ACPAtmospheric Chemistry and PhysicsACPAtmos. Chem. Phys.1680-7324Copernicus PublicationsGöttingen, Germany10.5194/acp-20-2161-2020Mutual promotion between aerosol particle liquid water and particulate
nitrate enhancement leads to severe nitrate-dominated particulate matter
pollution and low visibilityMutual promotion in haze eventsWangYuhttps://orcid.org/0000-0002-9480-3570ChenYinghttps://orcid.org/0000-0002-0319-4950WuZhijunzhijunwu@pku.edu.cnShangDongjieBianYuxuanhttps://orcid.org/0000-0002-5846-417XDuZhuofeiSchmittSebastian H.https://orcid.org/0000-0002-5491-3572SuRongGkatzelisGeorgios I.SchlagPatrickhttps://orcid.org/0000-0002-0206-8987HohausThorstenhttps://orcid.org/0000-0001-5722-6244VoliotisAristeidishttps://orcid.org/0000-0001-9710-9851LuKedinghttps://orcid.org/0000-0001-9425-9520ZengLiminZhaoChunshengAlfarraM. Ramihttps://orcid.org/0000-0002-3925-3780McFiggansGordonhttps://orcid.org/0000-0002-3423-7896WiedensohlerAlfredKiendler-ScharrAstridhttps://orcid.org/0000-0003-3166-2253ZhangYuanhangHuMinState Key Joint Laboratory of Environmental Simulation and Pollution
Control, College of Environmental Sciences and Engineering, Peking
University, Beijing, 100871, ChinaCentre for Atmospheric Science, School of Earth and Environmental
Sciences, The University of Manchester, Manchester, M13 9PL, UKLancaster Environment Centre, Lancaster University, Lancaster, LA1
4YQ, UKInternational Joint Laboratory for Regional Pollution Control, 52425
Jülich, Germany, and Beijing, 100871, ChinaCollaborative Innovation Center of Atmospheric Environment and
Equipment Technology, Nanjing University of Information Science and
Technology, Nanjing, 210044, ChinaState Key Laboratory of Severe Weather, Chinese Academy of
Meteorological Sciences, Beijing, 100081, ChinaInstitute for Energy and Climate Research IEK-8: Troposphere,
Forschungszentrum Jülich, 52425 Jülich, GermanyDepartment of Atmospheric and Oceanic Sciences, School of Physics,
Peking University, Beijing, 100871, ChinaNational Centre for Atmospheric Science, School of Earth and
Environmental Sciences, The University of Manchester, Manchester, M13 9PL,
UKLeibniz Institute for Tropospheric Research, 04318 Leipzig, Germanynow at: The Center of Urban Transport Emission Research and State
Environmental Protection Key Laboratory of Urban Ambient Air Particulate
Matter Pollution Prevention and Control, College of Environmental Science
and Engineering, Nankai University, Tianjin, 300071, Chinanow at: TSI GmbH, 52068 Aachen, Germanynow at: Guangdong Science and Technology Monitoring and Research
Center, Guangzhou, 510033, China
now at: NOAA Earth System Research Laboratory, Boulder, Colorado
80305, USAnow at: Cooperative Institute for Research in Environmental Sciences,
Boulder, Colorado 80309, USAnow at: Shimadzu Deutschland GmbH, 47269 Duisburg, Germany
As has been the case in North America and western Europe,
the SO2 emissions have substantially reduced in the North China Plain (NCP) in
recent years. Differential rates of reduction in SO2 and NOx
concentrations result in the frequent occurrence of particulate matter pollution dominated by nitrate
(pNO3-) over the NCP. In this
study, we observed a polluted episode with the particulate nitrate mass
fraction in nonrefractory PM1 (NR-PM1) being up to 44 % during
wintertime in Beijing. Based on this typical pNO3--dominated haze
event, the linkage between aerosol water uptake and pNO3-
enhancement, further impacting on visibility degradation, has been
investigated based on field observations and theoretical calculations.
During haze development, as ambient relative humidity (RH) increased from
∼10 % to 70 %, the aerosol particle liquid water
increased from ∼1µgm-3 at the beginning to
∼75µgm-3 in the fully developed haze period. The
aerosol liquid water further increased the aerosol surface area and volume,
enhancing the condensational loss of N2O5 over particles. From the
beginning to the fully developed haze, the condensational loss of
N2O5 increased by a factor of 20 when only considering aerosol
surface area and volume of dry particles, while increasing by a factor of 25 when
considering extra surface area and volume due to water uptake. Furthermore,
aerosol liquid water favored the thermodynamic equilibrium of HNO3 in
the particle phase under the supersaturated HNO3 and NH3 in the
atmosphere. All the above results demonstrated that pNO3- is
enhanced by aerosol water uptake with elevated ambient RH during haze
development, in turn facilitating the aerosol take-up of water due to the
hygroscopicity of particulate nitrate salt. Such mutual promotion between
aerosol particle liquid water and particulate nitrate enhancement can
rapidly degrade air quality and halve visibility within 1 d. Reduction
of nitrogen-containing gaseous precursors, e.g., by control of traffic
emissions, is essential in mitigating severe haze events in the NCP.
