Gas-Phase Ozone Oxidation of Monoterpenes: Gaseous and ...

236 JIANZHEN YU ET AL.
compounds) are quantified using the m/z 181 ion, products bearing COOH/OH
groups (type 2 compounds) are quantified using the m/z 73 and 75 ions, and for
those products that contain both carbonyl and COOH/OH groups (type 3 compounds),
the sum of three ions at m/z 181, 73 and 75 is used for their quantification.
The selection of surrogates on the basis of functional group types is justified by the
use of the above functional group specific ions for quantification. The estimated
calibration factors have a ∼±30% uncertainty. The other sources that contribute to
the uncertainty associated with product yields include uncertainties for collection
efficiency (∼ ±4%), extraction efficiency (∼ ±15%) and sample volume (±5%).
After considering all contributing sources, the uncertainty associated with product
yields is estimated to be ∼±50%.
Our calculation of gaseous product yields ignores wall loss processes, as we
estimate that the loss of an oxygenated product resulting from wall loss processes
is likely less than 15% at 300 min after its formation. Grosjean (1985) measured
the wall loss rates for five oxygenated species, biacetyl, pyruvic acid, o-cresol,
benzaldehyde, and benzoic acid in a Teflon chamber with a S/V of 3.8 m −1 .The
first three compounds had an unmeasurable wall loss rate while benzaldehyde and
benzoic acid had wall loss rates of 3.4 × 10 −4 and 10.8 × 10 −4 min −1 , respectively.
Hallquist et al. (1997) measured the wall loss rates for pinonaldehyde and
caronaldehyde to be (2.4–4.2) × 10 −3 min −1 in a borosilica glass reactor with a
S/V of 14.3 m −1 . If one assumes wall loss processes are similar on borosilica glass
and Teflon surfaces, one could calculate that the two dicarbonyls have a wall loss
rate of (3–5) × 10 −4 min −1 , comparable to that for benzoic acid (5 × 10 −4 min −1 ),
in the present Teflon reactor with a S/V of 1.8 m −1 . Using the wall loss rate for
benzoic acid, which has the highest wall loss rate among all the tested oxygenated
species, we estimate that about 86% of an oxygenated product remains in the gas
phase at 300 min after its formation, if we assume that wall loss is the only loss
process. A more detailed study of gas-phase product yields needs experimentally
determined wall loss rates for each product, which unfortunately can not be readily
accomplished as many products lack commercially available standards.
The organic aerosol mass generated in each experiment was obtained from the
aerosol volume measured by SEMS/CNC and assuming a SOA density of 1 g cm −3 .
The SOA yield (Y) was then calculated by
Y = M o
HC , (1)
where M o is the organic aerosol mass concentration (µg m −3 ) produced for
a given amount of hydrocarbon reacted, HC (µg m −3 ). The estimated uncertainty
in the measured SOA yield is roughly ±12%, with SOA density uncertainty
assumed to be ±10%.