Abstract
Human activities have been changing deposition rate of the atmospheric
reactive N over the last decades. The understanding of the process that
rules the accumulation and deposition of reactive nitrogen in the
environment still faces major gaps mainly in regions with lack of data
as South America. In this work, we evaluated the atmospheric dry
deposition of reactive nitrogen (Nr) using concentration measurements of
different chemical species, in six sampling sites with different
environmental characteristics in the state of São Paulo, Brazil, for a
sampling period of approximately 24 months (2015-2017). We used the
sampling system DELTA (Sampler for Atmospheric Sampling of Long Term),
and ion chromatography method to determine the concentration of gaseous
(NH3 and HNO3), and particulate (NH4+ and NO3-) chemical species in the
atmosphere. The N-NH3 (gaseous) was the dominant form of Nr in the
atmosphere at all sampling sites. The highest N-NH3 mean value was found
near the urban area of the municipality of São Paulo (SP, 1.58 ± 0.73
μgN m-3) and the lowest mean value in the most eastern sampling site
(0.26 ± 0.26 μgN m-3). The high values in the SP sampling site is
related to the intense traffic in the metropolitan area and, also to
waste management, industrial NH3, and human emissions as observed in
other urban areas around the world. The particulate N-NH4+ is the second
most common form of Nr in the São Paulo state atmosphere. The other
forms of Nr in the atmosphere, the gaseous HNO3 and particulate NO3,
represent only about 10% of the total Nr in the atmosphere each. The
total gaseous Nr concentration was a factor of 1.7 larger than
particulate Nr. The Nr deposition varied throughout the years. We did
not observe a pattern of variation linked to meteorological
characteristics of dry and wet season, as observed in other regions of
the globe. However, we found good correlation with wind speed higher
than 3.5 m s-1 and humidity. Our results provide the first spatial
analysis of Nr deposition using in situ data in a Latin American region
and will contribute to the understanding of nitrogen balance and to
improve Nr deposition modelling approaches. This study was supported by
the project Nitrogen cycling in Latin America: drivers, impacts and
vulnerabilities (Nnet, IAI/CRN3005 and FAPESP 2012/06416-1), PCI Program
of the MCTIC, and collaborators.