| Airborne particulate matter exerts great impacts on global climate change by changing radioactive forcing, atmospheric chemistry and formation of cloud and precipitation. It also plays the critical role in the adverse health effect of air pollution. Recently, China experienced increasing haze episodes with severe particulate pollution and continuous degradation of visibility. The haze pollution has now become a worldwide concern at home and abroad. However, most of the studies on formation mechanism of haze focused on PMio and PM2.5 (Particle with aerodynamic diameter Dp<10μm and <5μm, respectively) and few of them focused on submicron particulate matters (PMi, Dp<1 μm). Actually, due to the much smaller diameter, PMi is considered far more important than PMio and PM2.5 in the haze formation and adverse health effects. For example, particles with diameter of 0.1-1 μm have the highest light extinction ability, hence can severe degrade the visual range (or visibility). In addition, smaller particle can reach far deeper into human body (lung and even blood) than bigger ones, thus exert adverse impacts on health. According to these features, understanding the physical/chemical variability, evolution process, source and optical properties of PMi is critical in the investigation of haze formation mechanism and the prevention of atmospheric particulate pollution.An online semi-continuous analyzer of Monitoring for AeRosol and Gases (MARGA) was deployed to measure the water soluble ions (WSI) in PMi and associated gases at 1-h time resolution from late June in 2012 to late May in 2013. Firstly, by using various statistics and analytical methods, the chemical variability, seasonal/diurnal variation and source apportionments of PMi in shanghai were studied. Secondly, the impacts of temperature and relative humidity on the formation/evolution process of secondary inorganic species were studied and demonstrated. Based on the impacts of temperature and RH, gas-particle partitioning behavior of NH4NO3 and the influences on visibility/light extinction were investigated. Last but not the least,2 cases studies on the formation mechanism of the most serious haze pollution in 2013 in China were shown from the aspects of the physical and chemical variability and evolution process of PMi. The impacts of firework on air quality during Spring Festival in 2013 and 2014 were compared and demonstrated.The major results were summarized as follows:1. Chemical characteristics and sources apportionments of PMi in Shanghai:(1) Annual average and sources of major WSIs and associated gases:The PMi in Shanghai ranged from 1.50-190.10 ug/m3 with the average of 28.14±23.12μg/m3. Total Water Soluble Ions (TWSI) accounted for 60.81% of PMi mass loading. NH4+ SO42-、NO3-、Cl- were the major ions accounting for 22.78%,16.37%,13.52% and 3.20% of PMi mass loading, respectively. Source apportionments with PSCF showed the similar sources of PMi and its WSIs. Most of them came from Jiangsu Province, Midwestern Anhui, Northern Zhejiang and Southern Shandong. Sources of SO42- were much less and far away from Shanghai, while NO3- was from urban, sub-urban Shanghai and from regional source (e.g. Jiangsu, Shandong).(2) Seasonal variation of WSIs in PM1 and cluster analysis of back trajectory: Seasonal variation of PMi mass loading was winter>autumn>spring>summer. TWSI was the main component in PM1 in each season, with the average ranging from 56.80-64.62%. Contribution of TWSI decreased with the increasing of PMi mass loading in each season. However, Contribution of NO3- showed an increasing trend with the ascending PM1 mass concentration, exceeding that of SO42- in spring, autumn and ainter, and become the major WSI in PMi. Cluster analysis reveals that when controlled by the air mass from the sea, Shanghai experienced relative good air quality with SO42 dominated in PMi, especially in summer. In other seasons Continental air mass were more favored and often brought elevated PMi mass loading with higher NO3- fraction. The average RC/A of PM1 was 0.942±0.116, indicating the PMi in Shanghai was acidic. Seasonal variation of RC/A was spring>winter>autumn>summer. RC/A increased with the ascending PMi concentration. Source apportionments reveals that acidic PM1 was from the sea while more neutralized PMi was mainly from continent.(3) Diurnal variation of major WSI:NO3- peaked at around 7:00-8:00 AM and reached the lowest concentration at late afternoon in summer, autumn and spring. This was due to the accumulation during the night and the relative low-temperature and high-RH in ambient atmosphere, which facilitated the gas to particle partitioning of NH4NO3 in the early morning and strong dissociation with higher temperature at late afternoon. However, NO3- reached the peak at noon in winter, which was the result of strong gas phase oxidation of NO2 at noon to form HNO3 and hence reacted with NH3. SO42- reach the peak during the day, often at late afternoon, in four seasons. It was resulted from the gas phase oxidation of SO2+OH to form H2SO4, hence,(NH4)2SO42. Impacts of T and RH on formation/evolution of secondary WSIs:(1) Sulfur oxidation ratio (SOR) showed a clear seasonal variation:summer (0.64 ±0.20)> autumn (0.37±0.23)> spring (0.32±0.18)> winter (0.20±0.10). SOR increased with elevated RH, indicating the important role of aqueous phase oxidation for SO42- formation. At the same RH, higher temperature facilitated the gas phase oxidation of SO2 to form SO42-, this phenomenon was more significant in autumn.