Isotopic composition of nitrate and ammonium in São Paulo wet deposition

Date

2021-12

Authors

Chapa, Adrianna

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Abstract

Humans have altered the nitrogen cycle by emitting excess reactive nitrogen into the atmosphere causing ammonia (NH3) and nitrogen oxides (NO x) concentrations to increase exponentially over the last century. This excess nitrogen causes negative environmental and health effects by contributing to acid rain, decreased air quality, soil acidification, eutrophication, and a change in biodiversity. To better understand the nitrogen cycle, emission sources, and mitigation strategies, regional emissions of NH3 and NOx should be quantified. This study measured the stable nitrogen and oxygen isotope composition of nitrate (NO -) and ammonium (NH +) in wet deposition to estimate NH3 and NOx emission sources in the Ribeirão Preto region of São Paulo, Brazil from October 2018 through February 2020. The average NO - and N H + concentrations throughout the sampling period were 13.1 ± 10.0 μM and 31.0 ± 24.2 μM, respectively. The average δ15N-NO -, δ18O-NO - and δ15N-NH + values were -2.2 ± 4.5‰, 55.7 ± 7.8‰ and -4.0 ± 8.8‰, respectively. The δ15N-NO - values were corrected for isotope f ractionation which occurs during the NOx oxidation to HNO 3. The δ15N-NO - and δ15N-NH + values in wet deposition and δ15N values of NOx and NH 3 emission sources were applied to the Stable Isotope Analysis in R (SIAR) mixing model to estimate source contributions. The NOx emissions in Ribeirão Preto during the dry season were primarily from biogenic (9.8 ± 1.4%), biomass burning (49.8 ± 1.8%) and vehicle sources (40.4 ± 2.5%). During the wet season, percent contributions for biogenic and vehicular sources were 18.6 ± 5.3% and 81.5 ± 5.3%, respectively. NH3 emissions in the dry season were primarily from agriculture (i.e. livestock waste, fertilizers) (26.1 ± 9.5%, 26.4 ± 14.0%), vehicles (21.6 ± 8.2%), and biomass burning sources (25.9 ± 18.8%). In the wet season, livestock waste, fertilizers, and vehicles contributed 26.6 ± 14.4%, 16.2 ± 11.2% and 57.2 ± 23.3% of the NH3 emissions, respectively. Lower summer δ18O-NO3- values (55.6 ± 7.5‰) indicate NOx oxidation predominately through the OH radical pathway while increased winter δ18O-NO3- values (69.9 ± 1.7‰) imply increased influence of the ozone oxidation pathway. Results of this study aid in understanding the NOx and NH3 emission sources and atmospheric processes affecting air quality in São Paulo, Brazil.

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