Washout coefficient Λ is widely used as a parameter in washout models. These models describes overall HTO washout with rain by the first-order kinetic equation, while washout coefficient Λ  depends on the type of rain event and rain intensity and empirical parameters a, b.  It was shown recently that variations of published data of washout coefficient are significant. Thus Λ = 10−4 sec−1 for the light rain event (∼ 1 mm-hour−1) while Λ = 10−3 sec−1 for heavy rain (∼ 25 mm-hour−1). Canadian standard recommends washout coefficient of 1.8-10−4 sec−1, German standard gives 3.5-10−5sec−1, while published Japan data varies from Λ = (7.3 ± 4.1)-10−5 sec−1 at 2 mm hour−1 to Λ = 4.6-10−4 sec−1 for the same rain intensity. This means that further investigations of HTO washout process are required. One of the issues is determining the useful relationship between macroscopic parameter of HTO washout Λ and microscopic HTO exchange rate of HTO molecules in atmosphere and in the raindrop water. Approaches to address this issue have been presented elsewhere. It can be shown that the empirical parameters a, b  can be represented through the rain event characteristics using the relationships for molecular impact rate, rain intensity and specific rain water content while washout coefficient can be represented through the exchange rate K, rain intensity, raindrop diameter and terminal raindrop velocity.