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. A continuous source for the generation of pure HONO mixtures was developedand characterized, which is based on the Henry's law solubility of HONO inacidic aqueous solutions. With the help of a peristaltic pump, dilutednitrite and sulfuric acid solutions are mixed in a temperature-controlledstripping coil, which is operated with pure nitrogen or synthetic air at gasflow rates of 0.5–2 L min−1. Caused by the acidic conditions of theaqueous phase (pH ≈ 2.5), nitrite is almost completely convertedinto 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 &lt; 1 %). The HONO emission of the source perfectlycorrelates 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 HONOconcentrations 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'slaw constant of HONO in sulfuric acid solutions is used. For the calculationof the effective Henry's law constant, the acid dissociation equilibrium ofHONO / nitrite is used as a variable to adjust the theoretical HONOconcentration to the measured values. From the average of all experimentaldata the equilibrium of HONO / nitrite is described well bypKa=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 conservativeupper-limit accuracy of the HONO concentration of better 10 % isestimated. Thus, for the first time, a stable HONO source is developedwhich can be used for the absolute calibration of HONO instruments.

A source for the continuous generation of pure and quantifiable HONO mixtures