Photolysis of 4-Nitrophenol

After gaining some experience with our methods and instruments, we were able to really get to work on matching the literature data. In order to test the effects of other compounds (such as secondary organic aerosols or inorganics) on the lifetime of brown carbon, we had to achieve a good and reproducible baseline measure of how it reacted to sunlight on its own. To do this, we created a very dilute solution of 4-Nitrophenol (a type of brown carbon) and placed it in a quartz vial. The quartz lets more wavelengths of light through than glass or other materials. This vial was placed in front of a xenon arc lamp that simulated the wavelengths and energy of sunlight. We found that using a stir bar and backing the vial with reflective foil maximized the rate of photolysis. At different time intervals (1 hour time intervals shown in the following figure) we took aliquots of the solution and analyzed the sample using the UV/VIS spectrophotometer.


This plot of absorbance vs wavelength shows the changes in the solution’s lightwave absorbance over three hours. The curve flattens over time, showing the decrease of the presence of 4-Nitrophenol in its original form. Compare this chart with results from the literature (Photochemical processing of aqueous atmospheric brown carbon, R. Zhao et. all) shown below:


It took several weeks to perfect the photolysis experiment and maximize the rate of 4-Nitrophenol photolysis. Lower concentrations and lower pH levels increased the rate of photolysis. With the basics under control, we then turned to testing the effects of other compounds on the rate of brown carbon photolysis, a topic I will cover in my next blog entry.