Introduction
Aerosol particle hygroscopicity plays an important role in air quality
deterioration and cloud formation (Yu and Luo, 2009; Fitzgerald, 1973; Kreidenweis
and Asa-Awuku, 2014; Wang and Chen, 2019; McFiggans et al., 2006) and can also
directly influence aerosol measurements (Chen et al., 2018a). In
atmospheric environments influenced by anthropogenic activities, particulate
secondary inorganic compounds are often dominated by particulate sulfate and
nitrate (Heintzenberg, 1989), which originate from the oxidation
of sulfur dioxide (SO2) and nitrogen oxides (NOx) via multiple
chemical pathways (Calvert et al., 1985; Cheng et al., 2016; Wang et al.,
2016; Gen et al., 2019a, b). The abundance of secondary inorganic components
is one of the most important factors determining particle hygroscopicity
(Swietlicki et al., 2008), thereby
governing the aerosol liquid water content under ambient moist conditions.
Increased aerosol particle liquid water could accelerate secondary inorganic
and organic aerosol formation by decreasing the kinetic limitation of mass
transfer of gaseous precursors and providing more of a medium for multiphase
reactions (Mozurkewich and Calvert, 1988; Cheng et al., 2016; Wang et al.,
2016; Ervens et al., 2011; Kolb et al., 2010).
Sulfuric acid (H2SO4) is formed from the oxidation of SO2 via
gaseous and multiphase reactions. H2SO4 is subsequently fully or
partly neutralized by gaseous NH3 taken up on particles, resulting in
the formation of (NH4)2SO4 and/or NH4HSO4. Any
remaining NH3 is available to neutralize HNO3 to form particulate
NH4NO3 (Seinfeld and Pandis, 2006; and further excess
NH3 can neutralize any available HCl to form particulate NH4Cl).
Over the past several decades, substantial efforts have reduced emissions of
both SO2 and NOx, improving local and regional air quality all
over the world. For example, SO2 and NOx emissions were reduced by
82 % and 54 %, respectively, in the majority of European Environment Agency member
countries between 1990 and 2016 (https://www.eea.europa.eu/data-and-maps/indicators/main-anthropogenic-air-pollutant-emissions/assessment-4, last access: 15 February 2020).
In consequence, an increasing trend in the
NO3-/SO42- molar ratio was observed in long-term
measurements in Leipzig, Germany (Spindler et al., 2004),
and at some other European sites from the European Monitoring and Evaluation
Programme (EMEP; Putaud et al., 2004).
China also managed to reduce SO2 emissions by
75 % during 2007–2015 (C. Li et al., 2017), and they
declined by ∼15.1 % yr-1 during 2013–2017 (Vu et al., 2019), whereas NOx
emissions declined by only ∼10 % between 2011 and 2015
(de Foy et al., 2016) and by ∼4.3 % yr-1
during 2013–2017 (Vu et al.,
2019). Strict emission control reduced the PM2.5 mass concentration
and the corresponding chemical components in China significantly
(Vu et al., 2019). The annual mean PM2.5
mass loading decreased by 39.6 % during 2013–2017 in the
Beijing–Tianjin–Hebei region, and the SO42- and NO3-
mass concentrations in the PM2.5 declined by 40 % and 34 %,
respectively, during 2015–2017 in Beijing
(Vu et al., 2019). However, NH3 emissions
have been observed by satellites to have increased by ∼30 % from
2008 to 2016 over the North China Plain (NCP; Liu et al., 2018). The faster reduction rate of
SO2 compared with NOx emissions, in conjunction with elevated NH3
level, made it reasonable to switch the dominant inorganic component in fine
aerosol particles from sulfate to nitrate in recent years, similarly to
European countries (Sun et al., 2015; Hu et al., 2017, 2016; Wu
et al., 2018; Guo et al., 2014; Huang et al., 2014, 2010; Ge et
al., 2017; Q. Xu et al., 2019; Xie et al., 2019; Li et al., 2018). Field
measurements in Beijing show that the annually averaged
NO3-/SO42- molar ratio of NR-PM1 (nonrefractory
PM1) in 2012 (1.3–1.8; Sun et al., 2015) significantly
increased compared to that in 2008 (0.9–1.5; Zhang et al., 2013). Comparably, the
NO3-/SO42- molar ratio of PM2.5 in Beijing
increased substantially, from 1.5 before 2013 to 3.33 in 2017
(Q. Xu et al., 2019).
