Abstract. Total columns measured with the ground-based solar FTIR technique are highly variable in time due to atmospheric chemistry and dynamics in the atmosphere above the measurement station. In this paper, a multiple regression model with anomalies of air pressure, total columns of hydrogen fluoride (HF) and carbon monoxide (CO) and tropopause height are used to reduce the variability in the methane (CH 4 ) and nitrous oxide (N 2 O) total columns to estimate reliable linear trends with as small uncertainties as possible. The method is developed at the Harestua station (60 • N, 11 • E, 600 m a.s.l.) and used on three other European FTIR stations, i.e. Jungfraujoch stations. From the N 2 O total columns crude tropospheric and stratospheric partial columns were derived, indicating that the observed difference in the N 2 O trends between the FTIR sites is of stratospheric origin. This agrees well with the N 2 O measurements by the SMR instrument onboard the Odin satellite showing the highest trends at Harestua, 0.98 ± 0.28 % yr −1 , and considerably smaller trends at lower latitudes, 0.27 ± 0.25 % yr −1 . The multiple regression model was compared with two other trend methods, the ordinary linear regression and a Bootstrap algorithm. The multiple regression model estimated CH 4 and N 2 O trends that differed up to 31 % compared to the other two methods and had uncertainties that were up to 300 % lower. Since the multiple regression method were carefully validated this stresses the importance to account for variability in the total columns when estimating trend from solar FTIR data.

Abstract. Trends in the CO and C 2 H 6 partial columns (∼0-15 km) have been estimated from four European groundbased solar FTIR (Fourier Transform InfraRed) stations for the 1996-2006 time period. The CO trends from the four stations Jungfraujoch, Zugspitze, Harestua and Kiruna have been estimated to −0.45 ± 0.16 % yr −1 , −1.00 ± 0.24 % yr −1 , −0.62 ± 0.19 % yr −1 and −0.61 ± 0.16 % yr −1 , respectively. The corresponding trends for C 2 H 6 are −1.51± 0.23 % yr −1 , −2.11 ± 0.30 % yr −1 , −1.09 ± 0.25 % yr −1 and −1.14 ± 0.18 % yr −1 . All trends are presented with their 2-σ confidence intervals. To find possible reasons for the CO trends, the global-scale EMEP MSC-W chemical transport model has been used in a series of sensitivity scenarios. It is shown that the trends are consistent with the combination of a 20 % decrease in the anthropogenic CO emissions seen in Europe and North America during the 1996-2006 period and a 20 % increase in the anthropogenic CO emissions in East Asia, during the same time period. The possible impacts of CH 4 and biogenic volatile organic compounds (BVOCs) are also considered. The European and global-scale EMEP models have been evaluated against the Correspondence to: J. Mellqvist (johan.mellqvist@chalmers.se) measured CO and C 2 H 6 partial columns from Jungfraujoch, Zugspitze, Bremen, Harestua, Kiruna and Ny-Ålesund. The European model reproduces, on average the measurements at the different sites fairly well and within 10-22 % deviation for CO and 14-31 % deviation for C 2 H 6 . Their seasonal amplitude is captured within 6-35 % and 9-124 % for CO and C 2 H 6 , respectively. However, 61-98 % of the CO and C 2 H 6 partial columns in the European model are shown to arise from the boundary conditions, making the globalscale model a more suitable alternative when modeling these two species. In the evaluation of the global model the average partial columns for 2006 are shown to be within 1-9 % and 37-50 % of the measurements for CO and C 2 H 6 , respectively. The global model sensitivity for assumptions made in this paper is also analyzed.

Trends in the CO and C₂H₆ partial columns (~0–15 km) have been estimated from four European ground-based solar FTIR stations for the 1996–2006 time period. The CO trends from the four stations Jungfraujoch, Zugspitze, Harestua and Kiruna have been estimated to −0.45±0.16 % yr⁻¹, −1.00±0.24 % yr⁻¹, −0.62±0.19 % yr⁻¹ and −0.61±0.16 % yr⁻¹, respectively. The corresponding trends for C₂H₆ are −1.51±0.23 % yr⁻¹, −2.11±0.30 % yr⁻¹, −1.09±0.25 % yr⁻¹ and −1.14±0.18 % yr⁻¹. To find possible reasons for the CO trends, the global-scale EMEP MSC-W chemical transport model has been used in a series of sensitivity scenarios. It is shown that the trends are consistent with the combination of a 20 % decrease in the anthropogenic CO emissions seen in Europe and North America during the 1996-2006 period and a 20 % increase in the anthropogenic CO emissions in East Asia, during the same time period. The possible impacts of CH₄ and biogenic volatile organic compounds (BVOCs) are also considered. The European and global-scale EMEP model have been evaluated against the measured CO and C₂H₆ partial columns from Jungfraujoch, Zugspitze, Bremen, Harestua, Kiruna and Ny-Ålesund. The European model reproduces, on average the measurements at the different sites fairly well and within 10–22 % deviation for CO and 14–31 % deviation for C₂H₆. Their seasonal amplitude is captured within 6–35 % and 9–124 % for CO and C₂H₆, respectively. However, 61–98 % of the CO and C₂₆ partial columns in the European model are shown to arise from the boundary conditions, making the global-scale model a more suitable alternative when modeling theses two species. In the evaluation of the global model the average partial columns for year 2006 have shown to be within 1–9 % and 37–50 % for CO and C₂₆, respectively. The global model sensitivity for assumptions done in this paper is also analyzed

A multiple regression model has been used to estimate linear trends of the CH₄ and N₂O total columns measured with the ground-based solar FTIR technique at four European stations, i.e. Jungfraujoch (47° N, 8° E, 3600 m a.s.l.), Zugspitze (47° N, 11° E, 3000 m a.s.l.), Harestua (60° N, 11° E, 600 m a.s.l.) and Kiruna (68° N, 20° E, 400 m a.s.l.). The total columns were retrieved with a common method developed within the EU-project HYMN. Anomalies from air pressure, total columns of hydrogen fluoride (HF) and carbon monoxide (CO) and tropopause height were used in the regression model to reduce the time series variability and thereby estimate trustful trends. Significant positive CH₄ trends of 0.13–0.25% yr⁻¹ at the 2-σ level were found for all participating stations for the 1996–2009 period. The strongest trends were estimated at northern latitudes stations while slightly weaker trends were observed in the Alps. For the time period of 2007–2009 a strong increase in the CH₄ total column was observed for all stations with the strongest yearly growth at Kiruna (1.15 ± 0.17% yr⁻¹). Significant positive N₂O trends of 0.19–0.40% yr⁻¹ were found for all stations in the 1996–2007 period with the strongest trend at Harestua. From the N₂O data also crude tropospheric and stratospheric partial columns were derived, indicating that the observed difference in the N₂O trends between the FTIR sites is of stratospheric origin. This agrees well with the N₂O measurements by the Odin/SMR satellite showing the highest trends at Harestua 0.98 ± 0.28% yr⁻¹, and considerably smaller trends in the alp regions 0.27 ± 0.25% yr⁻¹. The multiple regression model was compared with two other trend methods, the ordinary linear regression and a Bootstrap algorithm. The multiple regression model estimated CH₄ and N₂O trends that differed by 12–31% compared to the other two methods. Since the trends estimated with the multiple regression model were carefully validated this stresses the importance to account for the atmospheric variability when estimating trends of CH₄ and N₂O total columns