However, when the influences of forests on hydrology are evaluated at the "airshed" or regional scale, water vapor generated from forests located in one watershed may be considered as water supply via precipitation inputs to watersheds located downwind (van der Ent et al. 2010).
Individual monthly average PM2.5 concentrations were calculated as the mean of the daily ambient PM2.5 estimates for the airshed in which the participant resided.
The lower atmosphere of the airshed is characteristically stable during dry seasons and there is a persistent inversion at about 3000 meters [ 17].
In time-series studies of the health effects of urban air pollutants, decisions must be made about how to characterize pollutant levels within the airshed.
We first used the multivariate receptor model, UNMIX (Henry 2000, 2003; Henry et al. 1999), to determine source signatures in the airshed.
With respect to PM2.5 organic carbon, we know that particles of both primary and secondary origin are present in the airshed, and that the fraction of particles of primary origin tends to be greater near the urban core.
For this health benefits estimate to be valid, the relationship between the concentrations at the central monitor and the rest of the airshed must be similar for both the observed (baseline) and alternative (policy/intervention) scenarios.
Each is picking up the same temporal signal (which is presumably a function of the total amount of pollution in the airshed), and thus the differences among the metrics appears to be an issue of calibration.
And while most would agree that this rescaling is necessary, between-study differences in the approach used to characterize air pollution levels within the airshed may be a source of heterogeneity among the estimates.
Assuming that the (log of the) health effect due to air pollution is proportional to the total amount of pollution within the airshed, it is not surprising that the rate ratio on a per unit basis would be lowest for the central monitor measurements.
Investigating the sensitivity of the estimates of between-city heterogeneity in multi-city studies to changes in the method of characterizing pollutant levels within the airshed could prove helpful in verifying that the apparent heterogeneity is not a consequence of monitor siting.
