ACPAtmospheric Chemistry and PhysicsACPAtmos. Chem. Phys.1680-7324Copernicus PublicationsGöttingen, Germany10.5194/acp-18-11779-2018Modelling studies of HOMs and their contributions to new particle formation and
growth: comparison of boreal forest in Finland and a polluted
environment in ChinaModelling studies of HOMQiXimengDingAijundingaj@nju.edu.cnhttps://orcid.org/0000-0003-4481-5386RoldinPontushttps://orcid.org/0000-0002-4223-4708XuZhengningZhouPutianhttps://orcid.org/0000-0003-0803-7337SarnelaNinaNieWeiHuangXinhttps://orcid.org/0000-0003-0922-5014RusanenAntonEhnMikaelhttps://orcid.org/0000-0002-0215-4893RissanenMatti P.https://orcid.org/0000-0003-0463-8098PetäjäTuukkahttps://orcid.org/0000-0002-1881-9044KulmalaMarkkuhttps://orcid.org/0000-0003-3464-7825BoyMichaelJoint International Research Laboratory of Atmospheric and Earth
System Sciences, School of Atmospheric Sciences, Nanjing
University, Nanjing, 210023, ChinaCollaborative Innovation Center
of Climate Change, Nanjing, 210023, ChinaDivision of Nuclear
Physics, Lund University, P.O. Box 118, 22100 Lund, SwedenInstitute for Atmospheric and Earth System Research/Physics,
Faculty of Science, University of Helsinki, P.O. Box 64, 00014 University of
Helsinki, Helsinki, FinlandAijun Ding (dingaj@nju.edu.cn)20August2018181611779117916March20185April20181August20182August2018This 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/18/11779/2018/acp-18-11779-2018.htmlThe full text article is available as a PDF file from https://acp.copernicus.org/articles/18/11779/2018/acp-18-11779-2018.pdf
Highly oxygenated multifunctional compounds (HOMs) play a key
role in new particle formation (NPF), but their quantitative roles in
different environments of the globe have not been well studied yet. Frequent
NPF events were observed at two “flagship” stations under different
environmental conditions, i.e. a remote boreal forest site (SMEAR II) in
Finland and a suburban site (SORPES) in polluted eastern China. The
averaged formation rate of 6 nm particles and the growth rate of 6–30 nm
particles were 0.3 cm-3 s-1 and 4.5 nm h-1 at SMEAR II
compared to 2.3 cm-3 s-1 and 8.7 nm h-1 at SORPES,
respectively. To explore the differences of NPF at the two stations, the HOM
concentrations and NPF events at two sites were simulated with the MALTE-BOX
model, and their roles in NPF and particle growth in the two distinctly
different environments are discussed. The model provides an acceptable
agreement between the simulated and measured concentrations of sulfuric acid
and HOMs at SMEAR II. The sulfuric acid and HOM organonitrate concentrations
are significantly higher but other HOM monomers and dimers from monoterpene
oxidation are lower at SORPES compared to SMEAR II. The model simulates the
NPF events at SMEAR II with a good agreement but underestimates the growth of
new particles at SORPES, indicating a dominant role of anthropogenic
processes in the polluted environment. HOMs from monoterpene oxidation
dominate the growth of ultrafine particles at SMEAR II while sulfuric acid
and HOMs from aromatics oxidation play a more important role in particle growth.
This study highlights the distinct roles of sulfuric acid and HOMs in NPF and
particle growth in different environmental conditions and suggests the need
for molecular-scale measurements in improving the understanding of NPF
mechanisms in polluted areas like eastern China.
Introduction
New particle formation (NPF), including the production of molecular
clusters and the subsequent growth of these clusters (Kulmala et al.,
2014), is a global phenomenon and has been observed under different
environmental conditions (Kulmala and Kerminen, 2008; Kulmala et al.,
2004; Zhang et al., 2012).). NPF can influence climate by contributing up to
50 % of cloud condensation nuclei (CCN) (Merikanto et al., 2009; Sihto
et al., 2011) and can have strong effects on air quality (Shen et al.,
2011; Yu et al., 2010; Guo et al., 2014).
Sulfuric acid has been commonly considered one of the main gas precursors
of NPF (Kulmala and Kerminen, 2008; Zhang et al., 2012). Recently, it was
found that highly oxygenated multifunctional compounds (HOMs) can participate
in the initial steps of NPF by stabilizing sulfuric acid
(Schobesberger et al., 2013; Riccobono et al., 2014; Kulmala et al., 2013).
