Sources and Source Processes of Organic Nitrogen Aerosols in the Atmosphere

Physical and chemical characteristics of organic aerosols sampled in the Cache Valley using an AMS are presented. Qualitatively, at least four distinct sources appear to contribute to the formation process of organic aerosol. Peaks representing these different contributions to aerosol are mass-to-charge (m/z) 57; a peak that is usually associated with hydrocarbon-like species; m/z 44 usually associated with oxygenated organic carbon, m/z 58 probably representing nitrogen-containing organic aerosol, and m/z 60 a key fragment associated with wood smoke combustion. Polycyclic aromatic hydrocarbons (PAHs) were also observed in the mass-to-charge range of 190-300 and perhaps lower. Mass distributions of the organic aerosols peaked in the accumulation mode size range with one exception. Distributions obtained using m/z 57 also appears in the ultra fine mode (~150 nm) and probably represents primary emissions from vehicles. Principal component analysis was applied to the data. Application of principal component analysis to the data resolves two meaningful components. Attempts to resolve m/z 44 and m/z 58 were not successful, perhaps because m/z 44 may be also produced by species with m/z 58. The two components are closely associated with m/z 44 and 57, the relative contribution by mass being 65% and 35% for components 1 and 2, respectively. Component 1 correlated well with the nitrate concentration, and appears largely due to oxygenated organic aerosol. This component is probably representative of secondary 29 reaction chemistry. Component 2 appears hydrocarbon-like, implying a source associated with primary organic aerosols, however it correlated weakly with carbon monoxide and nitrogen oxides associated with combustion of fossil fuel. There appeared to be no influence of temperature and relative humidity on the formation processes of the organic aerosol. Introduction Fine particulate matter (PM) is ubiquitous in the troposphere and the role that it plays in health, visibility, and global climate change has been a subject of interest by many researchers in the recent years (Jacobson et al., 2000; Seinfeld and Pankow, 2003; Kanakidou et al., 2005). Fine PM scatter solar radiation thus limiting visibility (Eldering and Cass, 1996; Watson, 2002). When inhaled, they can give rise to inflammation of respiratory tissues, heart complications, low birth weight, and even death (Dockery et al., 1993; Long et al., 2002; Parker et al., 2005). On a global scale, fine particles scatter or absorb incoming solar radiation and also modify cloud properties such as reflectivity and lifetime. These effects, the direct and indirect radiative forcing, can alter climate (Ravishankara, 1997; Jacobson et al., 2000; IPCC, 2007) Depending on the location, the organic fraction of particles accounts for ~10%70% of the total fine PM mass concentration (Murphy et al., 1998, Jacobson et al., 2000). The organic fraction is composed of a complex mixture made up of many individual compounds (Rudich, 2003) that exhibit a wide range of physical, chemical and thermodynamics characteristics (Saxena and Hildemann, 1996). The study of organic aerosols is also complicated due to lack of a single analytical method that can be used to 30 study the wide array of compounds (Turpin et al., 2000). Knowledge of the characteristics of these particles together with their sources is needed in order to fully understand the role that they play in local, regional and global air quality (McMurry, 2004). Organic aerosol sources can broadly be classified as primary and secondary (Jacobson et al., 2000; Kanakidou et al., 2005). Primary sources are those in which particles are emitted directly into the atmosphere (for example from vehicle exhaust) while secondary sources are those in which particles are formed when precursor volatile organic compounds react in the atmosphere to form low volatility products (e.g. Odum et al., 1996). These can either nucleate to form new particles or coat the surface of preexisting particles (Rudich, 2003). Primary and secondary organic aerosols can be emitted naturally from sources such as vegetation and sea sprays (Seinfeld and Pankow, 2003) or they can be from anthropogenic sources such as fossil fuel and biomass combustion (Jacobson et al., 2000). The relative contribution of primary and secondary organic aerosols to the total organic fraction has proven to be a challenge to many researchers, mainly because there is no direct chemical analysis method for studying these source and formation processes. Indirect methods have been employed for these purposes such as cluster analysis (Marcolli et al., 2006), custom principal component analysis (Zhang et al., 2005a) and positive matrix factorization (Lanz et al., 2007). Cache Valley is located in Northern Utah where occasional severe winter inversions occur that may last for several days that leads to a large accumulation of fine PM close to the surface. During January and February of 2004, Cache Valley experienced 31 one of the worst air quality problems in the country (Malek et al., 2006). This was due to prolonged temperature inversion periods facilitated by the valley geography, leading to several exceedances of the standard EPA regulatory limits (Silva et al., 2007). Mangelson et al. (1997) had previously conducted winter-time studies in this area using off-line filter based methods and they identified ammonium nitrate as the major component of the fine PM, with the organic aerosol fraction being the second major contributor. With data acquired by an AMS, this chapter reports on the characteristics of organic aerosols in Logan, Utah during the winter inversions of 2004.