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<p>The Weather Research and Forecasting model data assimilation (WRFDA) system, initially designed for meteorological data assimilation, is extended for aerosol data assimilation for the WRF model coupled with chemistry (WRF-Chem). An interface between WRF-Chem and WRFDA is built for the Regional Atmospheric Chemistry Mechanism (RACM) chemistry and the Modal Aerosol Dynamics Model for Europe (MADE) coupled with the Volatility Basis Set (VBS) aerosol schemes. This article describes the implementation of the new interface for assimilating <span class="inline-formula">PM_2.5</span> and <span class="inline-formula">PM₁₀</span> as well as four gas species (<span class="inline-formula">SO₂</span>, <span class="inline-formula">NO₂</span>, <span class="inline-formula">O₃</span>, and CO) on the ground. The effects of aerosol data assimilation are briefly examined through a month-long case study during the Korea–United States Air Quality (KORUS-AQ) period. It is demonstrated that the improved chemical initial conditions through the 3D-Var analysis can lead to consistent forecast improvements up to 26 %, reducing systematic bias errors in surface <span class="inline-formula">PM_2.5</span> (<span class="inline-formula">PM₁₀</span>) concentrations to 0.0 (<span class="inline-formula">−</span>1.9) <span class="inline-formula">µg m⁻³</span> over South Korea for 24 <span class="inline-formula">h</span>.</p>