Characterization of visibility and its affecting factors over Nanjing, China

Visibility, Air Pollution Index (API) and meteorological parameters over Nanjing during 2004 are analyzed using multiple statistic methods to study the characterization of visibility and relevant affecting factors. The mean value of visibility during the study year is 8.8 km, with 57.9% of daily average less than 10 km. Annual average visibility exhibits a significant diurnal variation with minimum of 6.6 km at 07:00 Local Time (LT) and maximum of 10.7 km at 16:00 LT. Seasonal variations is weak: 9.2 km in spring (March–May), 9.8 km in summer (June–August), 8.2 km in autumn (September–November), and 8.1 km in winter (December–February), respectively. No significant weekend effect in visibility is found. Moreover, wavelet analysis reveals the significant periodic variations in visibility, with major periods of synoptic scale (2–8 days), and quasi-two-week scale (8–17 days). Using a high pass filtering technique to eliminate the seasonal trend, correlation analysis and principal component analysis (PCA) are carried out to investigate the relations between visibility and relevant factors and to understand the dominated processes/causes to the synoptic changes in visibility. Our results highlight the significant impact of synoptic weather on air pollution and visibility in Nanjing.

Research Highlights
► The visibility over Nanjing is studied by using various statistical tools.
► Visibility in Nanjing is rather poor compared with other megacities in China.
► Significant periodic variations of visibility are found.
► Particulate matters and meteorological elements will contribute to visibility.
► Synoptic scale processes have obvious impact on visibility.