Because anthropogenic sulfur dioxide (SO
Atmospheric new particle formation (NPF) is regarded as an important source
of aerosol particles in terms of number concentrations, and the newly formed
particles can grow into a variety of sizes with different health and
climate effects. For example, particles larger than 50–80 nm may act as
cloud condensation nuclei (CCN), whereas those larger than 100 nm may
directly affect solar radiation (Kulmala and Kerminen, 2008; Kerminen et
al., 2012; Seinfeld and Pandis, 2012). Sulfuric acid (H
NPF events have been reported widely throughout the world, including in
severely polluted urban and rural areas in China that experience high sulfur
dioxide (SO
The long-term changes in NPF events under lower SO
In China, the earliest observation of NPF events started in approximately
2004 in Beijing (Wu et al., 2007). The comparison of tens of independent
experiments showed that the NPF frequency has remained relatively constant
until recent years, possibly due to the reduced production and reduced loss
rate of H
In this study, we analyzed the measurement data of particle number
concentrations, chemical compositions, trace gases, and meteorological
parameters collected at the summit of Mt. Tai (36.25
This study comprised seven intensive campaigns from 2007 to 2018, and the
details are summarized in Table 1. The duration of each campaign varied from
18 to 71 d. The measurement data obtained in the four campaigns in
2007, 2014, and 2015 have been reported by Gao (2008) and Lv et al.
(2018). Here, all of the available data were combined to examine the
effects of reduced SO
Summary of the seven observation campaigns at Mt. Tai.
All measurements were obtained using commercial instruments, which were
housed in a container and have been described in previous studies (e.g.,
Zhou et al., 2009; Lv et al., 2018). During the seven
campaigns, the particle number size distributions (PNSDs) were monitored
using a wide-range particle spectrometer (WPS; Model 1000XP, MSP Corporation, USA) at ambient relative humidity
(RH). Conductive tubes (TSI
Examples of NPF events in three categories. Black dots in the
figures are the fitted
The WPS instrument was calibrated and/or repaired every 1–2 years by its
vendor. The regular maintenance allowed the WPS to perform well based on
the recent comparison results of the WPS and a new scanning mobility
particle sizer (SMPS, Grimm, Germany) in the summer of 2020, as shown in Fig. S3 in the Supplement. The
regular calibration parameters included the DMA sample/sheath flow, LPS
sample/sheath flow, DMA/CPC pressure, DMA voltage, and DMA/ambient
temperature. Polystyrene latex (PSL) spheres (NIST, USA) with mean diameters of
100.7 and 269 nm were used for calibration. At the beginning of each
campaign, the zero points of the DMA, CPC, and LPS were checked using a
purge filter at the inlet. Sometimes the WPS operated improperly and the
data were excluded from the analysis (see Fig. S4 in the Supplement for the occasional
unexpected errors in three channels around 213 nm). In addition, we
reproduced the PM
The trace gases were monitored during each campaign. SO
The air mass back trajectories were calculated using the Hybrid Single
Particle Lagrangian Integrated Trajectory (HYSPLIT) model. The input
meteorological data (Global Data Analysis System (GDAS) data) were used with
a 1
In this study, particles with diameters smaller than 25 nm were defined as nucleation mode particles (Kulmala et al., 2012). Following the criteria proposed by Dal Maso et al. (2005) and Kulmala et al. (2012), three features had to be met for an event to qualify as NPF: (1) distinctly new nucleation mode particles must appear in the size distribution; (2) the new mode should prevail over a time span of hours; and (3) the new mode should show signs of growth. All three features are required for a day (00:00–23:59 LT) to be classified as an NPF day. Otherwise, the day is classified as a non-NPF day.
