Particle shrinkage events were identified during five out of the 14 NPF events.
Two shrinkage events were observed at the urban site, two at the downwind site and one at the coastal site. In the following, we present two examples of particle shrinkage, one where the Dmode and N10-25 were positively correlated and the other
negatively correlated. The evolution of particle number size distributions during the remaining three events can be found in Fig. A3.
On the 12 August 2010 at the urban site in the warm season, the start of morning NPF occurred at ~08:00 LT in the midst of traffic rush hours (Fig. 5). It was then immediately followed by intense particle growth from 11 to 51 nm until 12:30 LT.
The maximum N10-25 of 5.2104 cm-3 at 10:00 LT was preceded by a local minimum CS of 1.710-2 s-1, and it was correlated with a SO2 peak of 3.6 ppb. The median J1
and GRPARGAN were 216 cm-2 s-1 and 7.4 nm h-1, respectively. Between 12:30 and 16:00 LT, the Dmode and CS decreased from 32 to 11 nm and 3.110-2 to 1.510-2 s-1, respectively. It is notable that the shrinkage occurred during periods with the strongest atmospheric dilution (max. mixing height 590 m), highest ambient temperature (max.
34 °C) and lowest relative humidity (min. 61%). The continuous decrease of CS as well as CO and NOx during the shrinkage period suggests that the site was not impacted by local primary emissions. Meanwhile, the N10-25 and SO2 were increasing with time. The N10-25 reached another maximum of 5.1104 cm-3 near the end of the shrinkage. The GRMOD during the growth and shrinkage were 11 and -5.6 nm h-1, respectively. The winds of below 1.7 m s-1 were mild throughout the day. The winds were from the south before 12:00 LT, but then changed to westerly during the shrinkage period. The change of wind direction was accompanied with a slight increase of wind speed and the appearance of cloud clover, as indicated by the sudden drop of UVI at 14:00 LT. The maximum O3 was 51 ppb. The SO2 peak of 3.4 ppb during the shrinkage period was of particular interest as the rise of SO2 levels was simultaneously observed at three other air quality monitoring sites near the urban basin (Fig. A4). These results suggest that the SO2-enriched and low-CS air mass due
to atmospheric advection and mixing may in turn have caused another NPF event near the end of particle shrinkage period, while the enhanced atmospheric dilution and hot/dry ambient conditions favor the evaporation of semi-volatile species from the particles to the gas phase. The former explains the increasing N10-25 whereas the latter results in the decreasing Dmode.
On the 3 January 2009 at the coastal site in the cold season, the midday NPF between 10:30 and 13:00 LT was followed by intense particle shrinkage (Fig. 6).
During this period, the N10-25 increased rapidly from 1.0104 cm-3 to 2.5104 cm-3. The SO2 also increased from 2.1 to 4.0 ppb. Afterwards, the particles grew from 11 to 36 nm. The median J1 and GRPARGAN were 99 cm-2 s-1 and 15 nm h-1, respectively. Then the N10-25 gradually decreased to 0.6104 cm-3 by 19:00 LT and the Dmode decreased from 30 to 11 nm between 14:00 and 17:00 LT. The GRMOD during the growth and shrinkage were 7.8 and -5.1 nm h-1, respectively. The elevated CS, SO2, CO and NOx
at 16:00 LT indicate an impact of polluted air. However, it did not affect the apparent particle shrinkage. Although not measured on-site, a nearby monitoring site (8.3 km to the southeast) shows that it was a cloudless, sunny day with the maximum UVI of 4.1 between 12:00 and 13:00 LT. The maximum O3 was 47 ppb. The prevailing north-northeasterly winds were strong (> 4 m s-1), cool and stable throughout the event day.
It is also notable that the temperature increased from a minimum 12 °C to a maximum 20 °C between 12:00 and 16:00 LT. These results suggest that the strong dilution due to elevated wind speed and the large temperature rise were likely the driving forces leading to the evaporation of semi-volatile species from the particles to the gas phase.
However, unlike the previous example, the above particle shrinkage is characterized by a simultaneous decrease of Dmode and N10-25.
The above observations clearly show that new particle growth can be a reversible process under certain atmospheric conditions. For example, enhanced dilution and hot/dry ambient conditions can decrease the ambient vapor partial pressure or increase the equilibrium vapor pressure over the particle surface. As such, the condensed vapors tend to evaporate off the particle phase. At present it is unclear what were the evaporating vapors involved during the particle shrinkage. Potential candidates include semi-volatile ammonium nitrate (NH4NO3) and organics compounds due to their high volatility and abundance in ambient air. In central Taiwan, Fang et al.
(2006) showed that the major ionic species in ultrafine particles are SO42-, followed by NH4+ and NO3-. Furthermore, Lin et al. (2006) reported the annual average of NH3
was 12.3 ppb, with the highest average of 16.4 ppb observed in the summer. More recently, Bzdek et al. (2012) showed that 29-46% of the total mass growth of new particles were attributable to SO42- at an urban site in Wilmington, DE, USA. The remaining, more than 50%, new particle growth was due to NO3-, NH4+ and organics.
In addition, Riipinen et al. (2012) suggested that the reversible net condensation of gas-phase oxidation products is an important process governing organic vapor uptake by nanoparticles. The above studies demonstrate the potentially abundant and semi-volatile NH4+, NO3- and organics in the particle phase, and thus we speculate that they may have been involved in the particle shrinkage in the present study.
