| dc.description.abstract | During the summer of 2002, a modified tandem differential mobility analyzer
(TDMA) was used to examine the size-resolved hydration state of the ambient aerosol in
Southeast Texas. Although there were slight variations in the measured properties over
the course of the study, the deliquescent particles observed were almost always present as metastable aqueous solutions. A relative humidity (RH) scanning TDMA system was
used to measure the deliquescence/crystallization properties of ambient aerosol
populations in the same region. During August, sampling was conducted at a rural site in
College Station, and in September at an urban site near the Houston ship channel.
Measurements from both sites indicate cyclical changes in the composition of the soluble fraction of the aerosol, which are not strongly linked to the local aerosol source. The observations show that as temperature increases and RH decreases, the hysteresis loop
describing the RH-dependence of aerosol hygroscopic growth collapses. It is proposed
that this collapse is due to a decrease in the ammonium to sulfate ratio in the aerosol
particles, which coincides with increasing temperature and decreasing RH. This cyclical
change in aerosol acidity may influence secondary organic aerosol (SOA) production and
may exaggerate the impact of the aerosol on human health. The compositional changes
also result in a daily cycle in crystallization RH that is in phase with that of the ambient
RH, which reduces the probability that hygroscopic particles will crystallize in the
afternoon when the ambient RH is a minimum. During June and July of 2004 airborne
measurements of size-resolved aerosol hygroscopic properties were made near Monterey,
California. These were used to examine the change in soluble mass after the aerosol had
been processed by cloud. The calculated change in soluble mass after cloud-processing
ranged from 0.66 g m-3 to 1.40 g m-3. Model calculations showed these values to be
within the theoretical bounds for the aerosols measured. Mass light-scattering efficiencies
were calculated from both an averaged aerosol size distribution and from distributions
modified to reflect the effects of cloud. These calculations show that the increase in mass
light-scattering efficiency should be between 6% and 14%. | en |
