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-term 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 described well 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.
Highlights
Nitrous acid (HONO) is an important trace gas in the atmosphere which represents a major source of the OH radical (Kleffmann et al, 2005; Acker et al, 2006; Ma et al, 2013; Michoud et al, 2014; Gu et al, 2020), the detergent of the atmosphere
The resulting 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 (Park and Lee, 1988; Riordan et al, 2005; da Silva et al, 2006)
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
Summary
Nitrous acid (HONO) is an important trace gas in the atmosphere which represents a major source of the OH radical (Kleffmann et al, 2005; Acker et al, 2006; Ma et al, 2013; Michoud et al, 2014; Gu et al, 2020), the detergent of the atmosphere. HONO and its reaction products show mutagenic and carcinogenic properties (Pitts et al, 1978; Kirchner and Hopkins, 1991), which is especially important for indoor conditions, for which much higher levels of up to 90 ppb compared to the ambient atmosphere have been observed (Vecera and Dasgupta, 1994). The sources of HONO in the atmosphere are controversially discussed (Kleffmann, 2007), and among others, photochemical heterogeneous reactions have been proposed to explain unexpectedly high HONO concentrations during daytime (Stemmler et al, 2006; Zhou et al, 2011). The development of sensitive and selective HONO instruments is still of high interest, for which a stable and pure HONO source is very helpful.
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