Toward a better understanding of new particle formation

The creation of new atmospheric particles via nucleation is an important source of particles, and may influence climate by altering the aerosol size distribution. The objectives of my dissertation research were to better understand the process by which new particles are created (homogeneous nucleation), and how these particles are modified throughout their lifetime in the atmosphere. The approach combined field-measurements and observations with advanced instrumentation development and extensive data analysis.

In the laboratory, a Dry-Ambient Aerosol Size Spectrometer (DAASS) was constructed. The DAASS is an automated combination of aerosol sizing instruments and supporting equipment that measures aerosol size distributions from 10.9 nm to 10 µm at both ambient and dry relative humidities and was deployed during the MILAGRO field campaign. The design and construction of a Differential Mobility Analyzer from parts was also completed in order to provide the capability to perform Tandem DMA (or TDMA) measurements.

New particle formation events, occurring in both rural (Midwest United States) and urban (Mexico City) locations were analyzed. In the Midwest, the temporal pattern, frequency, associated meteorology and contributing factors were quantified for the first time in this location.

The urban observations were conducted in Mexico City, Mexico, as part of an international field campaign known as MILAGRO (Megacity Initiative: Local and Global Research Observations 2006). It was determined that new particle formation in Mexico City occurs following periods of decreased pre-existing aerosol surface area. These sharp decreases in pre-existing aerosol surface area are tied to the rapid ventilation of the Mexico City basin that occurs in the early afternoon as a result of its unique geographical setting in a mountain basin.

Number-based emission factors representative of Mexico City were determined using a method of signal peak identification in collocated SMPS and CO2 measurements. The emission factor as a function of time of day, day of week, and wind direction were examined. The overall emission factor is size resolved, and comparisons to other size resolved emission factors determined in other locations (such as Los Angeles) were performed.