A one-year comprehensive chemical characterisation of fine aerosol (PMsub2.5/sub) at urban, suburban and rural background sites in the region of Paris (France)

摘要

Studies describing the chemical composition of fine aerosol (PM_2.5) inurban areas are often conducted for a few weeks only and at one sole site,giving thus a narrow view of their temporal and spatial characteristics.This paper presents a one-year (11 September 2009–10 September 2010)survey of the daily chemical composition of PM_2.5 in the region ofParis, which is the second most populated "Larger Urban Zone" in Europe.Five sampling sites representative of suburban (SUB), urban (URB), northeast(NER), northwest (NWR) and south (SOR) rural backgrounds were implemented.The major chemical components of PM_2.5 were determined includingelemental carbon (EC), organic carbon (OC), and the major ions. OC wasconverted to organic matter (OM) using the chemical mass closuremethodology, which leads to conversion factors of 1.95 for the SUB and URBsites, and 2.05 for the three rural ones. On average, gravimetricallydetermined PM_2.5 annual mass concentrations are 15.2, 14.8, 12.6, 11.7and 10.8 μg m?3 for SUB, URB, NER, NWR and SOR sites,respectively. The chemical composition of fine aerosol is very homogeneousat the five sites and is composed of OM (38–47%), nitrate (17–22%),non-sea-salt sulfate (13–16%), ammonium (10–12%), EC (4–10%),mineral dust (2–5%) and sea salt (3–4%). This chemical composition isin agreement with those reported in the literature for most Europeanenvironments. On an annual scale, Paris (URB and SUB sites) exhibits itshighest PM_2.5 concentrations during late autumn, winter and earlyspring (higher than 15 μg m?3 on average, from December toApril), intermediates during late spring and early autumn (between 10 and15 μg m?3 during May, June, September, October, and November) andthe lowest during summer (below 10 μg m?3 during July andAugust). PM levels are mostly homogeneous on a regional scale, during thewhole project (e.g. for URB plotted against NER sites:slope = 1.06, r2=0.84, n=330), suggesting the importanceof mid- or long-range transport, and regional instead of local scalephenomena. During this one-year project, two thirds of the days exceeding thePM_2.5 2015 EU annual limit value of 25 μg m?3 were due tocontinental import from countries located northeast, east of France. Thisresult questions the efficiency of local, regional and even nationalabatement strategies during pollution episodes, pointing to the need for awider collaborative work with the neighbouring countries on these topics.Nevertheless, emissions of local anthropogenic sources lead to higher levelsat the URB and SUB sites compared to the others (e.g. 26% higher onaverage at the URB than at the NWR site for PM_2.5, during the wholecampaign), which can even be emphasised by specific meteorologicalconditions such as low boundary layer heights. OM and secondary inorganicspecies (nitrate, non-sea-salt sulfate and ammonium, noted SIA) are mainlyimported by mid- or long-range transport (e.g. for NWR plotted against URBsites: slope = 0.79, r2=0.72, n=335 for OM, andslope = 0.91, r2=0.89, n=335 for SIA) whereas EC isprimarily locally emitted (e.g. for SOR plotted against URB sites:slope = 0.27; r2=0.03; n=335). This database will serve as a basis for investigating carbonaceousaerosols, metals as well as the main sources and geographical origins ofPM in the region of Paris.