Comment on acp-2022-400

<strong class="journal-contentHeaderColor">Abstract.</strong> Daily air concentrations of inorganic nitrogen (<span class="inline-formula">N</span>) species, including gaseous <span class="inline-formula">HNO₃</span> and particulate-bound <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M3" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><mo>(</mo><mi mathvariant="normal">p</mi><mo>)</mo><msubsup><mi mathvariant="normal">NH</mi><mn mathvariant="normal">4</mn><mo>+</mo></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="37pt" height="15pt" class="svg-formula" dspmath="mathimg" md5hash="3c2158ed7289ae32d0adc93ca722cee7"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-22-14631-2022-ie00001.svg" width="37pt" height="15pt" src="acp-22-14631-2022-ie00001.png"/></svg:svg></span></span> and <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M4" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msubsup><mi mathvariant="normal">pNO</mi><mn mathvariant="normal">3</mn><mo>-</mo></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="31pt" height="16pt" class="svg-formula" dspmath="mathimg" md5hash="fd0105c3d3d0f3c3cdfb5e70ae97b01b"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-22-14631-2022-ie00002.svg" width="31pt" height="16pt" src="acp-22-14631-2022-ie00002.png"/></svg:svg></span></span>, and sulfur (<span class="inline-formula">S</span>) species, including <span class="inline-formula">SO₂</span> and <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M7" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msubsup><mi mathvariant="normal">pSO</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="35pt" height="17pt" class="svg-formula" dspmath="mathimg" md5hash="e519b2f49a0b298485293eca24eb1037"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-22-14631-2022-ie00003.svg" width="35pt" height="17pt" src="acp-22-14631-2022-ie00003.png"/></svg:svg></span></span>, and precipitation concentrations of <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M8" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msubsup><mi mathvariant="normal">NO</mi><mn mathvariant="normal">3</mn><mo>-</mo></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="25pt" height="16pt" class="svg-formula" dspmath="mathimg" md5hash="8a872e45f44a0fc3c08e466e371cfb3a"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-22-14631-2022-ie00004.svg" width="25pt" height="16pt" src="acp-22-14631-2022-ie00004.png"/></svg:svg></span></span>, <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M9" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msubsup><mi mathvariant="normal">NH</mi><mn mathvariant="normal">4</mn><mo>+</mo></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="24pt" height="15pt" class="svg-formula" dspmath="mathimg" md5hash="9641cdd414b305565815b5b604dabf23"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-22-14631-2022-ie00005.svg" width="24pt" height="15pt" src="acp-22-14631-2022-ie00005.png"/></svg:svg></span></span>, and <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M10" 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="8c898138530c760447165fe6cdc920bb"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-22-14631-2022-ie00006.svg" width="29pt" height="17pt" src="acp-22-14631-2022-ie00006.png"/></svg:svg></span></span>, have been routinely monitored by the Canadian Air and Precipitation Monitoring Network (CAPMoN) since 1983. Data at 15 rural sites from 2000–2018 were used to estimate dry and wet <span class="inline-formula">N</span> and <span class="inline-formula">S</span> deposition fluxes, which were then used to explore their spatiotemporal trends and assess ecosystem damage through a retrospective analysis of critical loads (CLs) exceedances. Total (dry <span class="inline-formula">+</span> wet) <span class="inline-formula">N</span> deposition ranged from 1.7–9.5 <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M15" display="inline" overflow="scroll" dspmath="mathml"><mrow class="unit"><mi mathvariant="normal">kg</mi><mspace linebreak="nobreak" width="0.125em"/><mi mathvariant="normal">N</mi><mspace linebreak="nobreak" width="0.125em"/><msup><mi mathvariant="normal">ha</mi><mrow><mo>-</mo><mn mathvariant="normal">1</mn></mrow></msup><mspace width="0.125em" linebreak="nobreak"/><msup><mi mathvariant="normal">yr</mi><mrow><mo>-</mo><mn mathvariant="normal">1</mn></mrow></msup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="68pt" height="15pt" class="svg-formula" dspmath="mathimg" md5hash="8159252732f421b4573c42dd7f30dc89"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-22-14631-2022-ie00007.svg" width="68pt" height="15pt" src="acp-22-14631-2022-ie00007.png"/></svg:svg></span></span> among the 15 sites, though dry deposition of <span class="inline-formula">NH₃</span> and some oxidized <span class="inline-formula">N</span> species were not included due to lack of monitoring data. Based on additional <span class="inline-formula">N</span> measurements in 2010 at one of the sites, annual total <span class="inline-formula">N</span> deposition may be