Variability of carbonaceous aerosols in remote, rural, urban and industrial environments in Spain: implications for air quality policy

Abstract

We interpret here the variability of levels of carbonaceous aerosols based on a 12 yr database from 78 monitoring stations across Spain specially compiled for this article. Data did not evidence any spatial trends of carbonaceous aerosols across the country. Conversely, results show marked differences in average concentrations from the cleanest, most remote sites (around 1 μg m^-3 of non-mineral carbon (nmC), mostly made of organic carbon (OC) with very little elemental carbon (EC), around 0.1 μg m^-3 ; OC / EC = 12-15), to the highly polluted major cities (8-10 μg m^-3 of nmC; 3-4 μg m^-3 of EC; 4-5 μg m^-3 of OC; OC / EC = 1-2). Thus, urban (and very specific industrial) pollution was found to markedly increase levels of carbonaceous aerosols in Spain, with much lower impact of biomass burning and of biogenic emissions. Correlations between yearly averaged OC / EC and EC concentrations adjust very well to a potential equation (OC = 3.37 EC^0.326 , R² = 0.8). A similar equation is obtained when including average concentrations obtained at other European sites (OC = 3.60EC^0.491 , R² = 0.7).

A clear seasonal variability in OC and EC concentrations was detected. Both OC and EC concentrations were higher during winter at the traffic and urban sites, but OC increased during the warmer months at the rural sites. Hourly equivalent black carbon (EBC) concentrations at urban sites accurately depict road traffic contributions, varying with distance from road, traffic volume and density, mixing-layer height and wind speed. Weekday urban rush-hour EBC peaks are mimicked by concentrations of primary gaseous emissions from road traffic, whereas a single midday peak is characteristic of remote and rural sites. Decreasing annual trends for carbonaceous aerosols were observed between 1999 and 2011 at a large number of stations, probably reflecting the impact of the EURO4 and EURO5 standards in reducing the diesel PM emissions. This has resulted in some cases in an increasing trend for NO₂ / (OC + EC) ratios as these standards have been much less effective for the abatement of NO_x exhaust emissions in passenger diesel cars. This study concludes that EC, EBC, and especially nmC and OC + EC are very good candidates for new air quality standards since they cover both emission impact and health-related issues.

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