Size Distribution and Diurnal and Seasonal Trends of Ultrafine Particles in Source and Receptor Sites of the Los Angeles Basin

Abstract

This paper presents results from a study conducted in two urban areas of southern California, Downey and Riverside, to examine the effect of different sources and formation mechanisms on the size distribution and temporal trends of ultrafine particles. Near-continuous data were collected for 5 months at each location. Our data clearly identified Downey as a source site, primarily affected by vehicular emissions from nearby freeways, and Riverside as a receptor site, where photochemical secondary reactions form a substantial fraction of particles, along with local vehicular emissions. In Downey, the diurnal trends of total particle number concentration and elemental carbon (EC) appear to be almost identical throughout the day and irrespective of season, thereby corroborating the role of primary emissions in the formation of these particles. This agreement between EC and particle number was not observed in Riverside during the warmer months of the year, while very similar trends to Downey were observed during the winter months in that area. Similarly, the size distribution of ultrafine particles in Downey was generally unimodal with a mode diameter of 30-40 nm and without significant monthly variations. The number-based particle size distributions obtained in Riverside were bimodal, with a significant increase in accumulation mode as the season progressed from winter to summer. During the warmer months, there was also an increase in sub-100-nm particles in the afternoon hours, between 2:00 p.m. and 4:00 p.m., that also increased with the temperature. The differences observed in the ultrafine particle distribution and temporal trends clearly demonstrated that mechanisms other than direct emissions play an important role in the formation of ultrafine particles in receptor sites of the Los Angeles Basin.

Seasonal and spatial trends in particle number concentrations and size distributions at the children's health study sites in Southern California

“…It should be noted that the proximity of that site to the ocean results in unusually higher relative humidity levels compared to the rest of the urban sites, with prolonged periods of night time and morning fog. The smaller summertime number median diameter could be due to the increased photochemical production of smaller particles, as observed by Kim et al (2002) and Wehner and Wiedensohler (2003). During the first week of October, union workers at the port of Long Beach went on strike.…”

“…During summer months, secondary aerosol formation is favored and new ultrafine particles may form as a result of the condensation of low-volatility products of photochemical reactions (largely organic compounds) onto stable, nanometer-size particles (O'Dowd et al, 1999;Kim et al, 2002;Sardar et al, 2004). Secondary aerosol formation is the most likely explanation for the diurnal trends in PN during the summer period at USC and Long Beach (Figures 4b and 5b, respectively) in which the peak particle concentrations during the afternoon period either coincide or slightly lag behind the peak in O 3 concentrations.…”

“…In addition to their direct emission in the atmosphere, particles may be formed as a result of photochemical reactions from gaseous precursors, including particulate sulfate formed from precursor sulfur dioxide, and secondary organic aerosols, formed from oxidation of aromatic hydrocarbons (Derwent et al, 2000). The secondary aerosol formation is largely governed by meteorological factors (Ma¨kela¨et al, 1997;Kim et al, 2002). The high degree of temporal variability of the meteorological parameters such as degree of solar radiation, atmospheric mixing depth, humidity and temperature F all contribute to the temporal variation in particulate number concentrations at a location.…”

“…Some recent continental sampling campaigns that measured size distributions include the Pittsburgh Air Quality Study (Stanier et al, 2004), the Atlanta PM Supersite program (Woo et al, 2001), and sampling campaigns in Los Angeles (Kim et al, 2002;Fine et al, 2004), Northern Europe (Ruuskanen et al, 2001), Tennessee (Cheng and Tanner, 2002), Brisbane, Australia (Morawska et al, 2002), the UK (Harrison et al, 2000), Estonia and Finland (Kikas et al, 1996) and Central Europe (Birmili et al, 2001). Most of these studies were con ducted in urban areas in which the vast majority of ultrafine PM originate from primary sources (Morawska et al, 1998;Harrison et al, 2000;Woo et al, 2001), thus their diurnal profiles match those of local vehicular sources.…”

“…The spatial aerosol characteristics at different locations of a city have also been examined. Kim et al (2002) identified periods of photochemistry and long-range advection as sources of ultrafine PM at two sites in Los Angeles Basin in addition to local vehicular emissions. Fine et al (2004) inferred sources of ultrafine particles at two different locations in the eastern portion of Los Angeles Basin.…”