underestimated by up to 32 %. Total <span class="inline-formula">N</span> deposition was dominated by wet <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">NO</mi><mn mathvariant="normal">3</mn><mo>-</mo></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="25pt" height="16pt" class="svg-formula" dspmath="mathimg" md5hash="8a28d0a06c83cc478fcb3953dfc5e6ea"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-22-14631-2022-ie00008.svg" width="25pt" height="16pt" src="acp-22-14631-2022-ie00008.png"/></svg:svg></span></span> and wet <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">NH</mi><mn mathvariant="normal">4</mn><mo>+</mo></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="24pt" height="15pt" class="svg-formula" dspmath="mathimg" md5hash="44642da34e3da1fffc83c720c465c894"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-22-14631-2022-ie00009.svg" width="24pt" height="15pt" src="acp-22-14631-2022-ie00009.png"/></svg:svg></span></span> deposition, which together comprised 71 %–95 %. Contributions to dry <span class="inline-formula">N</span> deposition were 40 %–74 % by <span class="inline-formula">HNO₃</span>, 11 %–40 % by <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M25" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msubsup><mi mathvariant="normal">pNH</mi><mn mathvariant="normal">4</mn><mo>+</mo></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="30pt" height="15pt" class="svg-formula" dspmath="mathimg" md5hash="42c8a81b640e5c5207ec2d76de191192"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-22-14631-2022-ie00010.svg" width="30pt" height="15pt" src="acp-22-14631-2022-ie00010.png"/></svg:svg></span></span>, and 5 %–25 % by <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M26" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msubsup><mi mathvariant="normal">pNO</mi><mn mathvariant="normal">3</mn><mo>-</mo></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="31pt" height="16pt" class="svg-formula" dspmath="mathimg" md5hash="c44bc829b1d2dd5dfebd4ff0afb4c92a"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-22-14631-2022-ie00011.svg" width="31pt" height="16pt" src="acp-22-14631-2022-ie00011.png"/></svg:svg></span></span>. Total <span class="inline-formula">S</span> deposition ranged from 1.3–8.5 <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M28" display="inline" overflow="scroll" dspmath="mathml"><mrow class="unit"><mi mathvariant="normal">kg</mi><mspace linebreak="nobreak" width="0.125em"/><mi mathvariant="normal">S</mi><mspace linebreak="nobreak" width="0.125em"/><msup><mi mathvariant="normal">ha</mi><mrow><mo>-</mo><mn mathvariant="normal">1</mn></mrow></msup><mspace linebreak="nobreak" width="0.125em"/><msup><mi mathvariant="normal">yr</mi><mrow><mo>-</mo><mn mathvariant="normal">1</mn></mrow></msup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="67pt" height="15pt" class="svg-formula" dspmath="mathimg" md5hash="db3a753ee79214b162c2d1348d11fbef"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-22-14631-2022-ie00012.svg" width="67pt" height="15pt" src="acp-22-14631-2022-ie00012.png"/></svg:svg></span></span> and was dominated by wet deposition of <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M29" 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="b70d5f4b00154e4d755a80f8e49b3806"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-22-14631-2022-ie00013.svg" width="29pt" height="17pt" src="acp-22-14631-2022-ie00013.png"/></svg:svg></span></span> and dry deposition of <span class="inline-formula">SO₂</span>. Relative percentages of wet and dry <span class="inline-formula">S</span> deposition were 45 %–89 % and 11 %–55 %, respectively. Acidic ion fluxes were greatest in southeastern Canada and were comparable among the west coast, prairie, remote, and eastern Canadian sites. Oxidized <span class="inline-formula">N</span> (dry <span class="inline-formula">HNO₃</span>, dry <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M34" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msubsup><mi mathvariant="normal">pNO</mi><mn mathvariant="normal">3</mn><mo>-</mo></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="31pt" height="16pt" class="svg-formula" dspmath="mathimg" md5hash="38cc5302f6d406a812f60ef70422f3c3"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-22-14631-2022-ie00014.svg" width="31pt" height="16pt" src="acp-22-14631-2022-ie00014.png"/></svg:svg></span></span>, wet <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M35" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msubsup><mi mathvariant="normal">NO</mi><mn mathvariant="normal">3</mn><mo>-</mo></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="25pt" height="16pt" class="svg-formula" dspmath="mathimg" md5hash="3408ea7bd52ee01c6f48215a5b5364fb"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-22-14631-2022-ie00015.svg" width="25pt" height="16pt" src="acp-22-14631-2022-ie00015.png"/></svg:svg></span></span>) deposition was greater than that of reduced <span class="inline-formula">N</span> (dry <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M37" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msubsup><mi mathvariant="normal">pNH</mi><mn mathvariant="normal">4</mn><mo>+</mo></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="30pt" height="15pt" class="svg-formula" dspmath="mathimg" md5hash="f78a16639738545719f76f703267bab2"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-22-14631-2022-ie00016.svg" width="30pt" height="15pt" src="acp-22-14631-2022-ie00016.png"/></svg:svg></span></span>, wet <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M38" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msubsup><mi mathvariant="normal">NH</mi><mn mathvariant="normal">4</mn><mo>+</mo></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="24pt" height="15pt" class="svg-formula" dspmath="mathimg" md5hash="dcfda7f7e9e6d10ee21653c7e18f979d"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-22-14631-2022-ie00017.svg" width="24pt" height="15pt" src="acp-22-14631-2022-ie00017.png"/></svg:svg></span></span>) in the early 2000s. In 2014–2018, reduced <span class="inline-formula">N</span> deposition surpassed that of oxidized <span class="inline-formula">N</span> in southeastern Canada. Total <span class="inline-formula">N</span> and <span class="inline-formula">S</span> deposition decreased significantly at a rate of <span class="inline-formula">−</span>0.03 to <span class="inline-formula">−</span>0.25 <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M45" display="inline" overflow="scroll" dspmath="mathml"><mrow class="unit"><mi mathvariant="normal">kg</mi><mspace width="0.125em" linebreak="nobreak"/><mi mathvariant="normal">N</mi><mspace linebreak="nobreak" width="0.125em"/><msup><mi mathvariant="normal">ha</mi><mrow><mo>-</mo><mn mathvariant="normal">1</mn></mrow></msup><mspace width="0.125em" linebreak="nobreak"/><msup><mi mathvariant="normal">yr</mi><mrow><mo>-</mo><mn mathvariant="normal">1</mn></mrow></msup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="68pt" height="15pt" class="svg-formula" dspmath="mathimg" md5hash="6082b3c1b0abf227bd451ea4b431411a"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-22-14631-2022-ie00018.svg" width="68pt" height="15pt" src="acp-22-14631-2022-ie00018.png"/></svg:svg></span></span> (<span class="inline-formula">−</span>1.1 % yr<span class="inline-formula">^−1</span> to <span class="inline-formula">−</span>3.3 <span class="inline-formula">% yr^−1</span>) and <span class="inline-formula">−</span>0.08 to <span class="inline-formula">−</span>0.66 <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M52" display="inline" overflow="scroll" dspmath="mathml"><mrow class="unit"><mi mathvariant="normal">kg</mi><mspace linebreak="nobreak" width="0.125em"/><mi mathvariant="normal">S</mi><mspace width="0.125em" linebreak="nobreak"/><msup><mi mathvariant="normal">ha</mi><mrow><mo>-</mo><mn mathvariant="normal">1</mn></mrow></msup><mspace width="0.125em" linebreak="nobreak"/><msup><mi mathvariant="normal">yr</mi><mrow><mo>-</mo><mn mathvariant="normal">1</mn></mrow></msup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="67pt" height="15pt" class="svg-formula" dspmath="mathimg" md5hash="41d8c5febce87ced38b47973f962892d"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-22-14631-2022-ie00019.svg" width="67pt" height="15pt" src="acp-22-14631-2022-ie00019.png"/></svg:svg></span></span> (<span class="inline-formula">−</span>3.5 % yr<span class="inline-formula">^−1</span> to <span class="inline-formula">−</span>6.6 <span class="inline-formula">% yr^−1</span>), respectively, among the sites. The weak declining trend in total <span class="inline-formula">N</span> deposition at the west coast site was consistent with the slower decline in <span class="inline-formula">NO_<i>x</i></span> emissions in western Canada. Reductions in total <span class="inline-formula">N</span> deposition were driven by its oxidized form as trends in reduced <span class="inline-formula">N</span> were non-significant. As a result, reduced <span class="inline-formula">N</span> contributions to total <span class="inline-formula">N</span> deposition increased on average from 42 % in 2000–2004 to 53 % in 2014–2018. Anthropogenic <span class="inline-formula">NO_<i>x</i></span> and <span class="inline-formula">SO₂</span> emissions reductions in both eastern Canada and eastern US were highly effective in reducing total oxidized <span class="inline-formula">N</span> and total <span class="inline-formula">S</span> deposition, respectively, in eastern Canada. Acidic deposition exceeded terrestrial CL at 5 of the 14 sites and aquatic CL at 2 of the 5 sites in the early 2000s. However, exceedances have been trending downwards and acidic deposition fluxes were mostly near or below CL after 2012 for the subset of sites assessed, which support recovery from historical acidification. Further assessments of CL exceedances are required in other Canadian regions susceptible to acidification and affected by elevated or increasing <span class="inline-formula">N</span> and <span class="inline-formula">S</span> emissions.

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