Cloud condensation nuclei in polluted air and biomass burning smoke near the mega-city Guangzhou, China – Part 2: Size-resolved aerosol chemical composition, diurnal cycles, and externally mixed weakly CCN-active soot particles

Abstract

Abstract. Size-resolved chemical composition, mixing state, and cloud condensation nucleus (CCN) activity of aerosol particles in polluted mega-city air and biomass burning smoke were measured during the PRIDE-PRD2006 campaign near Guangzhou, China, using an aerosol mass spectrometer (AMS), a volatility tandem differential mobility analyzer (VTDMA), and a continuous-flow CCN counter (DMT-CCNC).The size-dependence and temporal variations of the effective average hygroscopicity parameter for CCN-active particles (κ a ) could be parameterized as a function of organic and inorganic mass fractions (f org , f inorg ) determined by the AMS: κ a,p =κ org · f org + κ inorg · f inorg . The characteristic κ values of organic and inorganic components were similar to those observed in other continental regions of the world: κ org ≈0.1 and κ inorg ≈0.6. The campaign average κ a values increased with particle size from ∼0.25 at ∼50 nm to ∼0.4 at ∼200 nm, while f org decreased with particle size. At ∼50 nm, f org was on average 60% and increased to almost 100% during a biomass burning event.The VTDMA results and complementary aerosol optical data suggest that the large fractions of CCN-inactive particles observed at low supersaturations (up to 60% at S≤0.27%) were externally mixed weakly CCN-active soot particles with low volatility (diameter reduction <5% at 300 • C) andeffective hygroscopicity parameters around κ LV ≈0.01. A proxy for the effective average hygroscopicity of the total ensemble of CCN-active particles including weakly CCNactive particles (κ t ) could be parameterized as a function of κ a,p and the number fraction of low volatility particles determined by VTDMA (φ LV ): κ t,p =κ a,p −φ LV · (κ a,p −κ LV ).Based on κ values derived from AMS and VTDMA data, the observed CCN number concentrations (N CCN,S ≈10 2 -10 4 cm −3 at S = 0.068-0.47%) could be efficiently predicted from the measured particle number size distribution. The mean relative deviations between observed and predicted CCN concentrations were ∼10% when using κ t,p , and they increased to ∼20% when using only κ a,p . The mean relative deviations were not higher (∼20%) when using an approximate continental average value of κ≈0.3, although the constant κ value cannot account for the observed temporal variations in particle composition and mixing state (diurnal cycles and biomass burning events).Overall, the results confirm that on a global and climate modeling scale an average value of κ≈0.3 can be used for approximate predictions of CCN number concentrations in continental boundary layer air when aerosol size distribution data are available without information about chemical composition. Bulk or size-resolved data on aerosol chemical composition enable improved CCN predictions resolving regional and temporal variations, but the composition data need to be highly accurate and complemented by information about particle mixing state to achieve high precision (relative deviations <20%).

Assessment of cloud supersaturation by size-resolved aerosol particle and cloud condensation nuclei (CCN) measurements

“…Thus, we tested a range of relevant hygroscopicity parameters (κ) as specified in Appendices A and B and Table 1: (1) κ a as determined from the measured CCN efficiency spectra during out-of-cloud conditions (κ a = 0.19), (2) κ cut as determined from the observed total CCN concentration and the CN size distribution during out-of-cloud conditions (κ cut = 0.20), (3) a global average κ value for continental aerosol of κ mean = 0.3 (Pringle et al, 2010), and (4) κ AMS as derived from the aerosol chemical composition (κ AMS = 0.45). The large difference between κ AMS and κ a or κ cut can be attributed to a size dependence of particle hygroscopicity with an enhanced organic mass fraction for sub-100 nm particles, as observed in earlier studies (e.g., Gunthe et al, 2009Gunthe et al, , 2011Rose et al, 2011). The parameter κ AMS reflects the mass-weighted average hygroscopicity of the total aerosol, which is dominated by large accumulation-mode particles with an enhanced inorganic fraction (typically around 300 nm).…”

“…in which κ org and κ inorg are the hygroscopicity parameters for pure organic and inorganic substances (κ org = 0.1 and κ inorg = 0.6) Dusek et al, 2010;Rose et al, 2011). For the average AMS-derived hygroscopicity parameter we obtained κ AMS = 0.45.…”

“…As a result of long-term efforts and recent advances in atmospheric research, the CCN activity of atmospheric aerosol particles can be fairly well constrained by measurements and models (e.g., Kreidenweis, 2007, 2008;Gunthe et al, 2009;Kinne, 2009;Moore and Nenes, 2009;Juranyi et al, 2010;Pringle et al, 2010;Rose et al, 2010;Wex et al, 2010;Gunthe et al, 2011;Kim et al, 2011;Kulmala et al, 2011;Rose et al, 2011;Spracklen et al, 2011;Anttila et al, 2012;Christensen and Petters, 2012;Engelhart et al, 2012;Padró et al, 2012;Jurányi et al, 2013;Lance et al, 2013;Mikhailov et al, 2013;Petters and Kreidenweis, 2013). To fully describe the process of CCN activation and cloud droplet growth in the atmosphere, however, the supersaturation of water vapor in the cloud also needs to be known (e.g., Reutter et al, 2009;Pruppacher and Klett, 2010;Rosenfeld et al, 2012;Renno et al, 2013).…”

The scientific basis for a satellite mission to retrieve CCN concentrations and their impacts on convective clouds

“…When r e reaches ∼14 µm in the convective clouds, warm rain starts to form (e.g., Rosenfeld, 1999 and2000;Rosenfeld and Gutman, 1994;Gerber, 1996). As a result, if the cloud base temperature and pressure are known, the cloud depth at which r e reaches the 14 µm warm rain threshold (D p ) depends only on N a .…”

“…Fortunately, recent research has yielded considerable information about κ from both theoretical studies and laboratory and field measurements. This research shows that submicron aerosols, which constitute essentially almost all CCN, can usually be represented as linear mixtures of an organic component with κ ≈0.1 to 0.2 and an inorganic component with κ ≈0.6 (Gunthe et al, 2009; Rose et al, 2011). Where appropriate, an additional sea salt component with a κ ≈1.3 can be introduced.…”