4 Conclusions
This study provides the first systematic analysis for new particle formation (NPF) and growth events at four distinct types of environment (urban, coastal, mountain and downwind) in an air quality management district of subtropical central Taiwan. A total of 14 NPF and growth events were identified from October 2008 to January 2009 and from August 2010 to October 2010, and analyzed for aerosol characteristics, air
pollutant and meteorological conditions. Some NPF events were more or less limited to morning hours, whereas others occurred during midday hours. In either cases, the onset of NPF events coincided with decreases of CS and increases of SO2 during periods (09:00-13:00 LT) under enhanced atmospheric mixing and dilution. The increase of SO2 with enhanced dilution, in particular, suggests that the SO2 was not from local traffic emissions but likely from power plant or industrial plumes transported or mixed down from aloft to the observation sites. Nevertheless, the lower or comparable SO2 on event days than on non-event days suggests that SO2 was not a limiting factor for NPF.
Overall, the NPF events contributed significantly to the ambient ultrafine particles, with the maximum concentrations of 2.5-8.9104 cm-3, which are 2-4 times higher than that due to typical traffic emissions. The approximation-derived particle formation rate J10 and growth rates GRMOD were in the range of 4.4-30 cm-3 s-1 and 7.4-24 nm h-1, respectively. The nucleation rates J1 and growth rates GRPARGAN inverted from the measured aerosol size distributions were in the range of 40-253 cm-3 s-1 and 6.5-15 nm h-1, respectively.
A unique feature of our observations is the particle shrinkage following their prior formation and growth. In the five shrinkage events, their respective GRMOD ranged from -5.1 to -7.6 nm h-1. The shrinking and corresponding particle volume losses indicate that a notable fraction of the originally condensed chemical species was semi-volatile and had evaporated from the particle phase to gas phase under atmospheric conditions. In intense cases where particles shrank back to the smallest measurable size of ~10 nm, the new particle growth and shrinkage thereby created a unique “arch-like” shape in the size distribution contour plot. Time-series analyses of air pollutants
and meteorological conditions indicate that the particle shrinkage was related to air masses with enhanced atmospheric dilution, high ambient temperature and low relative humidity. Such atmospheric conditions favor the evaporation of semi-volatile species from the particle phase to the gas phase. Although chemical identification of the potential evaporating vapors was not possible in the present study, we speculate that the candidate vapors can be NH4+, NO3- and organics due to their semi-volatile nature and abundance in the study area. Chemical and volatility analysis of the nucleating and condensing/evaporating vapors is required in the future to infer more definitive mechanisms of NPF, particle growth and shrinkage that are relevant to climate change and public health.
A1 Eight consecutive morning NPF events with two particle shrinkage events Fig. A1 shows the diurnal variations of the number size distributions during eight consecutive morning NPF events at the urban site from the 10 to the 17 August 2010.
Among them there were two particle shrinkage events on the 12 and the 16 August 2010. As shown, the particles initially grew to sizes of 50-60 nm and then shrank back to the smallest measurable sizes of ~10 nm, creating a unique “arch-like” shape in the contour plot.
A2 Diurnal variations of mixing height, air pollutants, condensation sink and ultrafine particle number on non-event and event days
Fig. A2 presents the diurnal variations of the average mixing height, air pollutants (NOx, CO, SO2), condensation sink (CS), and 10-100 nm ultrafine particle number (N10-100) concentrations on non-event and event days. As shown, the differences of hourly pollutant levels and mixing height between the non-event and event days were not of statistically significance. Nevertheless, the diurnal trends of the hourly-average CS and N10-100 on events days deviated away from those on non-event days during late morning hours, between 09:00-12:00 LT. On event days, the 2 hours lag between the NOx and CO peaks at 08:00 LT and the N10-100 peak at 10:00 LT suggests that the elevated ultrafine particles was not emitted directly from motor vehicles but formed shortly afterwards in the atmosphere, i.e., via NPF. On non-events days, the diurnal variations of N10-100 were nearly identical to those of NOx and CO, indicating strong influence from traffic emissions.
A3 Three other particle shrinkage events
Fig. A3 shows the measured number size distributions, Dmode, N10-25 and CS during the three other particle shrinkage events on the (a) 16 August 2010, (b) 5 September 2010 and (c) 7 September 2010. As shown, the grown particles in the first event (a) shrank
back to the smallest measurable particle size of ~10 nm, whereas in the remaining two events (b and c) they shrank back to sizes of ~20 nm and then stopped. The N10-25
during the first and third events were increasing with time, whereas during the second event they were decreasing with time.
A4 Regional-scale SO2 impact
Fig. A4 shows the diurnal variations of SO2 at the present urban site (JM) and three nearby air quality monitoring sites on the 12 August 2010. As shown, the rise of SO2
was simultaneously observed at three other air quality monitoring sites near the urban basin, suggesting regional-scale SO2 impact occurred at 15:00 LT.
Acknowledgements. The authors wish to express their appreciation to the Editor V.-M.
Kerminen and anonymous referees for their helpful comments, and the Taiwan EPA for making the air quality monitoring sites available for the present study. The financial support from the Taiwan National Science Council (NSC97-2218-E-039-002-MY3 and NSC100-2628-E-039-001-MY3) and the China Medical University (CMU98-N1-26) are gratefully acknowledged.
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