Over the NCP region, heavy haze events are typically associated with
enhanced ambient RH levels. This leads to an increased aerosol liquid water
content (Wu et al., 2018), which enhances the particulate
nitrate formation by increasing the reactive uptake of precursors and the
thermodynamic equilibrium of ammonium nitrate (Cheng et al., 2016; Wang et
al., 2016, 2017; Yun et al., 2018; Yue et al., 2019). To date, a few
studies have reported aerosol liquid water content over the NCP region (Wang et
al., 2018; Bian et al., 2014; Cheng et al., 2016; Wu et al., 2018; Ge et al.,
2019). However, the observational and theoretical analysis of the
relationship between particulate nitrate enhancement and associated liquid
water during haze events in China has been infrequently reported
(Wu et al., 2018).
In this study, a self-amplification effect between particulate nitrate and
liquid water is demonstrated by examining a nitrate-dominated fine-particle
Beijing pollution episode. The facilitation of particulate nitrate
enhancement by abundant aerosol liquid water is subsequently theoretically
explored through the impacts of liquid water on thermodynamic equilibrium
and heterogeneous reactions. Finally, the corresponding impacts on the light
extinction coefficient and visibility degradation are estimated. These
results improve our quantitative understanding of the development of haze
events over the NCP and help to formulate emission reduction strategies, as
well as maybe also providing insights into other polluted regions.
Measurements and methodsLocation and instrumentation
Measurements were conducted within the framework of the BEST-ONE (Beijing
winter finE particle STudy – Oxidation, Nucleation and light Extinctions)
field campaign from 1 January to 5 March 2016, at the Huairou site
(40.42∘ N, 116.69∘ E), located in a rural environment,
north of Beijing, China. Detailed information about the sampling site was
described in Tan et al. (2018). A
weather station (Met One Instruments Inc., USA) was used to measure
meteorological parameters (ambient RH, temperature, wind speed, wind
direction), and detailed aerosol particle physical and chemical properties
were recorded using a suite of state-of-the-art instrumentation.
The hygroscopic growth factor (HGF) of submicrometer aerosol particles was
measured using a hygroscopicity tandem differential mobility analyzer
(H-TDMA, TROPOS, Germany; Wu et al., 2011; Massling et al., 2011; Wang et
al., 2018; Wu et al., 2016; Liu et al., 1978), and data retrieval followed the
TDMAinv method in Gysel et al. (2009). The
hygroscopicity parameter (κ) was estimated using the κ-Köhler approach (Petters and Kreidenweis,
2007; Köhler, 1936). Size-resolved NR-PM1 was recorded by an
Aerodyne high-resolution time-of-flight aerosol mass spectrometer
(HR-ToF-AMS, Aerodyne Research, Inc., USA; DeCarlo et al.,
2006). Regular calibration procedures followed as reported in
Jayne et al. (2000) and Jimenez et al. (2003),
and composition-dependent correction followed as in
Middlebrook et al. (2012). Gaseous HNO3 and NH3 were
measured using a gas–aerosol collector (GAC) coupled with ion chromatography
(IC; Dong et al., 2012). The mass concentration
of equivalent black carbon in aerosol particles
(Petzold et al., 2013) was recorded by a multiangle absorption photometer (MAAP, Model 5012, Thermo Fisher Scientific,
USA) with a laser wavelength of 670 nm (Petzold and
Schönlinner, 2004). Furthermore, particle number size distribution
(PNSD) in the size range of 3 nm–10 µm was measured
using a mobility particle size spectrometer (MPSS, Model 3776+3085
3775+3081, TSI, USA), following the recommendations described in
Wiedensohler et al. (2012), and an aerodynamic particle size spectrometer
(APS, Model 3021, TSI, USA; Wu et al., 2008; Pfeifer et al., 2016).
Detailed descriptions of the H-TDMA, HR-ToF-AMS and GAC–IC can be found in the
supporting information.
Estimation of aerosol particle liquid water
Given the absence of direct liquid water measurements, size-resolved liquid
water was calculated using the corresponding HGFs measured at RH =90 %
(50, 100, 150, 250, 350 nm in Stokes diameter), PNSD data (3 nm–10 µm) and meteorological parameters (RH, T),
following the method proposed in Bian et al. (2014), referred to
below as H-TDMA-derived liquid water. Briefly, the measured PNSD with 57
size bins was fitted using a four-mode lognormal distribution. The
classification of the four modes and the fitting results are shown in Table S1
and Fig. S4 in the Supplement. Good agreement between measured values and the fitted PNSD was
achieved, which indicates the reliability of the four-mode lognormal fitting
method. Based on four-mode lognormal fitting results, the particle number
size distribution and number fractions of each mode can be obtained. It has
been assumed that particles from the same mode have constant particle
hygroscopicity (κ). Under the assumption of constant particle
hygroscopicity in each mode (shown in Table S1), the κ values for
each mode (κ1, κ2, κ3) can be
calculated by Eq. (1) from the known number fraction of the fitted four modes
and the κ values of measured particle size from H-TDMA measurements.
