Week #7 Report

During this week, I continued my research on the chemical changes of biomass burning aerosol due to photolysis. And for this week’s post, I would like to introduce a sample with interesting chemical changes that I investigated.The mass spectra and elemental analysis for the sample is shown in the following two pictures.


For this specific sample, aerosols were collected from a controlled burning of chaparral on a filter. The filter was then cut into four pieces, and each piece got photolyzed under a simulated sunlight source for different number of days, which are shown with the mass spectra. One noticeable trend is that the aerosol got more and more oxidized along with the photolysis process. Such trend is indicated by the decreasing H/C and increasing O/C in the table. Notice that there seems to be a decrease in O/C from 1.5 to 3.5 days of experiment. This change is also shown in some experiments conducted on other samples, and the cause for such an unusual change is still uncertain.


Another interesting change in the mass spectra for the aging aerosol is: the intensity for the ion species CH3SO2+ keeps growing during the process of photolysis, as is shown in the second picture. Generally, the MS species CH3SO2+ is assigned to be a signature of marine sulfur source. However, a more probable explanation for this sample can be: the CH3SO2+ signal comes from the organosulfate species (esters formed from alcohol and sulfuric acid) formed during the esterification reactions induced by photolysis. One support for this explanation comes from an online aerosol mass spectrum for a burning of chaparral (the third picture), which shows a strong signal of sulfates species (the red sticks). We will need more chemical information of the sample as evidence for this speculation. If this explanation is proved accurate, this will be one of the first discoveries of organosulfate formation in biomass burning aerosol. Isn’t it exiting!