The initial time of an NPF event was defined as when new nucleation-mode
particles started to be observed (e.g.,
Three parameters are commonly used to evaluate NPF characteristics, viz.,
apparent formation rate (FR), growth rate (GR), and condensation sink (CS)
(Sihto et al., 2006; Kulmala et al., 2012). The apparent FR of new particles
is calculated based on nucleation-mode particles with sizes of 10–25 nm. The
GR is quantified by fitting the geometric median diameter of new particles
(
Another two metrics were applied to characterize the NPF events, i.e., the
net maximum increase in the nucleation-mode particle number concentration
(NMINP) and the maximum size of
Note that a few spikes were occasionally observed with a broader particle
number size distribution during the NPF period. These spikes were excluded
in the calculation of the FR, GR,
According to the different sizes of
In the absence of direct CCN measurements, the potential contribution of new
particles to the CCN population can be estimated from the particle number
size distribution (Lihavainen et al., 2003; Rose et al., 2017).
Theoretically, particles larger than 50 nm (i.e., 80 nm) can be activated as
CCN under quite high (moderate) supersaturation (Dusek et al., 2006; Petters
and Kreidenweis, 2007; Ma et al., 2016), and particles larger than 100 nm
can directly impact the climate by scattering and absorbing solar radiation
(Charlson et al., 1992; Seinfeld and Pandis, 2012). In this study, we
introduced three terms: the net increase in the NPF-derived CCN number
concentration (
The SP was calculated as described by Zhu et al. (2019):
The
In addition, the maximum geometric median diameter of the grown new
particles never exceeded 89 nm in spring 2007. Considering the log-normal
distribution of the grown new particles, the number concentration of grown
new particles with diameters
The proxy for the H
The contribution of H
During the seven campaigns, NPF events were observed on 106 of the 265 sampling days. As shown in Fig. 2, the NPF frequencies in the three seasons of different years were surprisingly almost the same, i.e., 50 % in the spring of 2007, 50 % in the summer of 2009, 49 % in the winter of 2017, and 51 % in the spring of 2018. However, the NPF frequency decreased to 42 % in the summer of 2014, 33 % in the fall of 2014, and 20 % in the summer of 2015. The low NPF frequencies were likely caused by perturbations from meteorological conditions. For example, there were 15 rainy days out of the 40 sampling days during the 2015 summer campaign, but only 3 rainy days out of the total 18 sampling days in the 2009 summer campaign. Moreover, the solar radiation averaged over the 2009 summer campaign was 1.4 times that of the 2015 summer campaign (Fig. S7 in the Supplement). These factors may have caused the NPF frequency in the 2009 summer campaign to be close to that in the other season campaigns but that of the 2015 summer campaign to be lower.
Occurrence frequencies of NPF events in different categories at Mt. Tai during the seven observation campaigns.
When Categories 1, 2, and 3 of the NPF events were examined separately, the
Category 1 NPF frequencies in the winter of 2017 (43 %) and the spring of
2018 (49 %) were significantly higher than those before (5 %–21 %;
We used four metrics, i.e., the apparent FR, NMINP, GR, and
Campaign average of the new particle formation rate (FR,
The NMINP showed a temporal variation pattern similar to that of the
apparent FR (Fig. 3b). The campaign average NMINP varied in a narrow range
of 3.8–5.1
The variations in GR were strongly seasonally dependent (Fig. 3c). Higher
GRs were generally observed in the summer campaigns, with the three campaign
averages in the range of 7.3–9.6 nm h
The
In summary, we found that the apparent FR and NMINP in the spring campaign
of 2018 were higher than those of 2007. The GR showed strong seasonal
dependence. The
Direct measurements of the CCN were not available; therefore, the potential
contributions of the grown new particles to the CCN population were
estimated using Eqs. (2)–(4). The contributions varied considerably
between campaigns (Fig. 4). In general, the NPF-derived CCNs were seasonally
dependent, i.e., the highest number concentrations occurred in summer,
followed by spring, fall, and winter. With an increase in the threshold
diameters, roughly corresponding to a decrease in supersaturation from
Campaign average of the net increase in the NPF-derived CCN number
concentration (
High SPs were found during the three summer campaigns in 2009, 2014, and
2015, with average SP
Figure 4c shows the percentage increase in the NPF-derived CCN relative to
the pre-existing CCN. The percentages were the highest in the summer of
2014, e.g., 6.8
H
Campaign average of SO
As the calculated CSs before the NPF events in the 2017 and 2018 campaigns
were higher than those in the 2007 and 2009 campaigns (Fig. 5b), CSs were
unlikely to be the cause for the lack of decreases in the NPF occurrence
frequency in 2017 and 2018. It has been reported that a low CS is not
necessary to promote NPF occurrence at altitudes higher than 1000 m
(Sellegri et al., 2019). Thus, other factors such as meteorological
conditions and biogenic precursors (e.g., amines and highly oxidized
organics) may overwhelm the effects of SO
We further conducted a few statistical tests to explore the association of
the apparent FR and NMINP with SO
Recall that the occurrence frequencies of NPF were also almost the same in
the spring of 2007 and 2018, at high values of 50 %–51 %, implying that
ambient factors in both campaigns favored NPF. Table 2 provides a
comprehensive comparison of the measured air pollutants of the two spring
campaigns. The decrease in the SO
Meteorological conditions and air pollutants during the formation and growth periods of new particles in the spring campaigns in 2007 and 2018.
