A Source for the Continuous Generation of Pure and Quantifiable HONO Mixtures

Abstract

Abstract. A continuous source for the generation of pure HONO mixtures was developed and characterized, which is based on the Henry’s law solubility of HONO in acidic aqueous solutions. With the help of a peristaltic pump diluted nitrite and sulfuric acid solutions are mixed in a temperature-controlled stripping coil, which is operated with pure nitrogen or synthetic air at gas flow rates of 0.5–2 L min-1. Caused by the acidic conditions of the aqueous phase (pH ≈ 2.5), nitrite is almost completely converted into HONO, which partitions to the gas phase limited by its known solubility in water. The source shows a fast time response of ~2 min (0–90 %) at higher concentrations and an excellent long-time stability (2σ noise <1 %). The HONO emission of the source perfectly correlates with the nitrite concentration from the sub-ppb range up to 500 ppb. The rate of NOx formation increases quadratically with the HONO concentration from non-detectable values at atmospheric relevant HONO concentrations reaching a NOx content of 1.6 % at 500 ppb. A general equation based on Henry’s law is developed, whereby the HONO concentration of the source can be calculated using measured experimental parameters, i.e. nitrite concentration, liquid flow rates, gas flow rate, pH of the solution and temperature of the stripping coil. In the equation, the known Henry’s law constant of HONO in sulfuric acid solutions is used. For the calculation of the effective Henry’s law constant, the acid dissociation equilibrium of HONO/nitrite is used as a variable to adjust the theoretical HONO concentration to the measured values. From the average of all experimental data the equilibrium of HONO/nitrite is well described by pKa = 1021.53/T – 0.449. The pKa of 3.0±0.1 (1σ) at 25 °C is in good agreement with the range of 2.8–3.28 published in former studies. A standard deviation between all measured and theoretical HONO concentrations of only ±3.8 % was observed and a conservative upper limit accuracy of the HONO concentration of better 10 % is estimated. Thus, for the first time, a stable HONO source is developed, which can be used for the absolute calibration of HONO instruments.

Comparison of two photolytic calibration methods for nitrous acid

“…The calibration uncertainty depends on the output stability of the HONO source and the quantification technique used. Villena and Kleffmann (2022) demonstrate using two separate techniques that overall calibration uncertainties can be well below 10 % (2σ ).…”

“…Consistent mixing between the HCl and the NaNO 2 powder is required. These calibrations also require substantial warmup times (often hours) to ensure source stability, though some recent versions report faster warmup periods (e.g., < 10 min reported by Villena and Kleffmann, 2022). High HONO concentrations (above 1 ppmv) are often produced, requiring dilution, though the temporary unrealistic HONO concentrations can lead to significant HONO loss by its self-reaction and inaccurate HONO quantification.…”

“…A recent, noteworthy version of this calibration improves upon this concentration issue and has the ability to produce [HONO] on the order of tens of parts per trillion by volume (Lao et al, 2020). The generated HONO can be quantified by various methods including theoretical calculation (Villena and Kleffmann, 2022), conversion to NO followed by chemiluminescence detection (Lee et al, 2012;Lao et al, 2020;Villena and Kleffmann, 2022), thermal conversion to NO 2 followed by NO 2 quantification (Gingerysty and Osthoff, 2020), and conversion to aqueous nitrite followed by derivatization and detection by UV-vis (Peng et al, 2020). The calibration uncertainty depends on the output stability of the HONO source and the quantification technique used.…”