| Haze events frequently appear in many regions of China, which are in part due to abundant secondary organic aerosols(SOA), in addition to primarily emitted particles and stagnant meteorological conditions. SOA accounts for a large fraction of atmospheric fine particles, of which dicarboxylic acids(Diacids) are abundant components. Diacids have a strong impact on physico-chemical properties of aerosols and play an important role in hygroscopic growth of particles, activation of cloud condensation nuclei and ice formation. Primary emissions of diacids from fossil fuel combustion and vehicle exhaust are minor, whereas diacids are largely derived from secondary formation through organic precursor oxidation. Diacids are often indicative of aerosol ageing. Information of compositions and spatial and temporal variations of diacids is imperative for understanding the role of SOA in the haze formation process.In this dissertation Xi’an and Chengdu cities were selected for investigating the spatial and temporal distributions and formation mechanisms of diacids under different meteorological conditions. Oxalic acid(C2) is the most abundant diacid in winter in Xi’an, followed by phthalic acid(Ph). Concentrations of C2 and Ph were higher in daytime than nighttime before the Spring Festival, while the other diacids exhibited opposite diurnal distribution. However, the diurnal ratios of diacids except azelaic acid(C9) all increased after the Spring Festival. Primary organic carbon(POC) decreased from 46μg m-3 to 27μg m-3 while the concentration of sulfate increased by a factor of about two. The relative abundance of diacids in OC increased from 2.2% to 3.9% and exhibited strong linear correlation with SO42-, indicating that the increases of temperature and RH are favorable for the aqueous phase production of diacids.Seasonal distributions of diacids in winter and summer were investigated in Xi’an with higher concentration of diacids in winter(1843±810ng m-3) and lower in summer(1259±781ng m-3). Mass concentrations of PM10, WSOC, OC and EC during the sampling period were 2-4 times higher in winter than summer, which may suggest stronger anthropogenic emissions in addition to decreased boundary layer height in the cold season. Diacids in clean and haze days showed higher concentrations in hazy days with higher ratios of C2/diacids, C3/C4, diacids/WSOC, suggesting more aged SOA in the haze periods. The equivalent ratio of anion to cation showed that aerosols were more acidic in haze periods. Lower Gly/m Gly ratio was observed in haze periods, which can in part be ascribed to faster transformation of Gly to large multifunctional compounds. Our results also suggest that ratio of Gly/m Gly is indicative of SOA aging.To further investigate the sources and formation mechanisms of SOA in the two cities, size distributions of diacids and related compounds in winter and summer in Xi’an and Chengdu were explored. C2, C3, C4, C5, C6 are mainly accumulated in the fine mode(<2.1μm) during winter, while they exhibit bimodal pattern in summer with two peaks in the fine(<2.1μm) and coarse(>2.1μm) modes, respectively. Higher temperature in summer is favorable for gas-to-particle partitioning of semivolatile diacids into the coarse particles, thus Ph mainly exists in the fine mode in winter but concentrates in the coarse mode in summer. t Ph is emitted from plastic and waste burning process and shows unimodal distribution in the fine mode. Gly, m Gly, Pyr and ωC2 all present bimodal pattern with a large peak in the fine mode and a relatively small peak in the coarse mode.High time resolution(1-hour) observation of PM2.5 aerosols in December, 2012 in Xi’an captured a haze event build-up process. Diacids, ketoacids and α-dicarbonyls showed similar hourly variations with PM2.5 concentration from the clean to the haze period, which continuously increased from the clean days to the haze days. However, the relative abundance of diacids and related compounds in PM2.5 was lower in haze period. The temporal variation of Gly/m Gly and C3/C4 ratios showed more oxidized OA in the clean period. Other markers also presented a similar trend. For example, levoglucosan, a biomass burning tracer, was much more abundant in the haze period. Moreover, increased Fe and Ca(8-10% to PM2.5 mass) was observed during the haze period, indicating that biomass burning and mineral dust are important to haze formation.Size-resolved aerosols in Chengdu were measured in winter on a day/night basis. Higher concentration of diacids was found in nighttime(1831±607ng m-3) than in daytime(1532±196ng m-3), but the ratio of C2/diacids showed no diurnal difference. Diacids, ketoacids and α-dicarbonyls were enriched in the fine mode both in day and night. Liquid water content(LWC) of the aerosol showed a significant increase at night, together with higher relative abundance of diacids and lower abundance of ketoacids and α-dicarbonyls in WSOC. Robust correlations of Gly/C2, C2/diacids with LWC suggest that the enhancement of LWC is favorable for oxidation of Gly and ωC2 to C2. Besides, LWC had better correlations with NO3-, NH4+ and WSON in nighttime samples. Higher abundance of K+ and Cl- in the fine mode particles and the pronounced positive correlations of K+ with WSOC, WSON and C2 indicate that organic aerosols are significantly affected by biomass burning.Size-resolved aerosols collected in 2009 and 2011 dust storm events, was observed with significant increase of SO42-, NO3-, NH4+ and diacids in the coarse mode aerosols. Total suspended particulate(TSP) samples were collected with hourly interval during a massive dust storm event from March 9th to 12 th in Xi’an, central China, along with the size-segregated sample collection. Extremely high concentration of diacids(5407ng m-3) was observed at the beginning hour of the dust storm event and decreased after the dust storm peak passing through the city. However, no obvious increase of ketoacids(17-101 ng m-3) and α-dicarbonyls(9.5-69 ng m-3) was found in the dust storm period. The influence of dust storm on organic compositions in urban areas was investigated, which shows that the photochemical degradation of high molecular weight(HMW) diacids is not an important pathway to produce LMW diacids. Size distribution of C2 presented a monomodal size distribution pattern with a peak pattern in the coarse mode(>2.1μm) in dust storm period and two equivalent peaks in the fine(<2.1μm) and coarse modes during non-dust storm period. Glyoxal(Gly), methylglyoxal(m Gly) and glyoxylic acid(ωC2) all showed a same bimodal pattern with a predominant peak in the coarse mode during dust storm period. C2 is 287μg g-1 in the desert source samples, which is lower than the concentration in Xi’an dust storm samples. Our results demonstrate that more than 60% of C2 is heterogeneously produced in dust storm period. The above results indicate an infant state of SOA chemistry on mineral dust particles in the urban areas near the Asian Gobi desert regions, which is helpful for understanding the chemistry evolution of dust particles during the long-range transport from the inland China to the Asian continental outflow region. |
PDF Full Text Request