Evaluation of the use of photochemical indicators to assess ozone—NOx—VOC sensitivity in the Southwestern Iberian Peninsula

Abstract

Understanding the chemical links between ozone (O3) and its two main precursors, nitrogen oxides (NOx) and volatile organic compounds (VOC), is important for designing effective photochemical smog reduction strategies. This chemical relationship will determine which precursor (NOx or VOC) emission reduction will be more effective for decreasing the ozone formation. Under certain conditions, ozone levels decrease as a result of a reduction in NOx emissions but do not respond significantly to changes in VOC emissions (NOx-sensitive condition), while under other conditions ozone concentrations decrease in response to reductions in VOCs and may even increase when NOx emissions are reduced (VOC-sensitive conditions). Indicator species can be used to assess the sensitivity of ozone to changes in the emissions of its precursors. These indicators are species or species ratios involved in ozone photochemistry which reflect the primary chemical process through which the ozone was formed. In this work we use the MM5-CAMx model system to explore the behaviour of various indicator species during two meteorological situations featuring different atmospheric conditions in a complex terrain area. The results show that indicators based on nitrogen compounds (i.e,. NOy and NOz) are suitable for defining the transition range from VOC- to NOx-sensitive chemistry, and that despite the uncertainties associated with the use of chemical indicators, the ratios O3/NOy and O3/NOz may provide a simple and useful way to summarize the response of ozone to changes in NOx and VOC emissions in Southwestern Spain.