Abstract
Abstract. For understanding and assessing aerosol–cloud interactions and their impact on climate, reliable measurement data on aerosol particle hygroscopicity and cloud condensation nuclei (CCN) activity are required. The CCN activity of aerosol particles can be determined by scanning particle size and supersaturation (S) in CCN measurements. Compared to an existing differential mobility analyzer (DMA) with CCN activity measurement, a broad supersaturation scanning CCN (BS2-CCN) system, in which particles are exposed to a range of S simultaneously, can measure the CCN activity with a high time resolution. Based on a monotonic relation between the activation supersaturation of aerosol particles (Saerosol) and the activated fraction (Fact) of the BS2-CCN measurement, we can derive κ, a single hygroscopicity parameter, directly. Here, we describe how the BS2-CCN system can be effectively calibrated and which factors can affect the calibration curve (Fact−Saerosol). For calibration, size-resolved CCN measurements with ammonium sulfate and sodium chloride particles are performed under three different thermal gradient (dT) conditions (dT=6, 8, and 10 K). We point out key processes that can affect the calibration curve and thereby need to be considered as follows: first, the shape of the calibration curve is primarily influenced by Smax, the maximum S in the activation tube. We need to determine appropriate Smax depending on the particle size and κ to be investigated. To minimize the effect of multiply charged particles, a small geometric mean diameter (Dg) and geometric standard deviation (σg) in number size distribution are recommended when generating the calibration aerosols. Last, Fact is affected by particle number concentration and has a decreasing rate of 0.02 per 100 cm−3 due to the water consumption in the activation tube. For evaluating the BS2-CCN system, intercomparison experiments between typical DMA-CCN and BS2-CCN measurements were performed with a laboratory-generated aerosol mixture and ambient aerosols. Good agreement of κ values between DMA-CCN and BS2-CCN measurements for both experiments shows that the BS2-CCN system can measure CCN activity well compared to the existing measurement method and can measure a broad range of hygroscopicity distributions with a high time resolution (∼1 s vs. a few minutes for a standard CCN activity measurement). As the hygroscopicity can be used as a proxy for the chemical composition, our method can also serve as a complementary approach for fast and size-resolved detection and estimation of aerosol chemical composition.
Highlights
Atmospheric aerosol particles affect global climate change in that aerosols alter the radiative balance by scattering and absorbing shortwave and longwave radiation directly
For the calibration of the BS2-cloud condensation nuclei (CCN) system, the goal is to determine the monotonic Fact − Saerosol relation, as discussed above. This can be obtained from size-resolved CCN measurement with pure calibration aerosols, e.g., ammonium sulfate and sodium chloride, in which hygroscopic properties are well known
Ammonium sulfate and sodium chloride, which are representative calibration aerosols for CCN counter (CCNC), are used under three different different thermal gradient (dT) conditions. It can be inferred from consistent results between the two calibration aerosols as well as reference curves for all three dT conditions that the experimental setting of the BS2-CCN system suggested in this study is appropriate and can apply to real measurements
Summary
Atmospheric aerosol particles affect global climate change in that aerosols alter the radiative balance by scattering and absorbing shortwave and longwave radiation directly. By serving as cloud condensation nuclei (CCN), atmospheric aerosol particles influence the radiative budget by modulating the microphysical structure, lifetime, and coverage of the clouds. The CCN activity, meaning the fraction at which aerosol particles can be activated to become CCN, can be determined by particle size and chemical composition at a given water vapor supersaturation (Charlson et al, 2001; Andreae et al, 2005, 2009; McFiggans et al, 2006; Cai et al, 2018) and can be parameterized by a single parameter, κ (Petters and Kreidenweis, 2007). The parameter κ can greatly simplify descriptions of the chemical composition effect in models
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
