Characteristics of Aerosol Pollution Under Different Visibility Conditions in Winter in a Coastal Mega-City in China

Haze refers to the weather phenomenon in which the average daily visibility is less than 10 km (7.5 km for remote sensing) due to suspended solid, liquid particles and water vapor in the atmosphere. It is closely related to meteorological conditions and air quality ^[1]. Haze events have adverse effects on visibility，human health, and global climate change^{[2, 3]}. Fine particulates (PM_2.5, aerodynamic equivalent diameter less than 2.5μm) are considered as important contributors to haze formation, due to their effects on light extinction ^[4]. Secondary inorganic species are the important components of PM_2.5, and have attracted wide attention due to their adverse impacts on visibility ^[5-6]. Due to the characteristics of atmospheric aerosols and other influencing factors, the meteorological parameters in different regions may be different, and the formation mechanism is also different. Previous studies also have pointed out that the characteristics and formation mechanisms could be different between haze and non-haze events^[7-8].

The Pearl River Delta region is one of the four regions with the strongest haze influence in China ^[9]. Shenzhen is one of China's megacities with a population of 12 million. Located west of the Pearl River Estuary and the Lingdingyang Bay, Shenzhen is a hub connecting Hong Kong with the inland cities of the Pearl River Delta. Shenzhen is located in a transitional zone of the East Asian monsoon system, where the southeaster summer monsoon from the South China Sea. With the rapid growth of local industrial fuel consumption and the number of motor vehicles, Shenzhen has experienced increased air pollution, reflected in the frequency of haze events ^[10]. The worst period was in 2004, during which there were 187 days with haze events. Compared with other parts of northern China, Shenzhen has seen significant improvement in air quality ^[11], but serious pollution events still occur occasionally in winter. Most existing studies were mainly based on sampling and off-line observations, which have limitations in providing further understanding of the characteristics and the roles played by major chemical species during haze events ^{[10, 12]}. There are few reports about the composition and formation mechanism of PM_2.5 in the haze formation process. In this study, a comprehensive monitoring campaign was carried out in Shenzhen to better understand the cause of haze events from Jan to Mar of 2016 through an online monitoring systems. This study aims to obtain a complete picture of pollution events and to understand formation mechanisms of secondary inorganic aerosols and their relationship with haze events.

The observation site (22°32'N, 114°0'E, altitude 63 meters) is located in Shenzhen Garden Expo Park.The surrounding region of this site is mainly commercial and living areas, without large industrial sources (Fig. 1). Ozone (O₃), sulfur dioxide (SO₂), nitric dioxide (NO₂) and carbon monoxide (CO) are measured using online commercial analyzers (Thermo Instruments, USA, TEI 49i, 43i, 42i, and 48i respectively) with the lowest detection limit of 1 ppb (10 seconds average time) and 0.50 ppb (300 seconds average time), 0.40 ppb (60 seconds average time), 0.04 ppm (10 seconds average time) respectively. These instruments are maintained, including zero and span calibrations weekly (each lasting for 30 minutes), and a multi-point calibration every month. PM_2.5 is measured by ambient Particulate Monitor (Grimm 180, Grimm Aerosol Technik GmbH & Co. KG, Germany) based on laser scattering theory, which can obtain the mass concentrations in different particle size segments.

Figure 1.  The sampling site in Shenzhen.

Hourly real-time concentrations of gases and particulate water-soluble inorganic ions in PM_2.5 are determined by the Monitor for Aerosols and Gases (MARGA, Applikon Analytical B. B. Corp., ADI2080, Netherlands). The details of MARGA system have been provided in Du et al. ^[13]. MARGA includes a sampling unit and an analytical unit. The sampling unit consists of two parts: one is a wet rotating denuder (WRD) for absorbing gas (HCl, HONO, SO₂, HNO₃ and NH₃) and the other is a steam jet aerosol collector (SJAC) for collecting particles. Ambient air is drawn through the WRD followed by the SJAC. Gaseous and particles components are collected for ion chromatographic (IC) analysis, respectively. The IC is continuously controlled by an internal calibration method using a standard lithium bromide (LiBr). In this work, the concentrations of trace gases (i. e., NH3) and water-soluble inorganic ions (i.e. ${\rm NH_4^ +}$, Na⁺, K⁺, Ca²⁺, Mg²⁺, Cl^-, ${\rm NO_3^ -}$, and ${\rm SO_4^{2 - }}$) in PM_2.5 were analyzed. During the observation period, the slope of 1.02 for regression and scattering of anions (AE)and cations(CE) (see (1) and (2) for calculation formula) (Fig. 2) (R²=0.99) indicated that the particles are neutral.

