Using Single Particle Measurements to Track Changes in Multiple Particle Populations

Atmospheric aerosols represent a major source of uncertainty in radiative forcing and exposure to fine particulate matter is correlated with increased mortality and morbidity around the world. Paramount to increasing our understanding of aerosols is developing increasingly precise methods to track aerosol composition and changes that come from interactions with other components in the atmosphere. Gas-phase exchange between aerosol populations via evaporation and condensation of semi-volatile organics can be a major mechanism of mixing between accumulation-mode particles with slow coagulation. This exchange may be impeded in highly viscous, semi-solid, or glassy particles due to diffusion limitations. Mixing experiments between distinct aerosol particle populations can help to elucidate mixing behavior of atmospherically relevant aerosols. 

First, we describe experiments on carefully prepared particle populations representing highly viscous or potentially “glassy” aged organic particles (nonvolatile sugars ¹³C-glucose, sucrose, and raffinose with ammonium sulfate seeds) and fresh biomass burning particles (erythritol with black carbon seeds) to develop a model phase space for organic aerosol systems and better understand when particle phase state impedes mixing. Our hypothesis is that these limitations are alleviated at some relative humidity threshold, which increases with decreasing ambient temperatures. We quantify the mixing state of these particle populations from 10 – 25 ºC and 5 – 90 % RH using an Aerosol Mass Spectrometer (AMS) combining Event Trigger (ET) and Soot Particle (SP) modes. The observed single particle mass spectra are aggregated in short time slices and used to perform a linear combination of relevant reference spectra to determine the contributions each constituent has on the resulting particle signal. Our results suggest that the nonvolatile sugar particles have little to no diffusive limitations to mixing at the conditions tested. 

Next, we describe experiments on carefully prepared particle populations representing aged organic particles – SOA formed from α-pinene with ammonium sulfate seeds as well as non-volatile ¹³C-glucose with sodium nitrate seeds – and fresh biomass burning particles – erythritol with black carbon seeds. We track the composition of all three particle populations simultaneously over the course of a mixing event. We then quantify the mixing state of these particle populations at 20 ºC and low RH using an Aerosol Mass Spectrometer (AMS) combining Event Trigger (ET) and Soot Particle (SP) modes. The observed single particle mass spectra are aggregated in short time slices and used to perform a linear combination of relevant reference spectra to determine the contributions each constituent has on the resulting particle signal. Our results show that the ¹³C-glucose particles have little to no limitation to mixing at the conditions tested, however the SOA formed from α-pinene take up none of the evaporated erythritol over the course of the mixing events.

Finally, we describe an experiment on carefully prepared particle populations – ¹³C-glucose coating ammonium sulfate seeds and sucrose coating black carbon seeds. We track the composition of these populations individually over the course of a mixing event at 20 ºC and low RH using an Aerosol Mass Spectrometer (AMS) combining Event Trigger (ET) and Soot Particle (SP) modes. We then quantify the mixing state of these particle populations with a new method extracting high resolution mass spectral data from the single particle data acquisition. The observed single particle mass spectra are aggregated in short time slices and used to perform a linear combination of relevant reference spectra to determine the contributions each constituent has on the resulting particle signal. We compare these results with those from previous studies where we used unit mass resolution (UMR) mass spectra. Our results show that the reference spectra are more distinct when we use high resolution data, but there is still “cross-talk” between the observed particle spectra during the mixing event.