Abstract
Naturally emitted reactive trace gases are thought to impact
tropospheric composition, predominantly through the emission and
chemistry of isoprene (C5H8). Other species are thought to play a less
important role. Here the GEOS-Chem model is used to compare the impacts
of isoprene and iodine emissions on present-day tropospheric
composition. Removing isoprene emissions leads to a 4.4% decrease in
tropospheric O3 burden, a smaller absolute change than the 5.7%
increase from removing iodine emissions. Iodine has a negligible impact
on global mean OH concentrations and methane lifetime (-0.2% and
+0.1%). Isoprene has a substantial impact on both (-7% and +6.5%).
Isoprene emissions and chemistry are seen as essential for tropospheric
chemistry models, but iodine is often not. We suggest iodine should
receive greater attention in model development and experimental research
to allow improved predictions of past, present and future tropospheric
O3.