ACPAtmospheric Chemistry and PhysicsACPAtmos. Chem. Phys.1680-7324Copernicus PublicationsGöttingen, Germany10.5194/acp-19-115-2019Atmospheric new particle formation in ChinaAtmospheric new particle formation in ChinaChuBiwubiwu.chu@helsinki.fihttps://orcid.org/0000-0002-7548-5669KerminenVeli-Mattihttps://orcid.org/0000-0002-0706-669XBianchiFedericohttps://orcid.org/0000-0003-2996-3604YanChaohttps://orcid.org/0000-0002-5735-9597PetäjäTuukkahttps://orcid.org/0000-0002-1881-9044KulmalaMarkkuhttps://orcid.org/0000-0003-3464-7825Institute for Atmospheric and Earth System Research/Physics, Faculty of Science, University of Helsinki, Helsinki, FinlandAerosol and Haze Laboratory, Beijing Advanced Innovation Center for Soft Matter Science and Engineering, Beijing University of Chemical Technology, Beijing, ChinaJoint International Research Laboratory of Atmospheric and Earth System Sciences, School of Atmospheric Sciences, Nanjing University, Nanjing 210023, ChinaBiwu Chu (biwu.chu@helsinki.fi)4January201919111513821June201813August20182December201813December2018This work is licensed under the Creative Commons Attribution 4.0 International License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/4.0/This article is available from https://acp.copernicus.org/articles/19/115/2019/acp-19-115-2019.htmlThe full text article is available as a PDF file from https://acp.copernicus.org/articles/19/115/2019/acp-19-115-2019.pdf
New particle formation (NPF) studies in China were summarized comprehensively
in this paper. NPF frequency, formation rate, and particle growth rate were
closely compared among the observations carried out at different types of
sites in different regions of China in different seasons, with the aim of
exploring the nucleation and particle growth mechanisms. The interactions
between air pollution and NPF are discussed, emphasizing the properties of
NPF under heavy pollution conditions. The current understanding of NPF cannot
fully explain the frequent occurrence of NPF at high aerosol loadings in
China, and possible reasons for this phenomenon are proposed. The effects of
NPF and some aspects of NPF research requiring further investigation are
also summarized in this paper.
Introduction
Atmospheric aerosols have adverse effects on human health and visibility,
and cause severe air pollution in many countries (Kaiser, 2005; Cheng et
al., 2011; Hand and Malm, 2007; Lelieveld et al., 2015). In addition, aerosol
particles influence the Earth's radiation balance due to their direct
extinction of light and their capability to serve as cloud condensation
nuclei (CCN) or ice nuclei (IN). These influences result in very high
uncertainties in predicting ongoing climate change (IPCC, 2013).
In order to understand these effects better, and especially to reduce the
uncertainty of evaluating their role in climate change, comprehensive
knowledge about the formation and growth of aerosol particles in the
atmosphere is required. Atmospheric new particle formation (NPF) is the
dominant source of atmospheric aerosol particles on a global scale in terms
of number concentrations and has attracted a wide range of attention for couple of
decades (Kulmala et al., 2004, 2013; Merikanto et al., 2009; Dunne et al., 2016).
Generally, NPF includes the following separate steps: (1) chemical reactions
in the gas phase to produce low-volatility vapour(s), (2) cluster formation
from gaseous vapours, (3) nucleation or barrierless nucleation,
(4) activation of clusters with a second group of vapours to form a critical
nucleated particle, and (5) subsequent condensational growth of nucleated
particles to detectable sizes or even larger (Kulmala et al., 2014). NPF starts from atmospheric
clustering. The key precursors of clusters have extremely low-volatility,
including sulfuric acid (Sipilä et al., 2010; Kirkby et al., 2011) and
highly oxygenated molecules (HOMs) (Bianchi et al., 2016; Tröstl et
al., 2016; Kirkby et al., 2016; Ehn et al., 2014). Molecular clusters seem to
be continuously generated almost everywhere and all the time
(Kulmala et al., 2017). These clusters can be further
stabilized by reacting with other gaseous compounds like amines, ammonia, and
HOMs, or through electrostatic interactions in the presence of ions
(Kirkby et al., 2016), after which they will grow to larger nanoparticles
or will be scavenged by existing surfaces. Therefore, there are two main
factors controlling whether NPF will be detected in the atmosphere. One is
how fast the clusters grow, while the other is how fast the clusters are
scavenged (McMurry and Friedlander, 1979; Kerminen et al., 2001; McMurry et
al., 2005; Kuang et al., 2010). Sulfuric acid and organics are the main
contributors to aerosol growth. Generally, condensation of sulfuric acid
gives an important, sometimes dominant, contribution to the initial growth,
while organics became more and more important as the particle size is
increased (Xiao et al., 2015; Kulmala et al., 2016b). High concentrations
of these growth contributors will help the nanoclusters grow to sizes large
enough be detected. Meanwhile, pre-existing aerosol particles act as a sink
for these precursors, small clusters, and newly formed particles and thereby
suppress the occurrence of an NPF event.
Several gas compounds and precursors have been shown to influence NPF under
conditions relevant to the atmosphere, such as SO2/H2SO4
(Sipilä et al., 2010), NH3 (Kirkby et al.,
2011; Kürten et al., 2016), amines (Almeida et al., 2013), volatile
organic compounds (VOCs)/HOMs (Riccobono et al., 2014; Ehn et al.,
2014; Bianchi et al., 2016), NOx (Wildt et al., 2014) and iodine species
(Sipilä et al., 2016). Meanwhile, many of these compounds are
responsible for secondary aerosol formation, which is very pronounced during
pollution episodes (R. Zhang et al., 2015; Guo et al., 2014). The
concentrations of these precursors and pre-existing aerosol particles can
both be high in polluted cities, especially in the developing countries like
China and India, and cause some special features in NPF events compared with
cleaner environments, which we cannot explain yet (Kulmala et
al., 2017). In China, rapid economic development and urbanization have
led to high emissions of various pollutants from coal combustion, motor
vehicle exhausts and various industrial emissions, and resulted in highly
complex air pollution. Besides high concentrations of fine particles
(PM2.5, particulate matter with diameters less than 2.5 µm), high
concentrations of SO2, NOx, NH3, and VOCs were observed in
frequent haze pollution episodes (Liu et al., 2013; Ye et al., 2011; Zou et
al., 2015; L. Wang et al., 2015). Due to a large proportion of energy supply
from coal combustion, the concentration of SO2 was thought to be the
highest in the world (Bauduin et al., 2016), with surface
concentrations in the range of a few ppb to over 100 ppb in northern China
(Sun et al., 2009; Li et al., 2007). The emissions and concentrations of
SO2 decreased in most regions of China in recent years
(Lu et al., 2010; S. W. Wang et al., 2015), but high concentrations (dozens of
or over 100 ppb) of SO2 are still being frequently observed during the
heating period in winter (L. Wang et al., 2015; Q. Zhang et al., 2015). Unlike
SO2, emissions of NOx are also closely related to traffic. NOx emissions
in China showed a decreasing trend from 2012 onwards, which appeared later
than SO2 (Ronald et al., 2017). Several studies have found that high
PM2.5 concentrations are strongly associated with the increasing
concentrations of NOx (Y. Wang et al., 2013; He et al., 2014; Ma et al.,
2018; Sun et al., 2016). NOx concentrations usually range from a few ppb to
dozens of ppb in Chinese cities, while during severe haze pollution episodes
NOx concentration in the city centre can be even higher than 300 ppb (He
et al., 2014; Sun et al., 2016). For the most important alkaline gas, i.e.
