Chlorophyll is utilized as a simple, inexpensive and environment-friendly ("green") colorimetric indicator for nitrogen dioxide (NO2) gas. A drastic color change from green to yellow was observed when chlorophyll, either dissolved in CH2Cl2 solution or absorbed into paper, was exposed to NO2 gas. Other gases such as CO2 and SO2 did not exhibit any color change with chlorophyll. Spectroscopic analysis showed nitration of chlorophyll as possible cause for the color change.