Abstract
Abstract. Aerosols influence the Earth radiative budget through scattering and absorption of solar radiation. Several methods are used to investigate aerosol properties and thus quantify their direct and indirect impacts on climate. At the Puy de Dôme station, continuous high-altitude near-surface in situ measurements and low-altitude ground-based remote sensing atmospheric column measurements give the opportunity to compare the aerosol extinction measured with both methods over a 1-year period. To our knowledge, it is the first time that such a comparison is realised with continuous measurements of a high-altitude site during a long-term period. This comparison addresses to which extent near-surface in situ measurements are representative of the whole atmospheric column, the aerosol mixing layer (ML) or the free troposphere (FT). In particular, the impact of multi-aerosol layers events detected using lidar backscatter profiles is analysed. A good correlation between in situ aerosol extinction coefficient and aerosol optical depth (AOD) measured by the Aerosol Robotic Network (AERONET) sun photometer is observed with a correlation coefficient around 0.80, indicating that the in situ measurements station is representative of the overall atmospheric column. After filtering for multilayer cases and correcting for each layer optical contribution (ML and FT), the atmospheric structure seems to be the main factor influencing the comparison between the two measurement techniques. When the site lies in the ML, the in situ extinction represents 45 % of the sun photometer ML extinction while when the site lies within the FT, the in situ extinction is more than 2 times higher than the FT sun photometer extinction. Moreover, the assumption of a decreasing linear vertical aerosol profile in the whole atmosphere has been tested, significantly improving the instrumental agreement. Remote sensing retrievals of the aerosol particle size distributions (PSDs) from the sun photometer observations are then compared to the near-surface in situ measurements, at dry and at ambient relative humidities. When in situ measurements are considered at dry state, the in situ fine mode diameters are 44 % higher than the sun-photometer-retrieved diameters and in situ volume concentrations are 20 % lower than those of the sun-photometer-retrieved fine mode concentration. Using a parameterised hygroscopic growth factor applied to aerosol diameters, the difference between in situ and retrieved diameters grows larger. Coarse mode in situ diameters and concentrations show a good correlation with retrieved PSDs from remote sensing.
Highlights
Over the last decade, aerosol studies have increased significantly due to the large uncertainty associated with their impact on climate in global models (IPCC, 2013)
The aerosol optical depth (AOD) is integrated over the whole atmospheric column while the aerosol in situ extinction is measured at one single altitude, they are strongly correlated (r2 = 0.82), indicating that the intermediate altitude of 1465 m, often at the interface between the mixing layer (ML) and the free troposphere (FT) where in situ measurements are performed is overall representative of the whole atmospheric column
When the Puy de Dôme station (PUY) site is in the ML (WCT > 1200), the fraction of the sun photometer AOD comprised in the ML (Eq 5) was calculated by using the ratio of ML/total backscatter measured by lidar following Eq (4)
Summary
Aerosol studies have increased significantly due to the large uncertainty associated with their impact on climate in global models (IPCC, 2013). Hervo et al (2014) showed important hygroscopic enhancement of aerosol optical properties at Puy de Dôme station (PUY) according different air mass origins over a 2-year period Both horizontal and vertical aerosol distributions determine the magnitude of the direct and indirect radiative effects and are still limits for general aerosol studies (Laj et al, 2009). The authors highlight the fact that taking into account the inhomogeneity of the atmosphere can significantly improve the correlation between groundlevel in situ measurements and remote sensing retrievals They show that correcting the sun photometer retrievals by the ML height decreases the discrepancy between measurements by about 10 %. We investigate the impact of the atmospheric structure and of the aerosol hygroscopicity on the agreement between the in situ and remote sensing measurement techniques
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