exposure_estimate_idw   Estimate the pollutant exposure using the
                        inverse distance weighting method
exposure_estimate_krige
                        Assess the environmental exposure using the
                        kringe method
expoure_estimate_simple
                        Assess the environmental exposure using the
                        simplest method: nearest monitoring site method
get_latlon_china        transform the address information into the
                        longitude and latitude
get_refrence_id_simple
                        Match the nearing monitoring site for each
                        individual
individual_data         The detailed information for each individual.
pollutant_data          The concentration of air pollutant at each time
                        point.
site_data               Monitoring sites.
timeseries_imput        Impute the missing value for the timeseries
                        using the linear interpolation
