International Conference on Air Pollution Control Benefit and Cost Assessment ICAPC2013 (ABSTRACT)
The reaction mechanism, measurement and simulation models of secondary particles in
atmospheric PM2.5: A review
SUN Yangyang1, CHEN Linghong1, ZHU Jie1, ZUO Lei1, JIAO Li2, GAO Xiang1, CEN Kefa1
1
2
State Key Laboratory of Clean Energy Utilization, Zhejiang University, Hangzhou 310027, China
Hangzhou Environmental Monitoring Center, Hangzhou 310007, China
ABSTRACT
The toxicity and damage to environment of secondary particles in atmospheric PM2.5 has been the focus for the
public in China. Reaction mechanism of secondary PM2.5 was reviewed including atmospheric photochemical
reactions of secondary organic carbon (SOA) between VOCs and oxidants such as OH, NOx, O3, transformation
mechanism of benzene, phenol and diolefin, and heterogeneous reactions of SO2 and NOx on mineral dust in
urban areas. Three theoretical models were illustrated to determine the contribution of secondary particles from
different types of emission sources. Chemical Mass Balance model (CMB) as source apportionment receptor
models calculates the linear summation of each source concentration contribution to compute the secondary
particles concentration of the receptor. For uncertain sources, statistical methods are adopted which can give birth
to other receptors such as positive matrix factorization method (PMF). Organic tracer method is introduced in
receptor models to study secondary organic particulates. In Community Multi-scale Air Quality (CMAQ)
modeling system, photochemical reactions are coupled with meteorological data and gridded emission inventories.
The methods to measure miscellaneous secondary particles were also discussed. Inorganic compound such as
sulphate, nitrate and chloride are analysed after water extraction with ion chromatography. Carbonaceous
component is divided into organic carbon (OC) and elemental carbon (EC), the latter is considered inert and
invariable in atmosphere. Thermal-Optical method is adopted for the OC/EC analysis. Other prevalent instrument
such as GC/MS is also introduced to measure different SOA. In the end, control strategies of secondary PM2.5
were discussed.
KEYWORDS
Secondary PM2.5, VOCs, OC/EC, reaction mechanism, measurement method, control strategy
ACKNOWLEDGEMENT
This work is supported by the National Basic Research Program of China (grants 2009CB219802), the National
Science Foundation (51206144), the Natural Science Foundation of Zhejiang province (LY12E06003), and the
Program of Introducing Talents of Discipline to University (B08026).
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International Conference on Air Pollution Control Benefit and Cost Assessment ICAPC2013 (ABSTRACT)
1. INTRODUCTION
Among all the atmospheric particulate matters, PM2.5, also called respiratory particles, refer to solid and liquid
particulate matters with aerodynamic diameters no more than 2.5 μm. PM2.5, especially the secondary particles of
it, has a significant impact on air quality. China has already become one of the most heavily polluted areas in the
word, occupying 16 out of 20 cities of the heaviest pollution according to a report of WHO for the year 2006.
Among the 522 air-monitored cities here in China, 40% of them cannot reach the second-level Air Quality
Standard, with 66.9% urban population exposed to the off-standard air. More and more combined pollutions such
as chemical smog and haze are appearing in cities of China.
Secondary particles of PM2.5 mainly transform from Volatile Organic Compounds (VOCs), SO2 and NOx.
According to a statistics of China 2010, the emission profile of SO2 amounts to 21851 thousand tons, with 12067
thousand tons of industrially emitted SO2, occupying 78%. mainly result from the combustion of fossil fuels.