Figure 2.  Scatter plots of anions and cations in PM_2.5.

Meteorology variables such as wind speed, wind direction, relative humidity (RH), and temperature are observed by MAW301 (Vaisala Corp. Finland). Atmospheric visibility is also observed by a PWD20 (Vaisala Corp. Finland). The station is local in Shenzhen National Basic Synoptic station, which is 50 meters away from the observation site of air pollutants.

The observation period is from January 1 to March 30, 2016. The MARGA data from January 31 to February 3, February 25, February 28-29, and March 30 are missing due to instrumental failure.

In this study, three different visibility conditions are defined: the daily visibility > 15km is defined as a relatively clean condition, denoted as V1; 5 km < the daily visibility < 7.5km with no precipitation is defined as haze day, denoted as V2; the daily visibility < 5km with no precipitation is defined as the heavy haze day, denoted as V3. Table 1 shows the average concentrations of several atmospheric pollutants (PM_2.5, SO₂, NO_x, and O₃) and the meteorological conditions (visibility, temperature, relative humidity and wind speed) under different visibility conditions. During haze events, the daily of PM_2.5 are higher than the average concentration during the observation period (33.1 ± 16.2 μg m^-3), was about 2 times of that during clear days. The highest PM_2.5 hourly concentrations exceed 50 μg m^-3, with a maximum value reaching 125 μg m^-3. The mean concentration of SO₂ changed slightly, during haze days compared to those during clear days. The concentration of NO₂ increased by 64% and 89%, respectively, during haze days and heavy haze day compared to those during non-haze days. In contrast, mean O₃ level were much higher during clear days than during haze days, implying lower atmospheric oxidation potential during haze events. As expected, unfavorable weather conditions (high RH, low wind speed) were among the causes of haze formation as well as in many other cities^[14-16].

Figure 3 shows the diurnal variation of the hourly averaged SO₂, NO_x, and PM_2.5 under different visibility conditions. For gas-phase compounds (SO₂, CO, and NO_x) are mainly affected by near-surface direct emissions, while O₃ is mainly affected by photochemical reactions. We observe very different diurnal variations between the two types of species. The concentrations of NO_x are relatively high during the morning and evening rush hours, and the concentration rapidly decreases around 10: 00 p. m.. In addition, the height of the atmospheric boundary layer (PBL) is also the main factor affecting the change of NO_x concentrations ^[17]. In the morning, the PBL is lower, and the NO_x concentrations are higher. With the gradual elevation of the PBL, the NO_x concentrations reach the lowest level at noon. In contrast, SO₂ shows one distinct peak, with peaks occurring at 18:00, because SO₂ is mainly affected by long-distance transport and elevation of the PBL ^[16]. As photochemical reaction is the main source of ozone ^[15], O₃ shows the highest concentrations at around noon.

Figure 3.  Diurnal variation of the hourly averaged SO₂, NO_x, O₃ and PM_2.5 under different visibility conditions for the whole measurement period.

Under different visibility conditions (V1, V2, and V3), the gas-phase compounds and PM_2.5 exhibit different behavior. The difference is mainly reflected in the magnitudes of concentrations. All gas and PM_2.5 (except for O₃) show higher concentrations under low visibility. In contrast, O₃ level in haze events presents a consistent low concentration and stable daily variation. The relatively low levels of O₃ under low visible conditions might be due to the decreased photochemical production. It should be noted that the concentration of O₃ in V3 is relatively higher than that in V2, due to ozone pollution at night. Because nocturnal low-level jets (LLJs) will enhance vertical mixing between the stable boundary layer and the residual layer, it will affect the vertical redistribution of O₃ ^[18].

