Subject: Light Intensity VS NO2 concentration

Hi.. I'm doing a school project on Light Intensity VS Nitrogen Dioxide 
Concentration. How my experiment works, is that I pass a ray of laser light 
through differing amounts of NO2 in a glass bottle (different amounts of 
copper are dissolved in nitric acid) and measure the light intensity out of 
the bottle by using an LDR connected in a circuit with a 100K ohm resistor and 
a 3V battery. These are my results:

Amount of Copper             Voltage out of LDR
0g                             0.225
0.2g                           0.255
0.4g                           0.325
0.6g                           0.613  

My experiments give a non- linear graph, that looks like the graph of equation 
y=mx^2+c. I have researched on this topic for the past month, and I haven't 
been able to find any information on why this sort of relationship is given. 
Also, after 0.6g, I tried dissolving 0.8g and 1.0g of copper in nitric acid 
and passed the laser light through, but the voltage goes right down again, 
giving these results:

0.8g                 0.35V
1.0g                 0.30V

I haven't found any information on why this happens either.

I'd appreciate it if you could give me some information on why light intensity 
varies like this according to the increasing gas concentration, or if you 
could point me in the right direction in the sites I should be looking at.

Thank You.

Re: Light Intensity VS NO2 concentration