Potential dual effect of anthropogenic emissions on the formation of biogenic secondary organic aerosol (BSOA)

Abstract

Abstract. The fraction of gasoline direct-injection (GDI) vehicles\ncomprising the total vehicle pool is projected to increase in the future.\nHowever, thorough knowledge about the influence of GDI engines on important\natmospheric chemistry processes is missing – namely, their contribution to\nsecondary organic aerosol (SOA) precursor emissions, contribution to SOA formation, and\npotential role in biogenic–anthropogenic interactions. The objectives of\nthis study were to (1)\xa0characterize emissions from modern GDI vehicles and\ninvestigate their role in SOA formation chemistry and (2)\xa0investigate\nbiogenic–anthropogenic interactions related to SOA formation from a mixture\nof GDI-vehicle emissions and a model biogenic compound, α -pinene.\nSpecifically, we studied SOA formation from modern GDI-vehicle emissions\nduring the constant-load driving. In this study we show that SOA formation\nfrom GDI-vehicle emissions was observed in each experiment. Volatile organic compounds (VOCs) measured\nwith the proton-transfer-reaction time-of-flight\nmass spectrometer (PTR-ToF-MS) could account for 19\u2009%–42\u2009% of total SOA mass generated in each experiment. This suggests that there were lower-volatility\nintermediate VOCs (IVOCs) and semi-volatile organic compounds (SVOCs) in the GDI-vehicle exhaust that likely contributed to SOA production but were not detected with the\ninstrumentation used in this study. This study also demonstrates that two distinct mechanisms caused by anthropogenic emissions suppress α -pinene SOA mass yield. The first suppressing effect was the presence of NOx . This mechanism is consistent with previous reports demonstrating suppression of biogenic SOA formation in the presence of anthropogenic emissions. Our results indicate a possible second suppressing effect, and we suggest that the presence of anthropogenic gas-phase species may have suppressed biogenic SOA formation by alterations to the gas-phase chemistry of α -pinene. This hypothesized change in oxidation pathways led to the formation of α -pinene oxidation products that most likely did not have vapor pressures low enough to partition into the particle phase. Overall, the presence of gasoline-vehicle exhaust caused a more than 50\u2009% suppression in α -pinene SOA mass yield compared to the α -pinene SOA mass yield measured in the absence of any anthropogenic influence.