(2) NO3- and Cl" mainly existed as NH4NO3 and NH4Cl in PMi, which are semi-volatile. NH4NO3 and NH4CI showed the similar phase change pattern under the influence of RH and temperature. When RH<50%, strong dissociation was favored, and the humidity variation in this RH range had little impacts on the equilibrium. Higher RH facilitated the gas to particle partitioning. When RH>70%, the transformation was enhanced by a factor of 1 than that at RH<30%. Low temperature also facilitated the gas to particle partitioning, when T<17℃, the partitioning was enhanced by more than 1 times of that at T=20℃.3. Semi-volatiles in PMi (particularly NH4NO3) exerts significant impacts on aerosol water content and optical properties:(1) During non-haze period, strong dissociation of NH4NO3 and NH4Cl were observed at 11:00-17:00 (LT). These strong dissociations was found to significantly decrease the correlation coefficient of PMi mass concentration vs visibility at RH<50%. It was likely due to the variation of particle diameter caused by the dissociations.(2) Water content of PMi was significantly correlated with NH4NO3 fraction, but showed no correlation with (NH4)2SO4. Higher NH4NO3 fraction often associated with higher RH (especially at 80-90%), the hygroscopic growth was mainly resulted from enhanced NH4NO3 fraction at high RH and facilitated the water uptake by PM1. The light extinction coefficient by PMi was enhanced by more than 2.75 times at RH of 80-90% than that of 50%. The relation of major components in PM1 and light extinction coefficient was quantified with stepwise Multiple Linear Regression (Stepwise MLR).4. Case study of typical haze pollution:(1) Shanghai experienced severe particulate pollution lasting for almost a month in January,2013.7 episodes were identified with the level of particle, RH and visibility. Based on the chemical characteristics, evolution process, these episodes were classified as multiple pollution, biomass/coal burning induced complicated pollution, nitrate dominant pollution and sulfate dominant pollution. Multiple pollution (episode-6) last for the longest time (144 h) among them, with the PM1 average concentration of 53.85 ±21.29 μg/m3. At the beginning, PM1 was dominated by sulfate transported from other region. With the development of the pollution, Shanghai was influenced by the mixture contribution of biomass/coal burning and local source with the continuous increasing of NO3-/SO42- mass ratio and significan elevation of BC, CO, K+ and Cl+. Episode-3 and Episode-7 were of the highest particulate pollution. Average PMi mass concentration was 97.84±29.98μg/m3 and 107.36±16.99 μg/m3, respectively. At the beginning of each episode, Shanghai was influenced by pollution emitted by biomass/coal burning, However heterogeneous reaction of HNO3 with KCl on the surface of PMi was observed with the development of these episodes, resulting in significant enhancement of NO3-.Nitrate dominant pollution (Episode-1,-4,-5) was contributed by local source and sulfate dominant pollution (Episode-2) was the result of long range transportation of aged aerosol.(2) Shanghai experienced the severest particulate pollution from Dec.5th-7th 2013. PM2.5 reached the historical record of 635.00 μg/m3 at 12:00-13:00 on Dec.6th. The episode was with extremely low wind speed (～0.5m/s), stable and high RH (-80%).The main contribution was the mixture of biomass burning and local source. Physical and chemical evolution of PMi during the episode was detailed and demonstrated. At 12:00-13:00 on Dec.6th, historical record of PM2.5 concentration was observed. The aerosol was highly aged with higher diameter and lower number concentration. Particles with diameter of 100-500 nm was dominant (56.61%) in the number concentration, showing 2 peaks (～40 nm and ～150 nm). Mass loading was mainly contributed by the large particle (diameter of 500-1000 nm) (54.96%). Mass loading also showed 2 peaks (648 nm and 524.4 nm). The air quality became better due to the influence of air mass from high altitude above the ocean at late Dec.6th.(3) Intensive firecracker/firework displays during Spring Festival released fine particles and gaseous pollution and deteriorated the air quality in Shanghai in recent 2 years. During the intensive discharge of firework in 2013 (0:00-1:00, Feb.10th), PM10, PM2.5, PMi reached as high as 789.52 ug/m3,408.76 μg/m3 and 190.10 ug/m3 respectively. Most significant variation was observed for K+ and Cl- in PMi, reaching 47.53 μg/m3 and 30.55 μg/m3,24.42 and 22.14 times as their average level during the whole period. SO42-, NO3-, NH4+ also showed significant changes. The situation in the Spring Festival in 2014 was much better, PM10, PM2.5, PMi were decreased by 54.23%, 25.41% and 34.75% of last year. Major WSI also dropped, compared with last year. After the intensive discharge, secondary inorganic was enhanced in PM1, this is likely relate to the heterogeneous oxidation on the existing particles which were released by firework and contained various trace metals. These particles also exert great impacts on light scattering, but with much shorter time, Highest light scattering coefficient for blue light (450 nm), green light (550 nm) and red light (700 nm) was observed with 2439 Mm-1、1919 Mm-1 respectively at 00:17 on Jan.31st,2014. However, they decreased more than 50% within 10 minutes. |
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