Observed and simulated time evolution of HCl, ClONO<sub>2</sub>, and HF total column abundances

Abstract

Abstract. Time series of total column abundances of hydrogen chloride (HCl), chlorine nitrate (ClONO2), and hydrogen fluoride (HF) were determined from ground-based Fourier transform infrared (FTIR) spectra recorded at 17 sites belonging to the Network for the Detection of Atmospheric Composition Change (NDACC) and located between 80.05° N and 77.82° S. By providing such a near-global overview on ground-based measurements of the two major stratospheric chlorine reservoir species, HCl and ClONO2, the present study is able to confirm the decrease of the atmospheric inorganic chlorine abundance during the last few years. This decrease is expected following the 1987 Montreal Protocol and its amendments and adjustments, where restrictions and a subsequent phase-out of the prominent anthropogenic chlorine source gases (solvents, chlorofluorocarbons) were agreed upon to enable a stabilisation and recovery of the stratospheric ozone layer. The atmospheric fluorine content is expected to be influenced by the Montreal Protocol, too, because most of the banned anthropogenic gases also represent important fluorine sources. But many of the substitutes to the banned gases also contain fluorine so that the HF total column abundance is expected to have continued to increase during the last few years. The measurements are compared with calculations from five different models: the two-dimensional Bremen model, the two chemistry-transport models KASIMA and SLIMCAT, and the two chemistry-climate models EMAC and SOCOL. Thereby, the ability of the models to reproduce the absolute total column amounts, the seasonal cycles, and the temporal evolution found in the FTIR measurements is investigated and inter-compared. This is especially interesting because the models have different architectures. The overall agreement between the measurements and models for the total column abundances and the seasonal cycles is good. Linear trends of HCl, ClONO2, and HF are calculated from both measurement and model time series data, with a focus on the time range 2000–2009. This period is chosen because from most of the measurement sites taking part in this study, data are available during these years. The precision of the trends is estimated with the bootstrap resampling method. The sensitivity of the trend results with respect to the fitting function, the time of year chosen and time series length is investigated, as well as a bias due to the irregular sampling of the measurements. The measurements and model results investigated here agree qualitatively on a decrease of the chlorine species by around 1% yr−1. The models simulate an increase of HF of around 1% yr−1. This also agrees well with most of the measurements, but some of the FTIR series in the Northern Hemisphere show a stabilisation or even a decrease in the last few years. In general, for all three gases, the measured trends vary more strongly with latitude and hemisphere than the modelled trends. Relative to the FTIR measurements, the models tend to underestimate the decreasing chlorine trends and to overestimate the fluorine increase in the Northern Hemisphere. At most sites, the models simulate a stronger decrease of ClONO2 than of HCl. In the FTIR measurements, this difference between the trends of HCl and ClONO2 depends strongly on latitude, especially in the Northern Hemisphere.

Satellite observations of stratospheric hydrogen fluoride and comparisons with SLIMCAT calculations

“…Chipperfield et al, 2015), this marked decline in the growth rate of HF is a particularly important marker for the success of the Montreal Protocol and should drop even further once HCFC-22 is phased out in developing countries over the coming years. Global trends calculated by SLIMCAT for the HALOE (1998HALOE ( -2005 and ACE-FTS (2004-2012 time series, 1.10 and 0.48 % year −1 , respectively, agree very well with observations; however, for the 1991-1997 HALOE period the model produces a value ∼ 20 % lower (4.01 % year −1 ). Again, the reason for this is not completely clear but is likely related to the ECMWF analysis used to drive the dynamics in the SLIMCAT calculation.…”

“…In general the agreement between observation and model is good, although the ACE-FTS measurements are biased high by ∼ 10 % relative to HALOE. The observed global HF trends reveal a substantial slowing down in the rate of increase of HF since the 1990s: 4.97 ± 0.12 % year −1 (1991-1997; HALOE), 1.12 ± 0.08 % year −1 (1998 HALOE), and 0.52 ± 0.03 % year −1 (2004-2012 ACE-FTS). In comparison, SLIMCAT calculates trends of 4.01, 1.10, and 0.48 % year −1 , respectively, for the same periods; the agreement is very good for all but the earlier of the two HALOE periods.…”

Radiative and dynamical contributions to past and future Arctic stratospheric temperature trends

“…The first period was chosen to allow a direct comparison with ERA-Interim. For future trends a starting point around 2000 is reasonable since the stratospheric halogen loading reached its maximum around that point and has been decreasing ever since (Kohlhepp et al, 2012). In the future, referring to A1B scenario, halogens continue to decrease.…”

The MUMBA campaign: measurements of urban, marine and biogenic air

“…In addition, the instrument can measure 13 C in CO 2 and retains the spectra, allowing post-analysis for other infrared active trace gases in highly polluted episodes (Kohlhepp et al, 2012). The instrument ran throughout the whole MUMBA campaign, with the only data interruption due to the cell temperature going above the range calibrated on 18 January 2013.…”