Most of the HOM dimers and the most oxidized monomers can be extremely low-volatility organic compounds (ELVOCs) (Kurtén et al., 2016) and most
likely contribute to the initial growth of newly formed particles (Trostl
et al., 2016). Ehn et al. (2014) showed that monoterpene oxidation is a
strong source of HOMs and these HOMs can explain the majority of the observed
particle growth from 2 nm up to 50 nm in boreal forest. Recent studies
(Jokinen et al., 2015; Trostl et al., 2016) showed that HOMs can enhance
atmospheric NPF and growth in most continental regions
and increase the CCN concentrations by applying a constant monoterpene HOM
yield (achieved from measurement) in a global model. Based on the HOM
formation theory described by Ehn et al. (2014), a detailed HOM formation
mechanism was applied (Öström et al.,
2017).
Currently, the role of HOMs in NPF has been mainly studied in specific
environment conditions with intensive observations available, such as the
SMEAR II station in a Nordic boreal forest (Yan et al., 2016; Dal Maso et
al., 2005). However, understanding the mechanisms of NPF is particularly
important from the perspective of air quality. As one of the most
economically invigorating and densely populated countries, China has measured
NPF events since the last decade (Shen et al.,
2011; Wu et al., 2007; Wang et al., 2017; Kivekas et al., 2009). Interestingly,
the NPF events were observed frequently in heavily polluted environments in
China (Kulmala et al., 2017; Wang et al., 2017). However, no measurements
of HOMs in China have been reported until now and the understanding of the roles of
HOMs in NPF is very limited. The SORPES station at Nanjing is one of the
“flagship” stations in the domain of the Pan-Eurasian Experiment (PEEX)
(Kulmala et al., 2015; Lappalainen et al., 2016), providing a completely
different environment in comparison to the remote boreal forest.
In this study, the NPF events at SMEAR II and SORPES, including the
formation rates, growth rates, and environmental conditions, were compared
first. Then, by using the new version of the MALTE-BOX model, the
precursor vapour gases (i.e. sulfuric acid and HOMs) and NPF at two sites were
simulated to deeply investigate the differences in NPF. This modelling study
will increase our understanding of NPF at an urban site in China and
examines whether the nucleation and HOM formation mechanisms, which are
intensively investigated at SMEAR II in Finland, can be used in a polluted
environment in China. In addition, applying a process model like MALTE-BOX,
to simulate HOM concentrations and their contribution to the growth of newly
formed particles at the two selected sites with different environmental
conditions, can validate whether a single HOM formation and nucleation
mechanism could be appropriate in global models.
Site (SMEAR II and SORPES) locations on a map of the emission inventory
of (a)SO2 and (b) monoterpenes (Sindelarova et
al., 2014; Granier et al., 2011) (emission inventory data are available at
http://eccad.aeris-data.fr, last access: 1 August 2018).
Materials and methodsSite and observation descriptions
SMEAR II station (Station for Measuring Forest Ecosystem–Atmosphere
Relations) is located in Hyytiälä, Finland (Fig. 1). The station is
a boreal forest site, which is surrounded by a Scots pine forest with high
monoterpene emissions. The SORPES station (Station for Observing Regional
Processes of the Earth System) is located in Nanjing, eastern China (Fig. 1)
(Ding et al., 2013, 2016). The station is a suburban site
and about 20 km northeast of downtown Nanjing. The aerosol number size
distributions were measured continuously with a differential mobility particle
sizer (DMPS) for the size range of 3–1000 nm at SMEAR II and 6–800 nm at
SORPES. The trace gases, including O3, SO2, and NOx (NO and
NO2), were measured with online analyzers (Thermo Fisher Scientific 49i,
43i, and 42i, respectively) at both sites. The meteorological parameters,
e.g. air temperature, relative humidity, and global radiation, were measured
with standard meteorological sensors. Volatile organic compounds (VOCs)
were observed using proton-transfer-reaction mass spectrometry (PTR-MS) at
SMEAR II continuously at different altitudes. The HOM monomers (molecules
with even mass in 300–450 Th), dimers (molecules with even mass in 452–620 Th),
organonitrate (represented by seven major molecules, i.e. C7H9O8NNO3-,
C10H15O6-11NNO3-), and sulfuric acid concentrations
were measured at SMEAR II with a chemical ionization
atmospheric-pressure-interface time-of-flight mass spectrometer (CI-APi-TOF)
(Jokinen et al., 2012) during spring 2013. At SORPES, VOCs were observed with
gas chromatography–mass spectrometry (GC-MS) from September to October in
2014 (Xu et al., 2017). A summary of the observations at the two
stations used in this study is provided in Table S1 in the Supplement. More details about the
two stations and measurements are described by Hari et al. (2013)
and Ding et al. (2016).
