Evidence of 1991-2013 decrease of biogenic secondary organic aerosol in response to SO₂ emission controls

Air quality policy to decrease fine particulate matter mass concentrations (PM_2.5) in the US has mainly targeted sulfate aerosol through controls on sulfur dioxide (SO₂) emissions. Organic aerosol (OA) instead of sulfate is now the dominant component of total PM_2.5. Long-term surface observations (1991-2013) in the Southeast US in summer show parallel decreases in sulfate (2.8-4.0% a^-1) and OA (1.6-1.9% a^-1). Decline of anthropogenic OA emissions is uncertain but is unlikely to fully explain this trend because most OA in the Southeast US in summer is biogenic. We conducted a 1991-2013 simulation with the GEOS-Chem chemical transport model including inventory decreases in anthropogenic SO₂, NO_x, and volatile organic compounds (VOCs) emissions, constant anthropogenic primary OA emissions, and a new mechanism of aqueous-phase SOA formation from isoprene. This simulation reproduces the observed long-term decreases of sulfate and OA, and attributes the OA decrease to decline in the OA yield from biogenic isoprene as sulfate decreases (driving lower aqueous aerosol volume and acidity). Interannual OA variability in the model (mainly driven by isoprene) is also well correlated with observations. This result provides support for a large air quality co-benefit of SO₂ emission controls in decreasing biogenic OA as well as sulfate.