Dominant Mechanisms that Shape the Airborne Particle Size and Composition Distribution in Central California

Abstract

The size and composition of ambient airborne particulate matter is reported for winter conditions at five locations in (or near) the San Joaquin Valley in central California. Two distinct types of airborne particles were identified based on diurnal patterns and size distribution similarity: hygroscopic sulfate/ammonium/nitrate particles and less hygroscopic particles composed of mostly organic carbon with smaller amounts of elemental carbon. Daytime PM 10 concentrations for sulfate/ammonium/nitrate particles were measured to be 10.1 µg m −3 , 28.3 µg m −3 , and 52.8 µg m −3 at Sacramento, Modesto and Bakersfield, California, respectively. Nighttime concentrations were 10-30% lower, suggesting that these particles are dominated by secondary production. Simulation of the data with a box model suggests that these particles were formed by the condensation of ammonia and nitric acid onto background or primary sulfate particles. These hygroscopic particles had a mass distribution peak in the accumulation mode (0.56-1.0 µm) at all times. Daytime PM 10 carbon particle concentrations were measured to be 9.5 µg m −3 , 15.1 µg m −3 , and 16.2 µg m −3 at Sacramento, Modesto, and Bakersfield, respectively. Corresponding nighttime concentrations were 200-300% higher, suggesting that these particles are dominated by primary emissions. The peak in the carbon particle mass distribution varied between 0.2-1.0 µm. Carbon particles emitted directly from combustion sources typically have a mass distribution peak diameter between 0.1-0.32 µm. Box model calculations suggest that the formation of secondary organic aerosol is negligible under cool winter conditions, and that the observed shift in the carbon particle mass distribution results from coagulation in the heavily polluted concentrations experienced during the current study. The analysis suggests that carbon particles and sulfate/ammonium/nitrate particles exist separately in the atmosphere

Simulating Particle Size Distributions over California and Impact on Lung Deposition Fraction

“…Coastal sites typically experience high concentrations of Na + and Cl − in the coarse-particle diameter range (Nolte et al 2008). Although the MOUDI observations at the coastal Bodega Bay site only covered fine-diameter particles (D aero < 1.8 µm), results in Figure 3 of Herner et al (2006) suggest that coarse Na + and Cl − were substantially under-predicted here. This underprediction is primarily due to the lack of enhanced sea-salt emissions from the coastal surf zone in CMAQv4.6.…”

“…Values for simulation parameters (e.g., breathing frequency, tidal volume) were chosen to match those of Asgharian et al (2001), except that oral rather than endo-tracheal breathing is simulated here. Also, particle density was set to 1.3 g cm -3 to approximate wood smoke particles (Herner et al 2006), although increasing the density to 1.8 g cm -3 had little impact on results.…”

“…Herner et al (2005) noted that this study appears to represent the largest measurement of size-composition distributions using cascade impactors outside of Los Angeles. Size-resolved observations of particle number concentration are also available from this study (Watson et al 2002;Herner et al 2006). Yet, to date, CMAQ simulations of particle size distributions have not been evaluated against these observations.…”

Factors affecting the indoor concentrations of carbonaceous aerosols of outdoor origin

“…3, the indoor behavior of BC aerosol is governed by physical penetration and deposition losses only. The measurements performed in Bakersfield (Herner et al, 2006) showed that both BC and OC fall in the same size range in the wintertime; thus if both species are governed by the same physical loss factors, both should have similar indoor/outdoor ratios. During other seasons, BC and OC may be present in different size ranges, which would alter the physical factors controlling the indoor-outdoor ratio.…”

“…It is necessary, therefore, to have some knowledge of the size resolved chemical composition of the aerosol in the Fresno area. Few such measurements exist; however, data of this type was collected in Bakersfield as part of the California Regional Particulate Air Quality Study conducted at the same time as the present study (Herner et al, 2006). Bakersfield is 124 miles southeast of Fresno, and experiences similar meteorology and air quality issues.…”