Characterization of Winter Haze Episodes and Their
Formation Mechanism in Guilin, China
L S Zhang
1
, S Zhong
1， *
, J Du
2
and Y S Li
2
1
College of Environment and Resources, Guangxi Normal University, Guilin
541004, China
2
Guilin Environment Monitoring Central Station, Guilin 541004, China
Abstract. Three heavy haze pollution episodes could be observed in Guilin in winter. The
sources and evolutionary mechanisms of the haze pollution episode that occurred from
December 23, 2014 to February 10, 2015 were analyzed. The meteorological parameters and
daily concentrations of PM
2.5
, sulfate, nitrate, and ammonium were measured. The results
showed that the mass concentrations of the sulfate, nitrate, and ammonium increased with
increasing PM
2.5
levels. Sulfate was the most abundant secondary water-soluble inorganic
species, which contributed to about 20% of the PM
2.5
mass concentration. The catalytic
oxidation of SO
2
to SO
4
2-
in the presence of NO
x
and Ca
2+
played a considerable role on the
severe winter haze episodes in Guilin.
1. Introduction
Haze is defined as a weather phenomenon that leads to atmospheric visibility less than 10 km due to
the moisture, dust, smoke, or vapor in the atmosphere. Since 1970s, the problem of haze was
generally concerned by researchers. Previous studies reported that the formation of haze was closely
linked to the meteorological and atmospheric conditions [1-2]. In addition, fine particulate matter
(PM) such as PM
2.5
, referring to the PM with an aerodynamic diameter less than or equal to 2.5 μm,
is the key factor for the increase in haze pollution. The increase of PM
2.5
concentrations due to
human activities was demonstrated to cause the formation of haze [3-4]. Also, high concentrations of
secondary species (i.e., SO
4
2−
, NO
3
−
, NH
4
+
) can be formed from the photochemical reactions of air
pollutants from anthropogenic sources), which accounts for the majority of severe hazes in many
cities [5-6].
Guilin is an international famous tourist city. The air quality is always concerned by public. In
recent years, the haze episode appeared frequently in the Guilin area owing to the rapid development
of industrial, transportation and construction industry, , which had a great impact on human’s life and
health as well as an adverse effect on Guilin’s tourist industry.
Previous studies investigated the characteristics of long-term change in haze events in Guilin and
discussed the effect of local weather conditions on the formation of haze [7-9]. However, no
systematic analysis of the formation mechanism of the heavy haze was performed. To the best of our
knowledge, no study reported the chemical components of aerosols during the haze evens in Guilin.
In this study, the heavy haze event occurring in winter was monitored. The possible formation
Zhang, L., Zhong, S., Du, J. and Li, Y.
Characterization of Winter Haze Episodes and Their Formation Mechanism in Guilin, China.
In Proceedings of the International Workshop on Environmental Management, Science and Engineering (IWEMSE 2018), pages 291-296
ISBN: 978-989-758-344-5
Copyright © 2018 by SCITEPRESS – Science and Technology Publications, Lda. All rights reserved
291

mechanisms of haze pollution were illustrated based on the continuous hour observation of PM
2.5
and
gaseous pollutants, and their chemical compositions.
2. Methods
The Sampling points were set up on the open rooftops of an office building (about 25 meters above
ground level) at Guilin Environment Monitoring Central Station, Guilin (Figure 1). The continuous
monitoring of mean hourly concentration of PM
2.5
was conducted by an aerosol particle mass
analyzer. The offline samples were collected daily by the quartz and teflon membrane from
December 23, 2014 to February 10, 2015 in parallel with the on-line data. Ion chromatography (IC)
was used to analyze water-soluble anions and cations.
Figure 1. Sampling locations at Guilin, China.
3. Results and discussion
3.1. Characteristics of meteorological conditions during the monitoring period
Figure 2 illustrates the temporal variations of visibility and major meteorological parameters
including wind speed, wind direction, temperature, and relative humidity during the observation
period. The temperature ranged from 4 ºC to 17 ºC with an average value of 10 ºC, while the relative
humidity was in the range of 31%-94% with an average value of 66%. The pressure was 1024.9 hPa.
The wind speed was 0.6-3.9 m/s with an average value of 1.7 m/s. The predominant wind direction
was north or northeast, accounting for 74.6% of observation days (50 days). There were 29 days of
precipitation, and the cumulative precipitation was 54.3 mm. Visibility was 0.80 km to 17.4 km, and
the average value is 6.04 km. However, only 5 days had a high visibility above 10 km. In this study,
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the haze episodes are defined according to the Chinese standard of “Observation and forecasting
levels of haze” (QX/T 113-2010), and the details are listed in Table 1. As a result, a total of 34 haze
days were observed without the influence of precipitation weather, which can be classified into 1
moderate haze day, 16 slight haze days, 17 mild haze days and 16 non-haze days.
Figure 2. Daily change of meteorological conditions in Guilin during observation period.
Table 1. Classification of haze episodes.
Classification
VR(km)
RH(%)
Weather
Number
Non-haze
≥10
/
/
16
Mild haze
5～10
RH≤80, or when RH=80～
95, PM
1
≥65 μg m
-3
or
PM
2.5
≥75 μg m
-3
Excluding bad
weather, such as
rain, snow and
dust storms
17
Slight haze
3～5
16
Moderate haze
2～3
1
Serious Haze
<2
/
Observation and forecasting levels of haze (QX/T 113-2010) – National standard (China)
3.2. Water soluble ions in PM
2.5
Concentrations of water soluble ions in PM
2.5
during the observation period are shown in Figure 3
and Figure 4. In this study, the continuous period with mean daily PM
2.5
mass concentration over 75
μg/m
3
is defined as haze episode (Grade II National Ambient Air Quality Standard, GB3095-2012).
According to this definition, three typical haze episodes were observed during the observation period,
i.e., December 23-26, 2014; December 28, 2014 to January 5, 2015, and January 10-27, 2015. The
Characterization of Winter Haze Episodes and Their Formation Mechanism in Guilin, China
293

