Experimental budgets of OH, HO₂, and RO₂ radicals and implications for ozone formation in the Pearl River Delta in China 2014

• Main Authors: Z. Tan, K. Lu, A. Hofzumahaus, H. Fuchs, B. Bohn, F. Holland, Y. Liu, F. Rohrer, M. Shao, K. Sun, Y. Wu, L. Zeng, Y. Zhang, Q. Zou, A. Kiendler-Scharr, A. Wahner
• Format: Article
• Language: English
• Published: Copernicus Publications 2019-05-01
• Series: Atmospheric Chemistry and Physics
• Online Access: https://www.atmos-chem-phys.net/19/7129/2019/acp-19-7129-2019.pdf

<p>Hydroxyl (OH) and peroxy radicals (<span class="inline-formula">HO₂</span> and <span class="inline-formula">RO₂</span>) were measured in the Pearl River Delta, which is one of the most polluted areas in China, in autumn 2014. The radical observations were complemented by measurements of OH reactivity (inverse OH lifetime) and a comprehensive set of trace gases including carbon monoxide (CO), nitrogen oxides (<span class="inline-formula">NO_<i>x</i>=NO</span>, <span class="inline-formula">NO₂</span>) and volatile organic compounds (VOCs). OH reactivity was in the range from 15 to 80&thinsp;<span class="inline-formula">s⁻¹</span>, of which about 50&thinsp;% was unexplained by the measured OH reactants. In the 3 weeks of the campaign, maximum median radical concentrations were <span class="inline-formula">4.5×10⁶</span>&thinsp;<span class="inline-formula">cm⁻³</span> for OH at noon and <span class="inline-formula">3×10⁸</span> and <span class="inline-formula">2.0×10⁸</span>&thinsp;<span class="inline-formula">cm⁻³</span> for <span class="inline-formula">HO₂</span> and <span class="inline-formula">RO₂</span>, respectively, in the early afternoon. The completeness of the daytime radical measurements made it possible to carry out experimental budget analyses for all radicals (OH, <span class="inline-formula">HO₂</span>, and <span class="inline-formula">RO₂</span>) and their sum (<span class="inline-formula">RO_<i>x</i></span>). The maximum loss rates for OH, <span class="inline-formula">HO₂</span>, and <span class="inline-formula">RO₂</span> reached values between 10 and 15&thinsp;<span class="inline-formula">ppbv h⁻¹</span> during the daytime. The largest fraction of this can be attributed to radical interconversion reactions while the real loss rate of <span class="inline-formula">RO_<i>x</i></span> remained below 3&thinsp;<span class="inline-formula">ppbv h⁻¹</span>. Within experimental uncertainties, the destruction rates of <span class="inline-formula">HO₂</span> and the sum of OH, <span class="inline-formula">HO₂</span>, and <span class="inline-formula">RO₂</span> are balanced by their respective production rates. In case of <span class="inline-formula">RO₂</span>, the budget could be closed by attributing the missing OH reactivity to unmeasured VOCs. Thus, the presumption of the existence of unmeasured VOCs is supported by <span class="inline-formula">RO₂</span> measurements. Although the closure of the <span class="inline-formula">RO₂</span> budget is greatly improved by the additional unmeasured VOCs, a significant imbalance in the afternoon remains, indicating a missing <span class="inline-formula">RO₂</span> sink. In case of OH, the destruction in the morning is compensated by the quantified OH sources from photolysis (HONO and <span class="inline-formula">O₃</span>), ozonolysis of alkenes, and OH recycling (<span class="inline-formula">HO₂+NO</span>). In the afternoon, however, the OH budget indicates a missing OH source of 4 to 6&thinsp;<span class="inline-formula">ppbv h⁻¹</span>. The diurnal variation of the missing OH source shows a similar pattern to that of the missing <span class="inline-formula">RO₂</span> sink so that both largely compensate each other in the <span class="inline-formula">RO_<i>x</i></span> budget. These observations suggest the existence of a chemical mechanism that converts <span class="inline-formula">RO₂</span> to OH without the involvement of NO, increasing the <span class="inline-formula">RO₂</span> loss rate during the daytime from 5.3 to 7.4&thinsp;<span class="inline-formula">ppbv h⁻¹</span> on average. The photochemical net ozone production rate calculated from the reaction of <span class="inline-formula">HO₂</span> and <span class="inline-formula">RO₂</span> with NO yields a daily integrated amount of 102&thinsp;<span class="inline-formula">ppbv</span> ozone, with daily integrated <span class="inline-formula">RO_<i>x</i></span> primary sources being 22&thinsp;<span class="inline-formula">ppbv</span> in this campaign. The produced ozone can be attributed to the oxidation of measured (18&thinsp;%) and unmeasured (60&thinsp;%) hydrocarbons, formaldehyde (14&thinsp;%), and CO (8&thinsp;%). An even larger integrated net ozone production of 140&thinsp;<span class="inline-formula">ppbv</span> would be calculated from the oxidation rate of VOCs with OH if <span class="inline-formula">HO₂</span> and all <span class="inline-formula">RO₂</span> radicals react with NO. However, the unknown <span class="inline-formula">RO₂</span> loss (evident in the <span class="inline-formula">RO₂</span> budget) causes 30&thinsp;<span class="inline-formula">ppbv</span> less ozone production than would be expected from the VOC oxidation rate.</p>