Environ. Sci. Technol. 2009, 43, 1884–1889
Effect of Relative Humidity on Gas/Particle Partitioning and Aerosol Mass Yield in the Photooxidation of p-Xylene ROBERT M. HEALY, BRICE TEMIME,† KRISTINA KUPROVSKYTE, AND JOHN C. WENGER* Department of Chemistry and Environmental Research Institute, University College Cork, Cork, Ireland
Received August 28, 2008. Revised manuscript received January 21, 2009. Accepted January 29, 2009.
The formation of secondary organic aerosol and gas/particle partitioning of carbonyl products from the photooxidation of p-xylene has been investigated as a function of relative humidity. Experiments were performed in an atmospheric simulation chamber at atmospheric pressure and ambient temperature in the presence of NOx. Aerosol yields increased by a factor of approximately two over the relative humidity range 5-75% and were found to correlate with initial water vapor concentration and hydroxyl radical (OH) concentration. The results indicate that an increase in relative humidity results in higher levels of HONO formation in the chamber which leads to increased OH concentration, a faster p-xylene decay rate, and higher aerosol mass yields. A recently developed denuder-filter sampling technique was used to investigate the gas/ particle partitioning behavior of the carbonyl photooxidation products. The identified products accounted for up to 18% of the aerosol mass formed. Dicarbonyls with at least one aldehyde functionality (e.g., glyoxal and methylglyoxal) exhibited gas/ particle partitioning coefficients several orders of magnitude higher than expected from absorptive partitioning theory, suggesting that reactive uptake and particle phase reactions are important processes for aerosol formation from these species. Experimental gas/particle partitioning coefficients were also found to be dependent on relative humidity, with every dicarbonyl exhibiting much lower values when the relative humidity was increased from 50% to 75%.
Introduction Aromatic hydrocarbons are a major class of air pollutants, typically accounting for 20-30% of the total volatile organic compounds detected in the urban atmosphere (1). The atmospheric oxidation of aromatic hydrocarbons is primarily initiated by reaction with the hydroxyl (OH) radical and leads to the formation of a range of secondary pollutants including ozone, nitrates, and secondary organic aerosol (SOA), which impact on human health and climate. As a result, a large number of laboratory studies have been performed on the atmospheric degradation of aromatic hydrocarbons in order to identify the oxidation products and investigate SOA * Corresponding author tel: +353 21 4902454; fax: +353 21 4903014; e-mail:
[email protected]. † Present address: Laboratoire Chimie et Environnement, Universite´ de Provence, Marseille, France. 1884
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formation (1-3). Despite these efforts, many aspects of the oxidation chemistry of the aromatics, and in particular the mechanisms for SOA formation, remain poorly understood. Laboratory studies of SOA formation from the photooxidation of aromatic hydrocarbons have mainly focused on determining the aerosol yield under a variety of reaction conditions (3). The aerosol yields have been shown to depend strongly on the amount of nitrogen oxides (NOx) in the system (4-7) and more recently on the oxidation rate of the precursor, which is largely controlled by OH concentration (7-10). Identification of the chemical species and processes responsible for SOA formation has proven to be a major challenge and there have only been a small number of studies on the composition of SOA produced from aromatic hydrocarbons. Forstner et al. (11) identified numerous compounds in the SOA produced from toluene, m- and p-xylene, while the particle-phase products resulting from the photooxidation of toluene have been reported on several occasions (12-15). More recently, Healy et al. identified a range of carbonyl compounds present in the SOA produced from the photooxidation of 1,3,5-trimethylbenzene (16). Although a large number of molecular products were identified in these studies, they only accounted for around 10-30% of the overall aerosol mass yield. Many of the species detected in SOA are first or second generation oxidation products that have simply undergone absorptive partitioning into the particle phase. Recent experimental evidence, however, indicates that a significant proportion of the mass of aromatic SOA also consists of oligomers or polymeric material formed as a result of heterogeneous or particlephase reactions of the oxidation products (17). The reactive uptake of carbonyl compounds, and in particular, the dicarbonyls glyoxal and methylglyoxal, is believed to play an important role in the formation of oligomers and growth of SOA in general (17-22). Reactive uptake of glyoxal onto particulate matter has also been shown to be dependent on water content (20, 21). The objective of this study was to investigate the effect of relative humidity on aerosol mass yield and gas/particle partitioning of the carbonyl products from the photooxidation of p-xylene. A series of simulation chamber experiments has been performed to obtain experimental gas/particle partitioning coefficients for the carbonyls and to provide insights into the oxidation products involved in SOA formation and their contribution to the chemical composition of the particle phase.
Experimental Section The p-xylene photooxidation experiments were performed in a 6500 L simulation chamber described in detail elsewhere (15, 16). The chamber was humidified by flowing purified air through the headspace of an impinger containing heated deionized (Milli-Q) water. p-Xylene and the hydroxyl radical precursor (NOx or HONO) were added to the chamber in a flow of purified air. HONO was generated by the dropwise addition of sulfuric acid (30% in water) to a 1% aqueous solution of sodium nitrite. Photooxidation was carried out for 6-8 h using 12 Philips TL05 lamps and 12 TL12 lamps. The concentrations of p-xylene and NOx were regularly monitored by GC-FID and a NOx analyzer (Thermo model 42i), respectively. Ozone was measured with a UV photometric O3 analyzer (Thermo model 49i). The formation and evolution of particles in the range 10-470 nm were monitored using a scanning mobility particle sizer (TSI model 3034) and temperature and relative humidity were measured using a dewpoint meter (Vaisala DM70). 10.1021/es802404z CCC: $40.75
2009 American Chemical Society
Published on Web 02/17/2009
TABLE 1. Reaction Parameters for the p-Xylene Photooxidation Experiments experiment
[HC]0 (ppbV)
[NO]0 (ppbV)
[NO2]0 (ppbV)
relative humidity (%)
average OH concentrationa (molecule cm-3)
∆[HC]b (µg m-3)
XYL_NOx_1 XYL_NOx_2 XYL_NOx_3 XYL_NOx_4 XYL_NOx_5 XYL_NOx_6 XYL_NOx_7 XYL_HONO_1
3001 3038 2951 2978 2992 2994 2983 2956
722 819 610 628 749 766 724 440
79 92 41 68 80 92 91 556e