NH3, there has been no national-scale measurement in China despite its
extensive emissions and increasing emission trend (Fu et al., 2015). High concentrations of
NH3 (maximum concentration higher than 100 ppb) (Z. Meng et al., 2015; Wen et
al., 2015; Meng et al., 2011, 2018; Pan et al., 2012,
2018) and strong correlations between the peak levels of fine particles and
large increases in NH3 concentrations (Liu et al., 2015; Ye et al.,
2011) were observed in the North China Plain. Unlike SO2, emissions of
NH3 are mainly from non-point sources difficult to control. Emission of
VOCs have a similar situation to NH3. The total emissions of VOCs in
China were estimated to be still increasing in recent years (Wei
et al., 2011; Wu et al., 2016; Zheng et al., 2018; Sun et al., 2018).
Observation data showed that the annual average mass concentration of total
non-methane hydrocarbons (NMHCs) was about 102µg m-3, or
dozens of ppb at urban and suburban sites in Chinese cities, which is much
higher than that in North America (H. Zhang et al., 2017; von Schneidemesser
et al., 2010; Parrish et al., 2009; Zou et al., 2015). HOMs can be formed from
anthropogenic VOCs (Molteni et al., 2018), although their
role in new particle formation is still not clear, yet they might play an
important role in NPF measured in Chinese megacities. High concentrations of
these gas precursors have resulted in high concentrations of secondary
inorganic and organic species in PM2.5 during haze formation (Yang
et al., 2011; Zhao et al., 2013; Dan et al., 2004; Duan et al., 2005; Wang et
al., 2012), but how this cocktail of high concentrations of SO2, NOx,
NH3, VOCs and particulate matter (or highly complex air pollution)
influence NPF remains highly uncertain.
Atmospheric NPF has been observed globally in almost all kinds of
environments (Kulmala et al., 2004, 2016b; Wang et al., 2017;
Manninen et al., 2010; Nieminen et al., 2018; Kerminen et al., 2018).
However, no uniform theory has been found that would explain the occurrence
and characteristics of NPF in different atmospheric environments. Generally,
NPF was observed less frequently than expected in pristine environments,
while more often than theoretically predicted in polluted cities
(Kulmala et al., 2017). In this study, we will summarize the
NPF studies conducted in China, focusing on the properties of the NPF events
in polluted regions and trying to figure out the possible reasons for the
frequent occurrence of NPF at high aerosol loadings. Recently, Wang et al. (2017)
summarized the techniques, recent advances, current bottlenecks
and future directions in studying NPF in China, while this study will
provide a more comprehensive summary of the characteristics of NPF and will
emphasize the interactions between air quality and NPF.
Map of observation sites involving NPF study in China. Most of these
observations sites were classified into four regions in this study, i.e. North
China Plain (NCP), Yangtze River delta (YRD), Pearl River delta (PRD) and western
China region (western).
Overview of NPF research in China
Field observation related to atmospheric NPF started around 2004 in China
(Wu et al., 2007). After that, observations concerning NPF were carried
out in the North China Plain (NCP), Yangtze River delta (YRD), Pearl River delta (PRD),
western Chinese cities like Lanzhou, Xi'an, and Urumqi, and coastal
cities as well as adjacent seas. NCP, YRD, and PRD are the most developed
regions in China and they all have a high population density. The air
pollution level decreases from NCP to PRD, or from north to south, among
these three regions (Zhang and Cao, 2015). The western Chinese cities
like Xi'an and Urumqi suffered from heavy air pollution. Xi'an was reported to
have a much higher concentration of fine particles than Beijing in the NCP
(Huang et al., 2014). In 2017, according to the reports of the
Xi'an Environmental Protection Bureau (http://xaepb.xa.gov.cn/ptl/def/def/index_982_4434_ci_trid_2861812.html,
last access: 12 October 2018), Xinjiang Department of Environmental
Protection (http://www.xjepb.gov.cn/xjepb/resource/cms/article/2012/268650/2017.pdf,
last access: 12 October 2018) and Beijing Municipal Environmental Protection
Bureau (http://www.bjepb.gov.cn/bjhrb/resource/cms/2018/05/2018051614522475279.pdf,
last access: 12 October 2018), the annual average PM2.5 concentrations
were 73 and 70 µg m-3 in Xi'an and Urumqi,
which were higher than that of Beijing (58 µg m-3).
A map of observation stations involving NPF studies in China is shown in
Fig. 1. These observation sites include urban and suburban sites like
Beijing, Shanghai, Nanjing, and Guangzhou; regional and rural sites like
Shangdianzi, Yufa, SORPES (Nanjing University), Backgarden, and Kaiping;
and mountain sites like Waliguan, Tai, Heng, and Huang, providing
information on aerosol size distribution in different environments. Besides
routine observations, comprehensive campaigns like PRIDE-PRD2004,
CAREBeijing-2006, and CAREBeijing-2008 were also carried out for a better
understanding of NPF and aerosol pollution in representative regions and
periods in China. Long-term observations of NPF are relatively rare in China,
and only a few studies reported NPF observations covering more than a
1-year period (Wu et al., 2007, 2011; Kivekäs et al., 2009; Yao et al., 2010;
Shen et al., 2011; Qi et al., 2015; Peng et al., 2017). The
relative short-period observations may not represent varying atmospheric
conditions, and therefore, the applicability of these observation results
may be limited to specific conditions.
Up to now, about 100 papers from about 20 groups have been published related
to NPF in China. Most of these studies focused on the characterization of
NPF events, such as the properties and time evolution of the particle size
distribution, particle formation and growth rate, and condensation sink.
Some of them also studied favourable conditions for NPF, including the
influences of relative humidity (RH), temperature, wind speed and direction,
and air mass origin. Few of these studies investigated NPF mechanisms
involving the nucleation participants, the growth contributors and the
scavenging process by preexisting aerosols, while some others investigated
various effects of NPF, especially the contribution of NPF to atmospheric CCN.
Parameters to characterize NPF events.
