Biogenic volatile organic compounds (BVOCs), including the monoterpene limonene, are a major source of secondary organic aerosol (SOA). While gas-phase oxidation initiates the dominant pathway for BVOC conversion to SOA, recent studies have demonstrated that biogenic hydrocarbons can also directly react with acidic droplets. To investigate whether mineral dust may facilitate similar reactive uptake of biogenic hydrocarbons, we studied the heterogeneous reaction of limonene with mineral substrates using condensed-phase infrared spectroscopy and identified the formation of irreversibly adsorbed organic products. For kaolinite, Arizona Test Dust, and silica at 30% relative humidity, GC-MS identified limonene-1,2-diol as the dominant product with total organic surface concentrations on the order of (3-5) × 10 molecules m. Experiments with O-labeled water support a mechanism initiated by oxidation of limonene by surface redox sites forming limonene oxide followed by water addition to the epoxide to form limonenediol. Limonene uptake on α-alumina, γ-alumina, and montmorillonite formed additional products in high yield, including carveol, carvone, limonene oxide, and α-terpineol. To model tropospheric processing of mineral aerosol, we also exposed each mineral substrate to gaseous nitric acid prior to limonene uptake and identified similar surface adsorbed products that were formed at rates 2 to 5 times faster than without nitrate coatings. The initial rate of reaction was linearly dependent on gaseous limonene concentration between 5 × 10 and 5 × 10 molecules cm (0.22-20.5 ppm) consistent with an Eley-Rideal-type mechanism in which gaseous limonene reacts directly with reactive surface sites. Increasing relative humidity decreased the amount of surface adsorbed products indicating competitive adsorption of surface adsorbed water. Using a laminar flow tube reactor we measured the uptake coefficient for limonene on kaolinite at 25% RH to range from γ = 5.1 × 10 to 9.7 × 10. After adjusting for reactive surface areas, we estimate uptake coefficients for limonene on HNO-processed mineral aerosol on the order of (1-6) × 10. Although this heterogeneous reaction will not impact the atmospheric lifetime of gaseous limonene, it does provide a new pathway for mineral aerosol to acquire secondary organic matter from biogenic hydrocarbons, which in turn will alter the physical properties of mineral dust.

Atmospheric processing of mineral aerosol by trace gases results in the formation of surface-adsorbed products that have the capacity to alter the chemical and physical properties of these airborne particulates. To investigate one potential impact of aerosol processing by biogenic volatile organic compounds (BVOCs), we investigated the heterogeneous decomposition of ozone on pure and monoterpene-processed kaolinite. We used a laminar flow reactor to measure O 3 reactive uptake coefficients on kaolinitecoated tubes as a function of relative humidity, O 3 concentration, and pre-exposure to gaseous limonene and αpinene. At 26% RH, kaolinite has a near equivalent of a monolayer of adsorbed water, and the ozone steady-state uptake coefficient was γ av = 2.9 × 10 −9 assuming the BET surface area. Pre-exposing kaolinite to limonene and α-pinene increased O 3 uptake coefficients by nearly 2 orders of magnitude to 2.1 × 10 −7 and 2.5 × 10 −7 , respectively. At all humidities studied (10−50% RH), O 3 uptake was at least 1 order of magnitude higher for monoterpene-processed kaolinite compared to that of pure kaolinite. This dramatic increase in O 3 reactivity is attributed to surface-adsorbed organics, namely limonenediol and α-terpineol, which contain alkene functionalities susceptible to ozonolysis. Increasing relative humidity decreased O 3 uptake for monoterpene-processed kaolinite consistent with competitive adsorption of water resulting in lower organic surface concentrations. These results demonstrate the significant impact adsorbed organics can have on O 3 uptake coefficients on mineral aerosol, which should be accounted for in atmospheric modeling studies.