Sensitivity of simulated ammonia fluxes in Rocky Mountain National Park to measurement time resolution and meteorological inputs

Gas phase ammonia (NH3) measurements were made in Rocky Mountain National Park (RMNP) at the base of Longs Peak. Biweekly passive NH3 data was collected at two heights on a National Ecological Observatory Network (NEON) tower site (40.275903, -105.54596). Continuous NH3 data was collected at a nearby National Park Service (NPS) shelter (approximately 500 meters north of the NEON tower), using ion mobility spectroscopy. Continuous NH3 data had a time resolution of 30 seconds. The sites were located at 2750 meters above sea level. All data are ambient concentrations reported as mixing ratios. The data was used, with a suite of micrometeorology measurements, to simulate bidirectional fluxes of NH3 above the RMNP forest. The tower is surrounded by lower montane forest, comprised of predominantly evergreen needleleaf species, including ponderosa pine, juniper, and Douglas fir. There are also groves of quaking aspen located in the region.

Methods
Biweekly NH3 Measurements

Biweekly NH3 ambient air concentration was measured using Radiello (https://radiello.com/) passive diffusion samplers. The Radiello sampling system includes a diffusive body and adsorbing cartridge, which is coated with phosphoric acid. NH3 (g) diffuses across the exterior diffusive body and is collected on the adsorbing cartridge as ammonium (NH4+) over two weeks. Collected ammonia (as NH4+) is extracted from the cartridge into deionized water and analyzed using ion chromatography (IC) [Li et al., 2016]. NH3 passive samples were collected in duplicate (σ = ±0.25 µg m-3) on top of the NEON tower (25.35 m-agl). Passive NH3 sampling methods have been shown to have a low bias when compared with other sampling methods, including University Research Glassware Denuders and Picarro Cavity Ringdown spectroscopy methods [Pan et al., 2020; Puchalski et al., 2011].

Continuous NH3 Measurements

NH3 (g) air concentration was also measured using an ion mobility spectrometer (IMS). Ion mobility spectroscopy separates ionized molecules based on their mobility through a carrier gas, under the influence of an electric field. The instrument used was the AirSentry II Point-of-Use IMS from Particle Measuring Systems (Boulder, CO). The instrument was in the National Park Service shelter (located at 40.278129, -105.545635) 500 meters north of the NEON site with an inlet located approximately 2 m above natural grassland. The sampling inlet was ¼” Teflon tubing, heated to 40 C to reduce NH3 loss to the sampling tube. Inlet length was kept as short as possible to further prevent NH3 loss. Particles were removed by a fiber filter at the tip of the inlet. Due to the high altitude of the site location, the instrument was zeroed to account for pressure differences upon installation. Multi-point calibrations were conducted at the beginning and end of sampling. Calibration was confirmed using a known concentration ammonia gas sample split between the instrument and a phosphoric acid-coated denuder where the NH3 collected by the denuder is extracted into deionized water and analyzed using ion chromatography. Zero measurements were made periodically by overflowing the inlet with ultra-high purity clean air. The AirSentry samples at a 30-second frequency. The limit of detection is 70 pptv.