ParameterDescriptionCalculated fromFRFormation rate of particlesTemporal variation of particle size distributionGRGrowth rateTemporal variation of particle size distributionCSCondensation sinkParticle size distributionCoagSCoagulation sinkParticle size distributionCcvCondensation vapour concentrationGRQSource rate of condensation vapoursCS and Ccv
The aerosol size distribution and its time evolution provide the basic
information for studying NPF. Many studies about NPF in China measured only
particles larger than 10 nm, while a few studies also measured particles
with diameters in the range of 3–10 nm. In recent years, an increasing
number of studies were carried out with measurements of sub-3 nanoparticles
(Xiao et al., 2015; Cai et al., 2017; Cai and Jiang, 2017; Jayaratne et al.,
2017; Dai et al., 2017; Lv et al., 2018; Yao et al., 2018), using a particle size
magnifier (PSM), a neutral cluster and air ion spectrometer (NAIS) or
a diethylene glycol scanning mobility particle spectrometer (DEG-SMPS). As
for gas-phase precursors, direct measurements of H2SO4 were
carried out with atmospheric pressure-ion drift-chemical ionization mass
spectrometer (AP-ID-CIMS) in a few studies (Yue et al., 2010; Zheng et
al., 2011; Wang et al., 2011), while other studies usually estimated
H2SO4 concentrations using different proxies related to SO2,
radiation, O3, and relative humidity (RH). Amines and ammonia are
crucial in NPF since they are able to stabilize sulfuric acid clusters by
forming acid-base complexes, yet there are very few NPF measurement
results related to these compounds in China (J. Zheng et al., 2015; Yao et
al., 2016, 2018). Measurement results on natural ions and neutral
compounds and clusters were recently reported by Yao et al. (2018), including both H2SO4 and HOMs, obtained using
an atmospheric pressure interface time-of-flight mass spectrometer (APi-TOF-MS)
and a nitrate-based chemical ionization–APi-TOF-MS (CI-APi-TOF-MS).
Research on these relevant gaseous compounds, like HOMs, or on air ions, is
still very limited in China. Comprehensive, long-term and high-quality
relevant measurements are required for a better understanding of the
nucleation and growth mechanisms of nanoparticles in China.
Characterization of NPF events in China
A few basic parameters were used to characterize NPF events, which are
listed in Table 1. Most of the NPF research in China calculated these
parameters, as listed in Table 2. In the following chapters, we will
summarize and discuss the measurement results of the frequency of NPF
events, new particle formation rate (FR), particle growth rate (GR) and the
related concentrations and source rate of condensation vapours. Although
there were differences in calculating these parameters by different groups,
we will not discuss much about the methodology, since the main purpose of
this paper is to provide an overview of the characteristics of NPF in China.
a EP is events percentage
(frequency).
b The average value and the range of the reported data in the
parenthesis. c These formation rates are the maximum FR during the
NPF event.
NPF frequency observed at different places in different seasons in
China. The number on top of each column indicates the references of the data: 1 is Cai et al. (2017), 2 is Leng et al. (2014), 3 is Zhu et al. (2013), 4 is Yue et al.
(2009), 5 is Zhang et al. (2011), 6 is Wang et al (2011), 7 is Peng et al. (2014),
8 is Yu et al. (2016), 9 is L. J. Shen et al. (2016a), 10 is Yue et al. (2013),
11 is Shen et al. (2016b), 12 is Dai et al. (2017), 13 is Jayaratne et al. (2017),
14 is Xiao et al. (2015), 15 is Tan et al. (2016), 16 is Wang et al. (2013b), 17 is Wu
et al. (2007), 18 is Gao et al. (2011), 19 is Gao et al. (2012), 20 is Guo et al.
(2012), 21 is X. H. Zhang et al. (2017), 22 is Herrmann et al. (2014), 23 is Peng et
al. (2017), 24 is Qi et al. (2015), 25 is Wang et al. (2013d), 26 is Liu et al.
(2008),
27 is X. J. Shen et al. (2016b), 28 is X. R. Zhang et al. (2017), 29 is Lv et al.
(2018), and 30 is Liu et al. (2014).
NPF frequency
The primary question in studying atmospheric NPF is whether it is taking
place or not, i.e. to identify NPF events. Unfortunately, there is no unique
mathematical criterion or definition for an NPF event. Dal Maso et al. (2005)
suggested criteria for justifying an NPF event: a
distinctly new mode of particles start in the nucleation-mode size range,
prevail over a time span of hours, and show signs of growth. The particle
growth is important for separating an NPF event from particles associated
with local emission sources like traffic, especially when the particle size
detection limit of the instruments is not low. In addition to NPF event
days, the days with an absence of particles in the nucleation-mode size
range are called non-event days. However, some days are not easily
classified as either events or non-events, so they are usually classified as
undefined days. Most NPF studies in China used similar methods, but
certainly subjective biases existed. A challenge that exists to identify
NPF is the interference of primarily emitted particles from local combustion
sources near the observation site. For example, the formation and rapid
growth of vehicular particles during the initial 1–2 s of exhaust cooling
and dilution processes frequently lead to a nucleation mode at 10–20 nm
(Vu et al., 2015; Lee et al., 2015). Spikes of particle number
concentration associated with combustion emissions were observed in many NPF
studies (Liu et al., 2014; D. W. Wang et al., 2014; Peng et al., 2017; Zhu et
al., 2017), but these spikes usually had some different characteristics from
those of the NPF events (D. W. Wang et al., 2014). The particle size
(Hofman et al., 2016), the ratio of number concentrations of in the
nucleation-mode particles to those of fine particles (Peng et al.,
2017; Jung et al., 2013), the time of duration of NPF events (Zhu et al.,
2017), and the correlation of the particle number concentration with other
gaseous pollutant concentrations and meteorology conditions (D. W. Wang et
al., 2014) were used to identify the contribution of primary emissions in
the burst in growth of particle number concentration. However, there are still
uncertainties in distinguishing the new-particle signal from the mixed
signals of newly formed particles and freshly emitted particles from
combustion, especially when NPF measurements were carried out with a
particle size detection limit larger than 10 nm. There is a possibility that
the growth of the vehicular emission of sub-10 nm particles may look like an
NPF event and therefore overestimate the NPF frequency. A recent
observation found a notable presence of traffic-originated nanocluster
aerosol particles in the size range of 1.3–3.0 nm in urban air
(Rönkkö et al., 2017), which might raise new questions
about the sources of nanocluster aerosol particles in semi-urban roadside
environments. In this study, as mentioned earlier, we will not pursue the
details of the justifying methods, but focus on the results of the measurement statistics.
NPF events were observed with quite different frequencies ranging from less
than 10 % to more than 50 % in different environments and different
seasons. In Fig. 2, we summarize the reported NPF event frequencies in
China according to the season, observation site type and region, but
ignoring observations of too short a period like less than 1 month.
