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<p><span id="page2148"/>High loadings of nitrate (<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M3" 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="a186e28964d6ae507e65dbc91f8b1f71"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="essd-13-2147-2021-ie00004.svg" width="25pt" height="16pt" src="essd-13-2147-2021-ie00004.png"/></svg:svg></span></span>) in the aerosol over China significantly exacerbate the air quality and pose a great threat to ecosystem safety through dry–wet deposition. Unfortunately, limited ground-level observation data make it challenging to fully reflect the spatial pattern of <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">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="e16cba38499a6a16cb1a10e488ec56da"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="essd-13-2147-2021-ie00005.svg" width="25pt" height="16pt" src="essd-13-2147-2021-ie00005.png"/></svg:svg></span></span> levels across China. Until now, long-term monthly particulate <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M5" 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="91b2e19ca239409a7665981c17575147"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="essd-13-2147-2021-ie00006.svg" width="25pt" height="16pt" src="essd-13-2147-2021-ie00006.png"/></svg:svg></span></span> datasets at a high resolution were still missing, which restricted the assessment of human health and ecosystem safety. Therefore, a unique monthly <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M6" 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="a33a7d42b70ca1fe513ac92c5832eec2"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="essd-13-2147-2021-ie00007.svg" width="25pt" height="16pt" src="essd-13-2147-2021-ie00007.png"/></svg:svg></span></span> dataset at 0.25<span class="inline-formula">^∘</span> resolution over China during 2005–2015 was developed by assimilating surface observations, satellite products, meteorological data, land use types and other covariates using an ensemble model combining random forest (RF), gradient-boosting decision tree (GBDT), and extreme gradient-boosting (XGBoost) methods. The new developed product featured an excellent cross-validation <span class="inline-formula"><i>R</i>²</span> value (0.78) and relatively lower root-mean-square error (RMSE: 1.19 <span class="inline-formula">µg N m⁻³</span>) and mean absolute error (MAE: 0.81 <span class="inline-formula">µg N m⁻³</span>). Besides, the dataset also exhibited relatively robust performance at the spatial and temporal scales. Moreover, the dataset displayed good agreement with (<span class="inline-formula"><i>R</i>²=0.85</span>, <span class="inline-formula">RMSE=0.74</span> <span class="inline-formula">µg N m⁻³</span>, and <span class="inline-formula">MAE=0.55</span> <span class="inline-formula">µg N m⁻³</span>) some unlearned data collected from previous studies. The spatiotemporal variations in the developed product were also shown. The estimated <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M16" 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="48a6d5724cc017ced9c974ab9a81c03a"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="essd-13-2147-2021-ie00008.svg" width="25pt" height="16pt" src="essd-13-2147-2021-ie00008.png"/></svg:svg></span></span> concentration showed the highest value in the North China Plain (NCP) (<span class="inline-formula">3.55±1.25</span> <span class="inline-formula">µg N m⁻³</span>); followed by the Yangtze River Delta (YRD) (<span class="inline-formula">2.56±1.12</span> <span class="inline-formula">µg N m⁻³</span>), Pearl River Delta (PRD) (<span class="inline-formula">1.68±0.81</span> <span class="inline-formula">µg N m⁻³</span>), and Sichuan Basin (<span class="inline-formula">1.53±0.63</span> <span class="inline-formula">µg N m⁻³</span>), and the lowest one in the Tibetan Plateau (<span class="inline-formula">0.42±0.25</span> <span class="inline-formula">µg N m⁻³</span>). The higher ambient <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M27" 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="54a63b90f99919b7f33388d68cde2f58"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="essd-13-2147-2021-ie00009.svg" width="25pt" height="16pt" src="essd-13-2147-2021-ie00009.png"/></svg:svg></span></span> concentrations in the NCP, YRD, and PRD were closely linked to the dense anthropogenic emissions. Apart from the intensive human activities, poor terrain condition might be a key factor for the serious <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M28" 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="da99f0b0265c157ce21f9580f34f8fd2"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="essd-13-2147-2021-ie00010.svg" width="25pt" height="16pt" src="essd-13-2147-2021-ie00010.png"/></svg:svg></span></span> pollution in the Sichuan Basin. The lowest ambient <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">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="f406d9210c9988b6f1f99fbfd13290fc"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="essd-13-2147-2021-ie00011.svg" width="25pt" height="16pt" src="essd-13-2147-2021-ie00011.png"/></svg:svg></span></span> concentration in the Tibetan Plateau was contributed by the scarce anthropogenic emission and favourable meteorological factors (e.g. high wind speed). In addition, the ambient <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M30" 