The Government of the Republic of Korea has enforced strict regulations to improve air quality since the early 2000s. The regulations are mainly focused on reducing vehicle emissions in the Seoul metropolitan region by conforming to the European emissions standards, replacing diesel buses with compressed natural gas buses, incentives for installing diesel exhaust after-treatment systems and buying eco-friendly vehicles. There was a 20% reduction in 2010s compared to the 2000s in terms of the mean concentrations of particulate matter (PM) with mean aerodynamic diameters of ≤10 μm (PM₁₀) and 2.5 μm (PM_2.5) during cold seasons (October through following February) although the decrease may not be entirely attributable to the regulations. The influences of other external factors such as transboundary transport of air pollutants and regional meteorological conditions cannot be neglected.

This study analyzes the change in the diurnal variations—two maxima at around 11 and 22 local time (LT) and two minima at around 6 and 16 LT—of air pollutant concentrations that may be closely related to the regulatory action in reducing local vehicle emissions. A reduction of over 40% for the amplitude of two PM concentrations at 11 LT was revealed when values from the 2010s were compared to those from the 2000s. There was a similar reduction for other vehicle exhaust gases including nitrogen dioxide, carbon monoxide, and sulfur dioxide. Additional analysis of the long-term trend in mixed layer height and surface wind speed showed that the change in environmental conditions in the diurnal time scale was either negligible or unfavorable for conditions which reduce PM concentrations. This study suggests that the mean concentration estimation may underestimate the regulatory effects, but the approach based on the diurnal variation may be a more accurate indicator.

自 2000 年代初以来，大韩民国政府一直执行严格的法规以改善空气质量。这些法规主要侧重于通过符合欧洲排放标准、用压缩天然气公交车取代柴油公交车、鼓励安装柴油尾气后处理系统和购买环保车辆来减少首尔大都市地区的车辆排放。与 2000 年代相比，2010 年代平均空气动力学直径≤10 微米（PM ₁₀）和 2.5 微米（PM _2.5）的颗粒物 (PM) 浓度降低了 20%) 在寒冷季节（10 月至次年 2 月），虽然下降可能并不完全归因于法规。大气污染物越境迁移、区域气象条件等其他外部因素的影响也不容忽视。

本研究分析了空气污染物浓度的日变化变化——两个最大值在当地时间 11 点和 22 点左右，两个最小值在 6 点和 16 点左右——可能与减少当地车辆排放的监管行动密切相关. 当将 2010 年代的值与 2000 年代的值进行比较时，显示 11 LT 时两种 PM 浓度的幅度降低了 40% 以上。其他车辆废气（包括二氧化氮、一氧化碳和二氧化硫）也有类似的减少。对混合层高度和地表风速长期趋势的进一步分析表明，环境条件在昼夜时间尺度上的变化对于降低 PM 浓度的条件要么可以忽略不计，要么不利。