Enhanced air pollution via aerosol-boundary layer feedback in China

TLDR

Severe air pollution episodes have been frequent in China, yet the root cause of the most severe events remains unclear despite high emissions. The study aims to show that high particulate matter concentrations stabilize the urban boundary layer, lowering its height and amplifying PM levels. The authors estimate the feedback strength by applying a new theoretical framework combined with ambient observations. The feedback is moderate below ~200 µg m⁻³ but becomes more effective at higher PM loadings, explaining why megacities experience severe episodes under stable synoptic conditions.

Abstract

Abstract Severe air pollution episodes have been frequent in China during the recent years. While high emissions are the primary reason for increasing pollutant concentrations, the ultimate cause for the most severe pollution episodes has remained unclear. Here we show that a high concentration of particulate matter (PM) will enhance the stability of an urban boundary layer, which in turn decreases the boundary layer height and consequently cause further increases in PM concentrations. We estimate the strength of this positive feedback mechanism by combining a new theoretical framework with ambient observations. We show that the feedback remains moderate at fine PM concentrations lower than about 200 μg m −3 , but that it becomes increasingly effective at higher PM loadings resulting from the combined effect of high surface PM emissions and massive secondary PM production within the boundary layer. Our analysis explains why air pollution episodes are particularly serious and severe in megacities and during the days when synoptic weather conditions stay constant.