Model descriptions
In this study we applied the MALTE-BOX model (the model to predict new
aerosol formation in the lower troposphere), a zero-dimensional model, which
includes several modules for the simulations of chemical and aerosol
dynamical processes (Boy et al., 2006). This model has been successfully
utilized in NPF analysis – for instance, reproducing OH radical and gaseous
sulfuric acid levels (Petäjä et al., 2009), validating various
plausible nucleation mechanisms and particle growth (Boy et al., 2007; S. Wang
et al., 2013), and identifying important factors influencing NPF occurrence
(Boy et al., 2006, 2008; Ortega et al., 2012).
The gas-phase chemistry was simulated using the Master Chemical Mechanism
version 3.3.1 (MCMv3.3.1, http://mcm.leeds.ac.uk/MCM/, last access: 1 August 2018) solved by
Kinetic PreProcessor (KPP) (Damian et al., 2002; Jenkin et al., 2003; Saunders et al., 2003). In addition, a new HOM
autoxidation mechanism, which is constructed based on the oxidation of
monoterpenes (Ehn et al., 2014), was added to MCM v3.3.1. This HOM
mechanism explicitly describes the HOM formation processes, i.e. ozone
oxidation of monoterpenes, intramolecular H shift, and O2 additions
(autoxidation) (Öström et al., 2017). Moreover, based on Molteni et
al. (2016), a simplified mechanism of HOM formation from the oxidation of
aromatics by OH was added to MCM v3.3.1. The aerosol dynamical processes
were simulated with the size-segregated aerosol model, UHMA (University of
Helsinki Multicomponent Aerosol model) (Korhonen et al., 2004). A fixed
sectional approach with 120 bins from 1 nm to 2.5 µm in diameter
was used. For the smallest size bin, the formation rates of newly formed
particles were estimated with the function of sulfuric acid and a
first-generation oxidation product of the included monoterpenes,
i.e. J1=k×[H2SO4][HOMnuc], where
HOMnuc was formed with a molar yield of 10-5 for each
monoterpene reacted with OH (Roldin et al., 2015). The kinetic coefficient
(k value) was set for each case to achieve the highest correlation compared
to the measured newly formed particles. Organic compounds with pure liquid
saturation vapour pressure less than 0.01 Pa were chosen as condensing vapours
in UHMA. The saturation vapour pressures of organic compounds in MCM v3.3.1
were estimated with the group contribution method by Nannoolal et al. (2008)
using the UManSysProp online system (Topping et al., 2016). The saturation
vapour pressures of HOMs were calculated using SIMPOL (Pankow and Asher, 2008) as
the method of Nanoolal et al. (2008) produces unrealistic estimates of vapour
pressures for multifunctional HOMs containing hydroperoxide or a peroxy acid
group (Kurteìn et al., 2016). H2SO4 was treated as a
non-volatile condensing vapour, which generally is a reasonable assumption at
typical atmospheric relative humidity and NH3 levels (Tsagkogeorgas
et al., 2017). The coagulation process, dry deposition process, and the
dilution of aerosol number concentration caused by boundary layer evolution
were estimated in the model as well. Further details about the MALTE model can be
found in Sect. S2 in the Supplement.
The measurement variables, i.e. meteorological conditions (e.g. air
temperature, relative humidity, pressure, and radiation), trace gases
concentrations (e.g. SO2, O3, NO, NO2, CO), and VOCs
(e.g. ethylene, ethane, propane, acetone, methyl vinyl ketone, n-Butane,
benzene, toluene, o-/m-xylene, 1,2,3/1,2,4-trimethylbenzene,
ethylbenzene, isoprene, and monoterpenes), were input into the MALTE-BOX model
every 10 min. As monoterpenes were not measured with GC-MS at SORPES,
monoterpene concentrations at SORPES were simulated using WRF-Chem, following
the method of Huang et al. (2016), in which it was shown that the MALTE-BOX
model worked well in NPF simulation with WRF-Chem output of VOCs. The
measured aerosol number size distribution was read into the model during the
first 5 h. The chemistry scheme was run with a spin-up time of 24 h
in order to achieve a realistic gas-phase composition before the aerosol
module was switched on.