κ=∑i=14κifi
Here, κi and fi represent the κ value and
the particle number fraction of the i mode. Then, the calculated κ
values for each mode and the derived number fraction of each size bin were
used to obtain the κ distribution for each size bin. Figure S5 shows
the comparison of the calculated size-resolved κ distribution and the
κ measured by the H-TDMA; the good agreement showed the reliability of
the method. Then, based on κ-Köhler theory
(Petters and Kreidenweis, 2007; Köhler, 1936), the
size-resolved HGFs at ambient RH were calculated. Finally, liquid water of
size-resolved particles can be derived by calculating the differentials
between the dry and wet PNSD of aerosol particles in Eq. (2):
liquid water=π6NjDp,j3HGFDp,RH3-1⋅ρw,
where j represents the bin number of measured PNSD and Nj and Dp,j
represent the number concentration and the diameter of dry particles of the
jth bin, respectively, while HGF and ρw are the hygroscopic
growth factor of aerosol particles and water density (1 g cm-3),
respectively.
Condensation rate of trace gases
The condensation rate (k) of trace gases (dinitrogen pentoxide,
N2O5, referred to as k_N2O5) was calculated
by the method of Schwartz (1986), shown in Eq. (3). In order to
illustrate the influences of the dry and wet PNSDs due to water uptake on the
condensation rate of gases, the PNSDs of the dry and wet particles (obtained
by applying the HGF estimated from the H-TDMA-derived liquid water method) were
used.
3k=4π3∫0∞r23Dg+4r3Cgγ-1r3dNdlogrdlogr,4Cg=3RTM,
where r represents the radius of the particles, and Dg represents the binary
diffusion coefficient evaluated following Maitland et al. (1981)
(1.18×10-5 m2 s-1). Cg is the kinetic velocity of the gas
molecules, calculated in Eq. (4). Here, R and M are the ideal gas constant
(8.314 kg m2 mol-1 K-1 s-2) and molar mass of the gas, respectively,
while T represents the ambient temperature. The expression dN/dlogr is the number size
distribution, and γ is the uptake coefficient of the gas.
The uptake coefficient of N2O5 was estimated following the method
proposed in Chen et al. (2018b) and
Chang et al. (2016) and the references therein. The influences
of RH, temperature, multiple inorganic particle compositions, secondary
organic aerosol (SOA) and primary organic aerosol (POA) are considered. The
uptake suppression effect of N2O5 due to the presence of SOA was
considered following the method in Anttila et al. (2006). Based on
our source apportionment using positive matrix factorization (SoFi tool,
ME2, Francesco Canonaco, PSI), two oxygenated organic aerosol (OOA) factors,
usually interpreted as SOA, and three POA factors were determined. The
fraction of SOA in the total organic aerosol (OA) was 60 %–90 % during the observed period, which is quite consistent with the
results of a previous study in Beijing (Huang et al.,
2014). Hence, 75 % was used as the ratio of SOA/OA in our model to
estimate the suppression effect of SOA on the uptake of N2O5
following the work of Anttila et al. (2006). The reaction of
chloride with N2O5 was not considered in this study due to its
limited mass concentration (on average 5 % of the PM1 mass
concentration during the marked haze period), which could cause minor
uncertainty in the k_N2O5 calculation. The detailed
information regarding the estimation γN2O5 is given in
Chen et al. (2018b), and influences of different
chemical components on γN2O5 are summarized in Table 1 of
Chen et al. (2018b).
Equilibrium of NH4NO3
The equilibrium dissociation constant of NH4NO3 (Kp) under dry
conditions was calculated as a function of ambient temperature
(Seinfeld and Pandis, 2006) in the following Eq. (5).
lnKp=84.6-24220T-6.1lnT298
Taking into account the associated liquid water, the equilibrium vapor
pressure of HNO3 and NH3 was calculated by employing the
Extended Aerosol Inorganic Model (E-AIM), H+-NH4+-SO42--NO3--H2O (Clegg et al., 1998),
using HR-ToF-AMS data, NH3 from GAC–IC and meteorological parameters
(RH, T). In this calculation, a simplified ion pairing scheme was performed
to ensure the ion balance of the input chemical composition following the
method in Gysel et al. (2007).
Light extinction coefficient and visibility calculation
The size-resolved chemical composition of the NR-PM1 from the HR-ToF-AMS, the mass
concentration of equivalent black carbon from the MAAP, PNSD data and the
H-TDMA-derived liquid water were used to calculate the light extinction
coefficient (including light absorption and scattering) and visibility
degradation of size-resolved particles by the Mie scattering theory
described in Barnard et al. (2010). Here, the
size-resolved equivalent black carbon mass concentration was inferred from the
particle mass size distribution measurement by a single-particle soot
photometer at Peking University Atmosphere Environment Monitoring Station (PKUERS). The method of redistribution of liquid water and
HR-ToF-AMS data has been described in the supporting information (Text S1 in the Supplement,
HR-ToF-AMS introductory section). Thus, with the redistributed datasets as
the input into the Mie scattering theory, the light extinction coefficient for
atmospheric particles in the absence and presence of liquid water with a
size range of 100–2500 nm in Stokes diameter can be derived.
