The diurnal cycle of particle, size, composition, and density in Sacramento, California, during CARES

 

Authors

Alla Zelenyuk-Imre — Pacific Northwest National Laboratory
Dan Imre — Imre Consulting
Timothy Daniel Vaden — Pacific Northwest National Laboratory
Josef Beranek — Pacific Northwest National Laboratory

Category

Aerosol Properties

Description

A central objective of the Carbonaceous Aerosol and Radiative Effects Study (CARES) was to characterize all aspects related to organics in aerosols. To this end, a range of instruments measured loadings, size distributions, compositions, densities, CCN activities, and optical properties of particles in Sacramento, CA during June 2010. We present the results of measurements conducted by our single particle mass spectrometer, SPLAT. SPLAT measured the size, composition, and density of individual particles with diameters between 50 to 2000 nm. Each day, SPLAT measured the size of ~2 million particles and the compositions of ~350,000 particles. In addition, SPLAT, combined with a DMA, measured the density of individual particles twice per day: in the morning and mid-afternoon. Preliminary analysis of the data shows that under most conditions, the particles were relatively small (below 200 nm), and the vast majority were composed of oxygenated organics mixed with various amounts of sulfates. Analysis of the mass spectra shows that the oxygenated organics in these particles are the oxidized products of biogenic volatile organic precursors. In addition, we detected and characterized fresh and processed soot particles, biomass burning aerosol, organic amines, sea salt—fresh and processed—and a small number of dust and other inorganic particles commonly found in the urban environment. SOA mixed with sulfates were the vast majority of particles at all times, while the other particle types exhibited episodic behavior. The data show a reproducible diurnal pattern in SOA size distributions, number concentrations, and compositions. Early in the morning the particle number concentrations are relatively low, and the particle size distributions peak at ~70 nm. 80 nm particles have a density of 1.3 g cm-3, and the density of larger particles (200 nm) is 1.6 g cm-3. The mass spectra show that the smaller particles are composed of organics mixed with ~10% sulfates, and larger ones contain mostly sulfate with a small amount of organics. As biogenic emissions are processed, nucleation events lead to a large increase in the concentrations of very small particles. As the day progresses, particle number concentrations increase and particles grow. By mid-afternoon, the density of 80 to 200 nm particles is ~1.3 g cm-3, and particles are composed of oxygenated organics mixed with a ~10% sulfate. A detailed analysis of the mass spectra shows that there are two types of SOA particles. Interestingly, we find evidence to suggest that in both particle types, a large fraction of the intensity in peaks at m/z 44 and 73 is related to surface compounds.