Nitrogen's stable isotopes as a proxy to determine ammonium sources in PM using a Monte Carlo's simulation

During cold periods (from November to April) in France, PM concentrations regularly exceed the standard prescribed 50 μg.m-3 limit, which about 50 % in mass is composed on ammonium nitrate (NH4NO3) (Rouïl et al. 2015). The continental scale of such events is a threat to human health and decrease significantly the quality of our environment. The ammonium (NH4 +) is suspected to come from different sources, especially from the agricultural ammonia (NH3) but there is no direct evidence of it. The INACS program aims to investigate the sources proportion in the PM using several methods. This work tries to distinguish and quantify the source apportionment using nitrogen stable isotopes from ammonium nitrate in the PM since previous studies (Felix et al. 2014) highlighted the high level of correlation between the NH4 + concentration and the δ15N of NH4 +. From 2011 to 2013, PM were collected at 7 stations in France at daily sampling rate. δ15N of NH4 + was measured at these sites, ranging from ambient rural to urban background. Besides these isotopic measurements on ambient aerosol samples, the isotopic characterization of different emissions sources (traffic, biomass burning, and agricultural activities) was performed to obtain their specific source isotopic signatures. To the best of our knowledge, this database constitutes probably the most extensive dataset studied so far, with more than 500 δ15N of NH4 + aerosols' observations. In order to take into account the isotopic variability of the sources, a stochastic Monte Carlo's simulation (MCS) was conducted. This approach has in the past demonstrated its applicability for source apportionment method (Sheesley et al. 2011). Applying MCS to our dataset gives a probability distribution function (PDF) for each source and observation day as shown in Fig. 1 for the 2nd March 2013 in a French rural background site..