Due to lack of measurements of aerosol particle morphology and mixing states,
we assume particles are spherical as described in
Barnard et al. (2010). To perform Mie calculations,
the complex reflective index of each component is given in Table 1 of
Barnard et al. (2010) and references therein. This
method shows good agreement with measurements in Mexico City and is
consistent with the regional atmospheric chemistry model WRF-Chem. Here,
Ext_550nm_wet and Ext_550nm_dry represent the calculated light extinction
coefficient for particles in the presence and absence of liquid water at an
incident light wavelength of 550 nm. The corresponding visibility
degradation (VIS) for dry and wet particles was calculated from the light
extinction coefficient following the Koschmieder Eq. (6).
VIS=3.912Ext_550nm
Results and discussionNitrate-dominated fine particulate matter pollution
Figure 1 illustrates a summary of the chemical composition of NR-PM1,
ambient RH, size distribution of and total aerosol particle liquid water, and size
distribution of and total aerosol surface area concentration during the period
of 29 February to 5 March 2016 in the BEST-ONE campaign. During this
period, polluted episodes occurred under stagnant meteorological conditions
with low wind speed (Fig. S6) and elevated ambient RH (Fig. 1a). As
seen in the haze period shown in Fig. 1, an obvious increase in NR-PM1 was
observed. The secondary inorganic components (sulfate, nitrate and ammonium)
were dominant components of the NR-PM1, accounting for up to 73 %
during the haze period. Particularly, nitrate was the major contributor to
the secondary inorganic components and accounted for up to ∼44 %
of NR-PM1 mass, while sulfate contributed for ∼12 % on average.
The time series of (a) NR-PM1 chemical composition measured
by the HR-ToF-AMS and ambient RH (solid red line), (b) size-segregated
aerosol particle liquid water and the total mass concentration of liquid
water with an aerodynamic diameter smaller than 1 µm (solid red
line), and (c) size-segregated aerosol particle surface area and total aerosol
particle surface area without considering particle hygroscopic growth during
29 February to 5 March 2016.
In recent decades, severe haze events with high aerosol mass loading
occurred frequently in Beijing during wintertime (Hu et al., 2016, 2017; Sun et al., 2014, 2015). To mitigate the air pollution,
the Beijing government implemented strict emission controls. The total mass
loading of particulate matter has reduced substantially in recent years
(http://sthjj.beijing.gov.cn/, last access: 15 February 2020). With decreasing PM mass
concentration, the mass fraction of particulate nitrate during these haze
events in Beijing was enhanced substantially. In 2014, the highest fraction of
nitrate in PM1 was reported as ∼20 % and increased to
∼35 % in 2016 (W. Xu et al., 2019), which is
comparable to the ratio (44 %) in this study. The particulate nitrate
became more dominant in secondary inorganic compounds other than particulate
sulfate with the air quality improvement over the NCP.
As one of the main hydrophilic compounds in atmospheric aerosol particles,
the ability of water uptake is comparable between deliquescent
(NH4)2SO4 and NH4NO3 particles with the same sizes and
ambient RH (Kreidenweis and Asa-Awuku, 2014; Wu et al., 2016; http://umansysprop.seaes.manchester.ac.uk/, last access: 15 February 2020). However, compared to
(NH4)2SO4, NH4NO3 particles have a lower
deliquescence RH (62 %, 298 K) than (NH4)2SO4 (80 %, 298 K; Kreidenweis and Asa-Awuku, 2014) and easily liquefy
(Y. J. Li et al., 2017). In addition, NH4NO3
particles are semivolatile; the co-condensation of semivolatile compounds
and water (Topping et al., 2013; Hu et al., 2018) could be significant.
Therefore, the switching from sulfate-dominated to nitrate-dominated aerosol
chemistry may impact on aerosol water uptake. The interaction between
aerosol particle liquid water and particulate nitrate formation and
visibility degradation should be reconsidered.
Mutual promotion between liquid water and particulate nitrate
enhancement
Lu et al. (2019) conducted a box model to calculate the
potential particulate nitrate formation during the same investigated period
of the BEST-ONE project. They found out that HNO3 from daytime
photooxidation of NO2 was the major source of the particulate nitrate
(>75 %), whereas the contribution of the N2O5 pathway
was lower than 25 % (Lu et al., 2019). In the following
discussion, the enhancement of particulate nitrate during the haze period
is elucidated by theoretical calculations of the condensational loss rate of
N2O5 and the thermodynamic equilibrium of NH4NO3 and
HNO3. In particular, the role of aerosol water uptake in particulate
nitrate formation is comprehensively investigated.
