In essence, the statement of Proposition 2 is that, under certain conditions, meteorological normalization estimates the expectation of the response variable (such as PM 10 ) conditioned only on the non-meteorological variables (such as the Julian date in ). It is these certain conditions that (may) justify the usage of meteorological normalization, and 110 the violation of which may yield a meteorological normalization M (t) that behaves unexpectedly, and even undesirably.