Based on the same statistics mentioned, the primary sources for industrial SO2 are electric and petroleum
industries, with emission profiles of 8898 and 6222 thousand tons respectively, contributing 52.6% and 36.3% of
the total industrial SO2 emission[1]. SO2 and NOx from urban vehicles are the major reasons for combined
pollutions in city areas, but the statistical data are still unavailable for emission inventory of vehicle-emitted
VOCs countrywide. More and more attentions are drawn to the role that VOCs play in the formation of
photochemical pollutions, which is also a major problem heckling cities abroad. VOCs are the major precursors
for PM2.5 secondary particles, and they are usually emitted in large amounts with complicated emission sources
widely distributed, including incomplete combustion, evaporation of oil solvent, industrial process and etc. Table
1 shows the emission profile of VOCs of several major countries, indicating that the production and usage of
industrial solvent and vehicles are the contributing emission sources for VOCs. Figure 1 shows the emission
amounts of SO2 NOx and VOCs in China.
Table 1 Anthropogenic emissions of several countries and their respective weightings
Countries
or areas
Emission of VOCs
(in million tons)
First emission source
Weighting
Second emission
source
43.8%
Vehicles
20.9%
Usage of products
China
23.11
containing VOCs
USA
12.8
——
——
——
——
0.776
Stationary area source
40%
Vehicles
26%
California,
USA
European
Solvent, product usage, commercial organization, family emissions and
7.412
Union
Japan
vehicles account for 76%
0.79
Painting
37.3%
fuels (evaporating)
Notes: The emission data for USA are collected from statistics of 2012, while others 2010.
2
18.9%
International Conference on Air Pollution Control Benefit and Cost Assessment ICAPC2013 (ABSTRACT)
Figure 1 Annually emission amount of SO2 NOx and VOCs in China
2. Formation principles of secondary particles
Generally, after emitted into the atmosphere from emission sources, primary pollutants go through complicated
chemical and photochemical reactions, and finally form secondary particles under the combined actions of
horizontal and vertical transportation and diffusion, gaseous chemical transformations, cloud liquid processes,
equilibrium and coagulation of aerosol production, dry and wet subsidence and many other factors.
2.1 Photochemical reactions of VOCs in atmosphere
According to statistics of 2005, alkane, unsaturated hydrocarbon, benzene series account for 20%, 21%, and 30%
respectively in the VOCs emission of China. Usually, Reactive Organic Gases (ROGs) are those VOCs with
transformation efficiency more than 10% under the current concentration of atmospheric oxidants. Anthropogenic
ROGs mostly come from off-gas of vehicles, such as benzene series, phenols, diolefines. Volatile aromatic
compounds are the leading anthropogenic precursors for Secondary Organic Aerosols (SOA), 50%~70% SOA in
the urban air come from benzene or its derivatives[2]. For gaseous organic compounds, transformations into SOA
take places through two steps: the first is the gaseous reactions between ROGs and •OH, NOx, O3 and other
atmospheric oxidants; the second is reversible distribution of semi-volatile secondary products between gas
phases and solid phases. The formation of SOA is subjected to many factors such as other inorganic and organic
components, temperature, light and humidity. Figure 2 shows the mutual transformation of free radicals in
atmosphere[3].
The transformations of aromatic compounds are started by reactions with •OH, where addition reactions play
the major role and the dehydrogenation (replacing the substituent or the hydrogen atoms at C⎯H bonds of benzene)
only makes up approximately 10%[4]. Take toluene as an example, the addition reactions between toluene and •OH
are quite complex, for •OH can be added to ortho-position, meta-position, para-position and ipso–position,
therefore form four different adducts. Two kinds of reactions including benzene conservation and benzene
decomposition are followed as shown in Figure 3[5]. With NO available, the major products of reactions between
OH-benzene adducts and O2 are cresol, toluene oxide, butenedial, ethylglyoxal etc. For different position of the
O⎯O Bridge, the decomposition products can also be glyoxal and methylbutenedial. Jiang et al.[6] and Kamens et
al.[7] discovered in the study of benzene reactions that the actual distribution modulus (Kp) of phenolic products is
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International Conference on Air Pollution Control Benefit and Cost Assessment ICAPC2013 (ABSTRACT)
much higher than predicted, indicating that further reactions may continue in solid phase. The polymers formed by
solid reactions of aldehydes (e.g., aldehyde reacts with alcohol and forms hemiacetal) will cause more parental
aldehydes to enter solid phase, consequently increase SOA.