Generally, low frequencies were observed in remote clean environments like
above marginal seas (Liu et al., 2014), while there were no
significant differences among urban, suburban and rural or regional sites.
Although higher NPF event frequencies were sometimes observed in an urban
site compared with a rural site in the same region (Yue et al., 2013),
NPF was usually found to be a regional phenomenon in China. For example, NPF
in the Beijing urban area always coincided with NPF at a regional site
120 km away (Wang et al., 2013b). Shen et al. (2018) observed regional NPF
in the NCP with a horizontal extent larger than 500 km and found that
large-scale regional NPF was favoured by a fast transport of northwesterly
air masses. Despite the similar frequencies, much higher FR (by 220 %) and
GR (by 50 %) were observed at the Beijing urban site than at the
corresponding regional background site (Yue et al., 2009; Wang et al.,
2013b). The corresponding values of a source rate of condensation vapours (Q),
condensation vapour concentration (Ccv), and condensation sink (CS) were also
larger at Beijing than those at the regional site Yufa by 40 %, 40 %,
and 60 %, respectively (Yue et al., 2009). These results indicated that
the higher pollution level in Chinese cities usually resulted in stronger
NPF events compared to rural areas. As for different regions, there seemed to
be no significant differences in the NPF event frequency between NCP, YRD
and western Chinese cities. PRD had a relatively lower NPF frequency compared
with these three regions, but the difference was not statistically
significant. Despite different pollution conditions in different regions of
China, there is a lack of long-term NPF observations, which limits our
knowledge about the relationship between the level of air pollution and the
occurrence of NPF. Air pollutants and meteorological features are usually
studied together with nanoparticles and their precursors. By comparing the
pollution character between NPF events and other days, the primary factors
affecting NPF events might be identified. Cai et al. (2017) found that
the Fuchs surface area (which is a representative parameter of coagulation
scavenging based on kinetic theory and is proportional to CS) fundamentally
determined the occurrence of NPF events in Beijing. The Fuchs surface area
had a good correlation with the PM2.5 mass concentration, and no NPF
event was observed when the daily mean PM2.5 concentration was higher
than 43 µg m-3 in the winter of 2015 in Beijing
(Jayaratne et al., 2017). However, in some cases, the CS
or the average coagulation sinks during NPF events were not significantly
lower compared to other times when new particles were not formed, indicating
that other factors, such as the precursor vapours and photochemical activity,
might also play an important role in driving NPF (Gong et al., 2010).
Besides the condensation sink, NPF events seemed not to be very sensitive to
the concentration levels of common gas pollutants in China, such as O3,
SO2, and NO2 (Zhu et al., 2013; An et al., 2015). It was
observed that SO2 concentrations were lower during the NPF event days
than during non-event days in the NCP (Herrmann et al., 2014) and Taiwan
(Young et al., 2013a), as well as during autumn and winter in the YRD (Qi
et al., 2015), whereas higher SO2 concentrations on NPF days were only
observed during spring and summer in the YRD (Qi et al., 2015; Yu et al.,
2016), during autumn in the PRD (Gong et al., 2010), and at mountain sites
(X. R. Zhang et al., 2017). Meanwhile, based on the empirical parameter
developed to judge whether NPF will occur or not, the exponent of SO2
in this empirical parameter was quite small, indicating that there is usually
enough SO2 for NPF to occur under heavily polluted conditions
(Herrmann et al., 2014). Similar results for sulfuric acid were reported
and it was found that sulfuric acid concentrations were not significantly
higher (even lower, sometimes) on NPF days compared with non-event days
(Qi et al., 2015; Xiao et al., 2015; Cai et al., 2017). Overall, the
previous results seem to suggest that SO2 was not a limiting factor for
NPF in China, and a similar conclusion might also be made for sulfuric acid.
However, higher SO2 concentrations could increase the probability of
occurrence of NPF events at a mountaintop site (Lv et al., 2018).
Besides, NPF might have different patterns in an environment with abundant
SO2 or not. Stronger nucleation but weak growth of particles was
observed with high concentrations of SO2 in polluted air masses
characteristic of urban (heavy traffic emission) or power-plant plumes, in
spite of similar CS with lower concentrations of SO2 (Gao et al.,
2009; Yue et al., 2010).
NPF event frequencies were different between the different seasons. In
northern China, spring is usually the season with the highest frequency of NPF
events, which is probably due to the typically low CS, relatively high solar
radiation intensity, and low temperature and RH (Shen et al., 2011; Wu et
al., 2007). In the NCP of China, many studies observed that summer had the
lowest NPF event frequency, although the condensable vapour concentration was
the highest during summer months due to the enhanced photochemical process
(Shen et al., 2011; Wu et al., 2007; Yue et al., 2009). The lowest
frequency of NPF events during summertime in the NCP might be related to the
high temperatures and RH, together with the stagnant and polluted air masses,
which could cause a high CS (Wu et al., 2007). In the YRD region, high
NPF event frequencies were observed in spring and summer, although the
temperature and RH were high in summer (Zhu et al., 2013; Qi et al.,
2015). A low temperature favours NPF (Zhu et al., 2013),
but according to our summary, as shown in Fig. 2, low frequencies of the NPF
event were usually observed in winter, which might be due to the weak
solar radiation as well as typically high pollution levels at that time of
the year. In spite of an increasing number of aerosol size distribution
measurements in China, atmospheric NPF observations that cover the full
annual cycle are still quite limited. Meanwhile, the main reason for the
different NPF event frequencies in different seasons is still uncertain
because many factors influencing NPF, such as the radiation intensity,
temperature, relative humidity, wind properties, biogenic activity and
anthropogenic emissions, tend to be changed simultaneously.
The NPF event frequency can also be quite different in air masses from
different directions (Wu et al., 2007). Higher NPF event frequencies were
usually observed within relatively clean air masses having a low CS (Zhu
et al., 2013; An et al., 2015; Jayaratne et al., 2017; Peng et al., 2017).
However, in some cases, NPF events also occurred in polluted air masses. For
example, during the summer in Beijing, NPF was observed under low-wind-speed
conditions and this phenomenon usually coincided with a wind direction
change from north to south, where the air is more polluted (Zhang et al.,
2011). Similarly, in Hong Kong, NPF was usually observed when air masses
originated from the northwest to northeast directions (Guo et al., 2012).
