Consistency and Discrepancy between Visibility and PM(2.5) Measurements: Potential Application of Visibility Observation to Air Quality Study

High-quality measurements of air quality are the highest priority for understanding widespread air pollution. Visibility has been widely suggested to be a good alternative to PM(2.5) concentration as a measure. In this study, the similarities and differences between visibility and PM(2.5) measurements in China are checked and the results reveal the potential application of visibility observation to the study of air quality. Based on the quality-controlled PM(2.5) and visibility data from 2016 to 2018, the nonparametric Spearman correlation coefficient (ρ) values between stations for PM(2.5) and visibility-derived surface extinction coefficient (b(ext)) decrease as the station distance (R) increases. Some relatively low ρ values (<0.4) occur in regions characterized by the lowest (background) levels of PM(2.5) and b(ext) values, for example, the Tibetan and Yungui Plateau. The relatively lower ρ for b(ext) compared to PM(2.5) is probably caused by the predefined maximum threshold of visibility measurements (generally 30 km). A significant correlation between PM(2.5) and b(ext) is derived in most stations and relatively larger ρ values are evident in eastern China (Northeast China excluded) and in winter (the national median ρ is 0.67). The abrupt changes in specific mass extinction efficiency (α(ext)) imply a potentially large influence of alternation of visibility sensors or recalibrations on visibility measurements. The b(ext) data are thereafter corrected by comparison to the reference measurements at the adjacent stations, which leads to a three-year quality assured of visibility and b(ext) datasets.