During the 106 cases of NPF events, the apparent FR and NMINP showed a good
linear correlation (
Relationship between the FR and NMINP in 106 cases of NPF events at Mt. Tai in this study and in urban and marine atmospheres in previous studies (Man et al., 2015; Zhu et al., 2017, 2019; Ma et al., 2020). The half-solid markers can be fitted linearly in previous studies. The open markers show poor correlations.
Previous studies have reported that the BVOC emissions over the NCP have
increased in the last decade because of the afforestation and accelerating
global warming (Stavrakou et al., 2014; Ma et al., 2019). During our
observations, the total VOCs (including C
Based on the observations alone, the
Theoretically, the
Relationship between the GR and
In further analysis, we considered three situations of the new particle
growth. Type A (full marker in Fig. 7) represents that new particles
continuously grow to the size of
For Type A, the average GR and
In the case of Type B, the GR and
Type C was characterized by the largest GR, duration, and
The factors influencing the lower
However, uncertainties still exist, e.g., (1) the data were obtained in seven independent campaigns, each lasting 18–71 d, and the data size did not allow us to extend the conclusion to all the years from 2007 to 2018; and (2) the observations were conducted only at one site, alternating between the boundary layer and the free troposphere, and the generality of the conclusions on NPF events needs to be examined at more sites.
With an order of magnitude reduction in SO
We hypothesize that the NPF intensity increased unexpectedly with the
reduction in SO
The datasets related to this work can be accessed via
The supplement related to this article is available online at:
LX designed the research. JC and JG conducted the field observations in 2007, 2014, and 2015. XW, HL, YZ, ZG, TC, LW, PZ, and YS carried out the field measurements in 2009, 2017, and 2018. YZ analyzed the data and wrote the paper. XY, TW, and WW helped with the interpretation of the results. XY and LX revised the original manuscript. All authors contributed toward improving the paper.
The authors declare that they have no conflict of interest.
This work was funded by the National Key Research and Development Program of China (2016YFC0200500), the National Natural Science Foundation of China (41922051, 42075104, 41706122), Shandong Provincial Science Foundation for Distinguished Young Scholars (ZR2019JQ09), State Key Laboratory of Organic Geochemistry, GIGCAS (SKLOG-201914), and the Jiangsu Collaborative Innovation Center for Climate Change. We appreciate the NOAA Air Resource Laboratory for providing the HYSPLIT model and thank the staff of the Mt. Tai Meteorological Station for the help during the measurement campaigns.
This research was supported by the National Key Research and Development Program of China (grant no. 2016YFC0200500), the National Natural Science Foundation of China (grant nos. 41922051, 42075104 and 41706122), the Shandong Provincial Science Foundation for Distinguished Young Scholars (grant no. ZR2019JQ09), and the State Key Laboratory of Organic Geochemistry, GIGCAS (grant no. SKLOG-201914).
This paper was edited by Veli-Matti Kerminen and reviewed by four anonymous referees.