Statistics of observed formation rates of 6 nm particles
(J6), growth rates of 6–30 nm particles (GR), condensation sinks (CS),
O3, SO2 and NOx concentrations, radiation (Rad.), air
temperature (Temp.), and relative humidity (RH) from 09:00 to 15:00 LT on
NPF days at SMEAR II and SORPES. Note the statistical samples are the whole-year database of 2013 at SMEAR II and the whole-year database of 2014 at
SORPES.
SMEAR II SORPES AverageMedian25th75thAverageMedian25th75thJ6 (cm-3 s-1)0.30.10.060.32.31.613.5GR (nm h-1)4.52.82.05.68.78.06.510.4CS (10-2 s-1)0.180.140.080.243.02.72.13.6O3 (ppbv)36.136.629.641.844.743.328.059.1SO2 (ppbv)0.20.10.030.39.48.04.412.7NOx (ppbv)0.50.20.060.617.713.47.923.0Rad (W m-2)373383211519695720561876Temp. (∘C)6.76.9-0.815.119.420.914.525.1RH (%)5856427448453459ResultsComparisons of NPF at SMEAR II and SORPES
According to long-term observations, the frequency of NPF at SMEAR II is
23 %, with the highest value in spring months (about 40–50 %) (Nieminen
et al., 2014). Although the concentration of pre-exiting particles is high,
which inhibits NPF, the NPF occurs even more frequently in Chinese megacities
such as Nanjing. The frequency of NPF at SORPES is 44 %, with the highest
value also in spring (55 %) (Qi et al., 2015). As shown in Table 1, the averaged formation rate of 6 nm particles (J6) at SMEAR II is
0.3 cm-3 s-1 while the J6 at SORPES is 2.3 cm-3 s-1
on average, which is almost 7 times higher than at SMEAR II. The growth
rate of 6–30 nm particles is also higher at SORPES, with 4.5 nm h-1 at
SMEAR II compared to 8.7 nm h-1 at SORPES on average.
The environmental conditions during NPF at the two sites are substantially
different. First, the pre-existing particle loading is much higher at
SORPES than at SMEAR II. The condensation sink (CS) at SORPES is almost 20 times higher than at
SMEAR II (Table 1). High CS tends to inhibit the occurrence of NPF because of
the scavenging of cluster and the loss of gas-phase low-volatility compounds
(Kulmala et al., 2017). Second, the concentrations of atmospheric oxidant
such as ozone are higher at SORPES (Table 1). Moreover, the concentrations of
OH and NO3 radicals in the Yangtze River Delta urban area of China are higher than in the
clean area (S. Wang et al., 2013; Nan et al., 2017). Third, the
concentrations of anthropogenic pollutants and biogenic VOCs are distinctly different at the two stations. As an important gas
precursor of NPF, the SO2 concentration at SORPES is almost 50 times
higher than at SMEAR II (Table 1). The concentration of NOx,
which is believed to suppress NPF by reacting with peroxy radicals (Wildt
et al., 2014), is also much higher at SORPES. The concentrations of
anthropogenic volatile organic compounds (AVOCs) are much higher at SORPES
while the biogenic volatile organic compound (BVOC) concentrations
(e.g. monoterpene and isoprene) are higher at SMEAR II (Hakola et al.,
2012; Xu et al., 2017). Given such high anthropogenic VOCs at SORPES,
anthropogenic secondary organic aerosol (SOA) is one of the most important SOAs in a polluted area like
SORPES (Hu et al., 2017). As the BVOC emissions are quite high in
southern China (Fig. 1b), biogenic SOA formation might also be important at
SORPES through the interactions between biogenic and anthropogenic emissions,
especially when the air masses are from southern China under specific synoptic
weather (Zhang et al., 2017, 2016; Carlton et al., 2009). In
addition, the meteorological parameters during NPF at the two sites are also
different. Based on the statistics of 1 year of data provided in Table 1, the
global radiation and temperature are higher and relatively humidity is lower
at SORPES than at SMEAR II during the NPF events.
