Spectroscopic Characterization And Bioavailability Of Dissolved Organic Matter In Dry And Wet Deposition In Xiamen

Atmospheric deposition plays a key role in the global substance circle and energy transportation, in this case, the temporal and spatial variation of dissolved organic matter (DOM) in atmosphere is vital to discuss the role of atmospheric deposition in global carbon circle. In this study, regular rainwater samples were collected during March2011and May2012to study the bioavailability of DOM and fluorescent dissolved organic matter (FDOM). Distinction of dissolved organic carbon (DOC) concentration, FDOM content and their bioavailability due to varied seasons and air mass trajectory types were discussed. In addition, the flux of rainwater DOC and biolabile DOC transported into the Xiamen Bay was evaluated. Referring to the conclusions provided by other studies, the global rainwater biolabile DOC flux and river water biolabile DOC flux into oceans were also assessed. During February2014, two rainwater samples were collected in order to study the degradation of rainwater DOC and FDOM during bacterial incubation experiments. Dry deposition samples were collected in Xiamen during May2013and February2014to discuss the spectroscopic characterization of water soluble organic compounds (WSOC) and its variation with seasons and air mass trajectory types. Main findings were:1. DOC concentration in rainwater collected in Xiamen ranged from9.50to931.63μM, with a volume-weighted average (VWA) concentration of124.60±1.70μM. DOC flux of rainwater into the Xiamen Bay was0.19×1010g C yr-1, which was1/13of the Jiulong River (the biggest runoff entering the Xiamen Bay) DOC flux (2.45×1010g C yr-1). Negative relationship was found between rainwater DOC concentration and rainfall amount. Sharp seasonal variation appeared in rainwater DOC concentration:highest in winter and then spring and summer, lowest in autumn. Also, DOC concentration varied in rain events with different air mass trajectory types: N-land>NE-mixed>S-oceanic>E-oceanic, indicating that rain events influenced by land had higher DOC concentration than marine rain events. During the bacterial incubation experiments which lasted for28d, the average percentage of biolabile DOC in Xiamen rainwater samples was as high as47±17%, with a range of9.21%to 90.94%. Rainwater biolabile DOC flux into the Xiamen Bay was0.08×1010g C yr-1, which was half of biolabile DOC flux of the Jiulong River. Considering the high bioavailability of rainwater DOC, the global rainwater biolabile DOC into ocean was36×1012g C yr-1, while the river biolabile DOC was60×1012g C yr-1. The percentage of biolabile DOC in rainwater samples collected in summer and autumn were higher than that in spring, while winter rainwater events had the lowest biolabile DOC percentage. No obvious difference of biolabile DOC percentage in rain events with different air mass trajectories was found. During the series bacterial incubation experiment, DOC concentration displayed a first-order exponential degradation with incubation time.2. Four fluorescent components were identified in regular rainwater samples collected in Xiamen using the excitation emission matrix spectroscopy combined with parallel factor analysis (EEMs-PARAFAC):humic-like components Cl(<250,325/436nm), C2(<250,320/386nm), C4(295/410nm) and protein-like component C3(270/326nm). IF of rainwater FDOM ranged from0.023R.U. to0.965R.U., with a VWA value of0.152±0.02R.U., positively correlated with DOC concentration and negatively correlated with rainfall amount. Different fluorescent components had different intensity, with a year average value of C1>C2>C4>C3, showing that humic-like components were the main fluorescent components. Seasonal variation of FDOM content in rainwater samples was obvious:with a highest value in winter, and then spring and summer, the lowest value in autumn. Rain events with different air mass trajectory types showed different IF:N-land>NE-mixed>S-oceanic>E-oceanic. During the regular bacterial incubation experiments, the trend of IF variation was complicated, ranging from-42%to55%. All in all, the average percentage of biolabile FDOM in rainwater samples collected in Xiamen was3±18%. With all fluorescent components, humic-like components Cl and C2were produced a little while C4was consumed obviously, on the contrary, protein-like component C3increased during the incubation period. The bioavailability of rainwater FDOM showed seasonal variations while no distinction caused by air mass trajectory types was found. During the incubation experiment operated continually, FDOM concentration represented a first-order exponential producing phenomenon, during which the humic-like component increased with incubation time while the protein-like component decreased with incubation time obeying a first-order exponential law.3. PM10concentration in Xiamen ranged from13.5to240.6μg m-3, with an average value of106.5±61.1μg m-3. Absorption coefficient α280of the WSOC from dry deposition ranged from0.1971to11.6550m-1, with an average value of3.4548±2.8026m-1. Spectral slope S275-295of WSOC from dry deposition ranged from0.0146to0.0221m-1, with an average value of0.0172±0.0020m-1. During different seasons, both PM10concentration and α280of WSOC represented as:winter> autumn>spring>summer. PM10concentration and α280value with different air mass trajectory types showed as:N-land>NE-mixed>E-oceanic>S-oceanic. S275-295was highest is spring, and then summer, then winter, lowest in autumn. Compared with PM10concentration and α280, S275-295showed an insignificant relationship with air mass trajectory types:S-oceanic=;NE-mixed-E-oceanic>N-land. Good positive relationship between α280and PM10concentration was found in dry deposition samples. Three fluorescent components were identified in WSOC from dry deposition samples collected in Xiamen by EEMs-PARAFAC:humic-like components Cl (<250,310/418nm), C2(<255,370/458nm) and protein-like component C3(270/374nm). IF of WSOC from dry deposition samples collected in Xiamen rangd from0.3042R.U. to2.5452R.U., with an average value of0.6454±0.5635R.U.. Fluorescent intensity of different fluorescent components represented as:C1>C2>C3, showing that humic-like components were the main components. Dry deposition samples sampled in different seasons showed varied FDOM concentration: winter>autumn>spring>summer. IF in WSOC from dry deposition events with different air mass trajectory types also varied:N-land>NE-mixed>S-oceanic> E-oceanic. Good positive relationship was found between IF as well as all fluorescent components and PM10concentration.