A new method to detect long term trends of methane (CH<sub>4</sub>) and nitrous oxide (N<sub>2</sub>O) total columns measured within the NDACC ground-based high resolution solar FTIR network

Abstract

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.

Five years of CO, HCN, C<sub>2</sub>H<sub>6</sub>, C<sub>2</sub>H<sub>2</sub>, CH<sub>3</sub>OH, HCOOH and H<sub>2</sub>CO total columns measured in the Canadian high Arctic

“…This is consistent with the decrease of tropospheric CO of −0.61 ± 0.16 % yr −1 observed at high-latitude sites between 1996 and 2006 (Angelbratt et al, 2011). This trend has been explained by the combination of a 20 % decrease in anthropogenic CO emissions in Europe and North America and 20 % increase in CO anthropogenic emission in east Asia.…”

Analysis of stratospheric NO<sub>2</sub> trends above Jungfraujoch using ground-based UV-visible, FTIR, and satellite nadir observations

“…A good agreement is found between SAOZ, FTIR, and satellite nadir observations with a decline in the stratospheric NO 2 column reaching 3 % decade −1 for both the 1990-2009 and 1996-2009 periods. This decrease is not consistent with the increase in N 2 O of about +2.5 % decade −1 reported globally (WMO, 2007) and in particular from FTIR observations at the Jungfraujoch station (Angelbratt et al, 2011). Since in the NO y chemistry, N 2 O is the source of NO y and then NO y is subsequently partitioned into family members, opposite NO 2 and N 2 O trends suggest a change in the NO y partitioning.…”

Renewed methane increase for five years (2007–2011) observed by solar FTIR spectrometry

“…Afterwards, a renewed strong increase can be observed, which has been persisting for 5 years now [mid 2006[mid , Sep 2011. The division into 3 time periods is in agreement with dedicated trend change points defined and used for methane in earlier work (Dlugokencky et al, 2003Angelbratt et al, 2011). Our trend analysis is based on the approach described in Gardiner et al (2008).…”

“…Retrievals of columnar methane from ground-based FTIR spectrometry in the mid-IR were used previously for trend studies (Zander et al, 1989;Angelbratt et al, 2011) and satellite validation (e.g., Sussmann et al, 2005). However, it became obvious only recently that the methane-column retrievals at high-humidity (low-altitude) sites may be dominated by water vapor (H 2 O/HDO)-CH 4 interference errors of up to 5 % .…”