Non-conventional techniques to monitor the aerosol vertical properties in the atmosphere over an Alpine station
H. Diémoz
1, T. Magri
1, G. Pession
1, C. Tarricone
1, M. Zublena
1, V. Marinelli
2and A.M. Siani
2Besides conventional techniques to monitor the amount and composition of particulate matter (PM) in the boundary layer, the Environmental Protection Agency (ARPA) of the Aosta Valley (Fig. 1) has started a research program to characterise the burden and radiative/microphysical properties of the total aerosol column in the whole atmosphere over an Alpine valley site. To this purpose, the following instruments:
a sun/sky photometer
an Advanced Lidar Ceilometer (ALC)
two optical particle sizers
have been installed at the same measuring station (“supersite” of Saint-Christophe, Aosta, Northern Italy, 45.74°N, 7.36°E, 560 m a.s.l.) and non-conventional techniques are employed.
1ARPA Valle d’Aosta, Dept. Air and Atmosphere, Saint-Christophe, 11020, Italy
2Sapienza – University of Rome, Dept. Physics, Rome, 00185, Italy
h.diemoz@arpa.vda.it
Introduction
A POM-02 sun/sky photometer (Fig. 2, left), included in the EuroSkyRad network (www.euroskyrad.net), measures the direct irradiance at 11 different wavelengths from the sun (every minute) and the diffuse radiance from the sky (every 10 minutes). By the Bouguer-Lamber-Beer law (direct component) and other retrieval algorithms (specific for the diffuse component), the measurements are inverted and the Aerosol Optical Depth (AOD) of the total atmospheric column (troposphere and stratosphere), Angstrom Exponent (AE), Single Scattering Albedo (SSA), volume distribution (Fig. 2, right), complex refractive index and phase functions are obtained.
Sun-sky Photometer
The optical particle sizers (OPS) provide the volume distribution (one of them also provides the mass distribution with the same accuracy as the gravimetric method) for several diameter classes. A comparison between the in-situ (mixing layer) aerosol properties measured by the OPSs and the columnar volume distribution estimated from the sun/sky photometer will be a valuable outcome from the measurement program.
In addition, the density vs size relation measured at ground can be used to estimate the vertical profile of aerosol mass concentration (from the number concentration retrieved by the ALC and the photometer) and to assess the impact of elevated aerosol layers sinking into the mixing layer on air quality at ground in compliance to the legislative PM limits.
Optical Particle Sizers
The supersite of ARPA will provide stimulating results for the atmospheric science, e.g.:
additional knowledge about the optical and microphysical aerosol properties at ground and in the upper atmospheric layers
information on vertical dynamics and mixing in a Alpine valley atmosphere, and impact on air quality
useful data for the validation of chemical transport models and satellites
assessment of the aerosol radiative forcing effects, using measurements from co-located UV and short-/long-wave radiometers.
Conclusions and further research
Fig.1 The measuring supersite of Saint-Christophe, Aosta (Aosta Valley).
Fig. 2 The sun/sky photometer (left), pointing to the sun, and an example of volume spectrum (right) during a Saharan dust event (high concentration at larger radii).
Whereas the photometer only allows the retrieval of average column properties, the Lufft CHM15k-Nimbus ceilometer, belonging to the Italian (Alice-net, www.alice- net.eu) and European (E-profile, www.eumetnet.eu/alc-network) networks, allows to map the aerosol vertical distribution. The instrument emits an impulsed laser at 1064 nm wavelength towards the zenith and records its backscatter (Fig. 3, upper panel) from the atmosphere, as a function of the range, up to 15 km altitude. The main targets of the laser beam are cloud droplets and aerosol particles. The vertical profile of the attenuated aerosol backscatter coefficient can be determined from the signal received by the LiDAR (Klett 1981; Klett 1985; Wiegner and Geiss, 2012), then both the aerosol extinction coefficient and number concentration vertical profiles can be retrieved by the Mie theory (Barnaba and Gobbi, 2001; Barnaba and Gobbi, 2004, Dionisi et al., 2015) and compared with the columnar data from the photometer. One of the several interesting case studies (i.e., advection of polluted air masses due to thermally-driven up-valley winds from the Po basin, Fig. 3) is presented as a good example of synergy between measurements and the output of a chemical transport 2D/3D eulerian Flexible Air quality Regional Model (FARM) driven by a nonhydrostatic limited-area atmospheric prediction model (COSMO) (Diémoz et al., 2016).
Advanced LiDAR Ceilometer (ALC)
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References
Fig. 3 Upper figure: backscattering profile (coloured background) from the ALC as a function of the time of the day (horizontal scale) and altitude (vertical scale). An advection of an aerosol-rich air mass from the Po Valley is visible in the afternoon (red area from the LiDAR and increase in both AOD and Angstrom Exponent). Lower panel: chemical transport models output, in agreement with measurements.
