The coefficient of interest, γ1, is the percentage effect of HNC on air pollution. The vector of covariates, xt, includes indicator variables for month of the year, day of the week, and hour of the day as well as interactions between weekends and hour of the day. In addition, xt includes weather variables including current and 1-hour lags of quartics in temperature, humidity, and wind speed. 14 Equation (1) is first estimated using least squares (OLS) for four different time windows rang- ing from 1986-1993 to 1989-1990. Windows smaller than two years are not considered because it becomes difficult to credibly control for seasonal variation. Limiting the.