The NPF classification and environmental conditions on each chosen
case day at SMEAR II and SORPES. Note that condensation sink, meteorological
conditions, and the concentrations of trace gases are from 09:00 to 15:00 LT.
To study the differences in NPF at SMEAR II and SORPES in depth, the 4
NPF days and 1 non-NPF day at each site were chosen for simulations with
MALTE-BOX (Table 2). In addition to the differences of NPF parameters and
environmental conditions at the two sites described above, monoterpene and
benzene concentrations on each day at the two sites are tabulated in Table 2.
Because of the high monoterpene emissions in southern China (Fig. 1), the
monoterpene concentrations are relatively high at SORPES, especially when the
air masses originate from the south. The averaged monoterpene concentration
on the
chosen days is 0.05 ppbv at SORPES compared to 0.13 ppbv at SMEAR II. The anthropogenic VOCs (e.g. benzene, Table 2) are higher at
SORPES, as a
suburban site, than at SMEAR II, with 0.54 ppbv of benzene concentration at SORPES
compared to 0.06 ppbv at SMEAR II on average. The averaged concentration of
aromatics (including benzene, toluene, o-/m-xylene,
1,2,3/1,2,4-trimethylbenzene, ethylbenzene) at SORPES on the chosen days was 1.2 ppbv.
Averaged simulated and measured diurnal cycles of
(a)H2SO4, (b) HOM non-nitrate monomers,
(c) HOM dimers, and (d) HOM organonitrates at SMEAR II and
SORPES.
The differences in simulated condensing vapours at two sites
As shown in Fig. 2a, similar to previous studies (e.g. Zhou et al., 2014), the
model underestimates the concentrations of sulfuric acid at SMEAR II
especially at night. The reasons for this discrepancy could be that there
are oxidants other than OH, and Criegee intermediate radicals lead to the
formation of sulfuric acid (Boy et al., 2013). Because of the detection limit of the CI-APi-TOF, the HOM non-nitrate
monomers, dimers, and organonitrates presented in Fig. 2b–d contain 7–14,
8–17, and 7–14 oxygen atoms, respectively. The model predicts the measured
diurnal cycle of HOM non-nitrate monomers at SMEAR II with good agreement.
For HOM dimers, the simulated concentrations are higher than the
measurements at night while they are slightly lower in the daytime when the NPF events
occur. For HOM organonitrate, although matching well with measurements in the
daytime, the simulation results have a stronger diurnal pattern, with much
lower concentrations than measurements at night. In general, the normalized
mean bias (NMB) values of sulfuric acid, HOM non-nitrate monomers, dimers,
organonitrates, and total HOM are -63.0 %, 11.1 %, 174.3 %, 8.0 %, and
38.3 %, respectively. Considering the uncertainties of the CI-APi-TOF in
measuring gas HOMs (estimated uncertainty up to a factor of 2–3) and the many
unknowns in their formations, the model provides an acceptable agreement
between simulated and measured vapour concentrations.
Although no measurements of sulfuric acid and HOM are conducted at SORPES, a
comparison of the simulated gas vapour concentrations at two sites can help
us to qualitatively understand the differences between the boreal forest and
polluted areas in China. As shown in Fig. 2a, the simulated sulfuric acid at
SORPES is 1 order of magnitude higher than at SMEAR II in the daytime. The high
value of sulfuric acid is mainly related to the extremely high SO2
concentrations and high atmospheric oxidation capacity at SORPES. Such high
simulated sulfuric acid concentration is consistent with the measurements
conducted in other urban sites in China, e.g. about
107 measurements cm-3
in Beijing (Z. B. Wang et al., 2013). The simulated HOM
non-nitrate monomer concentrations from monoterpene oxidation are lower at
SORPES (Fig. 2b) because of low values of monoterpene concentrations and high
condensation sink. The simulated HOM dimer concentrations are much lower at
SORPES than at SMEAR II while HOM organonitrate concentrations at SORPES are
1 order of magnitude higher than at SMEAR II (Fig. 2c, d). It is mainly
because high NO concentrations at SORPES suppress the HOM dimer formation but
contribute to the formation of HOM organonitrates.
