheric CH4 abundance (about 1999 to 2006 in the troposphere) leads to the conclusion that some of the observed changes in the measured chlorine trends above Kiruna may indeed be attributable to CH4. In contrast, at the Jungfraujoch, the CH4 influence may be ruled out by the peak in stratospheric inorganic chlorine occurring at the end of the 1990s.