An analysis based on the law of linear momentum conservation demonstrates unequivocally that the mass fraction is the scalar whose gradient determines gas diffusion, both molecular and turbulent. It illustrates sizeable errors in previous micrometeorological definitions of the turbulent gas flux based on fluctuations in other scalars such as the mixing ratio or density. In deference to conservation law, we put forth a new definition for the turbulent gas flux. Net gas transport is then defined as the sum of this turbulent flux with systematic transport by the mean flow. This latter, non-diffusive flux is due to the net upward boundary-layer momentum, a Stefan flow forced by evaporation, which is the dominant surface gas exchange. A comparison with the traditional methodology shows exact agreement between the two methods regarding the net flux, but with the novelty of partitioning gas transport according to distinct physical mechanisms. The non-diffusive flux is seen to be non-negligible in general, and to dominate turbulent transport under certain conditions, with broad implications for boundary-layer meteorology.

基于线性动量守恒定律的分析清楚地表明，质量分数是标量，其梯度决定了分子和湍流中的气体扩散。它说明了基于其他标量（例如混合比或密度）的波动，在湍流通量的先前微气象学定义中存在相当大的误差。根据守恒定律，我们为湍流提出了新的定义。然后，将净气体输送定义为该湍流与系统平均输送的湍流之和。后者的非扩散通量是由于净向上边界层动量所致，这是蒸发作用引起的Stefan流，这是主要的表面气体交换。与传统方法的比较表明，两种方法在净通量方面完全吻合，但是具有根据不同物理机制分配气体的新颖性。通常认为非扩散通量是不可忽略的，并且在某些条件下支配湍流传输，这对边界层气象学具有广泛的意义。