New particle growth and shrinkage observed in subtropical environments

Abstract

Abstract. We present the first systematic analysis for new particle formation (NPF), growth and shrinkage of new particles at four different sites in subtropical central Taiwan. A total of 14 NPF events were identified from 137 days of ambient measurements during a cold and warm season. The measured formation rates of 10 nm particles (J 10 ) and growth rates were in the range of 4.4-30 cm −3 s −1 and 7.4-24 nm h −1 , respectively. The onset of NPF events coincided with decreases of condensation sink (CS) and increases of SO 2 under enhanced atmospheric mixing and dilution. However, the lower or comparable SO 2 on event days than on non-event days suggests that SO 2 was not a limiting factor for NPF. On non-event days, the particle number concentrations were mostly driven by traffic emissions. We also observed shrinkage of new particles, the reversal of growth, during five out of the identified secondary formation. UFP particles events. In intense cases, the grown particles shrank back to the smallest measurable size of ∼ 10 nm, thereby creating a unique "arch-like" shape in the size distribution contour plot. The particle shrinkage rates ranged from −5.1 to −7.6 nm h −1 . The corresponding particle volume losses suggest that a notable fraction of the condensable species that contributed to growth was semi-volatile. The particle shrinkage was related to enhanced atmospheric dilution, high ambient temperature and low relative humidity, thus favoring the evaporation of semi-volatile species from the particulate phase to the gas phase. Our observations show that the new particle growth could be a reversible process, in which the evaporating semi-volatile species are important for the growth of new particles to sizes of environmental health concerns.

Observations of New Particle Formation, Subsequent Growth and Shrinkage during Summertime in Beijing

“…The occurrence of particle shrinkage has been reported in São Paulo (Backman et al, 2012), central Taiwan (Young et al, 2013), and Hong Kong (Yao et al, 2010). Particle shrinkage of preexisting Aitken mode particles and newly formed particles has also been reported in a recent study (Cusack et al, 2013).…”

“…Particle shrinkage always occurs after the nucleation and growth process, the mechanism of particle shrinkage is probably associated to the evaporation of condensed semivolatile species, especially when the condensation process or the chemical reactions involved in the growth of particles are reversible (Yao et al, 2010;Backman et al, 2012;Young et al, 2013). Thus, particle shrinkage is usually accompanied by a reduction of the total particle concentration (Backman et al, 2012).…”

“…The first peak was reached at 11:00 LT, with a maximum of 4813 cm -3 , whereas the second was reached at 16:30 LT, with a maximum of 4740 cm -3 . Considering that no nucleation occurred in that period, it seemed that nearly all the newly formed particles had shrunk back to small sizes after growing to Aitken mode, creating an "arch-like" shape in the size distribution contour plot (Young et al, 2013). The increase in number concentration of nucleation mode particles could not be induced by plume nucleation which has been reported by several recent studies (Wu et al, 2007) since the number concentration of particles in atmosphere was reducing at that period whereas plume nucleation would rapidly increase the particles number concentration within a very short time.…”

“…The black dotted line represented the temporal variation in GMD fitted with log-normal function. The favorable conditions for particle shrinkage remained highly uncertain, recent studies suggested that particle shrinkage was probably attributed to the atmospheric dilution and evaporation processes due to changes in meteorological conditions, mainly an elevation of wind speed and temperature (Cusack et al, 2013;Young et al, 2013). One thing those particle shrinkage events observed in various regions in common was that the events were more likely to occur in the afternoon during the warm season, indicating the intensive solar radiation and meteorological conditions would favor the evaporation of semi-volatile species.…”

A simple model for the time evolution of the condensation sink in the atmosphere for intermediate Knudsen numbers

“…The average condensation sink on new particle formation event days is generally lower as compared with nonevent days 20 (Kulmala et al, 2001;Birmili et al, 2003;Hyvönen et al, 2005;Wu et al, 2007;Asmi et al, 2011;Pikridas et al, 2012;Young et al, 2013;Kanawade et al, 2014;Qi et al, 2015;Vana et al, 2016). Indeed, a large condensation sink means that a relatively large surface of aerosol is available for condensation and coagulation with clusters serving as precursors of newly forming particles.…”