Uncertainty in preindustrial abundance of tropospheric ozone: Implicationsfor radiative forcing calculations

Recent model calculations of the global mean radiative forcing from troposp heric ozone since preindustrial times fall in a relatively narrow range, fr om 0.3 to 0.5 W m(-2) These calculations use preindustrial ozone fields tha t overestimate observations available from the turn of the nineteenth centu ry. Although there may be calibration problems with the observations, uncer tainties in model estimates of preindustrial natural emissions must also be considered. We show that a global three-dimensional model of tropospheric chemistry with reduced NOx emissions from lightning (1-2 Tg N yr(-1)) and s oils (2 Tg N yr(-1)) and increased emissions of biogenic hydrocarbons can b etter reproduce the nineteenth century observations. The resulting global m ean radiative forcing from tropospheric ozone since preindustrial times is 0.72-0.80 W m(-2), amounting to about half of the estimated CO2 forcing. Re duction in the preindustrial lightning source accounts for two thirds of th e increase in the ozone forcing. Because there is near-total titration of O H by isoprene in the continental boundary layer of the preindustrial atmosp here, isoprene and other biogenic hydrocarbons represent significant ozone sinks. The weak dr absent spring maximum in the nineteenth century observat ions of ozone is difficult to explain within our understanding of the natur al ozone budget. Our results indicate that the uncertainty in computing rad iative forcing from tropospheric ozone since preindustrial times is larger than is usually acknowledged.