Atmospheric measurements at Mt. Tai – Part I: HONO formation and its role in the oxidizing capacity of the upper boundary layer

<p>A comprehensive field campaign, with measurements of HONO and related parameters, was conducted in summer 2018 at the foot (150 m a.s.l.) and the summit (1534 m a.s.l.) of Mt. Tai (Shandong province, China). At the summit station, high HONO mixing ratios were observed (mean <span class="inline-formula">±</span> 1<span class="inline-formula"><i>σ</i></span>: 133 <span class="inline-formula">±</span> 106 pptv, maximum: 880 pptv), with a diurnal noontime peak (mean <span class="inline-formula">±</span> 1<span class="inline-formula"><i>σ</i></span>: 133 <span class="inline-formula">±</span> 72 pptv at 12:30 local time). Constraints on the kinetics of aerosol-derived HONO sources (NO<span class="inline-formula">₂</span> uptake on the aerosol surface and particulate nitrate photolysis) were performed and discussed, which enables a better understanding of the interaction of HONO and aerosols, especially in the polluted North China Plain. Various evidence of air mass transport from the ground to the summit level was provided. Furthermore, daytime HONO formation from different paths and its role in radical production were quantified and discussed.</p> <p>We found that the homogeneous reaction NO <span class="inline-formula">+</span> OH could only explain 8.0 % of the daytime HONO formation, resulting in strong unknown sources (<span class="inline-formula"><i>P</i>_un</span>). Campaigned-averaged <span class="inline-formula"><i>P</i>_un</span> was about 290 <span class="inline-formula">±</span> 280 pptv h<span class="inline-formula">⁻¹</span>, with a maximum of about 1800 pptv h<span class="inline-formula">⁻¹</span>. Aerosol-derived HONO formation mechanisms were not the major sources of <span class="inline-formula"><i>P</i>_un</span> at the summit station. Their contributions to daytime HONO formation varied from negligible to moderate (similar to NO <span class="inline-formula">+</span> OH), depending on the chemical kinetic parameters used. Coupled with sensitivity tests on the kinetic parameters used, the NO<span class="inline-formula">₂</span> uptake on the aerosol surface and particulate nitrate photolysis contributed 1.5 %–19 % and 0.6 %–9.6 % of the observed <span class="inline-formula"><i>P</i>_un</span>, respectively. Based on synchronous measurements at the foot and the summit station, an amount of field evidence was proposed to support the finding that the remaining majority (70 %–98 %) of <span class="inline-formula"><i>P</i>_un</span> was dominated by the rapid vertical transport from the ground to the summit level and heterogeneous formation on the mountain surfaces during transport.</p> <p>HONO photolysis at the summit level initialized daytime photochemistry and still represented an essential OH source in the daytime, with a contribution of about one-quarter of O<span class="inline-formula">₃</span>. We provided evidence that ground-derived HONO played a significant role in the oxidizing capacity of the upper boundary layer through the enhanced vertical air mass exchange driven by mountain winds. The follow-up impacts should be considered in regional chemistry transport models.</p>