Chemistry of Secondary Organic Aerosol Formation from Reactions of Monoterpenes with OH Radicals in the Presence of NO_x

Abstract

The oxidation of volatile organic compounds (VOCs), which are emitted to the atmosphere from natural and anthropogenic sources, leads to the formation of ozone and secondary organic aerosol (SOA) particles that impact air quality and climate. In the study reported here, we investigated the products of the reactions of five biogenic monoterpenes with OH radicals (an important daytime oxidant) under conditions that mimic the chemistry that occurs in polluted air, and developed mechanisms to explain their formation. Experiments were conducted in an environmental chamber, and information on the identity of gas-phase molecular products was obtained using online mass spectrometry, while liquid chromatography and two methods of functional group analysis were used to characterize the SOA composition. The gas-phase products of the reactions were similar to those formed in our previous studies of the reactions of these monoterpenes with NO 3 radicals (an important nighttime oxidant), in that they all contained various combinations of nitrate, carbonyl, hydroxyl, ester, and ether groups. But in spite of this, less SOA was formed in OH/NO x reactions and it was composed of monomers, while SOA formed in NO 3 radical reactions consisted of acetal and hemiacetal oligomers formed by particle-phase accretion reactions. In addition, it appeared that some monomers underwent particle-phase hydrolysis, whereas oligomers did not. These differences are due primarily to the arrangement of hydroxyl, carbonyl, nitrate, and ether groups in the monomers, which can in turn be explained by differences in OH and NO 3 radical reaction mechanisms. The results provide insight into the impact of VOC structure on the amount and composition of SOA formed by atmospheric oxidation, which influence important aerosol properties such as volatility and hygroscopicity.

Analytical methodologies for oxidized organic compounds in the atmosphere

“…83 In addition to these methods, Fourier-transform infrared spectroscopy (FTIR) has also provided similar qualitative information about functional group compositions. 151–155…”

Differences in Secondary Organic Aerosol Formation from α-Pinene Photooxidation in a Chamber with Purified Air and Ambient Air as Matrices: Preliminary Results

“…A previous study reported that α-pinene has the highest emissions among the monoterpenes, with an annual global emission of 27.2 Tg y −1 [10]. To assess the contribution of α-pinene to SOA formation, numerous laboratory experiments have been conducted on α-pinene photooxidation [11][12][13][14][15]. α-Pinene can react with oxidants such as hydroxyl radicals (OH), nitrate radicals (NO 3 ), and ozone (O 3 ) in the atmosphere to form low molecular weight carbonyls, such as alcohols and carboxylic acids, as well as high molecular weight dimers and other oxidation products, which are essential contributors to nucleation and SOA formation [13][14][15].…”

“…Previous studies have demonstrated that the formation of SOA from the photooxidation of α-pinene is influenced by temperature, relative humidity (RH), seed aerosols, nitrogen oxides (NOx), and sulfur dioxide (SO 2 ) [15][16][17]. NOx and SO 2 are prevalent anthropogenic pollutants in the atmosphere.…”

“…In regions with high levels of BVOC emissions, BVOC oxidation might be affected by NOx and SO 2 , which interfere with SOA formation [18]. NOx changes the fate of the RO 2 radical formed during α-pinene oxidation, inhibits the formation of low volatile organic compounds such as dimers, and promotes the formation of high volatile products, affecting SOA formation [15,19,20]. SO 2 could be oxidized to sulfuric acid, which promotes SOA formation through acid catalysis [21].…”

Advances in Atmospheric Chemical and Physical Processes

“…Papers in this virtual special issue include studies of the initiation reactions of VOC species by OH, Cl, − O 3 , , NO 3 , and HCO, as well as by direct photolysis . Chamber studies, in which the products of a VOC oxidation process are identified and quantified, are also prevalent; two studies quantify the formation of secondary organic aerosol (SOA) in the reaction of OH or NO 3 with monoterpenes, , while a combined experimental/theoretical study shows that methylmethanimine oxidation via the OH radical is unlikely to compete with atmospheric hydrolysis of this species . The initial products of oxidant/VOC reactions are often organic peroxy radicals, and studies of these species are represented in this issue.…”