Airborne flux measurements of ammonia over the southern Great Plains using chemical ionization mass spectrometry
Authors: Schobesberger Siegfried , D'Ambro Emma L. , Vettikkat Lejish , Lee Ben H. , Peng Qiaoyun , Bell David M. , Shilling John E. , Shrivastava Manish , Pekour Mikhail , Fast Jerome , Thornton Joel A. 
Published: January 2023

Abstract. Ammonia (NH3) is an abundant trace gas in the
atmosphere and an important player in atmospheric chemistry, aerosol
formation and the atmosphere–surface exchange of nitrogen. The accurate
determination of NH3 emission rates remains a challenge, partly due to
the propensity of NH3 to interact with instrument surfaces, leading to
high detection limits and slow response times. In this paper, we present a
new method for quantifying ambient NH3, using chemical ionization mass
spectrometry (CIMS) with deuterated benzene cations as reagents. The setup
aimed at limiting sample–surface interactions and achieved a 1σ
precision of 10–20 pptv and an immediate 1/e response rate of < 0.4 s,
which compares favorably to the existing state of the art. The sensitivity
exhibited an inverse humidity dependence, in particular in relatively dry
conditions. Background of up to 10 % of the total signal required
consideration as well, as it responded on the order of a few minutes. To
showcase the method's capabilities, we quantified NH3 mixing ratios
from measurements obtained during deployment on a Gulfstream I aircraft
during the HI-SCALE (Holistic Interactions of Shallow Clouds, Aerosols, and
Land-Ecosystems) field campaign in rural Oklahoma during May 2016. Typical
mixing ratios were 1–10 parts per billion by volume (ppbv) for the boundary layer and 0.1–1 ppbv in the lower free troposphere. Sharp plumes of up to
tens of ppbv of NH3 were encountered as well. We identified two of
their sources as a large fertilizer plant and a cattle farm, and our mixing
ratio measurements yielded upper bounds of 350 ± 50 and 0.6 kg NH3 h−1 for their respective momentary source rates. The fast
response of the CIMS also allowed us to derive vertical NH3 fluxes
within the turbulent boundary layer via eddy covariance, for which we
chiefly used the continuous wavelet transform technique. As expected for a
region dominated by agriculture, we observed predominantly upward fluxes,
implying net NH3 emissions from the surface. The corresponding analysis
focused on the most suitable flight, which contained two straight-and-level
legs at ∼ 300 m above ground. We derived NH3 fluxes
between 1 and 11 mol km−2 h−1 for these legs, at an effective
spatial resolution of 1–2 km. The analysis demonstrated how flux
measurements benefit from suitably arranged flight tracks with sufficiently
long straight-and-level legs, and it explores the detrimental effect of
measurement discontinuities. Following flux footprint estimations,
comparison to the NH3 area emissions inventory provided by the U.S.
Environmental Protection Agency indicated overall agreement but also the
absence of some sources, for instance the identified cattle farm. Our study
concludes that high-precision CIMS measurements are a powerful tool for
in situ measurements of ambient NH3 mixing ratios, and even allow for
the airborne mapping of the air–surface exchange of NH3.