O3
photolysis
O(1D)
RO2
O3+HC
NO
H2O
VOC+O2
RO
O2
photolysis
CO+O2
HO2
photolysis
·OH
O3
O3+peroxide or aldehydes
HONO
heterogeneous reaction
NO
O3
NO2
O2
NO2
NO3
N2O5
photolysis
photolysis
VOCs
aerosol
•OH
O(3P)
O2
HNO3
daytime
O3
nighttime
Figure 2 Mutual transformations of free radicals in the atmosphere
Also photochemical oxidations, cyclic diene and aliphatic diene react into dicarboxylic acid, is the focus in
SOA studying[5,8]. For other ROGs such as alkane in the atmosphere, they mostly react with •OH during daytime,
and with NO3 radicals during nighttime at relatively slow reaction rate[9,10]. Above two kinds of reactions both
undergo principal steps of dehydrogenation and transformation into alkyl radicals, which will be quickly oxidized
into RO2. The major difference is that nitrate groups might decompose and form NO2 in later reaction[11]. While
reactions between alkene and OH radicals are firstly through addition reactions at C=C bonds and
dehydrogenation at C-H bonds of alkyl substituent, and addition reactions play a major part in the small-molecule
noncyclic alkene.
For ROGs reactions in the second step, it is generally assumed that organic compounds will condense only
when the concentration exceeds its saturated vapour pressure, meaning that SOA will be formed by molecular
homogeneous nucleation. However, follow-up studies found that ROGs would also enter the solid phase through
adsorption, dissolution or absorption even when their gaseous concentration had not reached the saturated
stateError! Bookmark not defined.. Adsorption theory indicates that adsorptive capacity correlates with specific
surface area; while absorption theory indicates that gaseous organism enters the solid phase by absorption and
dissolution, and maintains the balance of absorption and desorption with its absorptive capacity proportional to the
mass of aerosols. PAHs and other organic matter with low polarity is primarily guided by absorption principles.
Besides this, solid reactions of aerosols can never be omitted. When the reaction time is shorter than the life
cycle (4~7 days) of particles in the flow laver, solid-phase reactions including accretion reaction[12,13,14] and
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International Conference on Air Pollution Control Benefit and Cost Assessment ICAPC2013 (ABSTRACT)
aerosol aging[15,16]must be paid attention.
CHO
CH3
CH3
CH2ONO2
OH
+HO2
O2
NO2
CH2·
O
*
CH3
CH3
OH
·OH
·OH
Abstraction
H
Addition
H
+O2
CH3
CH3
CH3
O
OH
O
H
O
H
OH
NO2
O
+O2
H
O
H
NO
O
+
O
H
O
H
+O2
O
O
H
OH
H
O
O
CH3
O
H
O
H
Figure 3 Reaction pathways for the reaction of ⋅OH with toluene
2.2 Photolysis of Vehicle-emitted NOx
NOx mainly comes from emission of motors, and its concentration is higher in urban areas with heavy off-gas
pollution. NOx in the convection layer can directly undergo photolysis by absorbing sunlight with wavelengths of
290~700nm[17].
NO2 +h  NO2* (excited phase)
400~700nm
(1)
NO 2 +h  NO+O  3 P 
290~420nm
(2)
O  3 P  +O 2 +M  O3 +M
(M=air)
(3)
NO+O3  NO2 +O2
(4)
M in above formulas are energy absorption molecules (usually O2 and N2). The rate of photolysis of NO2 is
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International Conference on Air Pollution Control Benefit and Cost Assessment ICAPC2013 (ABSTRACT)
rather high, and therefore NO2 can be decomposed into NO and O(3P) atoms by sunlight in relatively short time;
afterwards O(3P) reacts with atmospheric oxygen immediately and ultimately forms O3. If the convection layer is
clean enough, the reactions mentioned above will only undergo one cycle; in the presence of VOCs, one of the
pathways will be hampered by NOx reactions, resulting in the continual increase of NO2 and O3, and decrease of
NO.