mass concentration of PM
2.5
presented a characteristic saw-tooth pattern, indicating the repeatability
and complexity of the haze pollution processes.
During the monitoring period, the changes of mass concentrations in NO
3
-
, SO
4
2-
and NH
4
+
were
similar to the change of PM
2.5
, exhibiting a saw-tooth pattern. Considering the meteorological factors
obtained during December 23-26, 2014, the water absorption capabilities of NO
3
-
, SO
4
2-
and NH
4
+
increased due to the light rain on December 25, and their peak concentrations appeared. As the
rainfall was heavier to 2-3 mm on December 26-27, the concentrations of NO
3
-
, SO
4
2-
and NH
4
+
decreased quickly and reached the peak-valley values. During sunny days on Jan 1-2, 2015, the
levels of NO
3
-
and SO
4
2-
increased relatively quickly and accumulated in the presence of winter
inversion layer. Their peak concentrations appeared on January 4, and the level of SO
4
2-
reached 37.3
μg/m
3
. In addition, the peak concentration of fine PM also achieved. Then, all these substances
decreased due to the rainfall on January 5-6. However, the reduction was limited because the rainfall
was light. Accumulation of these substances was rebuilt in the cloudy days on January 8- 9 and
decreased again to the peak-valley value in a rainy day on January 10. Nevertheless, these substances
cannot be cleared continuously by rainfall, and were concentrated again in the serious pollution days,
which lasted for a long period of 10 days from January 14 to 23. This heavy-polluted process did not
end until the rainfall coming on January 27, and the concentrations of NO
3
-
, SO
4
2-
and NH
4
+
decreased correspondingly.
Figure 3. Variations of NO
3
-
, SO
4
2-
, NH
4
+
and PM
2.5
concentrations during the monitoring
period.
The changes of mass concentrations in Cl
-
, F
-
, Na
+
, K
+
, Mg
2+
and Ca
2+
showed a positive
relationship with the change in PM
2.5
, presenting a saw-tooth pattern as well. These water-soluble
ions were obviously affected by the weather conditions, especially the rainfall. The changing
processes of these ions during the observation period had similar features in comparison with NO
3
-
and SO
4
2-
.
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294

Figure 4. Varitions of Cl
-
, F
-
, Na
+
, K
+
, Mg
2+
, Ca
2+
and PM
2.5
concentrations during the
monitoring period.
3.3. Effects of pollutant emissions and meteorological factors
The monitoring results showed that rainfall only had a temporary effect on the reduction of NO
3
-
,
SO
4
2-
and NH
4
+
. Therefore, the pollutant emissions may significantly affect PM
2.5
pollution. A lower
pollutant emission intensity decreased PM
2.5
concentrations. In fact, a majority of SO
4
2-
and NO
3
-
in
PM
2.5
are produced by oxidation of SO
2
and NO
2
, respectively, while NH
4
+
is discharged from the
poultry industry, agriculture and vegetation decay. These substances are inorganic PMs generated
from the secondary reactions. Due to the Karst landscape in Guilin, the particulates normally contain
high concentrations of Ca
2+
, which has a catalytic effect on oxidation of SO
2
[6, 10]. Furthermore, the
formation of SO
4
2-
from SO
2
is promoted by the coexistence of NO
x
on the surface of fine particles in
the presence of Ca
2+
[11]. The relatively higher SO
4
2-
concentrations were found in aqueous phase of
particulate aerosols than NO
3
-
and NH
4
+
in this study. The results suggested that a catalytic oxidation
pathway of SO
2
to SO
4
2-
played an important role on the severe winter haze episodes in Guilin. As a
result, the local authorities should prioritize the reduction of air pollutant emissions, particularly
during the serious haze episodes. Once the generation of secondary inorganic aerosols was controlled,
the impact on growth of air PMs could be mitigated.
4. Conclusions
In this work, we investigated the daily variations of PM
2.5
and water soluble ions along with the
meteorological parameters during December 23, 2014 to February 10, 2015 in Guilin, and the factors
affecting the formation of haze episodes were discussed. The results showed that 34 haze days
occurred during the observation period. SO
4
2-
was the main water soluble ion in aqueous phase of
particulate aerosols in the typical three haze stages, suggesting that the emission of SO
2
significantly
contribute to haze formation in winter season in Guilin.
Acknowledgement
This study was supported by the Natural Science Foundation of Guangxi Zhuang Autonomous
Region (No. 2015GXNFDA139031); Scientific Research and Technology Development Plan of
Guilin (201601502). We would like to thank Guilin Environment Monitoring Central Station for the
data support.
Characterization of Winter Haze Episodes and Their Formation Mechanism in Guilin, China
295

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