N2O5 is an important gaseous precursor for particulate nitrate
formation via its hydrolysis to form HNO3 during the nighttime
(Brown et al., 2006). Liquid water can
enhance aerosol surface areas and volumes, thereby increasing the available
heterogeneous reacting medium. Across the development of the haze period, the
estimated liquid water increased from ∼1µg m-3 at
the beginning (2 March, 14:00–18:00 LT) to ∼75µgm-3 when the haze was fully developed (4 March,
04:00–08:00 LT). The total surface area and volume
concentrations of particles were increased by the liquid water by
2 %–3 % at the beginning and by up to ∼25 and
∼40 % in the fully developed haze compared to the dry
values, respectively (see Figs. S7 and S8). Additionally, from the
beginning to the fully developed haze, the uptake coefficient of
N2O5 was enhanced by a factor of 9 from 0.002 to 0.018, and the
k_N2O5 increased by a factor of 20 (dry particles),
while, considering the increased particle surface area and volume due to
water uptake, the respective value of enhanced k_N2O5 increased by a factor of 25 (Fig. 2a). Apart from providing an extra
reacting medium, the abundant liquid water can liquefy the aerosol particles
and may reduce any kinetic limitation of mass transfer for reactive gases
(Koop et al., 2011; Shiraiwa et al., 2011) and impact the thermodynamic
equilibrium of semivolatile compounds (Kulmala et al., 1993; Topping et
al., 2013) to contribute to secondary aerosol formation. Our previous study
provided the observational evidence that particles may have transitioned
from the solid phase to the liquid phase as RH increased from 20 % to
60 % during wintertime in Beijing (Liu et al., 2017). In
this study, the ambient RH increased from ∼10 % up to
70 % during the haze period, suggesting a likely transition of particles
from the solid to liquid phase. Such phase transition may facilitate
particulate nitrate formation by increasing the diffusion coefficients of
dissolved precursors.
The time series of the condensational loss rate of N2O5
(k_N2O5) with the calculation of the dry particle
number size distribution (PNSD) and wet PNSD during 29 February to 5 March 2016.
The comparison of the calculated temperature-dependent
dissociation constant of NH4NO3 (Kp; Seinfeld and
Pandis, 2006) in the absence of liquid water, the product of the equilibrium
vapor pressure of gaseous NH3 and HNO3 from E-AIM
(AIM_pNH3pHNO3), and the product of mixing ratios
of gaseous NH3 and HNO3 measured by GAC–IC (M_pNH3pHNO3). Here, Kp is colored by the ambient temperature ranging
265–293 K during 29 February to 5 March 2016.
The relationship between aerosol particle liquid water and
the molar ratio of particulate nitrate in the total nitrate,
mNH4NO3/(mHNO3+mNH4NO3)
(left y axis), during the nighttime 18:00–07:00+1 d (solid green triangle) and the daytime at 07:00–18:00 (solid red
triangle), and the mass concentration of particulate nitrate as a function of aerosol liquid water (right y axis) during the period of 29 February to 5 March 2016. Here,
particulate nitrate was measured by the HR-ToF-AMS, and the
HNO3 in the gas phase was measured by GAC–IC. Aerosol
liquid water was calculated using the H-TDMA-derived method.
The time series of chemical composition measured by the
HR-ToF-AMS (left y axis), calculated aerosol pH by ISORROPIA II (inner right y axis) and the molar ratio of particulate nitrate in the total nitrate
(gas + particle phase) shown on the outer right y axis during 29 February
to 5 March 2016.
The pH of the fine aerosol particles (left y axis) and the
molar ratio of particulate nitrate in the total nitrate
(gas + particle phase; right y axis) as a function of
NR-PM1 mass concentrations.
To illustrate the facilitation of particulate nitrate enhancement from
HNO3 in the presence of liquid water, we performed the theoretical
calculation of equilibrium between particulate NH4NO3 and gaseous
NH3 and HNO3 under dry and ambient conditions, respectively. The
dissociation constant of NH4NO3 (Kp) in dry conditions was calculated
using Eq. (5) without considering the influence of the liquid water. As
shown in Fig. 3, the equilibrium Kp in the dry conditions ranged from 0.06 to 4.61 ppb2 (275.3 to 291.5 K) during the haze period. Taking
account of the aerosol liquid water, the equilibrium vapor pressure of
HNO3 and NH3 over particles was calculated by the E-AIM Model II
(http://www.aim.env.uea.ac.uk, last access: 15 February 2020). Note that this calculation assumes negligible
interaction between dissolved organic components and the activity of
NO3-. In the presence of aerosol-associated water, the product of the
equilibrium vapor pressure of NH3 and HNO3 calculated according to the E-AIM
was 10 %–60 % lower than the equilibrium Kp in the dry
conditions during the marked haze period. This means the presence of
aerosol liquid water changed the equilibrium and would have favored the particulate
nitrate enhancement. However, the aerosol particles did not reach the
equilibrium between particulate NH4NO3 and the gases (NH3+HNO3) during the investigated period, as the measured product of the
NH3 and HNO3 partial pressure (2.55–9.63 ppb2)
was supersaturated compared to the equilibrium values in both dry and
deliquescent particles. In this case, the partitioning of gaseous NH3
and HNO3 in the atmosphere into the particle phase could be accelerated
and leads to particulate nitrate enhancement with increasing ambient RH.