Nitrous acid HONO is formed by heterogeneous reactions of NOx during nocturnal time, and later in the next
morning, photolysis of HONO happens under sunlight. Two possible photolysis reactions of HONO are as
follows[18]:
HONO+h  H  +NO2
HONO+h  HO  +NO
(5)
(λ=300~390nm)
(6)
If air is wet, HONO can be hydrolyzed into NO3-, and finally be turned into nitrates such as NH4NO3.
Multi-phase reactions of NOx on mineral surface have become the focus of recent studies. Börensen et al.[19] put
forward the formation principles of NO3- with active OH on surface of Al2O3 particles.
2.3 Process of formation of secondary particles in industrial SO2
SO2 mainly comes from industrial combustion of fossil fuels, and the formation of sulfates includes: oxidation
of clouds and liquids and heterogeneous reactions with other aerosols[20], while the principles of heterogeneous
reactions remain uncertain. Mineral dust is the major components of land aerosols, and accordingly the important
pathway for sulfate formation in the inland is the heterogeneous reactions of SO2 on mineral dust. Usher at al.[21]
took advantage of infrared transmission spectrum to study the reactions of SO2 on the surface of oxidants and
found that O2- and OH- were involved as follows:
O2（a）
+SO（
 SO32（a）
2 g）
(7)
OH（
a）
+SO（
 HSO（
2 g）
3 a）
(8)
or 2OH（a）
+SO（
 SO3（a）
+H 2 O
2 g）
(9)
-
2-
SO32- and HSO3- on the surface of oxidants are partially oxidized into SO42- under the interactions of oxygen
and adsorbed water, realizing the transformation from gaseous SO2 to solid SO42- particles.
3. Modelling methods of secondary particles
For the complexity of formation mechanism, reaction mechanism of VOCs and theories of secondary aerosol
nucleation progress still need perfection. Receptor models compute respective contributions of emission sources
to receptors without chemical mechanism, and have been widely used since it came out in the 1970s.
3.1 Chemical Mass Balance (CMB) model
Chemical Mass Balance model is one of the most typical receptor models, equation of mass conservation is as
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International Conference on Air Pollution Control Benefit and Cost Assessment ICAPC2013 (ABSTRACT)
follows:
n
Ci   fij S j  ei
(10)
j 1
Where j=1, 2, 3,……, n, Ci refers to the concentration of component i in the atmospheric particulate matter. fij is
the measured value of component i in the source j, Sj is the concentration contribution of source j, j is the number
of sources, i is the number of elements, while ei stands for the correction term. In the above formulas, Sj is the
ultimate variable that needs to be calculated.
The total concentration at the receptor site is the linear sum of contribution from each source. CMB is widely
applied in modelling contribution factors of key emission sources in municipal and rural places, such as motor
emissions, industrial coal combustion, and biomass combustion. The chemical mechanism of organic particles is
relatively simple: SO2 and NOx from combustion of fossil fuel and municipal solid waste and NH3 from fertilizer
and agriculture react in the atmosphere and produce sulfates and nitrates. Chen at al.[22] integrated data from
Interagency Monitoring of Protected Visual Environments (IMPROVE) and Speciation Trends Network (STN),
applied CMB into source apportionment in typical municipal and rural places in USA, and introduced the concept
of Effective Variance. The result showed that the percentage of sulfates and nitrates in PM2.5 is approximately
49~71%. With regard to organic particles, receptor models with organic tracers are usually chosen to identify the
diffusion process of organic matter with several designated tracers, and CMB are employed in subsequent steps.
Perrone et al.[23] modelled with CMB for northern Italian city of Milan and found a good agreement between
modelling and measured results, using levoglucosan as the tracer for wood burning to study the contribution of
wood burning in winter to organic pollutants. Contribution rates from secondary particles can also be identified
with receptor models, but constituent spectrum has to be established first to identify tracer matter. Atoms that can
be used as tracers in source apportionment are still limited, and therefore the method is still in the embryonic
stage.
