The major limiting factor for photosynthesis is 

The ${\mathrm{CO}}_2$ concentration in the atmosphere is only 0.03 to 0.04% by volume. But the optimum ${\mathrm{CO}}_2$ concentration for photosynthesis in ${\mathrm{C}}_3$ plants is 0.045% by volume. In ${\mathrm{C}}_3$ plants photosynthesis is limited by ${\mathrm{CO}}_2$ concentration. Only during cloudy days and plants growing in shades, light is the limiting factor.