The mean concentrations of water-soluble inorganic ions (WSIIs) during the observation period is 12.4±11.4 μg m^-3, accounting for 37% of PM_2.5 mass concentration. ${\rm SO_4^{2 - }}$ is the most abundant species in water-soluble inorganic ions, with an average of 5.1 ± 4.1 μg m^-3, followed by ${\rm NO_3^ -}$ (3.5±4.5 μg m^-3) and ${\rm NH_4^ +}$ (2.8±2.6 μg m^-3), accounting for 41%, 29% and 23% of the total concentrations, respectively. The sum of the three components accounts for 93% of the total concentration of the WSIIs, which is close to that of Beijing and Suzhou ^{[16, 19]}. Except for the three ions, the proportions of Na⁺ (0.10±0.17 μg m^-3), Cl^- (0.53±0.50 μg m^-3), K⁺ (0.14±0.31 μg m^-3), Mg²⁺ (0.02±0.05 μg m^-3) and Ca²⁺(0.15±0.11μg m^-3) are lower than 3%.

Figure 4.  Water-soluble inorganic ion species under different visibility conditions.

The average concentration of WSIIs in V1 was 10.4±5.1μg m^-3, while the concentration of WSIIs in V2 and V3 was 2.8 and 3.2 times as much as that in V1, respectively. The mean concentrations of ${\rm SO_4^{2 - }}$ and ${\rm NO_3^ -}$ during V1 are 3.40 μg m^-3 and 1.72 μg m^-3, respectively, accounting for 44.7% and 22.7%, and the ratio ${\rm NO_3^ -}$/${\rm SO_4^{2 - }}$ is 0.50. With decreasing visibility, the ratios of ${\rm NO_3^ -}$/${\rm SO_4^{2 - }}$ increased. During V2 and V3, the ratios of ${\rm NO_3^ -}$/${\rm SO_4^{2 - }}$ increases markedly, with 0.69 and 0.82 respectively, the corresponding concentrations of ${\rm NO_3^ -}$ and ${\rm SO_4^{2 - }}$ are 6.74 μg m^-3, 8.21 μg m^-3 and 9.73 μg m^-3, 10.06 μg m^-3. The ratio of ${\rm NO_3^ -}$/${\rm SO_4^{2 - }}$ during the pollution period is greater than that during the non-haze days, and the result is in agreement with the results in Guangzhou and Suzhou ^{[6, 20]}. In the present study, NO_x concentration greatly exceeded that of SO₂ during haze periods. Under high NO_x condition, concentration of OH and H₂O₂ were reduced, further decreasing the possibility of ${\rm SO_4^{2 - }}$ formation ^[6]. Thus, the elevation of ${\rm NO_3^ -}$ concentration under worse visibility conditions is greater than that of ${\rm SO_4^{2 - }}$ and contribute higher to the reduction of visibility. At the same time, the ratio of ${\rm NO_3^ -}$/${\rm SO_4^{2 - }}$ tend to be larger when air pollution became more serious. For examples, the ratio of ${\rm NO_3^ -}$/${\rm SO_4^{2 - }}$ during V1 (clean day), V2 (haze day) and V3 (heavy haze day) increases gradually.

${\rm NO_3^ -}$ and ${\rm SO_4^{2 - }}$ represent the secondary aerosol from transformation of the precursors of NO_x and SO₂ ^[9]. The study in the Yangtze River Delta showed that the emission ratio of NO_x/SO₂ for motor vehicles is 17.2-52.6, while the ratio of NO_x/SO₂ for stationary sources, such as factories, etc., is 0.527-0.804 ^[9]. Thus, the ratio of ${\rm NO_3^ -}/{\rm SO_4^{2 - }}$ is used as an important indicator of relative importance of mobile versus stationary sources of sulfur and nitrogen in atmosphere ^[21]. The ratio of ${\rm NO_3^ -}/{\rm SO_4^{2 - }}$ during the observation period in this study is 0.65 and the ratio of NO_x/SO₂ is 6.2, indicating that both the automobile exhaust and the stationary sources are very important in Shenzhen. The ratios of ${\rm NO_3^ -}/{\rm SO_4^{2 - }}$ in Shenzhen is greater than that of some other areas in China, such as Shanghai (0.43), Qingdao (0.35), Taiwan (0.2), Guiyang (0.13), Suzhou (0.59)^{[20, 22-26]}. At the same time, previous studies had shown that the ratios of ${\rm NO_3^ -}/{\rm SO_4^{2 - }}$ in Shenzhen in 2004 and in 2009 were 0.26 and 0.62 respectively ^[27], lower than the results in this study, indicating that the ratio of ${\rm NO_3^ -}/{\rm SO_4^{2 - }}$ had gradually increased. From 2004 to 2017, the number of car ownership in the Shenzhen area had continuously increased from 660, 000 to 3.4 million. The Pearl River Delta region had taken desulfurization measures starting from the"Eleventh Five-Year Plan", which may be one reason for the increase in the ratio of ${\rm NO_3^ -}/{\rm SO_4^{2 - }}$, indicating that the automobile exhaust emissions may have increasingly important impacts on pollution in Shenzhen.