3.2 Positive Matrix Factorization (PMF) model
Positive Matrix Factorization (PMF) employs weighting factors first to identify the errors in the chemical
components of particulate matters, and then nails down primary sources and their contribution rates using the
method of Least Squares. Compared with CMB, it does not require the detailed information of source components,
and therefore avoids the deviation resulting from the uncertainty of identified sources and the lack of unknown
sources. However, large quantities of data are needed for this method, for they are the determinants of solution.
Acquisition of highly synthetic data requires a monitoring network that operates continuously and analytical
ability to monitor different components of atmospheric particles with high quality. In the report of PM2.5
distribution characteristics in St Louis, Kim et al.[24] introduced PMF analytical method in detail, using algebraic
standard error to demonstrate time distribution patterns and Coefficients of Divergence (COD) to demonstrate the
difference in distribution behaviours of particles in different areas. In the modelling process of PMF, strengthening
the chemical analysis of secondary aerosols is the effective way to improve the modelling accuracy and extend the
receptor analysis of SOC in PM2.5.
3.3 CMAQ model
Receptor models calculate with statistics or coupled algorithms, which lack mechanism modelling of
photochemical processes in the atmosphere. Models-3 air quality system, which is put forward by US EPA, is
based on the “one atmosphere” principle and takes many factors in one single coordinate system. It realizes the
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International Conference on Air Pollution Control Benefit and Cost Assessment ICAPC2013 (ABSTRACT)
highly accurate modelling of a certain region by virtue of drawing small grids, and is typically composed of MM5
model which models physical aerodynamics, SMOKE model which pre-processes emission inventories, and
CMAQ model which simulates chemical transportation. CMAQ model is the central part of air quality systems,
and its conservation equation is as follows[25,26]:
.
ci
c 
   Uci    Di  i   Ri  c1 , c2 ,……, cn , T , t   Si ( x, t )
t

(11)
Where i=1, 2, 3,……, n, ci is the concentration of component i, U is the velocity vector of wind, Di is the
molecular dispersion coefficient of component i, Ri is the rate of concentration variation in the chemical reaction,
Si(x,t) is the magnitude of source of component i at coordinate x and at time t, ρ stands for air density, n stands for
number of assumed components in the model. The equation describes the formation, transportation and evolution
of components, with processing of constituents, meteorological conditions and atmospheric chemistry all included.
In CMAQ 4.7, Carbon Bond mechanism (CB05), updated in 2005, is employed to calculate input inventory such
as VOCs and PM2.5, and moreover isoprene, sesquiterpenes, benzene, glyoxal and methylglyoxal are added to the
precursors of secondary aerosols in the model. In previous versions of CMAQ, all SOA was treated as
semi-volatile. In CMAQv4.7, four types of nonvolatile SOA are simulated.
Pay et al.[27] simulated time and spatial distribution of particles in Spain. It set 4km×4km grids, and got
397×397 grids in total. The paper evaluated total emission of PM2.5 and carried out source apportionment of salt
aerosols, inorganic ions and other emitted particulate matter. Observation revealed that modelling results were in
good agreement with studies of other researchers. The biggest advantage of CMAQ is that it introduces the latest
atmospheric chemistry and photochemical mechanism, and significantly improves the accuracy by modelling all
factors under one atmosphere in the unified coordinate system. Marmur et al.[28] applied both CMAQ and CMB
models to study the health effect of PM2.5 in southern USA, and found that CMAQ was superior in the modelling
regional distribution by comparing the two results. We have to pay attention to the fact that as the most crucial
precursor of SOA, VOCs have extremely complicated components and reaction mechanisms still to be studied,
and therefore suitable chemical principles have to be chosen according to specific emission characteristics in
applying CMAQ, meanwhile it is necessary to compare the result with CMB model.