Owing to its highly hydrophilic nature, the increased ammonium nitrate
mass fraction leads to further water uptake. Such a mutual promotion of
particulate nitrate and aerosol liquid water enhancement becomes more
pronounced with the increasing pollution throughout the haze event owing to
the simultaneously increasing ambient RH. Consistently, a significant
co-increase of particulate nitrate and aerosol liquid water was observed
during haze development as shown in Fig. 4. At first, a steep increase in
particulate nitrate in the total nitrate mass ratio (from ∼12 %
to ∼98 %) was observed as the aerosol liquid water was enhanced
up to ∼20µgm-3. And then, the particulate
nitrate mass kept increasing with further increases in aerosol liquid water.
We observed that ∼98 % of nitrate was present in the particle phase when aerosol liquid water was higher than ∼20µgm-3. The function between the particulate nitrate fraction in the
total nitrate and the aerosol particle liquid water is given in Fig. 4. It is worth noting that N2O5
hydrolysis during the nighttime can contribute extra HNO3 in the wet
denuding method within the GAC–IC system. This effect explains the slight
underestimation of the particulate fraction during the nighttime when aerosol
liquid water is less than 10 µgm-3 (Fig. 4). However, the
general consistency of this function between daytime and nighttime
(Fig. 4) suggests a negligible influence of N2O5 interference on
our analysis during the investigated period.
Apart from aerosol liquid water, aerosol pH is also an important factor in
the particulate nitrate formation; a higher pH is favorable for the
equilibrium of HNO3 into the particle phase
(Nah et al., 2018). The pH of the fine aerosol
particles was calculated by ISORROPIA II (Fountoukis and
Nenes, 2007) during the investigated period. The model was running in
forward mode with the chemical composition of NR-PM1 (NO3-,
SO42-, Cl-, NH4+) and gas precursors (HNO3,
HCl, NH3) by GAC–IC as inputs. And the model was running in metastable
mode when assuming no solid existed in the system. Generally, the fine aerosol
particles became more acidic with pH dropping from ∼8 down to
∼4 when NR-PM1 mass concentration increased from
∼12 up to >300µgm-3 as shown in Figs. 5 and 6. This declining trend of pH is
not favorable for the HNO3 partitioning into the particle phase
(Nah et al., 2018). However, a clear enhanced
trend of the molar ratio of particulate nitrate in the total nitrate as a
function of NR-PM1 mass concentration was observed correspondingly (as
shown in Figs. 5 and 6). Therefore, in this case the increase in
aerosol liquid water is more likely to be the driving factor of particulate
nitrate formation compared to the influence of pH.
The time series of (a) the calculated total extinction
coefficient at a wavelength of 550 nm with the consideration of dry and wet
PNSD, referred to as Extinction coefficient_dry and Extinction
coefficient_wet and (b) calculated visibility with the
consideration of dry and wet PNSD, referred to as Visibility_dry
and Visibility_wet, respectively, during 29 February to 5 March 2016. The visibility degradation percentage is (Visibility_wet-Visibility_dry)/Visibility_dry,
representing the visibility degradation in the presence of liquid water.
(a) The size-segregated light extinction coefficient at a
wavelength of 550 nm for wet particles (Extinction
coefficient_wet) and (b) the size-segregated difference between
Extinction coefficient_wet and Extinction
coefficient_dry, representing the light extinction coefficient
difference with and without considering liquid water during 29 February to
5 March 2016.
The scheme of the mutual-promotion effect between aerosol liquid
water and particulate nitrate.
It is worth noting that a similar co-condensation effect between water vapor
and semivolatile organic components (Topping and McFiggans, 2012; Topping
et al., 2013; Hu et al., 2018) could promote the haze formation as well, for
which there may be some evidence in the current case. Such a co-condensation
effect will lead to the enhancement of semivolatile organic and inorganic
(e.g., nitrate) material with the increasing RH in a developing haze. The
associated water will favor partitioning of both HNO3 and semivolatile
organic materials into the particle phase depending on the organic solubility,
providing a linkage between the development of increasing organic and
inorganic particle mass.
The key role of liquid water on visibility degradation
Aerosol particles grow in size as ambient RH increases, further enhancing
their extinction coefficient and impacting visibility (Zhao et al.,
2019; Kuang et al., 2016). In this section, the size-resolved extinction
coefficient of aerosol particles was estimated, and the influences of liquid
water on the extinction coefficient and visibility were quantitatively
evaluated. As shown in Fig. 7a, the total light extinction coefficient of
dry and wet aerosol particles were enhanced by a factor of 4.3 and 5.4,
respectively, from the beginning to a fully developed haze. Correspondingly,
the calculated visibility without considering liquid water degraded
significantly from ∼10 km to less than 2 km within 48 h
during the marked haze period. The contribution of aerosol-associated
water to visibility impairment was negligible in the beginning (2 %),
while it was significant (up to 24 %) in the fully developed haze (Fig. 7b). This indicates that liquid water facilitated visibility degradation
during haze development.