At the summit of Mt Tai, a continental air mass passing through more polluted
areas also favoured NPF (Lv et al., 2018). Consecutive NPF events were
observed in the presence of strong biomass-burning plume at a downwind rural
site in the PRD (Wang et al., 2013d). Meanwhile, compared to the NPF events
taking place in clean air masses, the FR seemed to be lower and the GR
seemed to be higher in the NPF events taking place in a polluted air mass
plume (Qi et al., 2015). An observation in the North China Plain reported
that, when the air mass was transported from the polluted south area, the
average PM10 (PM with diameter less than 10 µm) concentrations
on NPF event days were higher than during the non-event days (Shen et
al., 2011). In addition, air masses from polluted northern China favoured the
occurrence of regional NPF, while clean air masses from the east usually caused
local NPF in Nanjing in the YRD region (Dai et al., 2017). These results
highlighted the complex relationship between air pollution and NPF. Many
factors, including pre-existing aerosols, organic pollutants and SO2,
are connected each other due to their similar emission sources, so it is not
easy to extract the influence of one factor on NPF. Furthermore, since
environments are complex and diverse, some other factors, such as the
concentration of OH radicals and topography, can also be important to NPF
and therefore deserve further investigation in both field observations and controlled experiments.
Formation rate
Due to the lack of measurements down to particle diameters of about 1.5 nm,
most atmospheric nucleation rates were inferred indirectly only by measuring
the particle formation rate at some larger size in most of the NPF studies
in China. The FR at larger sizes (the “apparent” particle formation rate)
can be related to the FR of critical clusters (the “real” nucleation rate)
by the Kerminen–Kulmala equation and its revised version (Lehtinen et al.,
2007; Kerminen and Kulmala, 2002), but the nuclei GR and coagulational
scavenging rate (CoagS or CS) are needed. Besides, the assumed coagulation
sticking probability of 1 for molecular clusters with pre-existing particles
in their collision and the unclear GR of sub-3 nm particles might result in
errors in the derivation of FR (Kulmala et al., 2017). We did
not convert the “apparent” particle formation rate into “real”
nucleation rate, but summarized FR calculated at different particle sizes in
this study (Fig. 3). The observed FR ranged from less than 0.1 cm-3 s-1
at particle sizes larger than 10 nm to about 103 cm-3 s-1
at particle sizes below 2 nm. At a certain particle size, the FR
could still differ by 2 orders of magnitude due to the different
environmental conditions. For example, many studies reported the FR of 3 nm
particles ranging from less than 1 to several tens of cm-3 s-1.
New particle FR observed at different places in different seasons in
China. The line between two data points indicates that a range of FR was
reported in the literature. The data are collected in the references in Table 2.
Due to the wide range of FR under different environmental conditions, it is
not easy to determine differences in FR between different site types,
regions or seasons. In principle, a higher CS causes a more rapid scavenging
of clusters and small particles, resulting in lower FR (Zhu et al.,
2014; Man et al., 2015). According to the equation developed by Herrmann et
al. (2014) based on the observation date in the YRD region, FR is also inversely
proportional to the CS. However, when NPF was studied at an urban site and a
nearby regional site at the same time, FR was usually higher at the urban
site in spite of the higher CS, indicating much more abundant precursors for
NPF in the polluted urban environment (Wang et al., 2013b). As for NPF at
a same observation site but in different seasons, the highest FR was
observed in summer in the NCP (Shen et al., 2011) and in spring in the YRD (Qi et al., 2015).
Particle GR observed in different seasons at a regional site in the YRD (a) and an urban and a regional site in the NCP (b) and some
measurement results of GR and sulfuric acid (SA) contribution to the GR in
different size ranges (c). The data are collected in the references
indicated in the figure.
Although the nucleation mechanism in different environmental conditions
remains unknown according to current knowledge, neutral clusters of sulfuric
acid, stabilized with additional vapours such as ammonia, amines, and HOMs should
play a key role in NPF (Kulmala et al., 2013, 2014). A
positive relationship between nucleation rate and the sulfuric acid
concentration (or H2SO4 proxy) was observed in many NPF studies in
China, although nucleation rates were rarely calculated using measurements
of particles in the size range of 1–3 nm. The fitted exponent between FR and
sulfuric acid concentration ranged from 0.65 to 2.4 (Cai et al.,
2017; Xiao et al., 2015; Dai et al., 2017), while sometimes even higher values
between 2.5 and 7 were found (Wang et al., 2011). These exponents were
observed to increase with an increasing CS in Beijing (Wang et al.,
2011). Besides sulfuric acid, organics, NH3 and amines were also found
to be important in atmospheric particle nucleation (Z. B. Wang et al., 2015).
As we mentioned earlier, although CS was much higher at urban sites, the FR
was usually higher at corresponding regional sites (Wang et al., 2013b).
Meanwhile, SO2 is a regional pollutant and its concentrations were
similar between regional sites and city areas (He et al., 2014; Ma et al.,
2018). These features indicate important roles of other gas precursors in
NPF in the air pollution complex of China. In fact, some observations showed
that the correlation between FR and NH3 was better than that between FR and
H2S4 (Xiao et al., 2015). According to the national ammonia observation
network, the overall average concentration of ammonia in China is much
higher than the values observed in the US. The seasonal maximum NH3
concentrations were observed in the summer and the most abundant
concentrations of NH3 were observed in the NCP region in China (Pan
et al., 2018). Compared to NH3, the amine measurements are more sparse
(J. Zheng et al., 2015; Yao et al., 2016), and direct information on amine
emissions is currently not available but these emissions have to be
estimated by assuming a fixed ratio or source-dependent ratios of amines to
total ammonia emissions in China (Mao et al., 2018). Dai et al. (2017)
proposed that plumes containing high concentrations of ammonia, amines or
HOMs produced from their observed VOCs led to strong local NPF events. The
observations made at the SORPES station in the YRD indicated that HOMs played an
essential role in the initial condensational growth of newly formed clusters
(Huang et al., 2016; Ding et al., 2016; Qi et al., 2018). Recently, Yao et
al. (2018) reported a long-term continuous observation for NPF in
urban Shanghai and observed 1 to 2 orders of magnitude higher FR than
typical values in the clean atmosphere. These observed FR were far higher
than those derived from H2SO4–H2O
or H2SO4–NH3–H2O mechanisms but close to those observed
in the H2SO4–DMA–H2O laboratory experiments, and coincided
with sulfuric acid clusters and sulfuric acid–dimethylamine (DMA) clusters.
These results suggested that H2SO4–DMA–H2O nucleation
played important roles in the NPF in Chinese megacities. Up to now, there are
still quite limited investigations into the relation between FR and organics,
NH3 and amines in China and it is certainly crucial for a better
understanding of NPF in polluted areas. Ion-induced pure organic
nucleation was proposed to be important according to chamber experiments
(Kirkby et al., 2016), but seems to have a minor role in the polluted
environment in China (Herrmann et al., 2014; Xiao et al., 2015; Yao et al.,
2018). This is understandable because the ion production rate is usually
much lower than FR in China.