Simulated diurnal cycles of HOMs formed from aromatics oxidation at
SORPES on each chosen day.
The simulated HOM monomer, dimer, and organonitrate concentrations presented
in Fig. 2 only refer to the HOMs formed from monoterpene oxidation, which
has been believed to be one of the main sources of HOMs and was considered in
the MALTE-BOX (Ehn et al., 2014). However, recent lab experiments show
that the aromatic hydrocarbons (e.g. benzene, toluene, o-/m-/p-xylene,
1,3,5-/1,2,3-/1,2,4-trimethylbenzene) oxidized by OH can lead to a
subsequent autoxidation chain reaction forming HOMs, which is believed to
contribute substantially to NPF in urban areas (Molteni et al., 2016). Therefore, according
to Molteni et al. (2016), a HOM molar yield of
3 % for the OH oxidation of the aromatic species was assumed and added
to MCM v3.3.1. The contributions of aromatics oxidation to the HOMs can
be ignored in remote boreal forest because of extremely low aromatics
concentrations. However, as shown in Fig. 3, the HOMs from aromatics oxidation
at SORPES can be above 108 cm-3, which is about 1 order of magnitude
higher than HOMs from monoterpene oxidation. HOM concentration from aromatics
oxidation on NPF days is obviously higher than non-event days, reflecting an
important role of HOMs in NPF. Such a high concentration of HOMs from aromatics
oxidation is caused by the high levels of aromatics and OH radicals in the
polluted urban environment and may contribute substantially to SOA
formation.
(a, c) Measured and (b, d) simulated particle
number size distribution at SMEAR II and SORPES, respectively. Note the
kinetic coefficient on each day is shown in Fig. 4b, d.
The simulations of aerosol size distributions at two sites
Figure 4 shows the variations in measured and simulated aerosol number size
distribution at SMEAR II and SORPES. The kinetic coefficients (k value) on
each day at both sites (tuned to cover the observed particle formation rates)
is shown in Fig. 4b and d. For the SMEAR II site, the model can capture both
the NPF events and non-NPF events with the same k value,
i.e. 1 × 10-18 m3 s-1. Comparing the observed and
simulated formation rates of 6 nm particles at SMEAR II (Table 3), the model
underestimated the formation rate on 1 May 2013 but overestimated the
formation rate on other NPF days. During event days, more than one banana shape
was simulated at SMEAR II, which is mainly because of the multi-peaks of
simulated sulfuric acid. For the SORPES station, the k value is higher than at
SMEAR II on average and with more discrepancies. The k value on 22
September 2014 is more similar to the value at SMEAR II but much lower
than on other chosen days. The variations in the k values can reflect the
variability in other unaccounted for compounds involved in the particle or
cluster formation and initial growth (Kuang et al., 2008). The much higher
k values at SORPES except on 22 September 2014 reflect that other
compounds, probably oxidation products of anthropogenic pollutants, can also
be
involved in the nucleation. Moreover, the model cannot simulate the high
formation rates observed at SORPES except on 22 September 2014 (Table 3).
The observed and simulated aerosol number size distributions
(a) at SMEAR II and (b) at SORPES. Note the observed and
simulated average (line) and ±1 SD (shaded area) are in blue and red,
respectively.
The observed and simulated formation rates of 6 nm particles (J6)
and growth rates of 6–30 nm particles (GR) on chosen NPF days (MM/DD/YYYY) at each
site.
For simulations at the SORPES station, the brief formation mechanisms of HOMs
from aromatics were added to the MCM and the saturation vapour pressure of
HOMs were calculated by SIMPOL. However, even if we decrease the pure liquid
saturation vapour pressures of HOMs from aromatics oxidation by 2 orders of
magnitude, the model significantly underestimates the growth during the
event days, except on 22 September 2014. The simulated growth rates on 22
and 24 September and 4 and 6 October are 7.8, 3.3, 2.8, and 2.8 nm h-1, compared
to the observed growth rates with 9.9, 16.2, 14.9, and 12.9 nm h-1,
respectively (Table 3). These results indicate that under polluted
environmental conditions there must be some other important gas vapours that
are not accounted for in the model that contributes to the growth. Tao et
al. (2016) found that heterogeneous uptake of amines can effectively
contribute to particle growth of newly formed particles in the polluted Yangtze River Delta area
of China. Heterogeneous uptake of amines has not been included in the
MALTE-BOX and might be one of the possible reasons of the underestimation of
growth rate. Comparing the observed and simulated number size
distribution (Fig. 5), the simulated aerosol size distributions were in good
agreement with measurements at SMEAR II, but the simulated number
concentrations in the size range below 200 nm at SORPES are extremely lower
than the observation. One reason is that primary particle emission is an
important source of ultrafine particles in urban areas (Qi et al., 2015),
but not accounted for in the model. Another reason is that current chemistry
mechanisms and the accounted for VOCs in the model dramatically underestimate
SOA formation in polluted areas. In addition to the
monoterpene-formed SOA, the MALTE-BOX model also considers the isoprene and
anthropogenic SOA. However, the mechanisms of SOA formation, especially for
the anthropogenic SOA, are still unclear and other unconsidered
anthropogenic gas vapours in the modelling studies may also contribute to
SOA formation.
