Browsing by Subject "OMI"

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  • Sundström, A. -M.; Nikandrova, A.; Atlaskina, K.; Nieminen, T.; Vakkari, V.; Laakso, L.; Beukes, J. P.; Arola, A.; van Zyl, P. G.; Josipovic, M.; Venter, A. D.; Jaars, K.; Pienaar, J. J.; Piketh, S.; Wiedensohler, A.; Chiloane, E. K.; de Leeuw, G.; Kulmala, Markku (2015)
    Proxies for estimating nucleation mode number concentrations and further simplification for their use with satellite data have been presented in Kulmala et al. (2011). In this paper we discuss the underlying assumptions for these simplifications and evaluate the resulting proxies over an area in South Africa based on a comparison with a suite of ground-based measurements available from four different stations. The proxies are formulated in terms of sources (concentrations of precursor gases (NO2 and SO2) and UVB radiation intensity near the surface) and a sink term related to removal of the precursor gases due to condensation on pre-existing aerosols. A-Train satellite data are used as input to compute proxies. Both the input data and the resulting proxies are compared with those obtained from ground-based measurements. In particular, a detailed study is presented on the substitution of the local condensation sink (CS) with satellite aerosol optical depth (AOD), which is a column-integrated parameter. One of the main factors affecting the disagreement between CS and AOD is the presence of elevated aerosol layers. Overall, the correlation between proxies calculated from the in situ data and observed nucleation mode particle number concentrations (N-nuc) remained low. At the time of the satellite overpass (13: 00-14: 00 LT) the highest correlation is observed for SO2/CS (R-2 D 0.2). However, when the proxies are calculated using satellite data, only NO2/AOD showed some correlation with N-nuc (R-2 D 0.2). This can be explained by the relatively high uncertainties related especially to the satellite SO2 columns and by the positive correlation that is observed between the ground-based SO2 and NO2 concentrations. In fact, results show that the satellite NO2 columns compare better with in situ SO2 concentration than the satellite SO2 column. Despite the high uncertainties related to the proxies calculated using satellite data, the proxies calculated from the in situ data did not better predict N-nuc. Hence, overall improvements in the formulation of the proxies are needed.
  • Heikkilä, Anu; Makelä, Jakke Sakari; Lakkala, Kaisa; Meinander, Outi; Kaurola, Jussi; Koskela, Tapani; Karhu, Juha Matti; Karppinen, Tomi; Kyro, Esko; De Leeuw, Gerrit (2016)
    The two Brewer spectrophotometers of the Finnish Meteorological Institute at Jokioinen and Sodankyla have been operated according to the highest levels of the WMO/GAW (World Meteorological Organization/Global Atmosphere Watch) recommendations with rigorous quality control and quality assurance. The calibration of the instruments is based on annual recalibrations of primary standard lamps in the VTT MIKES Metrology National Standards Laboratory in Finland and an exhaustive measurement program with measurements of standard and working lamps in the on-site optical laboratories. Over the years, the maintenance of the calibration has produced data sets of approximately 2000 lamp scans for both instruments. An extensive re-examination of the lamp measurements and the response of the spectrophotometers was carried out. The primary standard lamps were found to age on an average rate of 0.3% per burn. The responsivity at wavelength 311 nm was found to exhibit both long-term and short-term changes. The overall long-term change was declining. In addition, abrupt changes of as large as 25% were detected. The short-term changes were found to fluctuate on time frames shorter than the interval between the measurements of the primary standard lamps. This underlines the importance of the use of more frequently measured working standard lamps.