Synthesis of carboxylic acid and dimer ester surrogates to constrain the abundance and distribution of molecular products in α-pinene and β-pinene secondary organic aerosol

Abstract

Liq. chromatog./neg. electrospray ionization mass spectrometry [LC/(-)ESI-MS] is routinely employed to characterize the identity and abundance of mol. products in secondary org. aerosol (SOA) derived from monoterpene oxidn. Due to a lack of authentic stds., however, com. terpenoic acids (e.g., cis-pinonic acid) are typically used as surrogates to quantify both monomeric and dimeric SOA constituents. Here, we synthesize a series of enantiopure, pinene-derived carboxylic acid and dimer ester homologues. We find that the (-)ESI efficiencies of the dimer esters are 19 to 36 times higher than that of cis-pinonic acid, demonstrating that the mass contribution of dimers to monoterpene SOA has been significantly overestimated in past studies. Using the measured (-)ESI efficiencies of the carboxylic acids and dimer esters as more representative surrogates, we det. that mol. products measureable by LC/(-)ESI-MS account for only 21.8 ± 2.6% and 18.9 ± 3.2% of the mass of SOA formed from ozonolysis of α-pinene and β-pinene, resp. The 28- 36 identified monomers (C₇₋₁₀H₁₀₋₁₈O₃₋₆) constitute 15.6-20.5% of total SOA mass, whereas only 1.3-3.3% of the SOA mass is attributable to the 46-62 identified dimers (C₁₅₋₁₉H₂₄₋₃₂O₄₋₁₁). The distribution of identified α-pinene and β-pinene SOA mol. products is examd. as a function of carbon no. (n_C), av. carbon oxidn. state (OS_C), and volatility (C*). The obsd. order of magnitude difference in (-)ESI efficiency between monomers and dimers is expected to be broadly applicable to other biogenic and anthropogenic SOA systems analyzed via (-) or (+) LC/ESI-MS, and demonstrates that the use of unrepresentative surrogates can lead to substantial systematic errors in quant. LC/ESI-MS analyses of SOA.