Formation of p-cymene from the Oxidation of Biogenic Sources during NACHTT 2011

A comprehensive suite of vertically-resolved volatile organic compound (VOC) measurements were conducted at the Bolder Atmospheric Observatory (BAO) in Erie, Colorado from 18 February to 13 March 2011 as part of the Nitrogen, Aerosol Composition, and Halogens on a tall tower (NACHTT) campaign. The main purpose of this experiment was to investigate and quantify the contribution of monoterpene oxidation to the secondary production of aromatic hydrocarbons. A limited number of studies have been performed regarding this area of research, but recent laboratory experiments have suggested that BVOC oxidation should be considered as an important source to the production of aromatic hydrocarbons. Gratien et al. (2011) established that p-cymene is a product of the oxidation of α-pinene in air and its yield is greater at higher RH and in the presence of acids. The BAO tower was an ideal location to conduct research because of the potential gas phase precursor sources and the comprehensive suite of measurements during NACHTT. The vertically-resolved measurements have provided insight on biogenic emissions and their subsequent chemical transformations. Two different scenarios were observed in the individual vertical profiles. Scenario 1 suggests normal conditions on 25 February 2011. The vertical profiles increase with height from 50-150 meters, but decrease from 150-250 meters. Scenario 2 can be observed on 6 March 2011; the vertical profiles show an increase in p-cymene and a decrease in α-pinene and other aromatics indicating the formation of p-cymene from chemical processing in the atmosphere. Both scenarios have an acidic atmosphere, but the colder temperatures and higher relative humidity on 25 February 2011 is facilitating particle formation as opposed to the heterogeneous processing of α-pinene to form p-cymene. The drier air and ample surface area observed on 6 March 2011 allows the heterogeneous reactions to take place, which facilitates the formation of p-cymene. 1.0 Introduction Volatile organic compounds (VOCs) are emitted into the atmosphere from both biogenic and anthropogenic sources [1]. Emissions of VOCs from biogenic sources are approximately1150 Tg/year-1 carbon per year and anthropogenic sources constitute about 140 Tg per year [2,3]. Biogenic sources include monoterpenes, which are an important class of biogenic volatile organic compounds (BVOCs) emitted from vegetation [4,5]. Other biogenic sources include isoprene, sesquiterpenes, and oxygenated hydrocarbons [3,6]. Among the monterpenes are the gases α-pinene, p-cymene, and β-pinene, with α-pinene being the most abundant monterpene [3,7]. In the troposphere, these compounds react with hydroxyl radicals (OH), ozone (O3), and nitrate radicals (NO3) to form an array of oxidation products [8]. Globally, monoterpenes constitute 10-50% of BVOC emissions from vegetation with approximately ~30-130 Tg/year [3,9]. It has been widely assumed that aromatic hydrocarbons only arise from anthropogenic sources; however, studies have shown that they have also been recently associated with biogenic emissions [10,11,12,13,14]. The compounds p-cymene and toluene are not only released from motor vehicle exhaust, solvent evaporation, and industrial wastewater, but direct emissions from