Intercomparison of in situ measurements of ambient NH3: instrument performance and application under field conditions
Authors: Twigg Marsailidh M. , Berkhout Augustinus J. C. , Cowan Nicholas , Crunaire Sabine , Dammers Enrico , Ebert Volker , Gaudion Vincent , Haaima Marty , Häni Christoph , John Lewis , Jones Matthew R. , Kamps Bjorn , Kentisbeer John , Kupper Thomas , Leeson Sarah R. , Leuenberger Daiana , Lüttschwager Nils O. B. , Makkonen Ulla , Martin Nicholas A. , Missler David , Mounsor Duncan , Neftel Albrecht , Nelson Chad , Nemitz Eiko , Oudwater Rutger , Pascale Celine , Petit Jean-Eudes , Pogany Andrea , Redon Nathalie , Sintermann Jörg , Stephens Amy , Sutton Mark A. , Tang Yuk S. , Zijlmans Rens , Braban Christine F. , Niederhauser Bernhard 
Published: November 2022

Abstract. Ammonia (NH3) in the atmosphere affects both the environment and human
health. It is therefore increasingly recognised by policy makers as an
important air pollutant that needs to be mitigated, though it still remains
unregulated in many countries. In order to understand the effectiveness of
abatement strategies, routine NH3 monitoring is required. Current
reference protocols, first developed in the 1990s, use daily samplers with
offline analysis; however, there have been a number of technologies developed
since, which may be applicable for high time resolution routine monitoring
of NH3 at ambient concentrations. The following study is a
comprehensive field intercomparison held over an intensively managed
grassland in southeastern Scotland using currently available methods that are
reported to be suitable for routine monitoring of ambient NH3. In total, 13 instruments took part in the field study, including commercially
available technologies, research prototype instruments, and legacy
instruments. Assessments of the instruments' precision at low concentrations
(< 10 ppb) and at elevated concentrations (maximum reported
concentration of 282 ppb) were undertaken. At elevated concentrations, all
instruments performed well and with precision (r2 > 0.75). At
concentrations below 10 ppb, however, precision decreased, and instruments
fell into two distinct groups, with duplicate instruments split across the
two groups. It was found that duplicate instruments performed differently as a result of differences in instrument setup, inlet design, and operation of the instrument. New metrological standards were used to evaluate the accuracy in determining
absolute concentrations in the field. A calibration-free CRDS optical gas standard (OGS, PTB, DE) served as an instrumental reference standard, and
instrument operation was assessed against metrological calibration gases
from (i) a permeation system (ReGaS1, METAS, CH) and (ii) primary standard gas mixtures (PSMs) prepared by gravimetry (NPL, UK). This study suggests that, although the OGS gives good performance with respect to sensitivity and
linearity against the reference gas standards, this in itself is not enough
for the OGS to be a field reference standard, because in field applications, a closed path spectrometer has limitations due to losses to surfaces in
sampling NH3, which are not currently taken into account by the OGS.
Overall, the instruments compared with the metrological standards performed
well, but not every instrument could be compared to the reference gas
standards due to incompatible inlet designs and limitations in the gas flow
rates of the standards. This work provides evidence that, although NH3 instrumentation have
greatly progressed in measurement precision, there is still further work
required to quantify the accuracy of these systems under field conditions.
It is the recommendation of this study that the use of instruments for
routine monitoring of NH3 needs to be set out in standard operating
protocols for inlet setup, calibration, and routine maintenance in order
for datasets to be comparable.