Abstract: [Objective] The primary objective is to investigate the patterns of variation in atmospheric visibility and its affecting factors over Qiandao Lake National Forest Park, in order to obtain data for improving forest tourism and recreation.[Method]Based on monitoring data collected from December 1, 2015 to November 30, 2016, variation of atmospheric visibility and its correlations with the affecting factors (AQI, concentrations of PM₁₀, PM_2.5, SO₂, NO₂, CO, O₃, wind speed, temperature, dew point temperature, sea level pressure, relative air humidity) over Qiandao Lake National Forest Park were investigated.[Result]The annual average value of atmospheric visibility in Qiandao Lake National Forest Park was 12.84 km; other than PM_2.5, which exceeded the grade Ⅰ standards slightly, annual average concentrations of the remained air pollutants met the Chinese Ambient Air Quality Standards Grade Ⅰ, indicating a good air quality of Qiandao Lake National Forest Park. Atmospheric visibilities were better in summer and autumn, but worse in spring and winter; for diurnal variation, the visibility peaked at dusk (17:00) and showed the lowest value in the early morning (5:00). The ratings of atmospheric visibility were categorized by very bad (<4 km), bad (4-10 km), good (10-20 km) and very good (≥ 20 km), and its monthly statistics showed a distinct seasonal pattern; in spring and winter, the frequencies of very bad and bad ratings were high while those of good and very good were low. In summer and autumn, the frequencies of good and very good were high while those of very bad and bad ratings were almost none. Negative correlations between atmospheric visibility and the concentrations of PM₁₀, PM_2.5, SO₂, CO, NO₂, humidity as well as air pressure were found with correlation coefficients of -0.29, -0.33, -0.26, -0.16, -0.35, -0.42, -0.11, respectively, while positive correlations were detected between atmospheric visibility and temperature, wind speed and dew point temperature, the correlation coefficient were 0.39, 0.20, 0.19, respectively. K-means clustering showed the higher visibility in summer was consistent with higher temperature and wind speed as well as lower humidity and air pressure, the lower visibility in winter and spring was associated with low temperature, high humidity and high sea level pressure. The regression equation constructed by the (dry extinction coefficient) extinction coefficient and the AQI (air quality index) and the concentration of PM₁₀, PM_2.5, SO₂, CO, NO₂ were established (P<0.001). The result showed the contribution rates (R²) of AQI and the concentrations of PM₁₀ and PM_2.5 to atmospheric visibility were higher than the concentrations of other air pollutants, indicating higher impacts of AQI and the concentrations of PM₁₀ and PM_2.5 on visibility.[Conclusion]The atmospheric visibility in Qiandao Lake National Forest Park was high, indicating a good air quality of Qiandao Lake National Forest Park. The atmospheric visibility showed obvious seasonal and diurnal variation. High correlations between atmospheric visibility and AQI, concentrations of PM₁₀ and PM_2.5, temperature, wind speed and relative air humidity was found. The study of atmospheric visibility and its affecting factors in Qiandao Lake National Forest Park showed a fundamental influence of air quality on atmospheric visibility, and atmospheric visibility can be an effective indicator of air quality, except rain, fog and other special weathers.

Key words: forest park, atmospheric visibility, air pollutants, meteorological factors, regression equation