The averaged retroplume (footprint residence time) from 09:00 to
15:00 LT on (a) 22 September, (b) 24 September,
(c) 26 September, (d) 4 October, and (e) 6 October 2014.
Only the NPF event on 22 September 2014 was simulated in good agreement
with measurements because this day had the lowest condensation sink and
highest aromatics concentrations among the chosen NPF cases at SORPES. Figure 6
presents the footprints of all the cases at SORPES. The air mass on 22
September 2014 was from a marine area. A previous study shows that these marine
air masses have the lowest accumulation mode particle concentrations and
therefore NPF occurs frequently (Qi et al., 2015). Although having
the lowest condensation sink, the aromatics concentration on this day was
still quite high, which was most probably due to a local petrochemical
industrial area. The air masses on 24 September and 6 October were from
northern China and brought air pollutants to Nanjing (Fig. 6b, e). On 4
October, the air masses had similar retroplumes to those on 22 September but with more
local origin (Fig. 6). Holiday effects in China (national holiday with more
family vacations during 1–7 October) caused the high NOx and
anthropogenic VOC concentrations on this day (Xu et al., 2017).
The NPF and growth were suppressed by high NOx
concentrations and therefore cannot be simulated by the current MALTE-BOX model.
The relative contributions of precursor vapours to the growth of
sub-100 nm particles at (a) SMEAR II and (b) SORPES.
The differences of relative contributions of precursor vapours to particle growth
at two sites
Figure 7 shows the averaged relative contributions of precursor vapours to the
growth of sub-100 nm particles from 09:00 to 15:00 LT during the four
chosen NPF days at SMEAR II and on 22 September 2014 at SORPES. Only the NPF
event on 22 September 2014 was presented at SORPES because the current MALTE-BOX
model can only capture the shape of NPF on this day. At SMEAR II, the growth
of ultrafine particles was dominated by HOM from monoterpene oxidation, which
is consistent with the previous study by Ehn et al. (2014). HOM monomers
contribute most to the growth at SMEAR II as they have high concentrations
and relatively low saturation vapour pressures.
The relative contributions of precursor vapours to the growth of particles at
SORPES are quite different from those at SMEAR II. First, through the higher
gas-phase sulfuric acid concentration at SORPES (as shown in Fig. 2),
sulfuric acid has huge contributions to the growth of ultrafine particles at
SORPES while playing a minor role in the growth at SMEAR II. Second, high NO
concentration at SORPES switches the formation of HOM non-nitrate monomers
and dimers to the formation of HOM organonitrates. As under the same oxygen-to-carbon ratio the saturation vapour pressures of organonitrates were higher
than non-nitrate monomers and dimers, the HOMs from monoterpene oxidation
contribute less to the growth at SORPES in general. Third, at SORPES, HOMs
from aromatics oxidation play a dominant role in the growth of ultrafine
particles because of high aromatics concentrations. Dai et al. (2017)
conducted the simultaneous measurements near a petrochemical industrial area
in Nanjing and found that the anthropogenic VOCs have significant
contributions to both the nucleation and the growth. This is also consistent
with the previous study at SORPES finding that higher growth rates were observed
when the air masses were from the Yangtze River Delta area with high anthropogenic VOC
emissions (Qi et al., 2015).
