Limited Secondary Organic Aerosol Production from Acyclic Oxygenated Volatile Chemical Products

Abstract

Volatile chemical products (VCPs) have recently been identified as potentially important unconventional sources of secondary organic aerosol (SOA), in part due to the mitigation of conventional emissions such as vehicle exhaust. Here, we report measurements of SOA production in an oxidation flow reactor from a series of common VCPs containing oxygenated functional groups and at least one oxygen within the molecular backbone. These include two oxygenated aromatic species (phenoxyethanol and 1-phenoxy-2-propanol), two esters (butyl butyrate and butyl acetate), and four glycol ethers (carbitol, methyl carbitol, butyl carbitol, and hexyl carbitol). We measured gas- and particle-phase products with a suite of mass spectrometers and particle-sizing instruments. Only the aromatic VCPs produce SOA with substantial yields. For the acyclic VCPs, ether and ester functionality promotes fragmentation and hinders autoxidation, whereas aromatic rings drive SOA formation in spite of the presence of ether groups. Therefore, our results suggest that a potential strategy to reduce urban SOA from VCPs would be to reformulate consumer products to include less oxygenated aromatic compounds.

Keywords: NOx; PM2.5; aerosol mass yield; oxidation flow reactor; secondary organic aerosol; volatile chemical product.

Figure 1

(Top) Gas-phase product distribution for oxidation of phenoxypropanol (n_C = 9) and (bottom) hexyl carbitol (n_C = 10) measured using an I^– chemical ionization mass spectrometer. Panels on the left show product peak area (as symbol area) vs carbon and oxygen number (n_C, n_O). Panels on the right show product peak area vs volatility (log c⁰). Volatility classes (extremely, low, semivolatile, and intermediate volatility organic compounds) are shown as colored regions. Calibrated parameterizations defining the ranges differ for aromatics such as phenoxypropanol and aliphatics such as hexyl carbitol. The points above the black contour line in the left panel and to the left of the black contour line in the right panel correspond to lower volatility products that easily enter the particle phase; these are in the LVOC and ELVOC volatility classes, indicating significant aerosol formation.

Figure 2

Average f₄₃ vs f₄₄ plot for SOA formed from aromatic VCPs under high- and low-NO_x conditions. For both species, the extent of oxidation (indicated by a higher ratio of f₄₄ to f₄₃) was greater under low-NO_x conditions. The red and blue dashed lines represent the region in which ambient oxidized OA typically falls. Representative error bars (1σ) are shown for low NO_x phenoxypropanol.

Figure 3

SOA mass spectra for phenoxypropanol under low-NO_x (top) and high-NO_x (bottom) conditions. The graphs contain the fraction of signal vs fragment size (m/z) and are colored based on the chemical family. Purple fragments represent oxygenated species with either one oxygen (dark) or greater than one oxygen (bright) while blue species are nitrogen containing. The graphs indicate a low amount of nitrogen-containing species overall and a lower oxygenation for high NO_x products.

Figure 4

Difference between the high NO_x fraction and low NO_x fraction of SOA from phenoxypropanol, colored by family. Positive numbers indicate higher fractions under high-NO_x conditions, and negative numbers indicate higher fractions under low NO_x. Embedded in the graph is the calculated angle between high and low NO_x vectors. Angles closer to 90° show a substantial difference between high and low NO_x, whereas angles closer to 0° show no difference. The signal for the mass fragments greater than 58 (the red vertical dashed line) has been multiplied by a factor of 10.