Secondary Organic Aerosol Formation and Organic Nitrate Yield from NO3 Oxidation of Biogenic Hydrocarbons

The secondary organic aerosol (SOA) mass yields from NO3 oxidation of a series of biogenic volatile organic compounds (BVOCs), consisting of five monoterpenes and one sesquiterpene (α-pinene, β-pinene, Δ-3-carene, limonene, sabinene, and β-caryophyllene), were investigated in a series of continuous flow experiments in a 10 m3 indoor Teflon chamber. By making in situ measurements of the nitrate radical and employing a kinetics box model, we generate time-dependent yield curves as a function of reacted BVOC. SOA yields varied dramatically among the different BVOCs, from zero for α-pinene to 38–65% for Δ-3-carene and 86% for β-caryophyllene at mass loading of 10 μg m–3, suggesting that model mechanisms that treat all NO3 + monoterpene reactions equally will lead to errors in predicted SOA depending on each location’s mix of BVOC emissions. In most cases, organonitrate is a dominant component of the aerosol produced, but in the case of α-pinene, little organonitrate and no aerosol is formed.

zero air through a trap containing N 2 O 5 crystals, submerged in an isopropyl alcohol and dry ice bath held at a temperature of -65 to -60°C. The temperature settings depended upon daily trap condition, so before each experiment, flows and temperatures were tuned to achieve the desired N 2 O 5 concentration as measured by cavity ringdown spectroscopy (CRDS) and then kept constant for the remainder of that experiment. After N 2 O 5 concentrations had reached steady state in the chamber, BVOC was introduced to the inlet flow. In all cases except β-caryophyllene, the BVOC was introduced using a constant flow of a quantified ppm-level gas standard (balance N 2 ) that was subsequently diluted by the 40 liters per minute zero air flow to obtain the desired final chamber concentrations of ~20-60 ppb. Cylinder concentrations were verified using a cryofocused GC-FID system calibrated against a NIST-certified butane/benzene gas standard. In the case of β-caryophyllene, a small flow (4-18 mL/min) of zero air was fed through a heated trap held at 30-77°C containing the liquid hydrocarbon into the chamber input zero air flow. In this case, the β-caryophyllene concentration was verified by sampling 200 mL of the BVOC + air mixture onto a two stage adsorbent cartridge (filled with Tenax T and Carbograph B) and analyzed by thermo-desorption GC-MS-FID.

Wall loss analysis
The total chamber flow (Q) was 40 liters per minute (or 0.667 L s -1 ), and the total volume (V) was 10,000 L. The resulting β is plotted against particle diameter (d p ).
These wall loss rates were used to correct all SMPS data collected during chamber experiments. Size-dependent rates were applied to each raw size distribution to determine the number of particles lost to the walls at each time step. It was assumed that particles were lost to the walls irreversibly and no longer able to act as a gascondensation reservoir. Thus, to get accurate gross mass yields, these losses were cumulatively added back to the size distributions.

Determination of ΔVOC: method 2: Model constrained
For this determination of ΔVOC, the model (see equation 3 in the main text for general structure) is initiated with an empty chamber. N 2 O 5 is added for 24 hours to achieve steady-state concentrations matching the initial [N 2 O 5 ] and [NO 3 ] observed for each experiment, using wall loss rate constants that were optimized for the set of all experiments. At hour 24, we begin adding BVOC to the chamber at the rate known from experiment conditions, with continuing addition of N 2 O 5 at the same rate as previously. The reactions included are shown below, with a table of rate constants. The model is run at 295 K and 0.8 atm (Boulder, CO).
The greatest uncertainty is in the RO 2 rate constants, for which we followed Ziemann and Atkinson [1] in combination with this useful structure-function relationship page on the MCM: http://mcm.leeds.ac.uk/MCM/categories/saunders-2003-4_6_5-genmaster.htt?rxnId=12891 We assume our RO 2 radical to all be tertiary, with an enhancement of 2 orders of magnitude due to the beta-NO 3 functional group, which we assume to have the same effect as a beta-hydroxy substitution. We further assume that no HO 2 will be generated in this system (via NO 3 +RO 2 reactions), because the RO2 are all tertiary.

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Organonitrate production Figure S7. Top: Full time series of organonitrate measurements for β-pinene high concentration experiment (#2), in which BVOC was added at 5:20. Bottom: fraction of total aerosol mass that is organonitrate, assuming a mono-nitrate with MW ~ 230 g/mol. Figure S8. Schematic of nitrogen balance, to aid interpretation of Table 3