Ruochong Xu1,2, Ximeng Qi 1,2, Guoqing Dai1,2, Haoxian Lin1,2, Jiachun Shi1,2, Chengxu Tong1,2, Peng Zhai1,2, Caijun Zhu1,2, Lei Wang1,2, Aijun Ding1,2

Joint International Research Laboratory of Atmospheric and Earth System Sciences, School of Atmospheric Sciences, Nanjing University, Nanjing 210023, China
Jiangsu Provincial Collaborative Innovation Center for Climate Change, Nanjing 210023, China

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Xu, R., Qi, X., Dai, G., Lin, H., Shi, J., Tong, C., Zhai, P., Zhu, C., Wang, L. and Ding, A. (2020). A Comparison Study of Indoor and Outdoor Air Quality in Nanjing, China. Aerosol Air Qual. Res.,


  • Simultaneous indoor and outdoor O3 , CO, PM2.5 measurements in Nanjing, China.
  • Outdoor meteorological conditions influence the indoor-outdoor relationships.
  • Indoor O3 and PM2.5 can get 30 ppb and 131.6 µg m–3 during outdoor pollutions.
  • A simplified indoor-outdoor mass-balance equation was established.


Because humans spend much of their time in indoor microenvironments, the air quality indoors has attracted significant attention. This study simultaneously conducted real-time indoor observations and comprehensive outdoor measurements of O3, CO and PM2.5 in suburban Nanjing in eastern China from September to December 2018. The O3, CO and PM2.5 exhibited median indoor concentrations of 3.2 ppb, 395.8 ppb and 17.3 µg m3, respectively, based on measurements in a room with closed windows and relatively regular daily indoor human activity, and median outdoor values of 30.5 ppb, 386.1 ppb and 37.2 µg m3, resulting in median indoor-outdoor (I/O) ratios of 0.14, 1.01 and 0.46. Moreover, the indoor concentrations traced the outdoor variations with correlations of approximately 0.68, 0.82 and 0.82 for the O3, CO and PM2.5, respectively. During pollution episodes, the indoor O3 and PM2.5 concentrations reached about 30 ppb and 130 µg m3, respectively. Both meteorological conditions (e.g., wind speed or relative humidity) and human activity indoors influenced the relationships between the indoor and outdoor concentrations, including the I/O ratios and time lag. A simplified indoor-outdoor mass-balance equation was developed to simulate the indoor concentrations, and the predictions fitted the observed data for most of the testing period, especially after considering the human activity indoors and limited penetration of particles. This study enhances our understanding of the indoor-outdoor relationships for gaseous and particulate matter concentrations in polluted areas such as the Yangtze River Delta and highlights the urgent need for improving indoor air quality in the megacities of China.

Keywords: Air quality; Indoor measurements; I/O ratio; Yangtze River Delta.

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