Conclusions
Higher-frequency formation rates and growth rates of new particle formation
(NPF) events were observed at SORPES, a suburban site in eastern China,
compared to SMEAR II, a boreal forest site in Finland. To quantitatively
understand the differences in NPF at the two sites, the condensing vapours
(i.e. sulfuric acid and HOM) and particle number size distributions were
simulated by a new version of the MALTE-BOX model with the comprehensive HOM
formation mechanism based on monoterpene oxidation and a simplified mechanism
of HOM formation from aromatics oxidation.
The model was proven to work well on simulating the sulfuric acid and HOMs
from monoterpene oxidation by comparing them with measurements at SMEAR II.
Comparing the simulated sulfuric acid and HOMs from monoterpene oxidation at
two sites, the sulfuric acid and HOM organonitrate concentrations were much
higher while the concentrations of HOM non-nitrate monomers and dimers were
lower at SORPES than at SMEAR II. High concentrations of HOMs from aromatics
oxidation were simulated at SORPES. The differences in gas vapours (sulfuric
acid and HOMs) at two sites are mainly because of the substantially higher SO2,
NO, aromatics concentration, and condensation sink at SORPES. The model can
simulate the particle number size distributions on NPF and non-NPF days with
same kinetic coefficient at SMEAR II. However, the k value is more divergent
at SORPES, which means the mechanism of nucleation in polluted urban areas is more
complicated. HOMs from monoterpene oxidation contribute more to the growth at
SMEAR II while the sulfuric acid and HOMs from aromatics play dominant roles
in the growth of newly formed particles at SORPES. This study highlights
that sulfuric acid and HOM concentration and their relative contributions to growth are distinct at different environmental conditions.
In summary, this study gives an example comparing the simulations of NPF and
particle growth in different environmental conditions using the MALTE-BOX
models with advanced chemical mechanisms. This study demonstrates that the
current model has a limited capacity in reproducing NPF and the growth rate in
polluted environments like eastern China. To improve the understanding of
NPF and SOA formation in the polluted environment, intensive, long-term
field measurements of HOMs with a CI-APi-TOF, combined with various measurements
of gaseous precursors, oxidants, clusters, and aerosol particles are needed
in the future. Further developments of the box model based on more
quantitative chamber studies are also needed. These efforts will help build
a universal chemical mechanism applicable for different (either clean or
polluted, anthropogenic or biogenic dominated) environmental conditions in the
world and further improve the capability of global air quality and climate
models.
The data of the SMEAR II station (including meteorological, trace gas, VOCs,
aerosol size distribution) are available at https://avaa.tdata.fi/web/smart (Junninen et al., 2009),
and data of SORPES (meteorological, trace gas, VOCs, aerosol size
distribution) are available from the corresponding author upon request
before the SORPES database is opened publicly. Emission data are available
at http://eccad.sedoo.fr/eccad_extract_interface/ (Granier et al., 2011).
The supplement related to this article is available online at: https://doi.org/10.5194/acp-18-11779-2018-supplement.
AD and MB designed the study. XQ performed the model simulations,
the analysis and the first draft of this paper. PR, MPR and
ME contributed to the construction of HOM formation mechanism. XQ, PZ and AR contributed
to the MALTE-BOX setup. ZX and XH contributed to the simulations of WRF-Chem. XQ, ZX and WN provided
the measurement data at SORPES and NS provided the measurement data
at SMEAR II. All authors commented and edited the paper.
The authors declare that they have no conflict of
interest.
This article is part of the special issue “Pan-Eurasian Experiment (PEEX)”. It is not associated with a
conference.
Acknowledgements
This study was supported by the Ministry of Science and Technology of China
(2016YFC0200500; 2016YFC0202000), the National Natural Science Foundation of
China (41725020, 41505109, 41675145, 91544231), Kunshan municipal scientific
project (KD2016005), the European Research Council (ERC) under the European
Union's Horizon 2020 research and innovation program (grant agreement numbers
638703(COALA), 742206 (ATM-GTP), and 227463 (ATMNUCLE)), and the Academy of
Finland Center of Excellence program (272041). The numerical modelling was
carried out on the Blade cluster system in the High-Performance Computing
& Massive Data Center (HPC & MDC) of the School of Atmospheric Sciences,
Nanjing University. The authors would like to thank the CSC – China
Scholarship Council for the joint PhD grant and thank Theo C. Kurten
for suggestions on the paper.
Edited by: Dominick Spracklen
Reviewed by: two anonymous referees
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