Convection links biomass burning to increased tropical ozone: However, models will tend to overpredict O₃

Abstract

Biomass burning throughout the inhabited portions of the tropics generates precursors which lead to significant local atmospheric ozone pollution. Several simulations show how this smog could be only an easily observed, local manifestation of a much broader increase in tropospheric ozone. We illustrate basic processes with a one‐dimensional time‐dependent model that is closer to true meteorological motions than commonly used eddy diffusion models. Its application to a representative region of South America gives reasonable simulations of the local pollutants measured there. Three illustrative simulations indicate the importance of dilution, principally due to vertical transport, in increasing the efficiency of ozone production, possibly enough for high ozone to be apparent on a very large, intercontinental scale. In the first, cook‐then‐mix, simulation the nitrogen oxides and other burning‐produced pollutants are confined to a persistently subsident fair weather boundary layer for several days, and the resultant ozone is found to have only a transient influence on the whole column of tropospheric ozone. In the second, mix‐then‐cook, simulation the effect of typical cumulonimbus convection, which vents an actively polluted boundary layer, is to make a persistent increase in the tropical ozone column. Such a broadly increased ozone column is observed over the the populated “continental” portion of the tropics. A third simulation averages all emission, transport, and deposition parameters, representing one column in a global tropospheric model that does not simulate individual weather events. This “oversmoothing” simulation produces 60% more ozone than observed or otherwise modeled. Qualitatively similar overprediction is suggested for all models which average significantly in time or space, as all need do. Clearly, simulating these O3 levels will depend sensitively on knowledge of the timing of emissions and transport.

Medium-range mid-tropospheric transport of ozone and precursors over Africa: two numerical case studies in dry and wet seasons

“…L2 presents both high O 3 (more than 100 ppbv) and CO (up to 300 ppbv) Such values lye at the upper limit of the polluted type situation observed on monthly average (Sauvage et al, 2005). Considering the mixing ratio of O 3 and CO, L2 cannot be explained by local pollution and is more representative of mature airmass loaded with pollutants few days ago (Chatfield and Delany, 1990). As in Sect.…”

On the origin of tropospheric O<sub>3</sub> over the Indian Ocean during the winter monsoon: African biomass burning vs. stratosphere-troposphere exchange

“…Graedel and Crutzen, 1993). Not only will O 3 be produced close to the source regions of its precursors, but as the precursors are transported, it will also be produced during advection (Chatfield and Delaney, 1990;Pickering et al, 1992). This so-called "mix-then-cook" mechanism, in combination with O 3 removal that occurs at the surface, causes O 3 mixing ratios to be generally higher in the free troposphere than at the surface.…”

Tropospheric ozone climatology at two Southern Hemisphere tropical/subtropical sites, (Reunion Island and Irene, South Africa) from ozonesondes, LIDAR, and in situ aircraft measurements

“…The reason for this positive trend for the upper layer is an interesting issue and we draw two hypotheses: first, the influence of the increase of biomass burning and pollution can play a role: the trend values in the upper troposphere at Reunion could be related to the complex dynamical mechanisms which are necessary for Reunion Island tropospheric ozone to be influenced by biomass burning: injection of pollutants in the free troposphere by convection, mix-thencook scheme and ozone production during westerly transport (Chatfield and Delany, 1990;Baldy et al, 1996). The second hypothesis is an increase in stratosphere-troposphere exchange which could be induced by climate change.…”