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

<p>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<span class="inline-formula">⁻¹</span>. Caused by the acidic conditions of the aqueous phase (pH <span class="inline-formula">≈</span> 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 <span class="inline-formula">∼</span> 2 min (0 %–90 %) at higher concentrations and an excellent long-term stability (2<span class="inline-formula"><i>σ</i></span> noise <span class="inline-formula">&lt;</span> 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 NO<span class="inline-formula">_<i>x</i></span> formation increases quadratically with the HONO concentration from non-detectable values at atmospheric relevant HONO concentrations reaching a NO<span class="inline-formula">_<i>x</i></span> 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 <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M8" display="inline" overflow="scroll" dspmath="mathml"><mo>/</mo></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="8pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="fb147fccdcf98a9911cf3d26a8f6dc33"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="amt-15-627-2022-ie00001.svg" width="8pt" height="14pt" src="amt-15-627-2022-ie00001.png"/></svg:svg></span></span> 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 <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M9" display="inline" overflow="scroll" dspmath="mathml"><mo>/</mo></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="8pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="880d1b22cfae9b4167ff115d05c6894c"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="amt-15-627-2022-ie00002.svg" width="8pt" height="14pt" src="amt-15-627-2022-ie00002.png"/></svg:svg></span></span> nitrite is described well by p<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M10" display="inline" overflow="scroll" dspmath="mathml"><mrow><msub><mi>K</mi><mi mathvariant="normal">a</mi></msub><mo>=</mo><mn mathvariant="normal">1021.53</mn><mo>/</mo><mi>T</mi><mo>-</mo><mn mathvariant="normal">0.449</mn></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="118pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="0fd499b7d72d4db1dbb1ebab2eeddf65"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="amt-15-627-2022-ie00003.svg" width="118pt" height="14pt" src="amt-15-627-2022-ie00003.png"/></svg:svg></span></span>. The p<span class="inline-formula"><i>K</i>_a</span> of 3.0 <span class="inline-formula">±</span> 0.1 (1<span class="inline-formula"><i>σ</i></span>) at 25 <span class="inline-formula">^∘</span>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 <span class="inline-formula">±3.8</span> % 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.</p>