Enhanced nitrate contribution to light extinction during haze pollution in Chengdu: Insights based on an improved multiple linear regression model.

In recent years, the annual mean concentration of PM has decreased in Chengdu, China; however, atmospheric visibility has not improved accordingly. Low-visibility events occurred even when the PM mass concentrations were below the national ambient air quality secondary standard (daily mean concentration, 75 μg/m). In this study, the non-linear relationship between PM and visibility was analyzed under different NO mass fractions in PM based on 2-year field observation data. The results indicated that NO formation contributed to particulate pollution events and reduced atmospheric visibility. Multiple linear regression was used to propose a localized reconstruction equation for the light-scattering coefficient. According to the maximum likelihood estimation method and log-transformed residuals, the mass scattering coefficients (MSEs) of organic matter (OM), NHNO, and (NH)SO in Chengdu were 7.42, 3.83, and 3.80, respectively. OM and NHNO contributed to more than 50% of the light-extinction coefficient (b). NHNO was the main pollutant causing the substantial increase in b. Chengdu has a high relative humidity (annual mean 70%), and under such conditions, the contribution of NHNO to b was considerably enhanced through hygroscopic growth and heterogeneous reactions. This study estimated the localized MSEs of OM, NHNO, and (NH)SO in Chengdu and emphasized that effective control measures to reduce nitrate and its precursors could simultaneously ameliorate air quality and visibility in humid regions with poor atmospheric visibility.