|dc.description.abstract||The dense pollutants from active biomass burning (BB) in Indochina and Asian mainland are transported by prevailing westerly every spring. During long-range transport, secondary aerosol produced from aerosol precursors through various chemical reactions accompanying with primary aerosol play an important role in climate change by scattering and absorbing solar radiation and/or forming cloud condensation nuclei. This study observed atmospheric aerosol under prevailing westerly at Lulin Atmospheric Background Station (LABS, 2,862 m a.s.l.) in Taiwan from March to April in 2011. The objectives are to understand dynamic variations of aerosol properties of different air masses, to infer possible mechanisms affecting variations of aerosol properties during cloud-fog and ultrafine particle formation events, and to estimate aerosol pH values of different air masses.
The BB, free troposphere, and anthropogenic are the three major types of backward trajectories at LABS during the observation period. The diurnal variation of CO was weakening, while NOy, aerosol absorption and scattering coefficients were keeping regular diurnal patterns when under the influence of BB. This may indicate the influence of local valley breeze on LABS. Through more representative nighttime data, the concentrations of PM2.5 and PM10 from BB relative to background air are estimated to increase to 3.5 and 3.6 folds, respectively, while the increases are 3.6 and 3.8 folds, respectively, when adding local valley breeze transport to BB. This shows that coarse particles are slightly increased with the addition of local pollution to the transported BB plume.
The well correlated short term SO42-、NO3-、NH4+, and K+ concentrations measured by Particle-Into-Liquid-Sampler coupled to an Ion Chromatograph (PILS-IC) plus higher values of Nitrogen Oxidation Ratio (NOR) in cloud-fog events implies aerosol properties not only affected by the integration of BB type of air masses with local valley breeze but also by the heterogeneous or photochemical reactions. In addition, the reduced relative humidity (RH) and increased O3 concentration during early morning is considered to be the effect of lowering boundary layer. Meanwhile, the increased concentrations of Ca2+ and Mg2+ accompanies with high ambient temperature are inferred to be caused by mountain breeze during the night.
Aerosol number size spectra measured by Scanning Mobility Particle Sizer (SMPS) are dominated by accumulation mode for cloud-fog events, which is consistent with the size ranges of major chemical species of BB aerosol. This demonstrates the effect of BB plume on cloud-fog events. In contrast, aerosol number size spectra are presented with both accumulation and Aitken modes to show the occurrence of nucleation during the events of ultrafine particle formation. It is noted that NO2- is observed by PILS-IC to evidence the occurrence of photochemical reactions because HNO3 or HONO can be formed from photochemical reactions of NO2 in cloud-fog droplets. The peak diameter of Aiken mode is around 30 nm, which indicates photochemical reactions may occur near LABS because the peak size is small. During the observation period, only three events of ultrafine particle formation occurred, which implies most photochemical reaction sites are not near LABS.
Finally, aerosol pH values from various air masses were modeled through AIM2 model to reveal in a range of 0 to -2. Compared with literature values, the pH range of LABS aerosol is apparently lower, which is thought due to the influence of transported water-soluble ions and atmospheric RH. Employing multiple linear-regression analysis on factors influencing aerosol pH values, SO42- and NO3- are found having negative effect, while RH and NH4+ are positive.||en_US|