The relationship between the ratio of ${\rm NO_3^ -}/{\rm SO_4^{2 - }}$ and wind direction during haze events is illustrated in Fig. 5. The result shows that the ratio of ${\rm NO_3^ -}/{\rm SO_4^{2 - }}$ is relatively large, when the sea breezes (southerly wind and westerly winds). But when the land breezes (northerly and easterly wind), the ratio of ${\rm NO_3^ -}/{\rm SO_4^{2 - }}$ is relatively lower. It indicates that the land and sea breeze could affect the formation of pollution in Shenzhen, though the regional transportation from the inland area of Pearl River Delta is still important to the formation of the pollution events. To take the haze event (shown in Fig. 6) as an example, the haze event occur from January 1 to January 2, 2016. A weak northerly wind started and lasted from the night of December 31, 2015 to 10:00 a.m on January 1, 2016. Subsequently, the direction of wind turned to south and the sea breezes gradually dominated. As the wind changed, the ${\rm NO_3^ -}/{\rm SO_4^{2 - }}$ ratio changed. When the wind became sea breezes, the ratio of ${\rm NO_3^ -}/{\rm SO_4^{2 - }}$ was large. Previous studies have indicate that the land breezes (northerly winds) can transport the local pollutants in Shenzhen to the sea, and the sea breezes can transport the pollutants back to Shenzhen^[28]. The higher relative humidity was a beneficial factor for heterogeneous reactions when sea breezes occur. At the same time, high NO_x concentration further reduces the possibility of ${\rm SO_4^{2 - }}$ generation ^[6].

Figure 5.  Distribution of wind direction and the ratio of ${\rm NO_3^ -}$ (a), ${\rm SO_4^{2 - }}$ (b), ${\rm NO_3^ -}/{\rm SO_4^{2 - }}$ (c) on haze days.

Figure 6.  Time series of water soluble inorganic icons, wind speed, wind direction and RH from January 1 to January 2, 2016.

Sulfur oxidation rate (SOR=(n(${\rm SO_4^{2 - }}$)/(n(${\rm SO_4^{2 - }}$)+ n (SO₂))) and nitrogen oxidation rate (NOR=n(${\rm NO_3^ -}$)/(n(${\rm NO_3^ -}$) + n(NO₂))) can be used to estimate the transformation degree of sulfates and nitrates^[29]. During haze days, the values of NOR and SOR in Shenzhen are 2.5 and 2.2 times higher than those in clean days (V1), implying more significant transformation of sulfates and nitrates and more elevated secondary aerosols during haze days.

Gaseous SO₂ is converted to particulate sulfate through gas-phase oxidation by H₂O₂ and OH radical or aqueous reactions ^[30]. It has been reported in many studies that the oxidation of aqueous SO₂ catalyzed by transition metals are more efficient during winter haze, compared to gas-phase oxidation. Our measurements also suggest that aqueous oxidation is an important sulfate formation pathway in the region of study. As shown in Fig. 7, during haze events, the concentrations of RH, NH₃, and NO_x increase rapidly, especially RH and NOx. For example, from V1 to V2, RH and NO_x increase rapidly from 53% and 20.2 ppb to 77% and 39.8 ppb, respectively ^[31], indicating that high RH and the elevation of NH₃ concentration can provide suitable conditions for aqueous oxidation of SO₂. The high level of NO_x enhances the atmospheric oxidizing capability during hazy events. Therefore, the aqueous oxidation may be an important way to form sulfate in Shenzhen in winter.

