Optical properties and chemical composition of aerosol particles at an urban location: An estimation of the aerosol mass scattering and absorption efficiencies

Abstract

We investigated aerosol optical properties, mass concentration and chemical composition over a 1 year period (from March 2006 to February 2007) at an urban site in Southern Spain (Granada, 37.18°N, 3.58°W, 680 m above sea level). Light-scattering and absorption measurements were performed using an integrating nephelometer and a MultiAngle Absorption Photometer (MAAP), respectively, with no aerosol size cut-off and without any conditioning of the sampled air. PM10 and PM1 (ambient air levels of atmospheric particulate matter finer than 10 and 1 microns) were collected with two high volume samplers, and the chemical composition was investigated for all samples. Relative humidity (RH) within the nephelometer was below 50% and the weighting of the filters was also at RH of 50%. PM10 and PM1 mass concentrations showed a mean value of 44 19 mg/m3 and 15 7 mg/m3, respectively. The mineral matter was the major constituent of the PM10–1 fraction (contributing more than 58%) whereas organic matter and elemental carbon (OM+EC) contributed the most to the PM1 fraction (around 43%). The absorption coefficient at 550 nm showed a mean value of 24 9 Mm 1 and the scattering coefficient at 550 nm presented a mean value of 61 25 Mm 1, typical of urban areas. Both the scattering and the absorption coefficients exhibited the highest values during winter and the lowest during summer, due to the increase in the anthropogenic contribution and the lower development of the convective mixing layer during winter. A very low mean value of the single scattering albedo of 0.71 0.07 at 550 nm was calculated, suggesting that urban aerosols in this site contain a large fraction of absorbing material. Mass scattering and absorption efficiencies of PM10 particles exhibited larger values during winter and lower during summer, showing a similar trend to PM1 and opposite to PM10–1. This seasonality is therefore influenced by the variations on PM composition. In addition, the mass scattering efficiency of the major aerosol constituents in PM10 were also calculated applying the multilinear regression (MLR) analysis. Among all of them, the most efficient in terms of scattering was sulfate ion (7 1 m2g 1) while the least efficient was the mineral matter (0.2 0.3 m2g 1). On the other hand, we found that the absorption process was mainly dominated by carbonaceous particles.