Airborne flux measurements of ammonia over the southern Great Plains using chemical ionization mass spectrometry

[摘要] Ammonia (NH 3 ) is an abundant trace gas in theatmosphere and an important player in atmospheric chemistry, aerosolformation and the atmosphere–surface exchange of nitrogen. The accuratedetermination of NH 3 emission rates remains a challenge, partly due tothe propensity of NH 3 to interact with instrument surfaces, leading tohigh detection limits and slow response times. In this paper, we present anew method for quantifying ambient NH 3 , using chemical ionization massspectrometry (CIMS) with deuterated benzene cations as reagents. The setupaimed at limiting sample–surface interactions and achieved a 1 σ 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 sensitivityexhibited an inverse humidity dependence, in particular in relatively dryconditions. Background of up to 10 % of the total signal requiredconsideration as well, as it responded on the order of a few minutes. Toshowcase the method's capabilities, we quantified NH 3 mixing ratiosfrom measurements obtained during deployment on a Gulfstream I aircraftduring the HI-SCALE (Holistic Interactions of Shallow Clouds, Aerosols, andLand-Ecosystems) field campaign in rural Oklahoma during May 2016. Typicalmixing 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 totens of ppbv of NH 3 were encountered as well. We identified two oftheir sources as a large fertilizer plant and a cattle farm, and our mixingratio measurements yielded upper bounds of 350  ±  50 and 0.6 kg NH 3  h −1 for their respective momentary source rates. The fastresponse of the CIMS also allowed us to derive vertical NH 3 fluxeswithin the turbulent boundary layer via eddy covariance, for which wechiefly used the continuous wavelet transform technique. As expected for aregion dominated by agriculture, we observed predominantly upward fluxes,implying net NH 3 emissions from the surface. The corresponding analysisfocused on the most suitable flight, which contained two straight-and-levellegs at ∼  300 m above ground. We derived NH 3 fluxesbetween 1 and 11 mol km −2  h −1 for these legs, at an effectivespatial resolution of 1–2 km. The analysis demonstrated how fluxmeasurements benefit from suitably arranged flight tracks with sufficientlylong straight-and-level legs, and it explores the detrimental effect ofmeasurement discontinuities. Following flux footprint estimations,comparison to the NH 3 area emissions inventory provided by the U.S.Environmental Protection Agency indicated overall agreement but also theabsence of some sources, for instance the identified cattle farm. Our studyconcludes that high-precision CIMS measurements are a powerful tool forin situ measurements of ambient NH 3 mixing ratios, and even allow forthe airborne mapping of the air–surface exchange of NH 3 .

[学科分类] 内科医学