Different physicochemical behaviors of nitrate and ammonium during transport: a case study on Mt. Hua, China

<p>To understand the chemical evolution of aerosols in the transport process, the chemistry of PM<span class="inline-formula">_2.5</span> and nitrogen isotope compositions on the mountainside of Mt. Hua (<span class="inline-formula">∼1120</span> m above sea level, a.s.l.) in inland China during the 2016 summertime were investigated and compared with parallel observations collected at surface sampling site (<span class="inline-formula">∼400</span> m a.s.l.). The PM<span class="inline-formula">_2.5</span> exhibited a high level at the mountain foot site (MF; average <span class="inline-formula">76.0±44.1</span> <span class="inline-formula">µg m⁻³</span>) and could be transported aloft by anabatic valley winds, leading to the gradual accumulation of daytime PM<span class="inline-formula">_2.5</span> with a noon peak at the mountainside sampling site (MS). As the predominant ion species, sulfate exhibited nearly identical mass concentrations at both sites, but its PM<span class="inline-formula">_2.5</span> mass fraction was moderately enhanced by <span class="inline-formula">∼4</span> % at the MS site. The ammonium variations were similar to the sulfate variations, the chemical forms of both of which mainly existed as ammonium bisulfate (<span class="inline-formula">NH₄HSO₄</span>) and ammonium sulfate (<span class="inline-formula">(NH₄)₂SO₄</span>) at the MF and MS sites, respectively. Unlike sulfate and ammonium, nitrate mainly existed as ammonium nitrate (<span class="inline-formula">NH₄NO₃</span>) in fine particles and exhibited decreasing mass concentration and proportion trends with increasing elevation. This finding was ascribed to <span class="inline-formula">NH₄NO₃</span> volatilization, in which gaseous <span class="inline-formula">HNO₃</span> from semi-volatile <span class="inline-formula">NH₄NO₃</span> subsequently reacted with dust particles to form nonvolatile salts, resulting in significant nitrate shifts from fine particles into coarse particles. Such scavenging of fine-particle nitrate led to an enrichment in the daytime <span class="inline-formula">¹⁵N</span> of nitrate at the MS site compared with to the MF site. In contrast to nitrate, at the MS site, the <span class="inline-formula">¹⁵N</span> in ammonium depleted during the daytime. Considering the lack of any significant change in ammonia (<span class="inline-formula">NH₃</span>) sources during the vertical transport process, this <span class="inline-formula">¹⁵N</span> depletion in ammonium was mainly the result of unidirectional reactions, indicating that additional <span class="inline-formula">NH₃</span> would partition into particulate phases and further neutralize <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M21" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msubsup><mi mathvariant="normal">HSO</mi><mn mathvariant="normal">4</mn><mo>-</mo></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="33pt" height="16pt" class="svg-formula" dspmath="mathimg" md5hash="199dd3dfc9db731a72551032f2dfcb05"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-22-15621-2022-ie00001.svg" width="33pt" height="16pt" src="acp-22-15621-2022-ie00001.png"/></svg:svg></span></span> to form <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M22" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msubsup><mi mathvariant="normal">SO</mi><mn mathvariant="normal">4</mn><mrow><mn mathvariant="normal">2</mn><mo>-</mo></mrow></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="29pt" height="17pt" class="svg-formula" dspmath="mathimg" md5hash="47e678c4680581e78f7a83f3b1df9ebc"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-22-15621-2022-ie00002.svg" width="29pt" height="17pt" src="acp-22-15621-2022-ie00002.png"/></svg:svg></span></span>. This process would reduce the aerosol acidity, with a higher pH (<span class="inline-formula">3.4±2.2</span>) at the MS site and lower ones (<span class="inline-formula">2.9±2.0</span>) at the MF site. Our work provides more insight into physicochemical behaviors of semi-volatile nitrate and ammonium, which will facilitate the improvement in the model for a better simulation of aerosol composition and properties.</p>