Missing Measurements of Sesquiterpene Ozonolysis Rates and Composition Limit Understanding of Atmospheric Reactivity

Emissions of biogenic reactive carbon significantly influence
atmospheric
chemistry, contributing to the formation and destruction of secondary
pollutants, such as secondary organic aerosol and ozone. While isoprene
and monoterpenes are a major fraction of emissions and have been extensively
studied, substantially less is known about the atmospheric impacts
of higher-molecular-weight terpenes such as sesquiterpenes. In particular,
sesquiterpenes have been proposed to play a significant role in ozone
chemical loss due to the very high ozone reaction rates of certain
isomers. However, relatively little data are available on the isomer-resolved
composition of this compound class or its role in ozone chemistry.
This study examines the chemical diversity of sesquiterpenes and availability
of ozone reaction rate constants to evaluate the current understanding
of their ozone reactivity. Sesquiterpenes are found to be highly diverse,
with 72 different isomers reported and relatively few isomers that
contribute a large mass fraction across all studies. For the small
number of isomers with known ozone reaction rates, estimated rates
may be 25 times higher or lower than measurements, indicating that
estimated reaction rates are highly uncertain. Isomers with known
ozone reaction rates make up approximately half of the mass of sesquiterpenes
in concentration and emission measurements. Consequently, the current
state of the knowledge suggests that the total ozone reactivity of
sesquiterpenes cannot be quantified without very high uncertainty,
even if isomer-resolved composition is known. These results are in
contrast to monoterpenes, which are less diverse and for which ozone
reaction rates are well-known, and in contrast to hydroxyl reactivity
of monoterpenes and sesquiterpenes, for which reaction rates can be
reasonably well estimated. Improved measurements of a relatively small
number of sesquiterpene isomers would reduce uncertainties and improve
our understanding of their role in regional and global ozone chemistry.