Nitrate pollution of the karstic groundwater is an increasingly serious problem with the development of Guiyang, the capital city of Guizhou Province, southwest China. The higher content of NO3- in groundwater compared to surface water during both summer and winter seasons indicates that the karstic groundwater system cannot easily recover once contaminated with nitrate. In order to assess the sources and conversion of nitrate in the groundwater of Guiyang, we analyzed the major ions, delta(15)N-NH4+, delta(15)N-NO3-, and delta(18)O-NO3- in surface and groundwater samples collected during both summer and winter seasons. The results show that nitrate is the major dominant species of nitrogen in most water samples and there is a big variation of nitrate sources in groundwater between winter and summer season, due to fast response of groundwater to rain or surface water in the karst area. Combined with information on NO3- /Cl-, the variations of the isotope values of nitrate in the groundwater show a mixing process of multiple sources of nitrate, especially in the summer season. Chemical fertilizer and nitrification of nitrogen-containing organic materials contribute nitrate to suburban groundwater, while the sewage effluents and denitrification mainly control the nitrate distribution in urban groundwater.

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Organic peroxides play a vital role in the formation, evolution, and health impacts of atmospheric aerosols, yet their molecular composition and fate in the particle phase remain poorly understood. Here, we identified, using iodometry-assisted liquid chromatography mass spectrometry, a large suite of isomer-resolved peroxide monomers (C 8−10 H 12−18 O 5−8 ) and dimers (C 15−20 H 22−34 O 5−14 ) in secondary organic aerosol formed from ozonolysis of the most abundant monoterpene (α-pinene). Combining aerosol isothermal evaporation experiments and multilayer kinetic modeling, bulk peroxides were found to undergo rapid particle-phase chemical transformation with an average lifetime of several hours under humid conditions, while the individual peroxides decompose on timescales of half an hour to a few days. Meanwhile, the majority of isomeric peroxides exhibit distinct particle-phase behaviors, highlighting the importance of the characterization of isomer-resolved peroxide reactivity. Furthermore, the reactivity of most peroxides increases with aerosol water content faster in a low relative humidity (RH) range than in a high RH range. Such non-uniform water effects imply a more important role of water as a plasticizer than as a reactant in influencing the peroxide reactivity. The high particle-phase reactivity of organic peroxides and its striking dependence on RH should be considered in atmospheric modeling of their fate and impacts on aerosol chemistry and health effects.

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Atmospheric proteinaceous matter is characterized by ubiquity and potential bioavailability. However, little is known about the origins, secondary production processes, and biogeochemical role of proteinaceous matter in wet deposition. Precipitation samples were collected in suburban Guiyang (southwestern China) over a 1 year period to investigate their chemical components, mainly including dissolved combined amino acids (DCAAs), dissolved free AAs (DFAAs), and nonleachable particulate AAs (PAAs). Glycine was most abundant in the DFAAs, while the dominant species in DCAAs and PAAs was glutamic acid (including deaminated glutamine). The total DCAA, DFAA, and PAA concentrations peaked on average in spring (min. in summer). On average, the contribution of DCAA-nitrogen (median of 3.44%) to dissolved organic nitrogen was 5-fold higher than that of DFAA-nitrogen (median of 0.60%). Correlation analyses of AAs with ozone, nitrogen dioxide, and the quantitative degradation index suggest that DC­(/F)­AAs are linked with both abiotic and biological degradation of proteinaceous matter. Moreover, the high FAA scavenging ratios indicate the presence of postdepositional degradation of atmospheric proteinaceous matter. Further, the positive matrix factorization results suggest that the degradation of atmospheric proteinaceous matter markedly contributes to DCAAs and DFAAs in precipitation. Overall, the results suggest that the secondary processes involved in the degradation of atmospheric proteinaceous matter significantly promote direct bioavailability of AA-nitrogen.

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