> Recent Northern Hemisphere stratospheric HCl increase due to atmospheric circulation changes

E. Mahieu, M. P. Chipperfield, J. Notholt, T. Reddmann, J. Anderson, P. F. Bernath, T. Blumenstock, M. T. Coffey, S. S. Dhomse, W. Feng, B. Franco, L. Froidevaux, D. W. T. Griffith, J. W. Hannigan, F. Hase, R. Hossaini, N. B. Jones, I. Morino, I. Murata, H. Nakajima, M. Palm, C. Paton-Walsh, J. M. Russell , M. Schneider, C. Servais, D. Smale and K. A. Walker, Recent Northern Hemisphere stratospheric HCl increase due to atmospheric circulation changes, Nature , 515, 104-107, doi:10.1038/nature13857 [pdf]

The abundance of chlorine in the Earth's atmosphere increased considerably during the 1970s to 1990s, following large emissions of anthropogenic long-lived chlorine-containing source gases, notably the chlorofluorocarbons. The chemical inertness of chlorofluorocarbons allows their transport and mixing throughout the troposphere on a global scale, before they reach the stratosphere where they release chlorine atoms that cause ozone depletion. The large ozone loss over Antarctica was the key observation that stimulated the definition and signing in 1987 of the Montreal Protocol, an international treaty establishing a schedule to reduce the production of the major chlorine- and bromine-containing halocarbons. Owing to its implementation, the near-surface total chlorine concentration showed a maximum in 1993, followed by a decrease of half a per cent to one per cent per year, in line with expectations. Remote-sensing data have revealed a peak in stratospheric chlorine after 1996, then a decrease of close to one per cent per year in agreement with the surface observations of the chlorine source gases and model calculations7. Here we present ground-based and satellite data that show a recent and significant increase, at the 2-sigma level, in hydrogen chloride (HCl), the main stratospheric chlorine reservoir, starting around 2007 in the lower stratosphere of the Northern Hemisphere, in contrast with the ongoing monotonic decrease of near-surface source gases. Using model simulations, we attribute this trend anomaly to a slowdown in the Northern Hemisphere atmospheric circulation, occurring over several consecutive years, transporting more aged air to the lower stratosphere, and characterized by a larger relative conversion of source gases to HCl. This short-term dynamical variability will also affect other stratospheric tracers and needs to be accounted for when studying the evolution of the stratospheric ozone layer.

Figure caption : Comparison of merged GOZCARDS satellite HCl observations (by HALOE, ACE-FTS and Aura/MLS) with SLIMCAT model runs for Northern Hemisphere and Southern Hemisphere mid-latitude lower (46 hPa) and upper (7 hPa) stratosphere.


> Retrievals of formaldehyde from ground-based FTIR and MAX-DOAS observations at the Jungfraujoch station and comparisons with GEOS-Chem and IMAGES model simulations

B. Franco, F. Hendrick, M. Van Roozendael,J.F. Müller, T. Stavrakou, E. A. Marais, B. Bovy, W. Bader, C. Fayt, C. and Hermans, C. and Lejeune, B. and Pinardi, G. and Servais, C. and E. Mahieu, Retrievals of formaldehyde from ground-based FTIR and MAX-DOAS observations at the Jungfraujoch station and comparisons with GEOS-Chem and IMAGES model simulations, Atmos. Meas. Tech. Discuss., 7, 10715-10770, 2014 [pdf][Interactive Discussion]

As a ubiquitous product of the oxidation of many Volatile Organic Compounds (VOCs), formaldehyde (HCHO) plays a key role as a short-lived and reactive intermediate in the atmospheric photo-oxidation pathways leading to the formation of tropospheric ozone and secondary organic aerosols. In this study, HCHO profiles have been successfully retrieved from ground-based Fourier Transform Infrared (FTIR) solar spectra and UV-Visible Multi-AXis Differential Optical Absorption Spectroscopy (MAX-DOAS) scans recorded during the July 2010-December 2012 time period at the Jungfraujoch station (Swiss Alps, 46.5 N, 8.0 E, 3580 m a.s.l.). Analysis of the retrieved products has revealed different vertical sensitivity between both remote sensing techniques. Furthermore, HCHO amounts simulated by two state-of-the-art Chemical Transport Models (CTMs), GEOS-Chem and IMAGESv2, have been compared to FTIR total columns and MAX-DOAS 3.6-8 km partial columns, accounting for the respective vertical resolution of each ground-based instrument. Using the CTMs outputs as intermediate, FTIR and MAX-DOAS retrievals have shown consistent seasonal modulations of HCHO throughout the investigated period, characterized by summertime maximum and wintertime minimum. Such comparisons have also highlighted that FTIR and MAX-DOAS provide complementary products for the HCHO retrieval above the Jungfraujoch station.

Figure caption : Monthly-mean column abundances of HCHO displayed on a one-year time base, according to the FTIR and MAX-DOAS retrievals (a and b, respectively) above Jungfraujoch from July 2010 to December 2012. The circle, triangle and square dots correspond to the monthly means from 2010, 2011 and 2012, respectively. The thick curves correspond to a running mean fit to the daily-mean columns displayed on a oneyear time base. The shaded areas represent the 1-sigma SD associated to the running mean curves. The HCHO amounts calculated from the smoothed CTMs profiles are displayed in each frame as running mean fit (solid thick curve). Similar figure, but for the HCHO concentration derived from MAX-DOAS and the CTMs within the 3.6-8km near-surface layer, is drawn in (c).



