Impacts of monocyclic aromatics on regional and global tropospheric gas-phase chemistry

Aromatic compounds in the troposphere are reactive towards ozone
(O₃), hydroxyl (OH) and other radicals. Here we present an
assessment of their impacts on the gas-phase chemistry, using the
general circulation model EMAC (ECHAM5/MESSy Atmospheric Chemistry). The
monocyclic aromatics considered in this study comprise benzene, toluene,
xylenes, phenol, styrene, ethylbenzene, trimethylbenzenes, benzaldehyde
and lumped higher aromatics bearing more than 9 C atoms. On a global
scale, the estimated net changes are minor when aromatic compounds are
included in the chemical mechanism of our model. For instance, the
tropospheric burden of CO increases by about 6 %, and those of OH,
O₃, and NO_x (NO + NO₂) decrease between
2 % and 14 %. The global mean changes are small partially because of
compensating effects between high- and low-NO_x regions. The
largest change is predicted for glyoxal, which increases globally by 36
%. Significant regional changes are identified for several species. For
instance, glyoxal increases by 130 % in Europe and 260 % in East Asia,
respectively. Large increases in HCHO are also predicted in these
regions. In general, the influence of aromatics is particularly evident
in areas with high concentrations of NO_x, with increases up
to 12 % in O₃ and 17 % in OH. Although the global impact of
aromatics is limited, our results indicate that aromatics can strongly
influence tropospheric chemistry on a regional scale, most significantly
in East Asia.