Ozone in the Tropical Troposphere

The aim of the research presented here is to acquire knowledge of the past, present, and future composition, stability, sensitivity, and variability of the troposphere. We focus mostly on the tropical regions because it has received little attention so far, measurements here are scarce, and large changes are expected to occur in the future. Special attention is given to ozone as it plays a key role in tropospheric photochemistry. Not only is it a greenhouse gas, it is also the most important precursor of the hydroxyl radical (OH) which is responsible for the removal of many trace gases from the troposphere. Furthermore, ozone is an important indicator for atmospheric transport and photochemistry. The involvement of ozone in many different processes also renders it an excellent compound to test our knowledge of coupled transport-chemistry systems. In Chapter 2, we present the measurements of ozone performed since the inauguration (1999) of a new ozone monitoring station in Paramaribo, Suriname. This station was started under the Research on Atmospheric Dynamics and Chemistry in Suriname (RADCHiS) project. The choice for this location was partly due to the historical ties with Suriname, but also because of the unique location of Paramaribo with respect to the Atlantic Ocean, the equator (northern hemisphere), and the Inter Tropical Convergence Zone (ITCZ). Because the ITCZ passes Suriname twice per year the station samples both the meteorological northern and southern hemispheres. This leads to strong contrast with nearby southern hemisphere stations (Ascension and Natal) during February and March. Chapter 3 describes the variability of ozone in the tropics, and our ability to reproduce this variability (on time scales of months to years). For that purpose we use a model simulation that spans the period 1979-1993, and that includes trends of anthropogenic emissions, and day-to-day variability of meteorology. The model calculated seasonal cycle of ozone in the tropics shows that we are able to reproduce observations for stations in the remote Pacific Ocean quite well, but that the more polluted Atlantic Ocean is more problematic. At times, the model underestimates ozone by more than 30%. Furthermore, our results show that the model realistically reproduces the changing convection patterns in the Walker circulation during the positive phase of the El Nino-Southern Oscillation (ENSO). These results are in good agreement with satellite observations, even though other influences on ozone that change during ENSO (e.g. biomass burning in dry areas) were not included in our model simulation. This work presents the first ENSO signal in a multi-year model simulation of ozone. To better understand, and possibly solve, the large underestimate of ozone over the Atlantic Ocean we have investigated the zonal distribution of ozone in more detail in Chapter 4. Since ozone monitoring stations in the tropics are not abundant, and usually only span a few years, we have used satellite observations of ozone (1979-1992) to compare our model to. We introduce a method to compare model and measurements quantitatively and systematically, and then use this method to identify times and regions where model underestimates are largest. Chapter 5 treats the stability of photochemistry in the tropics, and the largely unchanged levels of OH since industrialization. We focus specifically on OH levels, which are strongly coupled to ozone levels because together with water vapor and sunlight, they determine OH production rates. Our calculations show that global OH has remained constant through a 50% increase in primary OH production, and a 75% increase in secondary production. Locally, the OH distribution has changed substantially since industrialization with increases of more than 200% in urban areas, and decreases of more than 30% in remote marine areas.