4. Measuring principles and methods of secondary particles
PM2.5 secondary particles include various sulfates, nitrates and SOA. Measurements include quantitative
analysis of particle components and inspection of tracers in secondary reactions. Secondary organic matters are
usually measured with Organic Carbon (OC) method. It assumes that element carbon (EC) originates from
combustion, and stays inactive in the atmosphere as primary particles. OC/EC in primary particles of a specific
region is fixed, and therefore if measurement shows that the actual ratio exceeds the value, it means that there are
extra OC emerged, and we treat this part of OC as SOA. SOC can be approximated using the following
formula[29]:
OCsec  OCtot  EC (OC / EC ) pri
(12)
Where OCsec is secondary organic particles, OCtot is the total carbon measured, EC is element carbon, and
(OC/EC)pri stands for the ratio of primary organic carbon to element carbon.
4.1 Thermal-optical method
Thermal-optical method is widely recognized as a method for analysing SOC. Let a laser beam transmit a filter
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International Conference on Air Pollution Control Benefit and Cost Assessment ICAPC2013 (ABSTRACT)
membrane which adsorbs organic particles, and the absorption rate of element carbon and transmission rate of the
laser can be used to infer the variation of carbonaceous component qualitatively. This method is often termed
Thermal-Optical Transmission (TOT). Yin & Harrison[30]applied this method to analyze OC in urban and rural
atmospheric particles and their experimental procedures are as follows:
Heat quartz-fiber membrane with organic matters from ambient temperature to 700℃ under non-oxidant
atmosphere (helium atmosphere, during which process partial decomposition of OC and carbonization take place),
and let the decomposition products pass through MnO2 oxidation furnace blown by the carrier gas helium, and
then mix up the oxidant products CO2 and H2 with catalysts for the formation of CH4. CH4 can be finally
identified quantitatively with Flame Ionization Detector (FID). The next step is to heat filter membrane under the
mixed atmosphere of helium and O2 from 550℃ to 850℃, during which process the remaining carbon will be
oxidized to CO2 and then be transformed into CH4 with H2 for FID detection. Throughout the whole process, a
laser beam is projected onto the filter membrane. The laser intensity changes during this progress, and when
transmission intensity comes back to the original value, the point it reaches is the breakpoint of OC and EC.
If we correct the breakpoint of OC and EC with optical reflection, it is Thermal-Optical Reflection (TOR). The
breakpoint is the key to differentiate between these two methods. Figure 4 illustrates the principle of TOT and
TOR. Cheng et al.[31] analysed 333 samples in southern USA with TOT and TOR respectively, and results
indicated that the two methods were comparable in terms of total carbon measurement, but the TOT obtained a
slightly higher value of 10-20% than TOC. It is reported that the breakpoint of TOR comes before that of TOT,
and the reason lies in the fact that oxidation takes place from outside to inside for particulate matters on the
surface of quartz filter membrane. Due to the adsorption of membrane, when OC evaporates in the non-oxygen
environment with high temperature, it will be first absorbed into the depth of membrane and then start to split. The
decomposed carbon in the depth will only result in the decrease of transmitted light, with no effect on the
reflection intensity. Thus, the reflected intensity will restore its original value ahead of transmitted intensity,
leading to the earlier arrival of the breakpoint for TOR, consequently causing the value of EC greater and the
value of OC smaller. Chow et al.[32] pointed out that difference in the pyrolysis temperature will also bring about
different results.
