A broadband cavity ringdown spectrometer for in-situ measurements of atmospheric trace gases

Abstract

Abstract. This paper describes a broadband cavity ringdown spectrometer and its deployment during the 2002 North Atlantic Marine Boundary Layer Experiment (NAMBLEX) to measure ambient concentrations of NO 3 , N 2 O 5 , I 2 and OIO at the Mace Head Atmospheric Research Station, Co. Galway, Ireland. The effective absorption path lengths accessible with the spectrometer generally exceeded 10 km, enabling sensitive localised "point" measurements of atmospheric absorbers to be made adjacent to the other instruments monitoring chemically related species at the same site. For the majority of observations, the spectrometer was used in an open path configuration thereby avoiding surface losses of reactive species. A subset of observations targeted the N 2 O 5 molecule by detecting the additional NO 3 formed by the thermal dissociation of N 2 O 5 . In all cases the concentrations of the atmospheric absorbers were retrieved by fitting the differential structure in the broadband cavity ringdown spectra using a methodology adapted from long path differential optical absorption spectroscopy. The uncertainty of the retrieval depends crucially on the correct treatment and fitting of the absorption bands due to water vapour, a topic that is discussed in the context of analysing broadband cavity ringdown spectra. The quality of the measurements and the retrieval method are illustrated with representative spectra acquired during NAMBLEX in spectral regions around 660 nm (NO 3 and N 2 O 5 ) and 570 nm (I 2 and OIO). Typical detection limits were 1 pptv for NO 3 in an integration time of 100 s, 4 pptv for OIO and 20 pptv for I 2 in an integration time of 10 min. Additionally, the concentrations of atmospheric water vapour and the aerosol optical extinction were retrieved in both spectral regions. A companion paper in this issue presents the time series of the measurements and discusses their significance for understanding the variability of short lived nitrogen and iodine compounds in the marine boundary layer.

Temporal variations of atmospheric CO<sub>2</sub> and CO at Ahmedabad in western India

“…Similarly to our observations of the afternoon mean concentrations of CO 2 , maximum values are also observed in April at Pondicherry (PON) and Port Blair (POB) with amplitudes of mean seasonal cycles at about 7.6 ± 1.4 and 11.1 ± 1.3 ppm respectively (Lin et al, 2015). Cape Rama (CRI), a costal site on the south-west coast of India shows seasonal maxima one month before our observations in March with an annual amplitude of about 9 ppm (Bhattacharya et al, 2009). The Sinhagad (SNG) site located over the Western Ghats mountain range, show much larger seasonal cycles with annual amplitude at about 20 ppm (Tiwari et al, 2014).…”

“…Hence, measurements over these regions are very helpful for understanding emissions growth as well as verifying the mitigation policies. The first observations of CO 2 , CO and other greenhouse gases started in February 1993 from Cape Rama (CRI: a clean site) on the south-west coast of India using flask samples (Bhattacharya et al, 2009). Since then, several other groups have initiated the measurements of surface-level greenhouse gases Sharma et al, 2014;Tiwari et al, 2014;Lin et al, 2015).…”

Impacts of bromine and iodine chemistry on tropospheric OH and HO<sub>2</sub>: comparing observations with box and global model perspectives

“…Photolysis rates use the FAST-JX scheme (Bian and Prather, 2002;Mao et al, 2010), with acetone photolysis rates updated by Fischer et al (2012). Stratospheric chemistry is based on LINOZ McLinden et al (2000) for O 3 and a linearised mechanism for other species as described by Murray et al (2012).…”

“…Photolysis rates of bromine and iodine compounds are calculated using the FAST-J radiative transfer model Bian and Prather, 2002;Mao et al, 2010). Wet and dry deposition are determined as for the standard GEOSChem model Wesely, 1989;Wang et al, 1998;Amos et al, 2012).…”

“…Simultaneous measurements of OH and HO 2 by laserinduced fluorescence (LIF) (Bloss et al, 2005a;Smith et al, 2006) and halogen species by a combination of DOAS (for BrO or IO, OIO and I 2 ) and broadband cavity ring-down spectroscopy (BBCRDS) (for OIO and I 2 ) (Bitter et al, 2005) during NAMBLEX enabled box model calculations to fully explore the impacts of halogens on the local composition. A box model without halogen chemistry was able to reproduce the NAMBLEX OH observations to within 25 %, but HO 2 observations were overestimated by up to a factor of 2 .…”