Overview
Numerous studies have demonstrated how ambient aerosols cause many adverse environmental effects, including negatively impacting human health, inducing climate change, and reducing visibility. Aerosols can be transported long distances, and therefore, it is extremely important to better understand the relative contributions of different sources (natural and anthropogenic) to ambient particles on both the local and regional scales. The meteorological conditions can influence how potential sources contribute to the overall PM2.5, causing the same location to experience different aerosol chemical compositions based upon the time of year. Many United States counties are currently on the EPA non-compliance list for PM2.5. In order for policymakers to set appropriate regulations that will allow these regions to achieve compliance, they must know the relative contributions of different PM sources to ambient air. The ultimate goal of many of our projects is to provide source apportionment of ambient aerosols and to determine how aerosol chemistry and sources vary spatially and temporally. An important and interesting aspect of one project (funded by the California Air Resources Board "CARB") is to determine how aerosols vary seasonally at the same location. The main purpose of the first phase of the project is to evaluate the prospects for achieving source apportionment of aerosols in California using a mobile laboratory being developed as part of this project. The first testing was done on a major study conducted in Riverside CA.
Biomass Burning
Oceans
Cars
Power Plants
Diesel
Dust
More on the CARB-Source Apportionment Study
Methods
As part of this project, we developed a mobile laboratory that hosts an array of gas and particle phase instruments, including aerosol time-of-flight mass spectrometry (ATOFMS). This laboratory was set-up and used for ambient measurements during the Summer and Fall of 2005 on the campus of the University of California, Riverside. By employing the single particle ATOFMS, mass spectral signatures from previous source characterization studies were used to source apportion the majority of PM2.5 in Riverside. The results from this study are being used to test and improve our data analysis techniques by incorporating and combining the measurements from the supporting gas, particle phase, and meteorological instruments.
Results
In order to understand how the chemical composition of the ambient particles varies in Riverside, California, we used two main data analysis approaches with the single particle mass spectral data: creating clusters of top chemical types based upon the degree of similarity among individual particles and matching the single particles to single particle source signatures, that were obtained in previous source characterization studies. Divided by size range, the clustering and matching results were plotted temporally to determine trends in the potential sources. In order to verify the accuracy of the techniques, the trends were compared with the time series of the other gas and particle phase instruments. For example, the ATOFMS apportioned heavy duty diesel vehicle (HDDV) exhaust particle contribution decreased in concentration on Sundays, which was consistent with the NOx and aethalometer readings which are known to track HDDV emissions. Most of the particles that did not match the fresh source signatures were heavily aged aerosols, as expected. Fortunately, with the results of thermodenuder measurements in Riverside, we have greater insight into the semivolatile fraction of the ambient aerosols and a better understanding of how the aging transforms the mass spectral signature of the freshly emitted source particles. As shown herein, such information is useful in the development of the appropriate data analysis approaches that can be used on “real” ambient aerosols, such as the masking of particular “aged” peaks in the mass spectral signature. With the co-location of several other research groups at the sampling site, we have been able to successfully strengthen our analysis approaches and source apportion the data collected in the mobile laboratory; the next step is to obtain scaled mass fractions of the different sources. With the success and new information learned during the Riverside study, we will use the developed approaches with a high level of confidence to apportion ambient aerosols and determine how the source contributions vary seasonally, as a function of size, and spatially in future sampling locations in California.