Seasonal cycle, size dependencies, and source analyses of aerosol optical properties at the SMEAR II measurement station in Hyytiälä, Finland

Abstract

Abstract. Scattering and absorption were measured at the Station for Measuring Ecosystem-Atmosphere Relations (SMEAR II) station in Hyytiälä, Finland, from October 2006 to May 2009. The average scattering coefficient σ SP (λ = 550 nm) 18 Mm −1 was about twice as much as at the Pallas Global Atmosphere Watch (GAW) station in Finnish Lapland. The average absorption coefficient σ AP (λ = 550 nm) was 2.1 Mm −1 . The seasonal cycles were analyzed from hourly-averaged data classified according to the measurement month. The ratio of the highest to the lowest average σ SP and σ AP was ∼1.8 and ∼2.8, respectively. The average single-scattering albedo (ω 0 ) was 0.86 in winter and 0.91 in summer. σ SP was highly correlated with the volume concentrations calculated from number size distributions in the size range 0.003-10 µm. Assuming that the particle density was 1.5 g cm −3 , the PM 10 mass scattering efficiency was 3.1 ± 0.9 g m −2 at λ = 550 nm. Scattering coefficients were also calculated from the number size distributions by using a Mie code and the refractive index of ammonium sulfate. The linear regression yielded σ SP (modelled) = 1.046 × σ SP (measured) for the data with the low nephelometer sample volume relative humidity (RH NEPH = 30 ± 9 %) and σ SP (modelled) = 0.985 × σ SP (measured) when RH NEPH = 55 ± 4 %. The effective complex refractive index was obtained by an iterative approach, by matching the measured and the modelled σ SP and σ AP . The average effective complex refractive index was (1.517 ± 0.057) + (0.019 ± 0.015)i at λ = 550 nm. The iterated imaginary part had a strong seasonal cycle, with smallest values in summer and highest in winter. The contribution of submicron particles to scattering was ∼90 %. TheÅngström exponent of scattering, α SP , Correspondence to: A. Virkkula (aki.virkkula@helsinki.fi) was compared with the following weighted mean diameters: count mean diameter (CMD), surface mean diameter (SMD), scattering mean diameter (ScMD), condensation sink mean diameter (CsMD), and volume mean diameter (VMD). If α SP is to be used for estimating some measure of the size of particles, the best choice would be ScMD, then SMD, and then VMD. In all of these the qualitative relationship is similar: the larger theÅngström exponent, the smaller the weighted mean diameter. Contrary to these, CMD increased with increasing α SP and CsMD did not have any clear relationship with α SP . Source regions were estimated with backtrajectories and trajectory statistics. The geometric mean σ SP and σ AP associated with the grid cells in Eastern Europe were in the range 20-40 Mm −1 and 4-6 Mm −1 , respectively. The respective geometric means of σ SP and σ AP in the grid cells over Norwegian Sea were in the range 5-10 Mm −1 and <1 Mm −1 . The source areas associated with high α SP values were norther than those for σ SP and σ AP . The trajectory statistical approach and a simple wind sector classification agreed well.

The first estimates of global nucleation mode aerosol concentrations based on satellite measurements

“…Second, both vapor condensation and light scattering are roughly proportional to the aerosol surface area distribution. The relatively good correlation between CS and aerosol light scattering coefficient has been confirmed by field measurements (Virkkula et al, 2011). Third, due to their similar dependence on the particle size, CS and AOD are expected to respond to changes in the ambient relative humidity in a similar manner.…”

South African EUCAARI measurements: seasonal variation of trace gases and aerosol optical properties

“…This combined with more sunlight would lead to a larger contribution of secondary organic aerosols (SOA) in summer than in winter. The seasonal cycle of ω 0 at Elandsfontein resembles that observed at some very different environments: an Arctic background site in Barrow, Alaska (Delene and Ogren, 2002) a boreal forest site in Finland (Virkkula et al, 2011) and several stations, even though not all, in the free troposphere of the Northern Hemisphere (Andrews et al, 2011). On the other hand, at two North American continental sites in Illinois and Oklahoma the seasonal cycle of ω 0 was different, with the summer medians lower than in winter (Delene and Ogren, 2002).…”

“…This dependence is not definite, as discussed, e.g. by Schuster et al (2006), Garland et al (2008), and Virkkula et al (2011).…”

“…The concentrations of industrially related trace gases were within a global context (Carmichael et al, 2003) relatively (Delene and Ogren, 2002), Hyytiälä, Finland (Virkkula et al, 2011), Lamont, USA (Delene and Ogren, 2002), Bondville, USA (Delene and Ogren, 2002), Elandsfontein, South Africa (this study), Granada, Spain (Lyamani et al, 2008), Guangzhou, China (Garland et al, 2008), Beijing, China (Garland et al, 2009). Error bars represent standard deviation, negative values are not shown.…”

“…They measured absorption with a MAAP so their σ AP was used as such. Virkkula et al (2011) presented aerosol optical data measured at the boreal forest site of SMEAR II in Finland. In this paper both scattering and absorption and respective Angström exponents were presented at three wavelengths so they were interpolated to 637 nm.…”