Growth rate
Growth of nanoparticles is crucial for NPF. The GR determines the size that
new particles can grow to before being scavenged; i.e. a higher GR results
in a larger particle diameter (Zhu et al., 2014; Man et al., 2015). There
are several methods that calculate GRs from the time variations of particle
size distributions, such as the appearance time method (Kulmala et al.,
2013) and mode-fitting method (Kulmala et al., 2012), or solving the
general aerosol dynamics equation (Pichelstorfer et al., 2018).
Regardless of the possible difference caused by using different calculation
methods, GRs reported in China varied a lot from an urban area to a rural
region and from spring to winter, ranging from a few nm h-1 to more
than 20 nm h-1 (Table 2). Generally, GRs at urban sites were found to
be higher than at their regional sites, as shown in Fig. 4a (Wang et
al., 2013b), which is also summarized by Kerminen et al. (2018). This is
probably caused by the more abundant condensing vapours in polluted cities,
although there are limited data on sulfuric acid and low-volatile organic
vapour concentrations in China. No significant differences were found among
the observations carried out in different regions in China in spite of the
different pollution levels. Lower GR was observed at a mountain site compared
to that in an urban area (H. L. Wang et al., 2014), but the GR of large
particles at mountain sites could be as high as about 10 nm h-1 (Nie
et al., 2014; H. L. Wang et al., 2014). For GRs in different seasons, higher GRs
were observed in summer than other seasons, indicating higher concentrations
of condensable vapours, which may be related to strong photochemical and
biological activities, as shown in Fig. 4a and b (Zhu et al., 2013; Shen
et al., 2011; Qi et al., 2015). GR is also dependent on the size of
the particles, with larger particles usually having a larger GR (Xiao et al.,
2015), which could also be inferred from the data summarized in Fig. 4c.
Sulfuric acid and organic vapours with low volatility were thought to be the
main contributors to the growth of particles formed by NPF. Generally,
sulfuric acid was thought to be the dominant contributor for the growth of
newly formed particles, but became less and less prominent for the growth of
larger particles (Xiao et al., 2015). For example, Xiao et al. (2015)
and Yao et al. (2018) calculated and estimated that sulfuric acid
was enough to explain the observed growth for particles smaller than 3 nm
but was insufficient to explain the observed growth rates of large
particles. They further calculated the relative contribution of sulfuric
acid to the particle growth in different particle size ranges. As shown in
Fig. 4c, these calculated contributions were 39 % and 29 % for the
size ranges of 2.39–7 and 7–20 nm, respectively, in urban Shanghai (Xiao
et al., 2015), 3 % to 14 % for the size range of 7–30 nm in urban
Beijing (Z. B. Wang et al., 2015), about 26 % for the size range of 5.5–25 nm
in suburban Hong Kong (Guo et al., 2012), and about 29 % during the
Beijing Summer Olympic period (Gao et al., 2012). Some
studies reported that H2SO4 had a negligible contribution to the
growth of particles larger than 10 nm (H. Meng et al., 2015; Liu et al.,
2014). The particle shrinkage (reversal in growth of particles size) was
reported in a few studies in China. The particle shrinkage could be due to
measuring particles present in different air masses during different times
of the day, or the evaporation of water and/or semi-volatile species in the
particles. If the air masses did not vary significantly, a similar shrinkage
rate to the growth rate in the NPF events might indicate a notable fraction
of semi-volatile species contributed to the growth (Young et al.,
2013b; Yao et al., 2010), which is consistent with organics being the main
contributor to the large particle growth. Yu et al. (2016) estimated
that a high concentration of extremely low-volatility organic compounds was
the key factor leading to a maximum in GR for very small particles
(1.4–3 nm) in urban Nanjing. Although the existence of local maxima in GR in the
sub-3 nm size range is highly sensitive to uncertainties in particle size
distributions, the results highlighted that detailed investigations for the
mechanisms of the initial growth steps of atmospheric NPF are needed (Yu
et al., 2016). On the other hand, Yue et al. (2010) proposed a dominant
role of sulfuric acid in the growth of new particles in sulfur-rich NPF
events. A model simulation study about NPF in Beijing also supported that
only small fraction of organics contributed to the growth of new particles,
and these organics were mainly O3 initiated (Wang et al., 2013a).
Besides sulfuric acid and organics, some studies reported a two-stage growth
of new particles in China, in which sulfuric acid and organics contributed
to the first-stage growth in the daytime, while NH4NO3 and organics
possibly contributed to the second-stage growth at night-time (Zhu et al.,
2014; Man et al., 2015; Liu et al., 2014). Tao et al. (2016) observed
higher levels of aminium in particles with relative smaller sizes, and
suggested that the heterogeneous uptake of amines by acid-base reactions
could effectively contribute to the particle growth during NPF events.
However, they only measured the particle chemical composition with a lowest
cut-off size of 56 nm, which may not be directly related to NPF. In fact,
measuring the chemical composition of nucleation-mode particles is still
quite challenging all over the world. To summarize, most studies observed a
slow GR for newly formed particles, with H2SO4 as the dominate
contributor, while other species, such as organics, would contribute more to
the particle growth as the particles grow to bigger sizes and also result in higher GR.
NPF under heavy air pollution
The heavy air pollution makes China quite a different environment for NPF
compared with western countries (Wang et al., 2017; Kulmala et al.,
2017; Yu et al., 2017). Generally, concentrations of particles and
condensable vapours in Chinese cities and regional background area are much
higher in China than that in North America or Europe (Shen et al.,
2016a, b; Wang et al., 2013b; Gao et al., 2009). The CS and small molecular
cluster and particle (1–3 nm) concentrations are about an order of
magnitude higher in China compared with European cities (Kulmala,
2015; Kontkanen et al., 2017). The occurrence frequencies of NPF events in
high aerosol-loading environments of China were higher than those in low
aerosol-loading environments (Peng et al., 2014). Meanwhile, the observed
FR was much higher, and the GR was also higher (but to a smaller extent
relative to FR) for NPF in China than that at rural/urban sites in western
countries (L. J. Shen et al., 2016). As pointed out by Cai et al. (2017),
previous FR calculations may still underestimate the
real nucleation rate due to underestimation or omission of coagulation among
particles in the nucleation mode with strong nucleation in China.
The influence of heavy air pollution on NPF might be identified by studying
NPF in periods with short-term strong air pollution control. Shen et al. (2016b)
investigated NPF during the Olympics in 2008 and during the APEC
meeting in 2015 in Beijing. They found that a higher NPF event frequency
coincided with the improved air quality during these important events
associated with temporary intense air pollution control actions compared to
a similar time of the year during 2010–2013. In spite of more frequent NPF
events (Yue et al., 2010; Zhang et al., 2011), the strength of NPF
decreased during these periods with temporary intense air pollution control
actions, characterized with lower FR and GR values (Shen et al., 2016b).