Figure 7.  Box plot of aerosol precursors (SO₂, NO_x), NH₃, O₃ and RH during V1 (red), V2 (blue) and V3 (yellow) and specify the quartiles represented and the meaning of the black dot and the white line in the boxes.

Gaseous is converted to particulate nitrate through gaseous oxidation of NO₂ by OH during daylight and the heterogeneous reaction of nitrate radical during nighttime ^[32]. By studying the relative relationship of ${\rm NH_4^ +}$ and ${\rm NO_3^ -}$ at different ${\rm SO_4^{2 - }}$ levels, we can understand the formation pathway of ${\rm NO_3^ -}$^{[5, 33]}. As shown in Fig. 8, the nitrate linearly increased with the increasing ammonium to sulfate molar ratio. An intercept of [${\rm NH_4^ +}$]/[${\rm SO_4^{2 - }}$] is 1.46 by fitting a linear regression. The value is comparable to that observed in Suzhou and Beijing, where the values were 1.51 and 1.5 respectively ^{[5, 20]}. This result indicates that nitrate formation via homogeneous reaction of HNO₃ with NH₃ became evident at [${\rm NH_4^ +}$]/[${\rm SO_4^{2 - }}$] =1.46. The excess ammonium [${\rm NH_4^ +}$]_exc ([${\rm NH_4^ +}$]_exc=（[${\rm NH_4^ +}$]/[${\rm SO_4^{2 - }}$] - 1.46）× [${\rm SO_4^{2 - }}$]) is defined as the amount of the ammonium concentration in excess at which nitrate formation became evident. The concentration of excess ammonium is greater than 0, with a linear correlation with the nitrate concentration, indicating that the gas-phase homogeneous reaction between the ambient ammonia and nitric acid is responsible for forming nitrate ^{[5, 20]}.

Figure 8.  Nitrate to sulfate molar ratio as a function of ammonium to sulfate molar ratio (left) and the relationship between molar concentrations of nitrate and excess ammonium (right).

Haze events frequently occurred in Shenzhen in 2016, with a total of nine haze days in total from January to March. The PM_2.5 concentration during haze days was higher than the average value during the entire observation, with the highest value reaching 125 μg m^-3. The high concentrations of NO₂ and lower levels of O₃ were observed during haze periods in comparison with clear days. Unfavorable meteorological conditions, such as low wind speed and high humidity, together with pollutants accumulation and secondary formation of aerosol are responsible for these haze formations.

The mean concentration of the WSIIs during the observation period was 12.4±11.4 μg m^-3, accounting for 37% of PM_2.5, among which ${\rm SO_4^{2 - }}$ was the highest element, followed by ${\rm NO_3^ -}$ and ${\rm NH_4^ +}$. The sum of the three components accounts for 93% of the total WSIIs. The sum of the major secondary ionic species during haze periods was about 2.8 times of that during clear days. During the haze period, ${\rm SO_4^{2 - }}$, ${\rm NO_3^ -}$ and ${\rm NH_4^ +}$ increased significantly, especially ${\rm NO_3^ -}$. The ratio of ${\rm NO_3^ -}$/${\rm SO_4^{2 - }}$ is 0.50 in clear days and tend to be larger when air pollution became more seriously. At the same time, compared with previous studies, it is found that this ratio has been gradually increasing, indicating that vehicle emissions have an increasingly impacts on pollution.

During haze events, the gas-phase and aerosol components showed higher concentrations, especially major aerosol components (${\rm SO_4^{2 - }}$, ${\rm NO_3^ -}$ and ${\rm NH_4^ +}$), which were mainly from secondary sources. When visibility became lower, the concentrations were higher. Furthermore, SOR and NOR was higher during the haze period than that in clean days, indicating efficient gas to particle conversion. Further analysis shows that high concentrations of sulfate might be explained by aqueous oxidation, but gas-phase homogeneous reactions might dominate the formation of nitrate.