Could be co2, but it's more likely to do with your light if the light is well above the lower shelf. The reason for this is because the intensity of light drops off as a square of the distance, meaning that light rays at a 2 foot distance have only 1/4 the intensity that they do at one foot. Seems strange to have that kind of energy loss so quickly, but people have discovered this to be the case. One squared being just one, 2 is four but this is energy loss so you think of it as one fourth.