Abstract. An analysis of the 22-yr ozone (O3) series (1988–2009) at the subtropical high mountain Izaña~station (IZO; 2373 m a.s.l.), representative of free troposphere (FT) conditions, is presented. Diurnal and seasonal O3 variations as well as the O3 trend (0.19 ± 0.05 % yr−1 or 0.09 ppbv yr−1), are assessed. A climatology of O3 transport pathways using backward trajectories shows that higher O3 values are associated with air masses travelling above 4 km altitude from North America and North Atlantic Ocean, while low O3 is transported from the Saharan continental boundary layer (CBL). O3 data have been compared with PM10, 210Pb, 7Be, potential vorticity (PV) and carbon monoxide (CO). A clear negative logarithmic relationship was observed between PM10 and surface O3 for all seasons. A similar relationship was found between O3 and 210Pb. The highest daily O3 values (90th percentile) are observed in spring and in the first half of summer time. A positive correlation between O3 and PV, and between O3 and 7Be is found throughout the year, indicating that relatively high surface O3 values at IZO originate from the middle and upper troposphere. We find a good correlation between O3 and CO in winter, supporting the hypothesis of long-range transport of photochemically generated O3 from North America. Aged air masses, in combination with sporadic inputs from the upper troposphere, are observed in spring, summer and autumn. In summer time high O3 values seem to be the result of stratosphere-to-troposphere (STT) exchange processes in regions neighbouring the Canary Islands. Since 1995–1996, the North Atlantic Oscillation has changed from a predominantly high positive phase to alternating between negative, neutral or positive phases. This change results in an increased flow of the westerlies in the mid-latitude and subtropical North Atlantic, thus favouring the transport of O3 and its precursors from North America, and a higher frequency of storms over North Atlantic, with a likely higher incidence of STT processes in mid-latitudes. These processes lead to an increase of tropospheric O3 in the subtropical North Atlantic region after 1996 that has been reflected in surface O3 records at IZO.

Abstract. We present lower/middle tropospheric columnaveraged CH 4 mole fraction time series measured by nine globally distributed ground-based FTIR (Fourier transform infrared) remote sensing experiments of the Network for the Detection of Atmospheric Composition Change (NDACC). We show that these data are well representative of the tropospheric regional-scale CH 4 signal, largely independent of the local surface small-scale signals, and only weakly dependent on upper tropospheric/lower stratospheric (UTLS) CH 4 variations. In order to achieve the weak dependency on the UTLS, we use an a posteriori correction method. We estimate a typical precision for daily mean values of about 0.5 % and a systematic error of about 2.5 %. The theoretical assessments are complemented by an extensive empirical study. For this purpose, we use surface in situ CH 4 measurements made within the Global Atmosphere Watch (GAW) network and compare them to the remote sensing data. We briefly discuss different filter methods for removing the local small-scale signals from the surface in situ data sets in order to obtain the in situ regional-scale signals. We find good agreement between the filtered in situ and the remote sensing data. The agreement is consistent for a variety of timescales that are interesting for CH 4 source/sink research: day-to-day, monthly, and inter-annual. The comparison study confirms our theoretical estimations and proves that the NDACC FTIR measurements can provide valuable data for investigating the cycle of CH 4 .Published by Copernicus Publications on behalf of the European Geosciences Union. E. Sepúlveda et al.: NDACC FTIR and GAW surface in situ tropospheric CH 4

