Relieved Air Pollution Enhanced Urban Heat Island Intensity in the Yangtze River Delta, China

Hao Wu,Chaoqun Ma,Tijian Wang,Yang Cao,Xiaodong Xie,Chong Liu,Qin’Geng Wang,Nicole Riemer

doi:10.4209/aaqr.2019.02.0100

Abstract

ABSTRACT The National Air Pollution Control Plan implemented by China in 2013 reduced the concentrations of air pollutants, especially PM2.5 (aerosol particles with an aerodynamic diameter equal to or less than 2.5 µm), between 2014 and 2017. This reduction in PM2.5 potentially affected the intensity of urban heat islands (UHIs), as the presence of fine particles can influence the energy balance of the earth-atmosphere system. In this study, the effect of the pollution control plan on the UHI intensity in the Yangtze River Delta, China, was investigated via observational analysis and numerical modeling. According to the observational data, the PM2.5 concentrations in the megacities of the Yangtze River Delta, viz., Shanghai, Nanjing, Hangzhou and Hefei, in 2017 were ~35 µg m–3, showing decreases of approximately 48.36%, 28.25%, 29.41% and 32.5%, respectively, compared to 2014. Furthermore, these reductions in the PM2.5 concentration correlated well with the strengthened diurnal intensity (increasing by up to 1 K) and the weakened nocturnal intensity (decreasing by up to 1 K) of the UHIs. Numerical simulations confirmed that this “seesaw effect” on the UHI intensity was due to the decrease in PM2.5 and the consequent increase in the downward surface shortwave radiation and the outgoing top-of-the-atmosphere longwave radiation. Thus, the Air Pollution Control Plan noticeably affected the UHI intensity by reducing PM2.5—a factor which should be considered in future studies on urban climate and environmental planning.

Highlights

Rapid industrialization and urbanization in the last decades have changed land cover, surface albedo and anthropogenic heat flux as well as air pollutant concentrations in China (Xia et al, 2014)
Our study reveals that the Air Pollution Control Plan of 2013 reduced PM2.5 concentrations in the Yangtze River Delta (YRD) by nearly 30% between 2014 and 2017 and increased the intensity of the urban heat islands (UHIs) during the day while decreasing it during the night—a phenomenon we named the “seesaw effect.”
The nocturnal outgoing TOA longwave radiation increased as the PM2.5 decreased, resulting in cooler surface temperatures

Summary

Introduction

Rapid industrialization and urbanization in the last decades have changed land cover, surface albedo and anthropogenic heat flux as well as air pollutant concentrations in China (Xia et al, 2014). UHI is a local phenomenon and distinct from largescale global warming trends (Jones et al, 1990; Peterson et al, 1999) It affects the boundary layer and the underlying surface energy balance (Lin et al, 2008), as well as wind patterns (Hjelmfelt, 2017), precipitation (Zhong et al, 2017) and sensible heat convection efficiency (Zhao et al, 2014). Human activities with the growing population result in the increase of air pollutant emissions (Wang et al, 2006) This results in an uneven distribution of air pollutants, including PM2.5, in the urban center and suburban district, which in turn feeds back on the UHI

Methods

Results

Conclusion