Modification and validation of a commercial dynamic chamber for reactive nitrogen and greenhouse gas flux measurements

Abstract. Reactive nitrogen compounds (NO, NO2, HONO, NH3 and others; Nr) play important roles in atmospheric processes, and their cascading impacts throughout the Earth system have adverse effects on both the environment and human health. The fluxes of these gases at the surface-atmosphere interface have been studied in isolation or in smaller subsets by micrometeorological techniques or chambers, but simultaneous observations of all Nr species alongside standard greenhouse gases (GHGs) as a function of time have not been reported. Here, a dual-dynamic chamber system was developed for Nr by modifying a commercially available system for GHG fluxes for use with destructive analyzers and to account for chemical changes. The resulting platform makes the measurement of Nr and, by extension other reactive gases, more widely accessible to the scientific community, as custom chambers do not need to be fabricated. System modifications to passivate surfaces were implemented, so that Nr gases like NO2 could be effectively transferred to standard gas analyzers, with an initial 36 % loss due to transformations ultimately minimized below analyzer detection limits (~10 %) under relevant atmospheric conditions. The modified 72 L chamber did not see a change in the baseline response times for GHGs or NO at a flow rate of 2 L min-1. They retained the same values as an ideal non-reactive trace gas (τ = 37–39 min versus 36 min. The modifications improved the transfer time constants of NO2, HONO, and NH3 by up to 2 min, but substantial surface interactions for NH3 remain. In all cases, a surface interaction term needs to be characterized for these gases to obtain accurate fluxes. Losses of NO2 and O3 by known gas phase reactions, or from deposition and reaction on pristine and aged chamber surfaces, were characterized across a range of environmentally relevant relative humidities (RH) and mixing ratios. The final dual-chamber system configuration includes a measurement and reference chamber, which are necessary to implement the corrections for surface effects and chemical transformations when accurately quantifying dynamic fluxes via a mass balance framework. Proof-of-concept measurements of Nr fluxes from agricultural soil samples under controlled lab conditions as a function of soil water content were able to quantify emissions of NO, NO2, HONO, NH3, and N2O simultaneously, when subject to fertilization experiments using urea, ammonium carbonate and bicarbonate, and ammonium nitrate. Unfertilized replicate agricultural soil samples showed variability in NO2 and HONO emissions when prepared with minimal disturbance to the soil structure, with values consistent with those reported by in-situ field measurements. These oppose maximum potential fluxes characterized in prior lab soil manipulations, particularly for HONO relative to NO. Last, fluxes were quantified with destructive gas analyzers in the field with the dual-chamber system on an in-use agricultural soil and included a urea-based fertilizer perturbation to stimulate microbial and chemical transformation and transfer Nr to the atmosphere. The resulting fluxes observed show good agreement with prior reports based on other flux techniques. The mass balance terms within the dual-chamber approach are fully inspected from the pilot deployment in the field, along with an error analysis, to aid in the uptake of this approach by the community.

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