Significantly mitigating PM_2.5 pollution level via reduction of NO_x emission during wintertime

Despite considerable decreases in fine particulate matter (PM_2.5) in Chinese megacities over the past decade, many second- and third-tier cities that distribute abundant industrial enterprises are still facing great challenges for PM_2.5 further reduction under the recent policy background of eliminating heavily-polluted weather. In view of core effects of NO_x on PM_2.5, the deeper reductions of NO_x in these cities are expected to break the plateau of PM_2.5 decline, however, the link between NO_x emission and PM_2.5 mass loading is currently lacking. Herein, we progressively construct an evaluation system for PM_2.5 productions based on daily NO_x emissions in a typical industrial city (Jiyuan), considering a sequence of nested parameters involving evolutions of NO₂ into nitric acid and then nitrate, and contributions of nitrate to PM_2.5. The evaluation system was subsequently validated to better reproduce real increasing processes for PM_2.5 pollution based on 19 pollution cases, with root mean square errors of 19.2 ± 16.4 %, suggesting the feasibility of developing NO_x emission indicators linked to goals of mitigating atmospheric PM_2.5. Additionally, further comparative results reveal that currently high NO_x emissions in this industrial city severely hinder the achievement of atmospheric PM_2.5 environmental capacity targets, especially in the scenarios of high initial PM_2.5 level, low planetary boundary layer height and long pollution duration. It is anticipated that these methodologies and findings would supply guidelines for further regional PM_2.5 mitigation, in which source-oriented NO_x indicators could also provide some orientations for industrial cleaner production such as denitrification and low nitrogen combustion.