Application of two modelling systems to estimate the impact of forest fires on air quality in an urban area

Smoke has to be considered one of the several disturbing effects of forest fires. Its impacts on air quality and human health can be important since large amounts of pollutants, like particulate matter (PM), carbon monoxide (CO), volatile organic compounds and nitrogen oxides, are emitted to the atmosphere. Currently, there is a growing awareness that smoke from wildland fires can expose individuals and populations to hazardous air pollutants, particularly in the wildland urban interface (WUI) due to the highest risk of human exposure. The study of smoke dispersion and chemistry in the analysis of WUI fires is a vital task in order to prevent the exposure of population to hazardous concentrations of air pollutants. In southern Europe several cities have been affected by air pollution episodes resulting from forest fires occurrences. In the summer of 1997 the city of Marseille, France, experienced an important air quality problem mainly related to the forest fires spreading at the Étoile Mountain. Athens is another city that has been facing several air quality problems related to forest fires emissions. In Portugal, the summer of 2003 was considered the most devastating of the last decade in terms of forest fires, and this is clearly reflected in the values measured by the air quality-monitoring networks, namely in some urban areas like Lisbon where forest fires spreading north of the city in the middle of September highly impacted the urban air quality. The Lisbon airshed, with a population of 3.5 million inhabitants, is the most important urban centre in Portugal. Because of its urban/wildland characteristics, high population density, and the high levels of pollutants registered, Lisbon region forest fires are a very interesting case for the study of the influence of forest fires emissions on urban air quality. Analysis of the measured air quality data enabled the identification of the 13th of September 2003 as the most critical day for high CO and PM concentration values in the Lisbon urban area. During this day, 12 fires were active in the Lisbon region, burning an area of about 500 ha of forest stands and shrubs. Numerical modelling allows the understanding of how pollutants emitted by a forest fire will be transported, dispersed, and will suffer chemical transformations in the atmosphere, estimating the resulting air pollutants concentration fields. The outcomes of these numerical tools could be useful to predict potential consequences namely on human exposure, air quality management strategies and international policy commitments. However, the use of a modelling system with confidence implies its previous evaluation, which can be done by comparison of its results with measured data and/or by inter-comparison of results from different modelling systems. In this scope, the main purpose of this paper is to estimate the effects of the 13th September 2003 forest fires on Lisbon air quality through the application of two modelling systems: (1) FARSITE (fire behaviour and fire growth simulator) coupled with LOTOS-EUROS (3D chemistry transport model); (2) ForeFire (fire spread model) incorporated in the online chemical transport model MESO-NH. This paper also aims to evaluate both systems performance inter-comparing their results and comparing them to the measured air pollutants concentration values in the Lisbon air quality monitoring network Both numerical systems were applied to the same simulation domains and using similar input data as meteorological conditions, land cover, and ignition points. The simulations started with the fire spread modelling (with FARSITE and ForeFire) for a smaller domain and a fine spatial and temporal resolution. Then results were used to estimate pollutants emissions that were inputted to the air quality modelling systems which were applied to a larger domain (100 km x 100 km, with 1 km2 resolution) in order to consider the transport and dispersion of pollutants along longer distances. Results confirm the strong impact of forest fires on the urban air pollution levels. The statistical quality indicators used to validate the modelling applications showed very good performance skills allowing applying this kind of systems to analyse the relation between forest fires and urban air pollution.