Production of HONO from NO 2 uptake on illuminated TiO 2 aerosol particles and following the illumination of mixed TiO 2 ∕ammonium nitrate particles

<p>The rate of production of HONO from illuminated TiO<span class="inline-formula">₂</span> aerosols in the presence of NO<span class="inline-formula">₂</span> was measured using an aerosol flow tube system coupled to a photo-fragmentation laser-induced fluorescence detection apparatus. The reactive uptake coefficient of NO<span class="inline-formula">₂</span> to form HONO, <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M7" display="inline" overflow="scroll" dspmath="mathml"><mrow><msub><mi mathvariant="italic">γ</mi><mrow class="chem"><msub><mi mathvariant="normal">NO</mi><mn mathvariant="normal">2</mn></msub><mo>→</mo><mi mathvariant="normal">HONO</mi></mrow></msub></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="56pt" height="12pt" class="svg-formula" dspmath="mathimg" md5hash="e965a9ac0719984a98bd7c86944994b2"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-21-5755-2021-ie00003.svg" width="56pt" height="12pt" src="acp-21-5755-2021-ie00003.png"/></svg:svg></span></span>, was determined for NO<span class="inline-formula">₂</span> mixing ratios in the range 34–400 ppb, with <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M9" display="inline" overflow="scroll" dspmath="mathml"><mrow><msub><mi mathvariant="italic">γ</mi><mrow class="chem"><msub><mi mathvariant="normal">NO</mi><mn mathvariant="normal">2</mn></msub><mo>→</mo><mi mathvariant="normal">HONO</mi></mrow></msub></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="56pt" height="12pt" class="svg-formula" dspmath="mathimg" md5hash="fd26b5e871b886faf9e57384d30def4e"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-21-5755-2021-ie00004.svg" width="56pt" height="12pt" src="acp-21-5755-2021-ie00004.png"/></svg:svg></span></span> spanning the range (9.97 <span class="inline-formula">±</span> 3.52) <span class="inline-formula">×</span> 10<span class="inline-formula">⁻⁶</span> to (1.26 <span class="inline-formula">±</span> 0.17) <span class="inline-formula">×</span> 10<span class="inline-formula">⁻⁴</span> at a relative humidity of 15 <span class="inline-formula">±</span> 1 % and for a lamp photon flux of (1.63 <span class="inline-formula">±</span> 0.09) <span class="inline-formula">×10¹⁶</span> photons cm<span class="inline-formula">⁻²</span> s<span class="inline-formula">⁻¹</span> (integrated between 290 and 400 nm), which is similar to midday ambient actinic flux values. <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M21" display="inline" overflow="scroll" dspmath="mathml"><mrow><msub><mi mathvariant="italic">γ</mi><mrow class="chem"><msub><mi mathvariant="normal">NO</mi><mn mathvariant="normal">2</mn></msub><mo>→</mo><mi mathvariant="normal">HONO</mi></mrow></msub></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="56pt" height="12pt" class="svg-formula" dspmath="mathimg" md5hash="e4944b59f202921cae73615fc76d94d6"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-21-5755-2021-ie00005.svg" width="56pt" height="12pt" src="acp-21-5755-2021-ie00005.png"/></svg:svg></span></span> increased as a function of NO<span class="inline-formula">₂</span> mixing ratio at low NO<span class="inline-formula">₂</span> before peaking at (1.26 <span class="inline-formula">±</span> 0.17) <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M25" display="inline" overflow="scroll" dspmath="mathml"><mrow><mo>×</mo><msup><mn mathvariant="normal">10</mn><mrow><mo>-</mo><mn mathvariant="normal">4</mn></mrow></msup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="32pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="ae2ff01d6ba96c5714e9e8f527947715"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-21-5755-2021-ie00006.svg" width="32pt" height="14pt" src="acp-21-5755-2021-ie00006.png"/></svg:svg></span></span> at <span class="inline-formula">∼</span> 51 ppb NO<span class="inline-formula">₂</span> and then sharply decreasing at higher NO<span class="inline-formula">₂</span> mixing ratios rather than levelling off, which would be indicative of surface saturation. The dependence of HONO production on relative humidity was also investigated, with a peak in production of HONO from TiO<span class="inline-formula">₂</span> aerosol surfaces found at <span class="inline-formula">∼</span> 25 % RH. Possible mechanisms consistent with the observed trends in both the HONO production and reactive uptake coefficient were investigated using a zero-dimensional kinetic box model. The modelling studies supported a mechanism for HONO production on the aerosol surface involving two molecules of NO<span class="inline-formula">₂</span>, as well as a surface HONO loss mechanism which is dependent upon NO<span class="inline-formula">₂</span>. In a separate experiment, significant production of HONO was observed from illumination of mixed <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M33" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><mi mathvariant="normal">nitrate</mi><mo>/</mo><msub><mi mathvariant="normal">TiO</mi><mn mathvariant="normal">2</mn></msub></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="61pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="e4d3fcd02f9990fcc9c76af506564a9c"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-21-5755-2021-ie00007.svg" width="61pt" height="14pt" src="acp-21-5755-2021-ie00007.png"/></svg:svg></span></span> aerosols in the absence of NO<span class="inline-formula">₂</span>. However, no production of HONO was seen from the illumination of nitrate aerosols alone. The rate of production of HONO observed from mixed <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M35" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><mi mathvariant="normal">nitrate</mi><mo>/</mo><msub><mi mathvariant="normal">TiO</mi><mn mathvariant="normal">2</mn></msub></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="61pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="90e10df1e58a7f6aa9187f016c0f5d64"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-21-5755-2021-ie00008.svg" width="61pt" height="14pt" src="acp-21-5755-2021-ie00008.png"/></svg:svg></span></span> aerosols was scaled to ambient conditions found at the Cape Verde Atmospheric Observatory (CVAO) in the remote tropical marine boundary layer. The rate of HONO production from aerosol particulate nitrate photolysis containing a photocatalyst was found to be similar to the missing HONO production rate necessary to reproduce observed concentrations of HONO at CVAO. These results provide evidence that particulate nitrate photolysis may have a significant impact on the production of HONO and hence NO<span class="inline-formula">_<i>x</i></span> in the marine boundary layer where mixed aerosols containing nitrate and a photocatalytic species such as TiO<span class="inline-formula">₂</span>, as found in dust, are present.</p>

Abstract licence: CC BY

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