On the summertime air quality and related photochemical processes in the megacity Shanghai, China

Ka Lok Chan; Shanshan Wang; Cheng Liu; Bin Zhou; Mark O Wenig; Alfonso Saiz-Lopez

doi:10.1016/j.scitotenv.2016.12.052

On the summertime air quality and related photochemical processes in the megacity Shanghai, China

Sci Total Environ. 2017 Feb 15:580:974-983. doi: 10.1016/j.scitotenv.2016.12.052. Epub 2016 Dec 14.

Authors

Ka Lok Chan¹, Shanshan Wang², Cheng Liu³, Bin Zhou⁴, Mark O Wenig⁵, Alfonso Saiz-Lopez⁶

Affiliations

¹ Shanghai Key Laboratory of Atmospheric Particle Pollution and Prevention (LAP(3)), Department of Environmental Science and Engineering, Fudan University, Shanghai 200433, China; Meteorological Institute, Ludwig-Maximilians-Universität München, Munich 80333, Germany.
² Shanghai Key Laboratory of Atmospheric Particle Pollution and Prevention (LAP(3)), Department of Environmental Science and Engineering, Fudan University, Shanghai 200433, China; Department of Atmospheric Chemistry and Climate, Institute of Physical Chemistry Rocasolano, CSIC, Madrid 28006, Spain; Key Lab of Environmental Optics & Technology, Anhui Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Hefei 230031, China. Electronic address: shanshanwang@fudan.edu.cm.
³ School of Earth and Space Sciences, University of Science and Technology of China, Hefei 230026, China; Center for Excellence in Urban Atmospheric Environment, Institute of Urban Environment, Chinese Academy of Sciences, Xiamen 361021, China; Key Lab of Environmental Optics & Technology, Anhui Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Hefei 230031, China. Electronic address: chliu81@ustc.edu.cn.
⁴ Shanghai Key Laboratory of Atmospheric Particle Pollution and Prevention (LAP(3)), Department of Environmental Science and Engineering, Fudan University, Shanghai 200433, China.
⁵ Meteorological Institute, Ludwig-Maximilians-Universität München, Munich 80333, Germany.
⁶ Shanghai Key Laboratory of Atmospheric Particle Pollution and Prevention (LAP(3)), Department of Environmental Science and Engineering, Fudan University, Shanghai 200433, China; Department of Atmospheric Chemistry and Climate, Institute of Physical Chemistry Rocasolano, CSIC, Madrid 28006, Spain.

PMID: 27986317
DOI: 10.1016/j.scitotenv.2016.12.052

Abstract

Summertime surface ozone (O₃) and related secondary formation of fine particles are the major air quality concerns in the megacity of Shanghai. We performed mobile Cavity Enhanced Differential Optical Absorption Spectroscopy (CE-DOAS) measurements to investigate the spatial distribution of on-road nitrogen dioxide (NO₂) concentrations along the Inner Ring Elevated Road (IRER) in Shanghai. The observations show a ratio of 4 between CE-DOAS averaged on-road NO₂ and the in-situ ambient measurements, illustrating the strong impact of vehicle emissions over the urban area. The air mass transport analysis suggests that the observed episodic ozone events arise from both the abundance of volatile organic compounds (VOCs) precursors in the sampled plume and the regional transport of ozone-rich air masses. Analysis of the sources of PM_2.5 shows that the secondary heterogeneous gas-to-particle conversion of sulfate and nitrate from sulfur dioxide (SO₂) and nitrogen oxides (NO_x) is the largest source of PM_2.5 contributing 44.8±9.2% of the total PM_2.5. Ozone-related photochemical formation of fine particles is estimated to contribute about 22.5±11.9% of the total PM_2.5, which is strongly facilitated by solar radiation in summer. According to our results, nitrous acid (HONO) is the major precursor of hydroxyl radicals (OH) accounting for 40% to 80% of the total OH production during daytime. A significant correlation is found between the HONO levels in the early morning and the daily O₃ and PM_2.5 levels. The summertime measurements indicate that the photolytic reaction of HONO after sunrise increased the abundance of daytime OH and oxidative capacity, resulting in an enhancement of ground level ozone and secondary organic aerosol formation. This study provides quantitative information to better understand photochemical formation of ozone and fine particles in Shanghai during summertime, which is useful for designing collaborative strategies to mitigate emissions of precursor pollutants.

Keywords: DOAS; HONO; O(3); PM(2.5); Photochemistry.