<p><strong>Abstract.</strong> Comprehensive chlorine heterogeneous chemistry is incorporated into the Community Multiscale Air Quality (CMAQ) model to evaluate the impact of chlorine-related heterogeneous reaction on diurnal and nocturnal nitrate formation and quantify the nitrate formation from gas-to-particle partitioning of <span class="inline-formula">HNO₃</span> and from different heterogeneous pathways. The results show that these heterogeneous reactions increase the

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... <span class="inline-formula">Cl₂</span> and <span class="inline-formula">ClNO₂</span> level (<span class="inline-formula">∼ 100</span>&amp;thinsp;%), which further affects the nitrate formation. Sensitivity analyses of uptake coefficients show that the empirical uptake coefficient for the <span class="inline-formula">O₃</span> heterogeneous reaction with chlorinated particles may lead to the large uncertainties in the predicted <span class="inline-formula">Cl₂</span> and nitrate concentrations. The <span class="inline-formula">N₂O₅</span> uptake coefficient with particulate <span class="inline-formula">Cl⁻</span> concentration dependence performs better in capturing the concentration of <span class="inline-formula">ClNO₂</span> and nocturnal nitrate concentration. The reaction of OH and <span class="inline-formula">NO₂</span> in the daytime increases the nitrate by <span class="inline-formula">∼15</span>&amp;thinsp;% when the heterogeneous chlorine chemistry is incorporated, resulting in more nitrate formation from <span class="inline-formula">HNO₃</span> gas-to-particle partitioning. By contrast, the contribution of the heterogeneous reaction of <span class="inline-formula">N₂O₅</span> to nitrate concentrations decreases by about 27&amp;thinsp;% in the nighttime, when its reactions with chlorinated particles are considered. However, the generated gas-phase <span class="inline-formula">ClNO₂</span> from the heterogeneous reaction of <span class="inline-formula">N₂O₅</span> and chlorine-containing particles further reacts with the particle surface to increase the nitrate by 6&amp;thinsp;%. In general, this study highlights the potential of significant underestimation of daytime concentrations and overestimation of nighttime nitrate concentrations for chemical transport models without proper chlorine chemistry in the gas and particle phases.</p>