where j references a city or location in China. TSPj is the average
annual ambient concentration of TSPs in city j over the period
1980–2000 and Yj is a measure of city j’s mortality or life expectancy
at birth. Nj is an indicator variable equal to 1 for locations
that are north of the Huai River line, f(Lj) is a polynomial in the
degrees north of the Huai River, and Xj is a vector of the demographic
and city characteristics, other than air quality, that are
associated with mortality rates (SI Appendix gives details)