Find in Library

<p>The anthropogenic emission of sulfur dioxide (<span class="inline-formula">SO₂</span>) over China has significantly declined as a consequence of the clean air actions. In this study, we have developed a new emission inversion system based on a four-dimensional local ensemble transform Kalman filter (4D-LETKF) and the Weather Research and Forecasting model coupled with Chemistry (WRF-Chem) to dynamically update the <span class="inline-formula">SO₂</span> emission grid by grid over China by assimilating the ground-based hourly <span class="inline-formula">SO₂</span> observations. Sensitivity tests for the assimilation system have been conducted firstly to tune four system parameters: ensemble size, horizontal and temporal localization lengths, and perturbation size. Our results reveal that the same random perturbation factors used throughout the whole model grids with assimilating observations within about 180 km can efficiently optimize the <span class="inline-formula">SO₂</span> emission, whereas the ensemble size has only little effect. The temporal localization by assimilating only the subsequent hourly observations can reveal the diurnal variation of the <span class="inline-formula">SO₂</span> emission, which is better than updating the magnitude of <span class="inline-formula">SO₂</span> emission every 12 h by assimilating all the observations within the 12 h window. The inverted <span class="inline-formula">SO₂</span> emission over China in November 2016 has declined by an average of 49.4 % since 2010, which is well in agreement with the bottom-up estimation of 48.0 %. Larger reductions of <span class="inline-formula">SO₂</span> emission are found over the a priori higher source regions such as the Yangtze River Delta (YRD). The simulated <span class="inline-formula">SO₂</span> surface mass concentrations using two distinguished chemical reaction mechanisms are both much more comparable to the observations with the newly inverted <span class="inline-formula">SO₂</span> emission than those with the a priori emission. These indicate that the newly developed emission inversion system can efficiently update the <span class="inline-formula">SO₂</span> emissions based on the routine surface <span class="inline-formula">SO₂</span> observations. The reduced <span class="inline-formula">SO₂</span> emission induces the sulfate and PM<span class="inline-formula">_2.5</span> surface concentrations to decrease by up to 10 <span class="inline-formula">µ</span>g m<span class="inline-formula">⁻³</span> over central China.</p>