The influences of liquid water on visibility degradation varied with aerosol
particle size. The size-resolved chemical composition data showed that the
inorganic species, mainly particulate nitrate, were dominant components in
the aerosol particles within the size range of 300–700 nm
(Fig. S3). Correspondingly, the particles in this size range contained
most of the liquid water (50 %–80 % of the total aerosol
liquid water content of PM1). According to the discussion in Sect. 3.2, the
mutual-promotion effect between liquid water and particulate nitrate can
promote their mass loading enhancement. Aerosol particles in this size range
experienced the most significant enhancement of light extinction due to
water uptake (Fig. 8a and b) and contributed 70 %–88 % of
the total extinction coefficient of the total NR-PM1 (Fig. S9). In
conclusion, the rapid particulate nitrate enhancement enhanced the aerosol
extinction coefficient during haze development, while the aerosol water
uptake further enhanced the visibility degradation by increasing the extinction
coefficient and promoting particulate nitrate enhancement.
It is worth noting that the enhanced dimming effect will further decrease the planetary boundary layer (PBL), which, in turn, depresses the dilution
of water vapor and particulate matter in the atmosphere, hence leading to a
higher RH and higher aerosol particle mass loading (Tie et al., 2017).
Such an effect is beyond the scope of this study.
Conclusions and implications
In this study, we observed a particulate-nitrate-dominated (up to 44 % of
nonrefractory PM1 mass concentration) particulate matter pollution
episode, which is typical during winter haze in Beijing, China. A clear
co-increase in aerosol particle liquid water and particulate nitrate was
observed, demonstrating the mutual promotion between them via
observation-based theoretical calculations.
As shown in Fig. 9, the water uptake by hygroscopic aerosols increased the
aerosol surface area and volume, enhancing the condensational loss of
N2O5 across particles and favoring the thermodynamic equilibrium of
HNO3 into the particle phase under the supersaturated ambient HNO3
and NH3. The enhanced particulate nitrate from the above pathways
increased the mass fraction of particulate nitrate, which had a lower
deliquescence RH than sulfate and resulted in more water uptake at lower
ambient RH (Kreidenweis and Asa-Awuku, 2014). Hence, the
increased aerosol particle surface area and volume concentrations due to
water uptake in turn facilitate particulate nitrate enhancement. Hence, a
feedback loop between liquid water and particulate nitrate enhancement is
built up. Therefore the enhanced particulate nitrate components can
accelerate the feedback compared with sulfate-rich pollution over the NCP
region in the past (Hu et al., 2016). This self-amplification can
rapidly degrade air quality and halve visibility within 1 d. Our results
highlight the importance of reducing the particulate nitrate and its
precursors (e.g., NOx) for the mitigation of haze episodes in the NCP region.
Code and data availability
The observational dataset of the BEST-ONE campaign can be accessed through
the corresponding author Zhijun Wu (zhijunwu@pku.edu.cn).
The E-AIM model can be accessed via: http://www.aim.env.uea.ac.uk/aim/aim.php (Clegg et al., 2020).
The supplement related to this article is available online at: https://doi.org/10.5194/acp-20-2161-2020-supplement.
Author contributions
ZW, YW and YC conceived the study. YZ, MH and AKS developed the
BEST-ONE field campaign program. YW, ZW, DS, ZD, SHS, RS,
GIG, PS, TH, KL, LZ, CZ, AKS, YZ and MH participated in
this campaign and collected the dataset. YW conducted aerosol particle
liquid water calculations under the guidance of YB and the thermodynamic equilibrium
of particulate ammonium nitrate under the guidance of GM. YC calculated the
uptake coefficient of N2O5, optical properties and visibility.
YW and YC cowrote the paper with inputs from all coauthors.
ZW, GM, AKS, SHS, GIG, PS, TH, AV and AW proofread and
helped improve earlier versions of the manuscript. All authors discussed the results.
Competing interests
The authors declare that they have no conflict of interest.
Special issue statement
This article is part of the special issue “Multiphase chemistry of secondary aerosol formation under severe haze”. It does not belong to a conference.
Acknowledgements
We thank Paul I. Williams for valuable
advice on the reaction constant of HNO3 and N2O5.
Financial support
This research has been supported by the National Natural Science
Foundation of China (grant nos. 41571130021 and 41875149), the Ministry of Science and
Technology of the People's Republic of China (grant no. 2016YFC0202801), the German
Federal Ministry of Education and Research (ID-CLAR), and the China Scholarship Council –
University of Manchester (PhD, full scholarship).
Review statement
This paper was edited by Jingkun Jiang and reviewed by two anonymous referees.
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