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Opis: Aerosol absorption coefficient measurements classically feature a very large uncertainty, especially given the absence of a reference method. The most used approach using filter-photometers is by measuring the attenuation of light through a filter where aerosols are being deposited. This presents several artifacts, with cross-sensitivity to scattering being most important at high single scattering albedo with the error exceeding 100%. We present lab campaign results where we have resuspended dust samples from different mid-latitude desert regions and measured the dust absorption and scattering coefficients, their mass concentration and the particle size distribution. The absorption coefficients were measured with two types of filter photometers: a Continuous Light Absorption Photometers (CLAP) and a multi-wavelength Aethalometer (AE33). The  dual-wavelength photo-thermal interferometer (PTAAM-2λ) was employed as the reference. Scattering coefficients were measured with an Ecotech Aurora 4000 nephelometer. The mass concentration was obtained after the weighting of filters before and after the sampling, and the particle size distribution (PSD) was measured by means of optical particle counters (Grimm 11-D).Measurements of the scattering with the nephelometer and absorption with the PTAAM-2λ we obtained the filter photometer multiple scattering parameter and cross-sensitivity to scattering as a function of the different sample properties. Moreover, by determining the mass concentration and the absorption coefficients of the samples, we derived the mass absorption cross-sections of the different dust samples, which can be linked to their size distribution as well as to their mineralogical composition.The focus of the JATAC campaign in September 2021 and September 2022 on and above Cape Verde Islands was on the calibration/validation of the ESA Aeolus satellite ALADIN lidar, however, the campaign also featured secondary scientific climate-change objectives. As part of this campaign, a light aircraft was set-up for in-situ aerosol measurements. Several flights were conducted over the Atlantic Ocean up to and above 3000 m above sea level during intense dust transport events. The aircraft was instrumented to determine the absorption coefficients using a pair of Continuous Light Absorption Photometers (CLAPs) measuring in the fine and coarse fractions separately, with parallel measurements of size distributions in these size fractions using two Grimm 11-D Optical Particle Size Spectrometers (OPSS). In addition, we performed measurements of the total and diffuse solar irradiance with a DeltaT SPN1 pyranometer.The combination of the absorption and PSD with source identification techniques enabled the separation of the contributions to  absorption by dust and black carbon. The atmospheric heating rate of these two contributions was determined by adding the irradiance measurements. Therefore, the integration of the results from the Using laboratory resuspension experiments  to interpret the airborne measurements is of great relevance for the determination  of the radiative effect of the Saharan Aerosol Layer as measured over the tropical Atlantic ocean.
Ključne besede: black carbon, mineral dust, Saharan dust, atmospheric heating rate, climate change, airborne measurements
Objavljeno v RUNG: 18.03.2024; Ogledov: 273; Prenosov: 2
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Opis: We experimentally quantified the impact of cloud fraction and cloud type on the heating rate (HR) of black and brown carbon (HRBC and HRBrC). In particular, we examined in more detail the cloud effect on the HR detected in a previous study (Ferrero et al., 2018). High-time-resolution measurements of the aerosol absorption coefficient at multiple wavelengths were coupled with spectral measurements of the direct, diffuse and surface reflected irradiance and with lidar–ceilometer data during a field campaign in Milan, Po Valley (Italy). The experimental set-up allowed for a direct determination of the total HR (and its speciation: HRBC and HRBrC) in all-sky conditions (from clear-sky conditions to cloudy). The highest total HR values were found in the middle of winter (1.43 ± 0.05 K d−1), and the lowest were in spring (0.54 ± 0.02 K d−1). Overall, the HRBrC accounted for 13.7 ± 0.2 % of the total HR, with the BrC being characterized by an absorption Ångström exponent (AAE) of 3.49 ± 0.01. To investigate the role of clouds, sky conditions were classified in terms of cloudiness (fraction of the sky covered by clouds: oktas) and cloud type (stratus, St; cumulus, Cu; stratocumulus, Sc; altostratus, As; altocumulus, Ac; cirrus, Ci; and cirrocumulus–cirrostratus, Cc–Cs). During the campaign, clear-sky conditions were present 23 % of the time, with the remaining time (77 %) being characterized by cloudy conditions. The average cloudiness was 3.58 ± 0.04 oktas (highest in February at 4.56 ± 0.07 oktas and lowest in November at 2.91 ± 0.06 oktas). St clouds were mostly responsible for overcast conditions (7–8 oktas, frequency of 87 % and 96 %); Sc clouds dominated the intermediate cloudiness conditions (5–6 oktas, frequency of 47 % and 66 %); and the transition from Cc–Cs to Sc determined moderate cloudiness (3–4 oktas); finally, low cloudiness (1–2 oktas) was mostly dominated by Ci and Cu (frequency of 59 % and 40 %, respectively). HR measurements showed a constant decrease with increasing cloudiness of the atmosphere, enabling us to quantify for the first time the bias (in %) of the aerosol HR introduced by the simplified assumption of clear-sky conditions in radiative-transfer model calculations. Our results showed that the HR of light-absorbing aerosol was ∼ 20 %–30 % lower in low cloudiness (1–2 oktas) and up to 80 % lower in completely overcast conditions (i.e. 7–8 oktas) compared to clear-sky ones. This means that, in the simplified assumption of clear-sky conditions, the HR of light-absorbing aerosol can be largely overestimated (by 50 % in low cloudiness, 1–2 oktas, and up to 500 % in completely overcast conditions, 7–8 oktas). The impact of different cloud types on the HR was also investigated. Cirrus clouds were found to have a modest impact, decreasing the HRBC and HRBrC by −5 % at most. Cumulus clouds decreased the HRBC and HRBrC by −31 ± 12 % and −26 ± 7 %, respectively; cirrocumulus–cirrostratus clouds decreased the HRBC and HRBrC by −60 ± 8 % and −54 ± 4 %, which was comparable to the impact of altocumulus (−60 ± 6 % and −46 ± 4 %). A higher impact on the HRBC and HRBrC suppression was found for stratocumulus (−63 ± 6 % and −58 ± 4 %, respectively) and altostratus (−78 ± 5 % and −73 ± 4 %, respectively). The highest impact was associated with stratus, suppressing the HRBC and HRBrC by −85 ± 5 % and −83 ± 3 %, respectively. The presence of clouds caused a decrease of both the HRBC and HRBrC (normalized to the absorption coefficient of the respective species) of −11.8 ± 1.2 % and −12.6 ± 1.4 % per okta. This study highlights the need to take into account the role of both cloudiness and different cloud types when estimating the HR caused by both BC and BrC and in turn decrease the uncertainties associated with the quantification of their impact on the climate.
Ključne besede: black carbon, brown carbon, cloud, atmospheric heating rate, climate change
Objavljeno v RUNG: 29.03.2021; Ogledov: 2336; Prenosov: 0
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