Due to the decreasing strength of NPF and also the reduction of the primary
emission source of fine particles, the number concentration of particles
decreased in spite of the increased frequency of NPF. The mean number and volume
concentrations of particles decreased by 41 % and 35 %, respectively, in
August 2008 during the Beijing Olympic compared with 2004–2007 (Wang et
al., 2013c). However, these temporary intense air pollution control actions
had a much smaller influence on Aitken-mode particles than on accumulation-mode
particles, according to the observations carried out during the APEC
meeting in 2015 in Beijing (Du et al., 2017).
NPF was observed more often in the high aerosol-loading environment of China
than we would expect based on the current understanding of nucleation and
particle growth (Peng et al., 2014; Kulmala et al., 2017). The ratio of
particle scavenging loss rate over condensational growth rate, which is
proportional to the ratio of CS to GR, was used as a criterion to predict
the occurrence of NPF events (McMurry et al., 2005; Kuang et al., 2010).
With much higher CS values in China than at European and American
sites, the difference in GR was not very obvious at the same site types
between China and other countries (Peng et al., 2014). It turned out that
NPF occurred frequently in megacities in China when the ratio of
CS (10-4 s-1) to GR (nm h-1) was above 200, whereas it only
occurred when this same ratio was less than 50 under clean and
moderately polluted conditions (Kulmala et al., 2017). As
shown in Fig. 5, most of the observation data reported ratios of
CS (10-4 s-1) to GR (nm h-1) between 200 and 500, while a few
were less than 200 but always higher than 50. More importantly, many studies
reported that NPF took place with this ratio higher than 500 at urban and
suburban sites. Such NPF events were able to take place in all regions in
China (NCP, YRD, PRD and western China), and during both winter and summer
seasons. There are several possible reasons for the higher threshold ratio
of CS to GR in highly polluted environment, including the overestimation of
particle losses due to assuming a coagulation sticking probability of 1, the
underestimation of GR in the sub-3 size range, and also unidentified
nucleation and growth mechanisms relevant to a polluted atmosphere
(Kulmala et al., 2017; Yu et al., 2017).
Particle GR as a function of CS in the NPF events in China. The solid
points are average data for a certain observation period, while the open points
are data for individual NPF days. The line between two data points indicates
that a range of GR and/or CS was reported in the literature. The data are
collected in the references in Table 2.
NPF mainly occurred when the PM2.5 concentration (CS) and gas pollutant
concentrations, such as NO2, CO and SO2, were both low (Wu et
al., 2007; Dai et al., 2017; Yu et al., 2016). These gas pollutants were
mainly from primary combustion emissions (De Gouw and Jimenez,
2009), whereas PM2.5, the main cause of haze, originated from both
primary emission and secondary formation, and the latter was thought to
dominate during haze events in China (Yang et al., 2011; Zhao et al.,
2013; Dan et al., 2004; Duan et al., 2005; Wang et al., 2012). NPF was found to
be concentrated on days with low RH in previous NPF studies in Beijing (Wu et
al., 2007; Yue et al., 2009). A possible reason for this would be that
photochemical reactions are faster on sunny days with strong solar radiation
and low RH. On the contrary, haze usually occurs at high RH when multiphase
processes contribute more to the aerosol mass (Sun et al., 2010; He
et al., 2014; B. Zheng et al., 2015; Cheng et al., 2016; Liu et al., 2017) and
the hygroscopic aerosols contribute more to light extinction compared
with low-RH conditions (Shi et al., 2014; Shen et al., 2015). NPF and haze
are either purely secondary processes or dominated by secondary pollution
processes, so there might be some common properties or internal relations
between them. With this in mind, besides the possible inaccurate estimation
for the GR and CS, as pointed out and estimated by Kulmala et al. (2017) and
further discussed in detail by Yu et al. (2017), several other possible reasons might also be related to the
frequent occurrence of NPF under heavy air pollution in China.
Secondary aerosols, including sulfate and organic aerosols, are still
underestimated in current air quality models (Xiao et al., 2015; Chen et
al., 2016; Hodzic et al., 2010), indicating unknown chemical and physical
processes that are important for secondary aerosol formation (Kulmala et al., 2014). These processes might create
oxidants or change the surface properties of aerosols, and thereby limit
their ability to take up condensable vapours and cause more frequent NPF
(Kulmala, 2015). The effects of a high percentage of inorganic
aerosol particles on the effectiveness of CS are unknown and may need to be
investigated in laboratory experiments. NPF was observed during dust
episodes in China, and both FR and GR were enhanced under dust conditions,
indicating that photo-induced, dust surface-mediated reactions might be important
for producing condensable vapours for NPF (Nie et al., 2014; Xie et al.,
2015; Kulmala et al., 2017). Heterogeneous photochemical processes inducing
new particle formation and growth might happen in the real atmosphere and
need to be further investigated. In addition to these, high concentrations
of sulfuric acid (106 molecules cm-3) were observed at night-time,
indicative of non-photochemical OH sources (Zheng et al., 2011). The
contribution of oxidation of SO2 by Criegee radicals (Welz et al.,
2012; Mauldin et al., 2012) and other possible surface-mediated reactions to
the formation of night-time sulfuric acid under complex air pollution conditions
in China need to be figured out as well.
The NPF leads directly to a burst of small nanoparticles and increases the
particle number concentration prominently. While NPF usually tends to occur
on clean days with low CS, particle number concentrations are usually much
higher on NPF event days than on non-event days (L. J. Shen et al., 2016; An et
al., 2015). Kulmala et al. (2016a) studied nucleation-, Aitken-
and accumulation-mode particle number concentrations separately in Nanjing in the YRD
regions of China, and estimated that the majority of the particles were of
secondary origin in all modes. NPF was found to be an important
influential factor on atmospheric aerosol number size distribution from
remote mountains to polluted cities (Du et al., 2012; L. J. Shen et al.,
2016, X. R. Zhang et al., 2017). NPF also changes the surface and volume size
distribution. An et al. (2015) observed that NPF events had a large
effect on Aitken- and nuclei-mode particle surface and volume
concentrations, while having limited contributions to accumulation- and
coarse-mode particles. NPF was observed to increase the proportions of
NH4+, SO42-, NO3-, K+ and Mg2+ in
nucleation- and Aitken-mode particles compared with those in the total
aerosol. Zheng et al. (2011) found that the calculated condensation rate
of H2SO4 correlated with the Aitken-mode sulfate mass
concentration but not with the accumulation-mode sulfate mass concentration.
With high concentrations of condensable vapours, newly formed particles have
the potential to grow quickly, which results in an increase in PM volume or
mass concentrations. In an episode with consecutive NPF events in the
presence of strong biomass burning in the PRD, the aerosol volume concentration
increased by 6.1 mm3 cm-3 in volume mass concentration per day or
about 10 µg m-3 per day in mass concentration, with organics and
sulfate accounting for 42 % and 35 %, respectively, of the particle mass
concentration (Wang et al., 2013d). Furthermore, it was estimated that
primary emissions and secondary formation provided 28 % and 72 % of particle
number concentration and 21 % and 79 % of mass concentration, respectively.
