Compromised Improvement of Poor Visibility Due to PM Chemical Composition Changes in South Korea

Abstract

Fine particulate matter (PM) significantly affects visibility, a sensitive indicator of air pollution. Despite a continuous decrease in the PM concentrations in South Korea, the public generally believes that PM air pollution has worsened over the past years. To explain this disparity, we analyzed the characteristics of recent visibility changes using observations of visibility and PM component data observed in Seoul, South Korea, from 2012 to 2018. A significant negative correlation (R = −0.96) existed between visibility and concentrations of PM, with an aerodynamic diameter ≤ 2.5 μm (PM2.5); a high PM2.5 concentration was the most important contributor to poor visibility. Annual mean PM2.5 concentrations in Seoul decreased by −5.1% yr−1 during 2012–2018, whereas annual mean visibility improved by 2.1% yr−1. We found that a lower improvement in visibility was associated with changes in the PM component. Among the PM components affecting poor visibility, contributions of ammonium nitrate (NH4NO3) significantly increased during 2012–2018 (from 48% in 2012 to 59% in 2018). Increases in NO3− aerosol concentrations were owing to SOx emission reduction and the resulting decreases in SO42− aerosol concentrations, which led to an increase in NH3 available for additional NH4NO3 production in the atmosphere. Despite decreased PM concentrations in Seoul, the change of PM components has compromised visibility improvement; thus, NO3− concentrations need to be reduced.