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International Conference on Air Pollution Control Benefit and Cost Assessment ICAPC2013 (ABSTRACT)
Laser(632nm)
Laser reflectance detector
Quartz light pipe
Sample oven with heater
Deposit side of 0.5cm2 quartz filter
CH4 ← CO2
Reducer
(Methanator)
CO2 ← C
Oxidizer
(Oxidation Oven)
Gas flow direction
Combustion at 100% He
Quartz light pipe
Flame
Ionization
Detector
(FID)
Pushrod
Thermocouple
Laser transmittance detector
Figure 4 Experimental configuration for thermal/optical reflectance (TOR ) and thermal/optical transmission
carbon analysis
4.2 Ion Chromatography (IC) and GC-MS
Inorganic matters are the major constituent of PM2.5, among which NO3- and SO42- need to be measured in
particular using Ion Chromatography[33]. The components of VOCs are extremely complicated, with aromatic
alkane, other alkane and alkene playing the major role in anthropogenic VOCs. GC-MS with high resolution has
good application in separating and testing SOA. Duan Fengkui and He Kebin[34]of Tsinghua University measured
PM2.5 SOA in Beijing with GC-MS, fulfilling the entire separation among normal alkane, polycyclic aromatics
and organic acid ester.
Table 2 Comparison between IC and GC-MS
Measurement methods
Ion Chromatography
-
Measurement objects
-
2-
+
Cl , NO3 , SO4 , Na ,
Mg2+, Ca2+
NH4+
GC-MS
+
,K ,
VOCs such as: normal alkane, polycyclic
aromatics and organic acid ester
Main steps
Sampling, Pre-processing and anion
measurement, Analysis and
checkout
Instrument and equipment, Processing of
samples, instrument analysis
Concentration range
1～10μg/L
10-10g
Characteristic
Off-line
Off-line
5 Control technologies of secondary particles
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International Conference on Air Pollution Control Benefit and Cost Assessment ICAPC2013 (ABSTRACT)
The crux of secondary particles removal lies in reducing the emission of precursors. In China air quality control
has always centred on dealing with dusts, SO2 and NOx, with insufficient attention to controlling VOCs.
Therefore most industrial emission sources of VOCs haven’t been regulated up to now. The following passages
mainly focus on introducing control technologies of VOCs.
5.1 VOCs control technologies
Basic ideas of source-tracing are used in our country[35]. The method studies emission of VOCs according to its
flow in social production activities. Emissions of VOCS happen through the whole process of production,
transportation and storage, technical process of manufacture from VOCs, usage and disposal of products
containing VOCs. The mainstream control techniques at home include: adsorption, catalytic combustion, thermal
incineration and biological technologies, among which adsorption technique makes up for over 50%, and catalytic
combustion technique accounts for about 30%. Compared to other treatment, adsorption has the characteristics of
high purification efficiency, simple equipment and low investment, and is the most widely applied[36].
5.2 Control measures of reducing emission of secondary particles[37]
5.2.1 Establishing emission factors of SO2, NOx and VOCs for domestic sources and complementing pollution
inventories
Emission factor, which requires nearly comprehensive data, is one principal method used in European and
American countries to estimate total pollution emission, and is recently popularly used here in China. Due to the
late establishment of inventory in China, data concerning emission factors of all types of sources are still lacking,
and therefore data abroad is commonly applied with correction to different situations. However, different
production conditions and social-economic development levels will lead to huge discrepancy between emission
factors and actual emission, thereby necessitating the establishment of emission factors based on domestic fuel
consumption and treatment level is currently the main task.
5.2.2 Framing emission standards and control policy for all sectors, identifying key targets for emission reduction
and inciting innovation of pollutant control
The monitor and study on pollutants start late in China, in addition, emission levels varies significantly in
different places, so it is hard to build an effective way to restrict emission. The key is to establish industrial
emission standards corresponding to the national conditions and eliminate outdated production and emission
reduction techniques. Seminars have been launched to set new standards for SO2 and NOx, but there is still a long
way to go for VOCs. We have always been the leading emitter in the world, and the total emission of VOCs is far
greater than that of SO2 and NOx, hence the control policy of VOCs should include: setting up emission standard
of VOCs for related industries, assessment system of clean production and technical regulation of treatment;
speeding up the establishment of standard measurement, technical regulation of monitor and standard of
monitoring instruments for air quality and VOCs from fixed sources; establishing admission regulations for sale
and usage of organic products, enforcing control on threshold of VOCs content in products, and building
declaration system of organic solvent usage for major industries.
