| dc.description.abstract | Biogenic environment, particularly forest, is among the main source regions of atmospheric aerosols. Changes in the natural formation of particulate matter in these regions may inherently lead to alteration in visibility conditions and in radiative forcing of climate. One of the principle causes of the deviation from the pristine condition is the transport of anthropogenic pollutants to the forestry areas. Probing the influence of the human-related activities on biogenic secondary organic aerosols may provide a projection of global aerosol budget in the coming years. In this study, a Thermo-Desorption Proton-Transfer-Reaction Time-of-Flight Mass Spectrometer (TD-PTR-TOF-MS) was developed to comprehensively characterize the particle phase organic markers to account the major sources of anthropogenic perturbation (e.g. urban) in a subtropical forest. PTR-TOF-MS as a detector of organic compounds in particle phase is highly beneficial due to its high sensitivity, a wide range of organic compounds detected and rapid mixing ratio quantification scheme.
The TD-PTR-TOF-MS was initially evaluated against standard compounds that are typically utilized as organic markers for source attribution for atmospheric aerosols (e.g. levoglucosan for biomass burning). The results demonstrated the linearity of the TD-PTR-TOF-MS signals against a wide range of mass loading of specific species on filters. However, it was found that significant fragmentation occurred in the drift tube of the PTR-MS. The instrument was further deployed to analyze a subset of submicron aerosol samples collected at the TARO (Taipei Aerosol and Radiation Observatory) in Taipei, Taiwan during August 2013. The inter-comparison with DRI thermo-optical carbon analyzer indicated that the TD-PTR-TOF-MS underestimated the mass of total organic matter (TOM) in aerosol samples by 27%. The underestimation was most likely due to the fragmentation in PTR drift tube. Besides, condensation loss of low vapor pressure species in the transfer components was also responsible for the underestimation to a certain degree. Nevertheless, it was showed that the sum of the mass concentrations of the major detected ion peaks correlated strongly with the TOM determined by DRI analyzer (R2 = 0.8578). Among the detected ions, phthalic acid and glutaric acid were identified based on the fragmentation pattern obtained from the mass spectra of the authentic substances. Accordingly, it was estimated that phthalic acid and glutaric acid contributed 7.0 and 9.4 %, respectively, to the TOM. The molecular fingerprints of anthropogenic tracers obtained in this urban site were deemed valuable as they can be used to account for the transport of pollutants to a biogenic site.
The TD-PTR-TOF-MS was then deployed at a subtropical forestry station, located in the Xitou Experimental Forest of National Taiwan University in central Taiwan, to probe the composition of organic aerosols (OA). The comprehensive identification of the major extracted peaks and the subsequent source apportionment using Positive Matrix Factorization (PMF) denoted three prominent source clusters of OA mass in Xitou forest. These include urban (30%), biogenic (7%) and biomass burning (7%). Urban and biomass burning tracers such as phthalic acid, levoglucosan, and succinic acid dominated the overall organic mass concentration which clearly indicated the impact of anthropogenic activities in the forest. Moreover, the presence of unnatural pollutants in the forest possibly inhibited the growth of particles through the scavenging of the nuclei particles. The elevated mixing ratio of the tracers observed in this study also hinted the role of the relative humidity (over 90% during the campaign) in the formation and partitioning of these compounds into the particle phase, thereby directly altering the fraction of the organic matter in the submicron particles.
In addition to the typical anthropogenic organic markers, the total mass concentration and molecular identity of specific organonitrates were analyzed using the TD-PTR-TOF-MS. Organonitrates (ONs) are the oxidation products of the volatile organic compounds in the presence of NOx. The peak of NO2+ (m/z 45.992) served as the surrogate of the sum of mixing ratio of organonitrates where isopropyl nitrate (IPN) was utilized to account the degree of fragmentation of ONs to NO2+. The results showed a linear response of TD-PTR-TOF-MS against a wide range of mass loading of ONs and 20% of the IPN dissociates to form NO2+. Analysis of the submicron particles from Xitou forest indicated that the organonitrates only accounted for 4% of the total organic matter, probably underestimated by 5 folds or more. Such inference was justified by the elevated relative humidity (> 90%) that caused the dissociation of ONs in the particle phase. The relationship of organonitrates to some environmental components revealed the impact of ONs in some of the crucial atmospheric processes in the forest. The association of organonitrates to the number concentration of nuclei mode particles (N100 nm) hinted the contribution of ONs to aerosol growth. Furthermore, an inferred branching ratio of 5.41% using the true mass concentration of organonitrates indicated that the ONs were a significant sink of NOx and suppressed the formation of ozone and organic oxidation products in Xitou forest through chain termination of the HOx cycle.
Overall, this study revealed that anthropogenic tracers (urban + biomass burning + organonitrate) contributed a significant fraction (?59%) of the OA burden in a subtropical forestry area, clearly indicating the strong influence of human-related activities. The characterization of the organic markers in aerosols using the TD-PTR-TOF-MS not only uncover the sources and formation of the secondary organic aerosols but also provided insights on perturbation of anthropogenic pollutants on critical atmospheric phenomena in a biogenic environment.
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