A miniDOAS instrument optimised for ammonia field measurements

Abstract

Abstract. We present a differential optical absorption spectroscopy (DOAS) instrument, called "miniDOAS", optimised for optical open-path field-measurements of ambient ammonia (NH3) alongside nitrogen oxide (NO) and sulfur dioxide (SO2). The instrument is a further development of the miniDOAS presented by Volten et al. (2012). We use a temperature-controlled spectrometer, a deuterium light source and a modified optical arrangement. The system was set up in a robust, field-deployable, temperature-regulated housing. For the evaluation of light spectra we use a new high-pass filter routine based upon robust baseline extraction with local regression. Multiple linear regression including terms of an autoregressive–moving-average model is used to determine concentrations. For NH3 the random uncertainty is about 1.4 % of the concentration, and not better than 0.2 µg m−3. Potential biases for the slope of the calibration are given by the precision of the differential absorption cross sections (±3 %) and for the offset by the precision of the estimation of concentration offsets (cref) introduced by the reference spectrum Iref. Comparisons of miniDOAS measurements to those by NH3 acid trap devices showed good agreement. The miniDOAS can be flexibly used for a wide range of field trials, such as micrometeorological NH3 flux measurements with approaches based upon horizontal or vertical concentration differences. Results from such applications covering concentration dynamics of less than one up to several hundreds of µg m−3 are presented.

Ammonia emissions from a grazed field estimated by miniDOAS measurements and inverse dispersion modelling

“…Since the initial miniDOAS publication ) the calibration procedure has been revised to correct a gas standard error in the conversion from ppm to µg m −3 . The corrected measurements presented in this study are a factor of 1.16 higher relative to the NH 3 concentrations presented by Sintermann et al (2016).…”

“…Deviations between the S1, S2 and S3 analysers were minor, and the coefficient of variation between them was determined to be 3.4 % (unpublished data). Sintermann et al (2016) have described this inter-comparison phase and the miniDOAS performance in detail; however, the authors compare only the miniDOAS sensors S2 and S3 as these sensors were fitted with all of the updated Swiss miniDOAS instrumental features discussed within that study. Since the input data had been filtered to remove conditions which do not meet the established criteria (u * < 0.1 m s −1 ), and instrumental uncertainty associated with the concentration measurements is very low, the principal uncertainties are associated with the modelled results, principally the input variables which could not be measured directly, such as R c and the predicted background concentration C b used for gap filling.…”

“…The miniDOAS instruments offer greater portability and a lower cost relative to prior DOAS instruments (Volten et al, 2012). The broadband and narrowband extinction of UV light (qual to absorption + scattering) is measured across the light path, and the concentration of different trace gases is determined by their respective absorption spectra (details in Sintermann et al, 2016). In the wavelength range used by the miniDOAS (204-230 nm) narrowband absorption is seen by NH 3 , sulfur dioxide (SO 2 ), and nitrogen oxide (NO), while other absorbers with broader absorption features are eliminated by high-pass filtering.…”

“…Reference spectra (I ref , see Sintermann et al, 2016) were determined for each instrument at the field site 1 week prior to the grazing experiment, where the three miniDOAS systems were configured to measure in parallel (measuring concentrations across the same open path). In order to provide the absolute concentration reference (c ref , see Sintermann et al, 2016) for the miniDOAS, a transect of three sets of passive sampler (ALPHA) triplicates (Tang et al, 2001) were placed along the path length, giving a time-integrated c ref measurement. The miniDOAS inter-comparison showed close agreement in the concentration levels between the three systems, where the coefficient of variation was 3.4 % (unpublished data).…”

Replacing the AMOR with the miniDOAS in the ammonia monitoring network in the Netherlands

“…That system inspired Sintermann and colleagues to build their own, modified miniDOAS system (Sintermann et al, 2016). They measured artificial sources, concentration differences due to grazing cattle, and the results of manure application to fields.…”