Air Pollution Index Case Study

3 Data
1. Air Pollution Index
In order to examine the relationship between subway expansion and air quality, I choose the daily Air Pollution Index to represent the air quality condition. The Air Pollution Index is a simple number that shows the level of air pollution in this city or area, and is based on five atmospheric pollutants, sulfur dioxide (SO2), nitrogen dioxide (NO2), suspended particulates (PM10), carbon monoxide (CO), and ozone (O3) measured at the monitoring stations throughout each city. The pollutants that Chinese government taking into account in Chinese API are the first three. The mechanics of the API level is that, an individual score is assigned to each pollutant based on their levels, and the final API is the highest of the three pollutant scores (The standard of scoring each pollutant is showed in Table 1).
Table 1: Transformation from Pollutant Concentration to API

For Beijing, there are two types of API data that can be access through the government’s website, one is the aggregate daily API released by the Ministry of Environmental Protection

The aggregate API is a number combined from all monitoring stations in Beijing, and is more easily to be manipulated by the government in order to give the public a false impression that the air pollution problem in Beijing is not as bad as it appears. Well the station-level API is different numbers monitored by 27 different monitoring stations that are located all around Beijing. Since the monitors are located in different locations, the index they monitored would be more representative, because different district of Beijing are different in industry structure, population, traffic condition and other factors that can influence the air pollution level. In addition, the station-level API is released by MEMC which is more like an environmental protection agency than a government department, thus the data can be more