Abstract
Fine particle (PM2.5) mass concentration and relative humidity (RH) are the primary factors influencing atmospheric visibility. There are some studies focused on the complex, nonlinear relationships among visibility, PM2.5 concentration, and RH. However, the relative contribution of the two factors to visibility degradation, especially for different aerosol types, is difficult to quantify. In this study, the normalized forward sensitivity index method for identifying the dominant factors of visibility was used on the basis of the sensitivity of visibility to PM2.5 and RH changes. The visibility variation per unit of PM2.5 or RH was parameterized by derivation of the visibility multivariate function. The method was verified and evaluated based on 4453 valid hour data records in Tianjin, and visibility was identified as being in the RH-sensitive regime when RH was above 75%. In addition, the influence of aerosol chemical compositions on sensitivity of visibility to PM2.5 and RH changes was discussed by analyzing the characteristics of extinction components ((NH4)2SO4, NH4NO3, organic matter, and elemental carbon) measured in Tianjin, 2015. The result showed that the fitting equation of visibility, PM2.5, and RH, separately for different aerosol types, further improved the accuracy of the parameterization scheme for visibility in most cases.
Highlights
Atmospheric horizontal visibility is defined as the distance at which the contrast of a black object with respect to its background is equal to the contrast threshold of human eyesight and is an indicator of atmospheric transparency [1]
The impact of chemical compositions on the sensitivity of visibility to PM2.5 and relative humidity (RH) was explored based on the online monitoring data of RH, visibility, PM2.5, and its major components in this study
The normalized forward sensitivity index method for identifying the dominant factors of visibility was used based on the functional equation among visibility, RH, and PM2.5 mass concentration given different aerosol types
Summary
Atmospheric horizontal visibility is defined as the distance at which the contrast of a black object with respect to its background is equal to the contrast threshold of human eyesight and is an indicator of atmospheric transparency [1]. There is an obvious, negative correlation between visibility and PM2.5 concentration [11–14]. A form of power function or exponential function between visibility and PM2.5 concentration was observed in many cities [11,15,16]. When the PM2.5 concentration was lower than a certain threshold, visibility increased quickly as the PM2.5 concentration declined [11,16–18]. The threshold corresponding to visibility < 10 km in Beijing, Xi’an, and Shanghai is 110, 88, and 65 μg/m3, respectively [11,16,17]. This difference may reduce the reliability of haze event identification when a fixed visibility threshold is employed [19]
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