A new approach of the normalization relationship between PM2.5 and visibility and the theoretical threshold, a case in north China

Both PM2.5 and visibility are important physical parameters to measure the air pollution condition. This paper first provided a new normalization approach for the relationship between PM2.5 and visibility in heavy pollutant region, which realized a mathematical calculation for two different physical parameters based on non-dimensional data process (also called Min-Max normalization). The normalization relationship showed a power function (y = axb) with high correlation coefficients. The inflection points of the maximum power function curvature composed the theoretical threshold range of PM2.5 for improving or deteriorating the visibility. In the case city of north China, the threshold range of PM2.5 was 112–121 μg·m−3 corresponding to the visibility range 4.5–5.9 km. Within the threshold range, the mean concentration of main pollutant components was 106 μg·m−3. The mean extinction coefficient, bext of 743.5 Mm−1, was mainly contributed by organic matter (34%), ammonium nitrate (19%), ammonium sulfate (18%) and elemental carbon (13%). When PM2.5 was below the range, the visibility increased significantly with the slow decrease of PM2.5. When PM2.5 exceeded the range, the visibility reduced seriously due to the saturated extinction contribution of the main pollutants. The new normalization approach theoretically revealed the significance of threshold and how to improve visibility by controlling PM2.5 at different pollution stages.