| dc.description.abstract |
During the Pittsburgh Air Quality Study (PAQS) aerosol size distributions between
3 nm and 680 nm were measured between July 2001 and June 2002. These distributions
have been analyzed to assess the importance of nucleation as a source of ultrafine
particles in Pittsburgh and the surrounding areas. The analysis shows nucleation on
50\% of the study days and regional-scale formation of ultrafine particles on 30\%
of the days. Nucleation occurred during all seasons, but it was most frequent in fall
and spring and least frequent in winter. Regional nucleation was most common on sunny
days with below average PM2.5 concentrations. Local nucleation events were usually
associated with elevated SO2 concentrations. The observed nucleation events ranged
from weak events with only a slight increase in the particle number to relatively
intense events with increases of total particle counts between 50,000 cm(-3) up to
150,000 cm(-3). Averaging all days of the study, days with nucleation events had number
concentrations peaking at around noon at about 45,000 cm(-3). This is compared to
work days without nucleation, when the daily maximum was 8 am at 23,000 cm-3, and
to weekends without nucleation, when the daily maximum was at noon at 16,000 cm(-3).
Twenty-four-hour average number concentrations were approximately 40\% higher on days
with nucleation compared to those without. Nucleation was typically observed starting
around 9 am EST, although the start of nucleation events was later in winter and earlier
in summer. The nucleation events are fairly well correlated with the product of {[}UV
intensity {*} SO2 concentration] and also depend on the effective area available for
condensation. This indicates that H2SO4 is a component of the new particles. Published
correlations for nucleation by binary H2SO4-H2O cannot explain the observed nucleation
frequency and intensity, suggesting that an additional component (perhaps ammonia)
is participating in the particle formation.
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