Similarly, Shen et al. (2011) observed that about 20 % of the NPF
events led to a measurable increase in the particle mass concentration, with
an average growth rate of about 4.9 µg m-3 h-1 for PM1
(PM with diameter less than 1 µm) during the period of the mass
concentration increase. Guo et al. (2014)
reported a case with NPF followed by the continuous growth and appearance of
haze pollution in Beijing and proposed that the efficient aerosol
nucleation and growth led to severe PM2.5 development.
In summary, NPF was found to be the main source of the particle number
concentration in the atmosphere, being able to dramatically increase
particle number concentrations in a relatively short time. NPF and
subsequent particle growth seem to also have a noticeable contribution to
the volume and mass concentration of nucleation- and Aitken-mode particles.
Although secondary formation of PM2.5 mass is the main cause of haze
compared with primary particle emissions, the accumulation of this secondary
aerosol mass usually occurs over several days following NPF. The
contribution of NPF to haze formation is still an open question.
Significance and future research directions for NPF study
The effects of NPF on air pollution and human health are crucial but highly
uncertain. As we mentioned above, the effect or contribution of NPF to haze
formation is still an open question. Answering this question might be
difficult using only field observations, so new laboratory experiments and
model simulations may need to be designed. In addition, interactions between
NPF, pollution and meteorological conditions should be studied further.
Heavy pollution could have significant feedbacks to meteorological
conditions in China. For a case study in the YRD, it was calculated that air
pollution resulted in a decrease in the solar radiation intensity by more
than 70 %, in the sensible heat by more than 85 % and
drop in temperature by almost 10 K (Ding et al., 2013). These effects
resulted in a decrease of the boundary layer height, which further increased
PM concentrations, forming a feedback loop (Petaja et al., 2016). On the
other hand, NPF occurring in a free troposphere may have a major impact on
the marine boundary layer particle concentrations due to the subsidence
(Clarke et al., 1998; Lin et al., 2007). When the aerosol loading was
high, the distance between the NPF peak and the planetary boundary layer became
larger (Quan et al., 2017). These interactions would be also crucial for
predicting NPF and air quality and for identifying the contribution of NPF
to air pollution. Rather than ground observations, multidimensional
measurements may need to be carried out in order to understand the
atmospheric process up to the free troposphere. Compared with the effect of
NPF to haze formation, the health effect of high number concentrations of
particles with diameters of several or tens of nanometres would be more
essential. NPF usually occurs around the same time period as people commute
to work. The effects of exposure to a high particle number concentration
environment should be investigated.
Atmospheric nucleation and subsequent growth of newly formed particles could
have significant effects on air quality and climate by contributing to CCN
(IPCC, 2013). NPF was calculated to enhance the CCN number significantly
with ratios ranging from 1.2 to 1.8 in Shanghai in the YRD region of China (Leng
et al., 2014). Considering both NPF and non-event days, the average
contributions of NPF events to potential CCN in the afternoon were
calculated to be 11 % and 6 % at urban sites and regional sites,
respectively (Peng et al., 2014). It seems that the enhancement of CCN
due to NPF in China on a regional scale was larger than that in Europe
(Shen et al., 2016a), which might be due to the combination of a higher
nucleation rate and quicker subsequent condensable growth associated with
higher pollution levels in China. NPF events were also found to have a
greater impact on CCN at polluted urban sites than at regional or rural
sites in China. For example, CCN number concentrations were observed to be
enhanced by a factor of 2–6 in background regions and by a factor of 5.6–8.7
in polluted regions during the NPF event days (Wang et al., 2013b; Shen et
al., 2016a). Nevertheless, the impact of NPF on the CCN number concentration
was found to depend on the location and individual character of each NPF
event, including different hygroscopic properties of particles and thus
different CCN activities during different NPF events, so Ma et al. (2016)
suggested not using a fixed parameter to predict the contribution of NPF to
CCN and Tao et al. (2018) emphasized the importance of real-time
measurements of hygroscopicity of particles.
During the past 15 years, a lot of NPF observations and related studies were
carried out in China but, as summarized by Wang et al. (2017), the
application of state-of-the-art instruments are still quite limited in
China. In recent years, an increasing number of studies utilized more
advanced instruments, such as PSM (Xiao et al., 2015; Dai et al., 2017; Yu
et al., 2016; Yao et al., 2018), NAIS (Jayaratne et al., 2017; Lv et al.,
2018), DEG-SMPS (Cai and Jiang, 2017; Cai et al., 2017) and
APi-ToF-MS/CI-APi-ToF-MS (Yao et al., 2018), greatly improving
our understanding about the nucleation and particle growth mechanisms in
China, especially in highly polluted environments. However, the lack of
continuous and comprehensive long-term observations, which should include
measurements of particle number size distribution preferably down to 1–2 nm
and vapours that potentially participate in NPF and subsequent particle
growth (H2SO4, ELVOCs, LVOC, ammonia and amines), still limits our
understanding of the mechanism of NPF in different environments in China.
Key participants and processes of NPF under complex air pollution conditions
in China still wait to be answered, and the unexpected NPF at high aerosol
loadings need to be explained. Contributions of different mechanisms to NPF
should be evaluated with the consideration of spatio-temporal difference, and
possibly also with the consideration of interannual variability in the
process of air pollution control in China. Long periods and comprehensive
observations would be the most important factor when investigating NPF
mechanisms in China, while laboratory experiments and model simulations
would also be very helpful and necessary. As suggested by Kulmala (2018),
grand environmental challenges, such as climate change, water
and food security as well as urban air pollution, are all linked and need to
be studied together. The effects of NPF in China on climate change and human
health are still poorly understood and should be evaluated quantitatively.
Although a global view is needed for these common challenges of mankind,
densely populated China will undoubtedly be a very important area in this
respect. Studying these effects will be essential for future studies of NPF
in China and will be important for a global effort for a better atmosphere on Earth.
All the data in the paper are presented in the references.
Additional data related to this paper may be requested from the corresponding
author: biwu.chu@helsinki.fi.
MK designed the study. BC collected the data. BC and VK led
the writing and data analysis with input from all co-authors. FB, CY, TP and
MK contributed to the editing of the paper.
The authors declare that they have no conflict of interest.
Acknowledgements
This work was supported by the Academy of Finland (1251427, 1139656, 296628,
306853, Finnish centre of excellence 1141135), and the EC Seventh Framework
Program and European Union's Horizon 2020 programme (ERC, project no. 742206,
ATM-GTP).
Edited by: Joachim Curtius
Reviewed by: two anonymous referees
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