Abstract
Ground level ozone concentration is dependent on its precursors, such as Nitrogen Oxides (NOx) and hydrocarbons (HCs) and meteorological parameters, most importantly air temperature. Positive ozone-temperature slope at average temperature is well-documented. However, how this relationship breaks at extremely high temperature in hotter climates is still debatable. As this could have implications for long term global modelling predictions, this paper explores evidence for a negative ozone-temperature slope during atypically high temperature events in Makkah, Saudi Arabia, where temperature levels as high as 50 degrees C are recorded. At temperature levels (15-42 degrees C) statistical analysis showed positive ozone-temperature slopes, however the slopes became negative at atypically high temperature levels (> 42 degrees C). Using data when hourly mean temperature was greater than 42 degrees C, Quantile Regression Model (QRM) showed negative ozone-temperature slopes. The negative slopes at quantile 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8 and 0.9 were -1.88, -5.83, -7.89, -7.08, -11.11, -15.00, -18.28, and -28.57 mu g/m(3)/degrees C, respectively. Mean slope determined by linear regression was -11.51 mu g/m(3)/degrees C. Furthermore, the negative slopes were stronger at higher quantiles of ozone distribution, indicating non-linearities in the association of ozone and temperature. Reduction in the levels of ozone precursors, such as total hydrocarbons (THCs) and nitrogen dioxide (NO2) is probably the most likely reason for the negative ozone-temperature slope at extremely high temperature. Previously concerns have been expressed that under the warming climate scenario increasing temperature may further increase ozone levels, particularly in urban areas during pollution episodes, however this study suggests the opposite at extremely high temperature in hot arid climatic conditions.