Joshua Klobas’ interest in the implications of climate change and the physical basis for the feedback mechanisms involved led him to the AirUCI organized research unit. He found an undergraduate opportunity in Prof. Nizkorodov’s Aerosol Photochemistry group and was fortunate enough to be given some degree of self-direction. Joshua’s project was a proof of concept of a method for measuring photo-induced dissociation of organic molecules. As the project matured, the tens of thousands of data points offered him the opportunity to learn new statistical techniques and treatments that he will be able to apply to many different types of problems in the future. Joshua is now pursuing a Ph.D. in chemical physics at Harvard University.

Atmospheric oxidation of volatile organic compounds from biogenic sources is responsible for a large fraction of the aerosol particles in the atmosphere. The resulting secondary organic aerosol (SOA) particles contribute to many processes implicated in climate change. A challenge in describing SOA-climate interactions is the constant evolution of SOA particles. Photodegradation by solar UV radiation is one of the most important processes in this evolution; therefore, it is important to know how efficient these processes are and understand the major mechanisms responsible for them. We describe a new quartz crystal microbalance (QCM) technique for characterizing SOA photodegradation. We detect the loss of mass in the process of UV irradiation of microgram quantities of SOA particles collected on a substrate using the high mass sensitivity of QCM. Limonene and other compounds were reacted with ozone and deposited directly on quartz crystal by impaction. A scanning mobility particle sizer provided quantitative information on the mass distribution and allowed the calculation of deposited mass. Photolysis experiments were conducted on films of SOA and representative compounds deposited on the quartz crystal. Mass loss was observed as a function of UV flux. This approach has promise for investigating photodegradation of aerosols and other environmental samples.

Sergey A. Nizkorodov School of Physical Sciences

Atmospheric particles have a disproportionally strong effect on the Earth’s climate despite their small abundance. Elevated concentrations of particulate matter pose significant health risks in heavily urbanized areas. The climate and health effects of atmospheric particles remain poorly understood because their chemical composition changes in complicated ways in response to various environmental factors. Joshua’s research addresses a question of whether particles can shrink in size in the presence of solar radiation. He succeeded in putting together a unique instrument to study this phenomenon and demonstrated that organic particles do become smaller when irradiated. This is an important finding because smaller particles scatter light less efficiently compared to larger ones, and they are also less efficient in nucleating cloud droplets.