On-road traffic emissions in a megacity

Abstract

A new annual bottom–up emission inventory of criteria pollutants and greenhouse gases from on-road mobile sources was developed for 2006 for the metropolitan area of Buenos Aires, Argentina, within a four-year regional project aimed at providing tools for chemical weather forecast in South America. Under the scarcity of local emission factors, we collected data from measuring campaigns performed in Argentina, Brazil, Chile and Colombia and compiled a data set of regional emission factors representative of Latin American fleets and driving conditions. The estimated emissions were validated with respect to downscaled national estimates and the EDGAR global emission database. Our results highlight the role of older technologies accounting in average for almost 80% of the emissions of all species. The area exhibits higher specific emissions than developed countries, with figures two times higher for criteria pollutants. We analyzed the effect on emissions of replacing gasoline by compressed natural gas, occurring in Argentina since 1995. We identified (i) a relationship between number of vehicles and a compound socioeconomic indicator, and (ii) time-lags in vehicle technologies between developed and developing countries, which can be respectively applied for spatial disaggregation and the development of projections for other Latin American cities. The results may also be employed to complement global emission inventories and by local policy makers as an environmental management tool.

Introduction

Megacities are very large urban agglomerations with populations that exceed 10 million inhabitants, differing from urban areas not only in population size but also in the scale of their economy, infrastructure and associated environmental impacts (Gurjar and Lelieveld, 2005). Rapid urbanization has resulted in increasing air pollution emissions, typically arising from transportation, energy production and industrial activities, concentrated in densely populated areas (Gurjar et al., 2008) and surpassing the limits of the megacities' physically occupied area, thus contributing significantly to air quality on a global scale through the long range transport of air pollutants (Gurjar and Lelieveld, 2005, Butler et al., 2008, Butler and Lawrence, 2009).

This work was developed within the framework of the inter-American project “South American Emissions, Megacities and Climate” aimed at developing a number of tools for the chemical weather forecast in the South American cities of Bogotá, Buenos Aires, Lima, Santiago and Sao Paulo. To the best of our knowledge this is the first inventory for the on-road transport categories of the metropolitan area of Buenos Aires that has estimated emissions of carbon dioxide (CO₂), methane (CH₄), nitrous oxide (N₂O), carbon monoxide (CO), nitrogen oxides (NO_x), non-methane volatile organic compounds (NMVOC), particulate matter (PM) and sulfur dioxide (SO₂) in two spatial disaggregation levels.

Road transport became a significant source of air pollution in the last century and is currently one of the largest emission sources in megacities with subsequent adverse effects on human health (Faiz, 1993, Colvile et al., 2001). In Argentina, road transport emissions accounted for 58% of total CO emissions in 2000 (Fundación Bariloche, 2005), share that is expected to rise because of the rising global demand for private mobility (Zachariadis et al., 2001). Real-world vehicle emissions are difficult to estimate because many factors are involved, and its accurate estimation is crucial for the formulation of efficient air quality management.

Emission inventories provide the necessary information required for air quality control policies, the development of future scenarios and modeling purposes (Parra et al., 2006, Zachariadis and Samaras, 2001, van Aardenne, 2002). There are two approaches for inventorying emissions: top-down and bottom up. Both methodologies share the same basic structure but present considerable differences regarding input data, assumptions and parameters (Reynolds and Broderick, 2000). The selection of the approach and the quality and consistency of the inventory depends almost entirely on the availability of the data and its degree of detail (Reynolds and Broderick, 2000, Tsilingiridis et al., 2002). It is futile to expect emission calculations to achieve results that are superior in accuracy to that of the original survey data (Sturm et al., 1996).

Differences are expected when comparing national emission inventories and specific megacity inventories. In the former, data on fuel consumption, technologies and installed abatement measures are applied uniformly over the country, while in the latter, the data employed is city-specific, the geographical units covered are smaller and the resulting emissions are distributed using more detailed maps of the emissions sources (Butler et al., 2008).

The metropolitan area of Buenos Aires (MABA) is composed of the city itself and 24 radially surrounding districts that are part of the province of Buenos Aires (Fig. 1), comprising an area of 3647 km². Being the 10th megalopolis in the world and the 3rd in Latin America it holds 32% of the total population in only 0.14% of the national territory, with a population density of 4600 inhabitants km⁻² and 2.4 million circulating vehicles. It is the greatest center of activities and the political, economical, and administrative center of the country, having the highest population density of the country, which diminishes from the center to its surrounding districts as poverty levels increase. In all its extent, the MABA is characterized by social and territorial inequities.

Three governmental levels coexist in the MABA (municipal, provincial and national) with different responsibilities and territorial limits within which each one may exercise its authority. Road transport regulation and management are key examples of this situation. For instance, emissions control for passenger vehicles is enforced at the municipal level in the city of Buenos Aires and at the provincial level in the 24 districts of the MABA; heavy-duty trucks are subject to national regulation, whereas light-duty trucks, being registered as passenger vehicles for private use, are also subject to municipal regulation; buses in the city of Buenos Aires are regulated at the national level while buses running in the provincial territory are subject to local regulation. This situation results in a juxtaposition of actors leading to diversity in sources of information and data. More importantly, this superimposing governmental levels combined with the fact that the MABA lies on a flat terrain with relatively good ventilation has been an excuse for a relatively poor air quality management in this highly populated urban zone, which lacks an official inventory of emissions of mobile sources. In addition, the availability of local and process-specific information on emissions represents a serious data gap, a common feature of many cities in developing countries (Guttikunda et al., 2005).