> Comparison of assimilation methods applied to a stratospheric transport model

Skachko, S., Q. Errera, R. Ménard, Y. Christophe, and S. Chabrillat, Comparison of the ensemble Kalman filter and 4D-Var assimilation methods using a stratospheric tracer transport model, Geosci. Model Dev. , 7, 339-377, 2014 [pdf][Interactive Discussion]

The Ensemble Kalman filter (EnKF) assimilation method is applied to the tracer transport using the same stratospheric transport model as in the 4D-Var assimilation system BASCOE. This EnKF version of BASCOE was built primarily to avoid the large costs associated with the maintenance of an adjoint model. The EnKF developed in BASCOE accounts for two adjustable parameters: a parameter controlling the model error term and a parameter controlling the observational error. The EnKF system is shown to be markedly sensitive to these two parameters. The performance of the EnKF and 4D-Var versions was estimated through the assimilation of Aura-MLS ozone observations during an 8 month period which includes the formation of the 2008 Antarctic ozone hole. To ensure a proper comparison, despite the fundamental differences between the two assimilation methods, both systems use identical and carefully calibrated input error statistics. We provide the detailed procedure for these calibrations, and compare the two sets of analyses with a focus on the lower and middle stratosphere where the ozone lifetime is much larger than the observational update frequency.

Figure caption : Monthly mean ozone analysis at 54.6 hPa from 4D-Var (left), EnKF (middle) and their abslute differences (right).


> Methanol at Jungfraujoch : long-term evolution and seasonal modulation

Bader, W. T. Stavrakou, J.-F. Müller, S. Reimann, C. D. Boone, O. Flock, B. Bovy, B. Franco, B. Lejeune, C. Servais, and E. Mahieu, Long-term evolution and seasonal modulation of methanol above Jungfraujoch (46.5N, 8E): optimization of the retrieval strategy, comparison with model simulations and independent observations, Atmos. Meas. Tech. 7, 4659-4692, 2014 [pdf][Interactive Discussion].

We present the first long-term time series of methanol total, lower tropospheric and upper tropospheric-lower stratospheric partial columns derived from the analysis of high resolution Fourier transform infrared solar spectra recorded at the Jungfraujoch station (46.5° N, 3580 m a.s.l.). Methanol does not exhibit a significant trend over the 1995–2012 time period, but a strong seasonal modulation characterized by maximum values and variability in June–July, minimum columns in winter and a peak-to-peak amplitude of 130%. In situ measurements performed at the Jungfraujoch and ACE-FTS occultations give similar results for the seasonal variation. The total and lower tropospheric columns are also compared with IMAGESv2 model simulations. There is no systematic bias between the observations and IMAGESv2 but the model underestimates the peak-to-peak amplitude of the seasonal modulations.

Figure caption : Seasonal modulation of methanol total columns. Dots with vertical lines represent the daily mean total columns over a 1-year time base and their associated standard deviation. The brown curve corresponds to a running mean fit to all data points with a 15-day step and a 2-month wide integration time. The area corresponds to the 1-sigma standard deviation associated to the running mean curve. Up and down blue triangles are monthly means of the model IMAGESv2 simulations. Upper frame shows monthly fractional difference between FTIR and IMAGESv2.


> Isoprene emissions over Asia 1979-2012 : bottom-up and top-down estimates

Stavrakou, T., Müller, J.-F., Bauwens, M., De Smedt, I., Van Roozendael, M., Guenther, A., Wild, M., and Xia, X.: Isoprene emissions over Asia 1979–2012: impact of climate and land use changes, Atmos. Chem. Phys. Discuss., 13, 29551-29592, 2013 [pdf] [Interactive Discussion]

The aim of this study is to investigate the temporal evolution of the isoprene fluxes in Asia over 1979-2012. to this purpose, we calculate the hourly emissions at 0.5x0.5 degree resolution using the MEGAN-MOHYCAN model driven by ECMWF-ERA Interim climatology. This study further incorporates (i) changes in land use, including the rapid expansion of oil palms, (ii) meteorological variability according to ERA Interim, (iii) long-term changes in solar radiation constrained by surface measurements, and (iv) recent experimental evidence that South Asian tropical forests are much weaker isoprene emitters than previously assumed. These effects lead to a significant lowering (factor of two) in the total isoprene fluxes (see Figure). Changes Changes in temperature and solar radiation are the major drivers of the interannual variability and emission trend. A positive annual flux trend of 0.2% and 0.52% is found over Asia and over China through 1979-2012, respectively. The bottom-up emissions are compared to field campaign measurements in Borneo and South China, and evaluated against top-down isoprene emission estimates constrained by GOME-2 HCHO columns through 2007-2012.


Figure caption : Annually averaged isoprene emission rates computed by the standard run (S0). S1 accounts for land use change effects, S2 also incorporates reduced emisison rates from tropical forests, and S4 includes both updates in solar radiation and in the representation of oil palm in Indonesia and Malaysia.




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