Air monitoring by researchers in 2000 found levels of volatile organic compounds — highly
reactive ozone precursors such as benzene, known as VOCs — were 10 to 100 times higher than what had previously been estimated.
From the Physical Science Basis: «Shindell et al. (2009) estimated the impact of
reactive species emissions on both gaseous and aerosol forcing species and found that
ozone precursors, including methane, had an additional substantial climate effect because they increased or decreased the rate of oxidation of SO2 to sulphate aerosol.