Abstract
Abstract. Atmospheric mineral dust influences Earth's radiative budget, cloud formation, and lifetime; has adverse health effects; and affects air quality through the increase of regulatory PM10 concentrations, making its real-time quantification in the atmosphere of strategic importance. Only few near-real-time techniques can discriminate dust aerosol in PM10 samples and they are based on the dust chemical composition. The online determination of mineral dust using aerosol absorption photometers offers an interesting and competitive alternative but remains a difficult task to achieve. This is particularly challenging when dust is mixed with black carbon, which features a much higher mass absorption cross section. We build on previous work using filter photometers and present here for the first time a highly time-resolved online technique for quantification of mineral dust concentration by coupling a high-flow virtual impactor (VI) sampler that concentrates coarse particles with an aerosol absorption photometer (Aethalometer, model AE33). The absorption of concentrated dust particles is obtained by subtracting the absorption of the submicron (PM1) aerosol fraction from the absorption of the virtual impactor sample (VI-PM1 method). This real-time method for detecting desert dust was tested in the field for a period of 2 months (April and May 2016) at a regional background site of Cyprus, in the Eastern Mediterranean. Several intense desert mineral dust events were observed during the field campaign with dust concentration in PM10 up to 45 µg m−3. Mineral dust was present most of the time during the campaign with an average PM10 of about 8 µg m−3. Mineral dust absorption was most prominent at short wavelengths, yielding an average mass absorption cross section (MAC) of 0.24±0.01 m2 g−1 at 370 nm and an absorption Ångström exponent of 1.41±0.29. This MAC value can be used as a site-specific parameter for online determination of mineral dust concentration. The uncertainty of the proposed method is discussed by comparing and validating it with different methods.
Highlights
Atmospheric dust often dominates PM10 aerosol mass concentrations in many regions of the world and is the second most abundant aerosol source at a global scale just after sea spray
The proposed VI method takes advantage of the concentration of coarse particles using a virtual impactor to enhance the coarse fraction in the sample and subtracts the absorption of the fine fraction
Real-time dust concentration of PM10 (Sect. 3.4) is derived by dividing the absorption of dust aerosols calculated in Sect. 3.3 with a mass absorption cross section (MAC) for dust calculated using filter-based chemical analyses
Summary
Atmospheric dust often dominates PM10 aerosol mass concentrations in many regions of the world and is the second most abundant aerosol source at a global scale just after sea spray. Dust particles modify the Earth’s radiation balance as they absorb and scatter light, affecting regional climate and precipitation regimes. The net radiative effect of atmospheric dust depends on the interplay between the heating of the atmosphere, due to the increased absorption of sunlight, and cooling due to scattering of sunlight back into space, leading to a direct radiative forcing for dust estimated around −0.1 ± 0.2 W m−2 (Myhre et al, 2013). Dust deposits on snow and ice increase the ion content in snow and snow water (Greilinger et al, 2018), and these exert a warming influence after deposition (Di Mauro et al, 2015).
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