Infrared stellar photometry from the Two Micron All Sky Survey ( 2MASS ) and spectral line imaging observations of ^ { 12 } CO and ^ { 13 } CO J = 1-0 line emission from the Five College Radio Astronomy Observatory ( FCRAO ) 14m telescope are analysed to assess the variation of the CO abundance with physical conditions throughout the Orion A and Orion B molecular clouds .
Three distinct A _ { v } regimes are identified in which the ratio between the ^ { 13 } CO column density and visual extinction changes corresponding to the photon dominated envelope , the strongly self-shielded interior , and the cold , dense volumes of the clouds .
Within the strongly self-shielded interior of the Orion A cloud , the ^ { 13 } CO abundance varies by 100 % with a peak value located near regions of enhanced star formation activity .
The effect of CO depletion onto the ice mantles of dust grains is limited to regions with A _ { v } > 10 mag and gas temperatures less than \sim 20 K as predicted by chemical models that consider thermal-evaporation to desorb molecules from grain surfaces .

Values of the molecular mass of each cloud are independently derived from the distributions of A _ { v } and ^ { 13 } CO column densities with a constant ^ { 13 } CO-to-H _ { 2 } abundance over various extinction ranges .
Within the strongly self-shielded interior of the cloud ( A _ { v } > 3 mag ) , ^ { 13 } CO provides a reliable tracer of H _ { 2 } mass with the exception of the cold , dense volumes where depletion is important .
However , owing to its reduced abundance , ^ { 13 } CO does not trace the H _ { 2 } mass that resides in the extended cloud envelope , which comprises 40-50 % of the molecular mass of each cloud .
The implied CO luminosity to mass ratios , M / L _ { CO } , are 3.2 and 2.9 for Orion A and Orion B respectively , which are comparable to the value ( 2.9 ) , derived from \gamma -ray observations of the Orion region .
Our results emphasize the need to consider local conditions when applying CO observations to derive H _ { 2 } column densities .