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Abstract. At the Izaña Atmospheric Research Center, highresolution mid-infrared solar absorption spectra have been recorded for more than 12 yr using Fourier Transform InfraRed (FTIR) spectrometers. We use the spectral fitting algorithm PROFFIT to retrieve long-term time series of methane (CH 4 ) from the measured spectra. We investigate the total column-averaged dry air mole fractions of methane (totXCH 4 ) obtained from a profile scaling and a profile retrieval, and apply two approaches for deriving the tropospheric column-averaged dry air mole fractions: firstly, we use the FTIR hydrogen fluoride (HF) total column amounts as an estimator for the stratospheric CH 4 contribution and a posteriori correct the totXCH 4 data of a profile scaling retrieval accordingly (troXCH 4 post ); secondly, we directly determine the tropospheric column-averaged dry air mole fractions of methane (troXCH 4 retr ) from retrieved CH 4 profiles. Our theoretical estimation indicates that the scaling retrieval leads to totXCH 4 amounts that are subject to a large smoothing error, which can be widely avoided by applying a profile retrieval (for the latter we estimate an overall precision of 0.41 %).We compare the different FTIR CH 4 data to Izaña's Global Atmospheric Watch (GAW) surface in-situ CH 4 data (CH 4 GAW ), which in the case of the Izaña Atmospheric Research Center high mountain observatory are very representative for the free tropospheric CH 4 amounts. Concerning totXCH 4 , the agreement between the FTIR data product and the in-situ measurement is rather poor documenting that totXCH 4 is not a valid free tropospheric CH 4 proxy, as it is significantly affected by the varying stratospheric CH 4 contribution and it rather follows the variation in the tropopause altitude. The a posteriori correction method as applied here only removes a part of this stratospheric CH 4 contribution. In contrast the profile retrieval allows for a direct estimation of the tropospheric column-averaged CH 4 amounts. Results of the profile retrieval analysis correlate well with the CH 4 GAW data (correlation coefficient of 0.60, FTIR-GAW scatter of 0.97 %), and both data sets show very similar annual cycles and trend behaviour for the 2001-2010 time period. Furthermore, we find a very good absolute agreement between the troXCH 4 retr and CH 4 GAW (mid-infrared FTIR/GAW scaling factor of 0.9987) suggesting that mid-infrared FTIR data can be well combined with the surface in-situ GAW data.Our study strongly supports the value of mid-infrared ground-based FTIR CH 4 profile retrievals as well as the robustness of the approach for achieving total and tropospheric column-averaged XCH 4 data of high quality.

At the Izaña Atmospheric Research Center high-resolution middle infrared solar absorption spectra have been recorded for more than 12 years using Fourier Transform InfraRed (FTIR) spectrometers. We use the spectral fitting algorithm PROFFIT to retrieve long-term time series of methane (CH₄) from the measured spectra. We investigate the total column-averaged dry-air mole fractions of methane (totXCH4) obtained from a scaling and a profile retrieval, and apply two approaches for deriving the tropospheric column-averaged dry-air mole fractions: firstly, we use the FTIR Hydrogen Fluoride (HF) total column amounts as an estimator for the stratospheric CH₄ contribution and a posteriori correct the totXCH4 data of a profile scaling retrieval accordingly (troXCH4_post), and secondly, we directly determine the tropospheric column-averaged dry-air mole fractions of methane (troXCH4_retr) from retrieved CH₄ profiles. Our theoretical estimation indicates that the scaling retrieval leads to totXCH4 amounts that are subject to a large smoothing error, which can be widely avoided by applying a profile retrieval (for the latter we estimate an overall precision of 0.41%).

We compare the different FTIR CH₄ data to Izaña's Global Atmospheric Watch (GAW) surface in-situ CH₄ data (CH_4GAW), which in the case of the Izaña Atmospheric Research Center high mountain observatory are well representative for the free tropospheric CH₄ amounts. Concerning totXCH4 the agreement between the FTIR data product and the in-situ measurement is rather poor documenting that totXCH4 is no valid free tropospheric CH₄ proxy instead it is significantly affected by the varying stratospheric CH₄ contribution and it rather follows the variation in the tropopause altitude. The a posteriori correction method only removes a part of this stratospheric CH₄ contribution. On the contrary the profile retrieval allows for a direct estimation of the tropospheric column-averaged CH₄ amounts. This troXCH4_retr data correlates well with the CH_4GAW data (correlation coefficient of 0.60, FTIR-GAW scatter of 0.97%) and both data show very similar annual cycles and trend behaviour (for the 2001–2010 time period). Furthermore, we find a very good absolute agreement between the troXCH4_retr and CH_4GAW (middle infrared FTIR/GAW scaling factor of 0.9987) suggesting that middle infrared FTIR data can be well combined with the surface in-situ GAW data.

Our study proves the significance of middle infrared ground-based FTIR CH₄ profile retrievals as well as the robustness of the approach for achieving total and tropospheric column-averaged XCH₄ data of high quality