physiology - Can I estimate leaf temperature from air temperature? What other information would be required?

Physiological measurements such as respiration rate and assimilation rate depend on temperature.

Most papers report tissue temperature (e.g. leaf temperature for leaf measurements), although some report air temperature.

My goal is to compare measurements in the literature. I use leaf temperature at time of measurement to adjust the rates to a common temperature. Currently, I assume that if leaf temperature is not available, it is the same as air temperature. (This is often assumed in papers where the leaf temperature was not measured).

Is there a way to estimate the difference between air temperature and leaf temperature? I can get fairly good estimates of other physiological traits (e.g. leaf thickness, albedo, stomatal conductance) as well as climate data (e.g. humidity, soil moisture content).

My model is:

$T_{leaf} = T_a + X + epsilon$

And I currently assume X = 0 in my calculations

Is there a better way to estimate X?

Under what conditions would X < 0? X > 0?