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="cbd0aa2bda73584a7a23dafea6b5761c"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="essd-13-2147-2021-ie00012.svg" width="25pt" height="16pt" src="essd-13-2147-2021-ie00012.png"/></svg:svg></span></span> concentration showed a marked increasing tendency of 0.10 <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M31" display="inline" overflow="scroll" dspmath="mathml"><mrow class="unit"><mi mathvariant="normal">µ</mi><mi mathvariant="normal">g</mi><mspace linebreak="nobreak" width="0.125em"/><mi mathvariant="normal">N</mi><mspace linebreak="nobreak" width="0.125em"/><msup><mi mathvariant="normal">m</mi><mrow><mo>-</mo><mn mathvariant="normal">3</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="65pt" height="15pt" class="svg-formula" dspmath="mathimg" md5hash="cd87d4cff21e042dd45f82a3e3f6c046"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="essd-13-2147-2021-ie00013.svg" width="65pt" height="15pt" src="essd-13-2147-2021-ie00013.png"/></svg:svg></span></span> during 2005–2014 (<span class="inline-formula"><i>p</i>&lt;0.05</span>), while it decreased sharply from 2014 to 2015 at a rate of <span class="inline-formula">−0.40</span> <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M34" display="inline" overflow="scroll" dspmath="mathml"><mrow class="unit"><mi mathvariant="normal">µ</mi><mi mathvariant="normal">g</mi><mspace width="0.125em" linebreak="nobreak"/><mi mathvariant="normal">N</mi><mspace width="0.125em" linebreak="nobreak"/><msup><mi mathvariant="normal">m</mi><mrow><mo>-</mo><mn mathvariant="normal">3</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="65pt" height="15pt" class="svg-formula" dspmath="mathimg" md5hash="f4285f72b01c6cc72cfbb9deae252f2e"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="essd-13-2147-2021-ie00014.svg" width="65pt" height="15pt" src="essd-13-2147-2021-ie00014.png"/></svg:svg></span></span> (<span class="inline-formula"><i>p</i>&lt;0.05</span>). The ambient <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M36" 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="4d13138f69382cea7eca33a7bac51ba9"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="essd-13-2147-2021-ie00015.svg" width="25pt" height="16pt" src="essd-13-2147-2021-ie00015.png"/></svg:svg></span></span> levels in Beijing–Tianjin–Hebei (BTH), YRD, and PRD displayed gradual increases at a rate of 0.20, 0.11, and 0.05 <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M37" display="inline" overflow="scroll" dspmath="mathml"><mrow class="unit"><mi mathvariant="normal">µ</mi><mi mathvariant="normal">g</mi><mspace linebreak="nobreak" width="0.125em"/><mi mathvariant="normal">N</mi><mspace linebreak="nobreak" width="0.125em"/><msup><mi mathvariant="normal">m</mi><mrow><mo>-</mo><mn mathvariant="normal">3</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="65pt" height="15pt" class="svg-formula" dspmath="mathimg" md5hash="ab9d013c508277983daa9c60aa504d33"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="essd-13-2147-2021-ie00016.svg" width="65pt" height="15pt" src="essd-13-2147-2021-ie00016.png"/></svg:svg></span></span> (<span class="inline-formula"><i>p</i>&lt;0.05</span>) during 2005–2013, respectively. The gradual increases in <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M39" 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="0ed2522f87147883b53f48851745fd62"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="essd-13-2147-2021-ie00017.svg" width="25pt" height="16pt" src="essd-13-2147-2021-ie00017.png"/></svg:svg></span></span> concentrations in these regions from 2005 to 2013 were due to the fact that the emission reduction measures during this period focused on the reduction of <span class="inline-formula">SO₂</span> emission rather than <span class="inline-formula">NO_<i>x</i></span> emission and the rapid increase in energy consumption. Afterwards, the government further strengthened these emission reduction measures and thus caused the dramatic decreases in <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M42" 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="742f2a785243506db9e8d8e9964694e9"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="essd-13-2147-2021-ie00018.svg" width="25pt" height="16pt" src="essd-13-2147-2021-ie00018.png"/></svg:svg></span></span> concentrations in these regions from 2013 to 2015 (<span class="inline-formula"><i>p</i>&lt;0.05</span>). The long-term <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M44" 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="6c9aef46e63985e949f52ba810262b4b"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="essd-13-2147-2021-ie00019.svg" width="25pt" height="16pt" src="essd-13-2147-2021-ie00019.png"/></svg:svg></span></span> dataset over China could greatly deepen the knowledge about the impacts of emission reduction measures on air quality improvement. The monthly particulate <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M45" 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="afc29ccf929971927e195b135fd5e45c"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="essd-13-2147-2021-ie00020.svg" width="25pt" height="16pt" src="essd-13-2147-2021-ie00020.png"/></svg:svg></span></span> levels over China during 2005–2015 are open access at <a href="https://doi.org/10.5281/zenodo.3988307">https://doi.org/10.5281/zenodo.3988307</a> (Li et al., 2020c).</p>