**1. Introduction**

Two major atmospheric contaminants are airborne particulate matter (PM) and nitrogen oxides (NOx). PM from both natural and anthropogenic sources affect human health and air quality and, although they are size-regulated by national and international environmental standards, its control remains a challenge. PM has been associated with cardiovascular and respiratory diseases and life-expectancy reduction [1]. Because of its small size, PM2.5 (PM < 2.5 mm in diameter) enter deep into the lungs and produce short- and long-time effects on the respiratory system,

generating oxidative stress, systemic inflammation, and neuroinflammation, among other ailments [1, 2]. In contrast to PM10 (PM < 10 mm in diameter), which is usually composed of resuspended dust from unpaved roads, industrial emissions, agricultural activity, pollen, and bacteria, PM2.5 is formed by combustion processes and the accretion of very small particles and/or condensation of gases on the surfaces of small particles [3]. PM2.5 and PM10 contain numerous chemical compounds, such as carbon, sulfates, nitrates, and ammonium, among others, depending on the emission location. Primary particles are emitted directly from sources and can be formed through gas-to-particle conversion known as secondary particles. The secondary inorganic PM fraction is mainly composed of ammonium nitrate and ammonium sulfate whose precursors are emitted as NOx, SO2**,** and NH3 and then converted to solids through chemical reactions (e.g., [4]).

In turn, nitrogen oxides (NOx), especially nitrogen dioxide (NO2), are strong atmospheric oxidants that enhance low atmospheric visibility, play a role in climate change, and are precursors of secondary contaminants [3]. As a result of anthropogenic activities, atmospheric nitrogen species and fluxes in urban environments far outweigh biogenic sources. For example, over 80% of nitrogen oxides (NOx) are originated by the combustion of fossil fuels for transportation, electricity generation, and industrial activities. This contrasts with the 4% emissions by agricultural and biogenic sources [5]. Other nitrogen species also occur as gases or in particles in the atmosphere. NH4 + salts contribute to the long-range transport of acidic pollutants due to day-scale atmospheric lifetime and after deposition, they can contribute to forest decline and soil acidification. Ammonia and nitric acid are the main precursors of nitrate aerosols. Nitric acid is produced in the atmosphere as an additional reaction product of NO2, from fossil fuel combustion, biomass burning, or from soil. NO2 can oxidize and also react to form HNO3 by pathways relied on the formation of NO3 − .

Mexico City Metropolitan Area is characterized by being one of the largest megacities in the world. In addition to its more than 20 million inhabitants and a fleet of over 6 million vehicles, the air quality of the metropolitan area (which includes Mexico City and adjacent municipalities of the States of Hidalgo and Mexico) is affected by industrial and vehicular activity in the north and northeast sectors of the city and by biomass burning from a nearby agricultural activity, which is then transported into the urban area (e.g., [6]). The city lies on a high-altitude plateau (2240 m above sea level) and, except for the NE and SE sectors, is surrounded by mountain chains, which preclude an efficient pollutant dispersion (**Figure 1**). As such, in 2010, the PM2.5 annual average concentration of 25 μg/m3 was over two times higher than the recommended annual average of 10 μg/m3 established by the World Health Organization (WHO), which clearly underscores the difficulty to reach the PM regulations [7]. Likewise, in 2010, the NO2 annual average of 55 μg/m3 was clearly greater than WHO's 40 μg/m3 . As a result of control enforcement policies during 2004–2008, PM2.5 average concentration decreased by 27% [7], and in 2014, there was a decrease in PM of 11% with respect to 2013. Control measures, such as banning lead from gasoline, the use of natural gas, and mandatory vehicle emission inspection, have consistently decreased PM emissions.

Some studies on Mexico City's atmosphere have reported that PM2.5 is mainly composed of 50% carbonaceous aerosol, suggesting an origin from incomplete combustion of fossil fuels and biomass burning, followed by sulfates, nitrates, ammonium, and geological material among others [8, 9]. The main secondary inorganic aerosol components in PM2.5 mainly occurred as ammonium sulfate [(NH4)2SO4] and ammonium nitrate [NH4NO3] due to the neutralization of atmospheric acids with gaseous

*The Use of Stable Isotopes to Identify Carbon and Nitrogen Sources in Mexico City PM2.5… DOI: http://dx.doi.org/10.5772/intechopen.107914*

**Figure 1.** *Study zone where depicting XAL and MER sampling sites in Mexico City Metropolitan Area.*

ammonia from agriculture, landfills, industries, biomass burning, and motor vehicles. In Mexico City, measurements made inside roadway tunnels showed a contribution of 8% of NH4 + emissions from on-road vehicles [10].

Stable nitrogen (15N and 14N) isotopes are valuable tools to trace the sources and transformations of airborne PM. Examples include tracing the sources of primary and secondary nitrogen in PM10 [11], nitrate accumulation in PM2.5 [12], and NOx contribution in nitrogen dry deposition [13]. On the other hand, stable carbon isotopes (13C and 12C) have been used in urban atmospheres to trace diesel and gasoline combustion, dust, soil and industrial emissions since these have distinct isotopic signatures [14–16]. Here, we use stable carbon isotopes and stable nitrogen isotopes of PM2.5 from a data set to elucidate sources and chemical processes between the nitrogen in particles and several species of atmospheric nitrogen at two contrasting sites during three dry season periods in Mexico City. The sources of PM2.5 are further studied using stable carbon isotopes. This chapter provides a baseline for δ 15N in Mexico City atmospheric particles as no previous data on N isotopes have been previously published.