5.2.3 Strengthening monitoring of pollutants by virtue of complementing regulating list of SO2, NOx and VOCs
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International Conference on Air Pollution Control Benefit and Cost Assessment ICAPC2013 (ABSTRACT)
Anthropogenic emission of SO2, NOx and VOCs is concerned with all aspects regarding industrial and
agricultural production. SO2 and NOx mainly come from industrial and traffic combustion of fossil fuels; the
sources for VOCs are rather complex and disperse widely. So it is of great importance to regulate key emission
sources, set up regulating list for areas and companies with heavy pollution and launch integrated pilot program of
pollution control for reducing pollutant emissions.
6 SUMMARIES
The study on atmospheric secondary particles is closely related with our life. Due to the intricate content of
atmospheric particles, approach is still unavailable for complete differentiation between primary and secondary
aerosols. Recently, there has been a trend to carry out source apportionment from precursors which include VOCs,
NOx and SO2. The attention has been paid to NOx and SO2 owing to issues of acid rain. Strict limitation has been
enforced since Chinese 11th five-year plan, and the policies need to be carried on strictly to alleviate pressure that
industrial production exposes on the environment, especially in light of the enormous base value of our emission.
The study on VOCs emission is still in the early stage, and basic data are still unavailable, necessitating the
establishment of studying measures of VOCs based on our national conditions. The summary is concluded on
three aspects as follows:
6.1 Enhancing the mechanism studies on photochemical reactions with VOCs in the atmosphere
With the deepening of studies on atmospheric reactions, our understanding on characteristics and formation
mechanism of SOA has been greatly improved. However, studies till now have only been concentrated on reaction
mechanism of specific gas precursors without taking the influence of secondary products into consideration, and
moreover there are still many limitations on particulate-phase aerosol reactions. Some substance is highly active
and therefore can only exist for a short time in the atmosphere, causing difficulty in identifying its content and
reaction mechanism. Due to the damage caused by analytical techniques, problems arise for the identification of
some trace-amount SOA. Smoke chamber is one important way to study secondary reactions. It can simulate
photochemical reactions by controlling parameters such as temperature, pressure, sunlight, humidity and wind
speed. Online monitoring technology makes it possible to monitor the real-time variation of content and
concentration in the reaction chamber, and therefore the method should be greatly promoted. Molecular tracer
method is an important way to establish organic content of pollution sources. We should build up data pool of
organic content for typical sources, enlarge the lists of organic tracers and develop and complement its application
in source apportionment.
6.2 Localizing Model-3 and employing CMAQ to domestic environmental decision-making
Model-3 simulates all factors including meteorology, geography and chemical principles under one single
system, and is highly credible with accurate distribution in three dimensions. In Model-3 system, emission sources
are simulated according to the resolution of American inventory. The very first step is to localize Models-3 due to
its strict requirements of gridded inventory. The job of localization includes new classification of inventory,
establishment of accurate emission factors, real-time monitor of key emission sources, establishment of
3-dimensinal data set for domestic atmosphere and etc. Pearl River Delta is one of areas covered by perfect air
monitoring network, and localization of Modes-3 has been initiated there. It is therefore crucial to take Pearl River
Delta as a reference to set up modelling and monitoring network to cover major regions, or even the whole nation.
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International Conference on Air Pollution Control Benefit and Cost Assessment ICAPC2013 (ABSTRACT)
6.3 Expediting establishment of domestic emission inventory of VOCs
The emission of VOCs of our country has far exceeded that of SO2 and NOx and has become one contributing
factor affecting urban environment. European and American countries have many experience and advances in
studying VOCs emission, and we are still borrowing their method of emission factor. However, considerable
errors arise due to the discrepancy of technical levels and national conditions between our country and western
countries. The suggestion is that we expedite the complement of emission factors of VOCs, employ source-tracing
method for analysing emission data, restrict avoidable errors in the estimation, and provide inventory support for
Models-3 application.
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