Determination of high-time resolution mineral dust concentration in real-time by optical absorption measurements

Mineral dust is an important natural source of aerosols and significantly influences air quality (Querol et al.,
Environ. Int., 2019) and the global radiation budget (Schepanski, Geosci., 2018). Frequent dust intrusions are
observed in the Mediterranean region (Ealo et al., Atmos. Chem. Phys., 2016; Pikridas et al., Atmos. Environ.,
2018) and Central Europe (Collaud Coen et al., Atmos. Chem. Phys., 2004; Schauer et al., Aerosol Air Qual.
Res., 2016), with high potential to cause exceedances of daily PM10 levels. To separate the influence of
anthropogenic and natural contribution to the PM10 levels, the new method was developed within the DNAAP
project (Detection of non-anthropogenic air pollution – http://www.aerosol.si/dnaap/).
Dust weakly absorbs light in the near ultra-violet and short wavelengths of the visible range, while the light
absorption of dust in longer wavelengths from the visible and near infra-red range is negligible. We used
filter-based photometer Aethalometer AE33 (Drinovec et al., Atmos. Meas. Tech., 2015) to measure the light
absorption at seven wavelengths, from 370 to 950 nm. The mineral dust is not the only light-absorbing aerosol
in the air. Black carbon (BC), a unique primary tracer for combustion emissions, strongly absorbs light across
the entire visual, near infra-red and near ultra-violet spectral range. Since optical absorption of mineral dust is
weaker than the optical absorption of black carbon, the coarse mode mineral particles have to be concentrated
using the high-volume virtual impactor (VI). The method is based on the optical absorption measurements of
the two sample streams, sampling particle size below 1 µm and sample stream with the concentrated coarse
mode particles, where mineral dust contribution is substantial. Experimental configuration includes two
Aethalometers AE33 with different size selective inlets: VI inlet for sampling coarse aerosol mode (mostly
mineral dust) and PM1 inlet for sampling fine mode of aerosols (mainly BC). The optical absorption of mineral
dust can be determined by subtracting the absorption of fine aerosol fraction (PM1) from the absorption of
aerosol sampled by the VI, taking into account the enhancement factor of VI setup (Drinovec et al., Atmos.
Meas. Tech., 2020). The mineral dust mass concentration is then calculated using mass absorption cross-section
(MAC) for dust which could be site and source-region specific.
The results from the measurement campaigns performed at six locations in the Mediterranean region will be
presented. The measurements took place in NE Spain (Barcelona – BCN, Montseny – MSY, Montsec – MSA),
on Cyprus (Nicosia – NI, Agia Marina Xyliatou – AMX), and in Slovenia (Ljubljana – LJ). Two year-long
datasets will be presented, focusing on the analyses of aerosol optical properties of PM1 and VI fractions. The
results were validated using low time resolution chemical specification of offline filters and a statistical
approach where dust was extracted from PM10 measurements for dust intrusions periods determined by models
and back-trajectory studies. For better understanding, helium ion microscopy (HIM) was applied to study the
microscopic differences between mineral dust and black carbon captured on the AE33 filter tapes.
This work was supported by SPIRIT Slovenia – Public Agency for Entrepreneurship, Internationalization,
Foreign Investments and Technology, project DNAAP.