『Abstract
The characterization of N isotopes in NO3-
is an effective method to determine NO3-
sources and NOx transformation mechanisms.
Xi'an city is located in a semi-arid to arid region and is representative
of most cities in the north and NW of China. Few studies have
been carried out on n isotopes in wet deposition of this region.
In this study, 29 wet deposition samples were collected from Xi'an
city in northwestern China between March 2008 and May 2009 and
analyzed for their volume-weighted NO3-
and NH4+ concentrations and n
isotopic compositions. Volume-weighted mean concentrations were
3.9μmol L-1 for NO3-
and 13.5μmol L-1 for NH4+.
The wet deposition samples showed pronounced seasonal variation
in the N isotopic composition of NO3-
with δ15N-NO3- ranging
from -2.01‰ to +9.2‰ over the course of a year, with a mean value
of +2.6±2.6‰. higher δ15N-NO3-
values were observed in the cool season (November-May, +3.8‰)
than in the warm season (June-October, +1.0‰). In autumn and winter
in particular, the mean δ15N-NO3-
values were clearly different (+0.9‰ and +6.7‰, respectively),.
Various factors are shown to lead to variation in the δ15N-NO3- values in wet deposition. One effect
is changing ratios of N derived from agricultural/soil sources
(low δ15N-NO3-, dominant
in the warm season) and fossil fuel-derived NOx
(high δ15N-NO3-, relatively
more important in the cool season). However, in the cool season
δ15N-NO3- unexpectedly
shows a strong negative correlation with SO2
concentration, which should correlate with fossil fuel combustion
inputs. This may be explained by interactions of N-species with
other atmospheric pollutants. The results also suggest that temperature
may be a significant factor affecting δ15N-NO3-. The NOx reacts
in different ways depending on temperature, leading to a correlation
of δ15N-NO3- with temperature.
Moreover, the effects of dilution cannot be neglected here, as
there is clear seasonal variation in precipitation. When combined
with meteorological parameters, the volume-weighted concentrations
of NO3- and NH4+
and air pollutant precursor data, the δ15N of NO3- in wet deposition can be used as
a reliable tool for the precise identification of NOx
transformation mechanisms in the environment and the fate of NOx emissions.』
1. Introduction
2. Experimental
2.1. Study site location and description
2.2. Sample collection and chemical analysis
2.3. Methodology and air pollution data
2.4. Isotopic analysis
3. Results and discussion
3.1. NO3- and NH4+
volume-weighted concentrations
3.2. nitrogen isotopic composition of nitrate
3.3. Relationships between NO3-
and NH4+ volume-weighted concentrations
and air pollutants and meteorological conditions
3.4. Relationships of δ15N-NO3
to NO3- and NH4+
volume-weighted concentrations and N sources
3.5. Relationships of δ15N-NO3-
values with air pollutants and meteorological conditions
3.5.1. Back trajectory analysis of δ15N-NO3- variations
3.5.2. δ15N-NO3-
value variation caused by NOx transformation
processes
3.5.3. δ15N-NO3-
variation caused by diffusion and dilution effects
3.5.4. δ15N-NO3-
value variation with other factors
4. Conclusions
Acknowledgements
References