Identification of Organic Acids in Secondary Organic Aerosol and the Corresponding Gas Phase from Chamber Experiments

IDENTIFICATION OF ORGANIC ACIDS IN SECONDARY ORGANIC AEROSOL AND THE CORRESPONDING GAS PHASE FROM CHAMBER EXPERIMENTS

R. Fisseha, J. Dommen, M. Sax (ETH Zürich and PSI), D. Paulsen, M. Kalberer (ETHZ), R. Maurer (Dionex), F. Höfler (Dionex), E. Weingartner, U. Baltensperger

Organic acids in the gas and aerosol phase from photo oxidation of 1,3,5-trimethylbenzene (TMB) in the presence of 300 ppb propene and 300ppb NOx in smog chamber experiments were determined using a wet effluent diffusion denuder/aerosol collector (WEDD/AC) coupled to ion chromatography (IC). Behind the IC, the samples were collected using a fraction collector, for identification of unresolved / unidentified organic acids with ion chromatography – mass spectrometry (IC-MS). In total, 20 organic acids were found with MS, of which 10 were identified. At the maximum aerosol concentration, organic acids comprised 20- 45% of the total aerosol mass.

1 INTRODUCTION Online IC Offline IC-MS Chamber experiments are now widely used to study Acetic acid Lactic Acid Oxalic Mass 171 the possible mechanisms of SOA formation. Kalberer acid Da. et al. [1] showed in chamber experiments that oli- gomerization of organic compounds occurs during Formic acid Pyruvic Acid Mass Mass 178 SOA formation and these oligomers make up a large 114 Da. Da. fraction of total SOA. Tolocka et al. [2] also showed the formation of oligomers in secondary organic aero- Pyruvic Succinic Acid Mass Mass 188 sol. Both papers indicated a substantial contribution acid 122 Da. Da. by organic acids. Detailed information about the gas Oxalic acid Malonic acid Mass Mass 190 and aerosol phase concentration of these organic 129 Da. Da. acids is therefore necessary to increase our under- standing of SOA formation. However, the measure- Malic acid Mass Citric acid ment of organic acids is a demanding task, as the 132 Da. analytical method must provide high time resolution Methyl maleic Mass Mass 234 and low detection limits. This work demonstrates that acid 166 Da. Da. direct coupling of ion chromatography-mass spectrometry (IC-MS) to wet effluent diffusion denuder/ Table 1: Organic acids identified using online IC and aerosol collector (WEDD/AC) is feasible and a suit- offline IC-MS measurement. able technique for measurement of organic acids in Total organic acid in the chamber and field measurements. 250 aerosol Total aerosol mass concentration 200 )

2 EXPERIMENTAL 3 g/m µ µ Samples from the PSI indoor smog chamber facility µ µ 150 were taken at a flow rate of 1.5l/min using the WEDD/AC, which is a homebuilt instrument for sam- 100 Concentration ( Concentration pling water-soluble gas phase and aerosol com- 50 pounds. The samples were then analyzed using ion chromatography (IC) with conductivity detection. The 0 0:00 1:00 2:00 3:00 4:00 5:00 6:00 7:00 8:00 9:00 effluent from the conductivity detector was collected 10:00 for later IC-MS analysis using a fraction collector. The TM Time after light is on MS used in this experiment was a MSQ standard Fig. 1: Comparison of the sum of organic acids mass spectrometer (Dionex). measured in the aerosol ( ) with the aerosol mass concentration derived from measured sub micrometer 3 RESULTS number sized distributions (SMPS) with the assump- -3 From the online IC measurement, only 4 organic acids tion of a particle density of 1.38 g m ( ). were identified. There were, however, unidentified and co-eluted peaks on the IC chromatograms. Later with 4 ACKNOWLEDGEMENT the offline IC-MS measurement, 16 organic acids This work was supported by the Swiss National Sci- were identified in addition to the ones that were identi- ence Foundation. fied with the online IC measurement (Table 1). For some masses, the corresponding organic acid is not 5 REFERENCES yet known; however, the number of acid groups was derived from their retention time in the IC. At the [1] M. Kalberer et al., Science 303 , 1659 (2004). maximum aerosol concentration the organic acids in [2] M. P. Tolocka et al., Environ. Sci. Technol. 38 , the aerosol contributed 25% of the total organic acids 1428 (2004). formed in the chamber and 20-45% of the aerosol mass in the sub micrometer size range (Figure 1) [3]. [3] R. Fisseha et al., Anal. Chem. 76 , 6535 (2004).