Cloud forming potential of secondary organic aerosol under near atmospheric conditions

Abstract

[1] Cloud droplets form by nucleation on atmospheric aerosol particles. Populations of such particles invariably contain organic material, a major source of which is thought to be condensation of photo-oxidation products of biogenic volatile organic compounds (VOCs). We demonstrate that smog chamber studies of the formation of such biogenic secondary organic aerosol (SOA) formed during photooxidation must be conducted at near atmospheric concentrations to yield atmospherically representative particle composition, hygroscopicity and cloud-forming potential. Under these conditions, the hygroscopicity measured at 95% relative humidity can be used reliably to predict the CCN activity of the SOA particles by assuming droplet surface tension of pure water. We also show that the supersaturation required to activate a given size of particle decreases with age. [2] Aerosol particles partially counteract the warming effect of greenhouse gases by modifying cloud radiative properties. Depending on their size and chemical composition, natural and anthropogenic aerosol particles may act as cloud condensation nuclei (CCN) and thereby change the cloud radiative properties and lifetime. The magnitude of this effect is the single biggest unknown in modeling radiative forcing (http://www.ipcc.ch.). Atmospheric aerosols often consist of (internal) mixtures of organic and inorganic substances. The cloud forming potential (i.e. the hygroscopic properties) of the inorganic constituents are generally well known, whereas not much is known on the hygroscopic properties associated with the organic matter, which typically contributes 20-50% to the mass of fine aerosol particles [Kanakidou et al., 2005]. Secondary organic aerosol (SOA), formed by photo-oxidation of volatile organic compounds (VOCs), is a major contribution to this organic fraction [Robinson et al., 2007]. In this study, we conducted smog chamber experiments on the SOA's hygroscopic properties and ability to act as CCN, using precursor concentrations approaching atmospheric levels. Hygroscopicity measurements in sub-and supersaturated conditions were compared in order to investigate potential surface tension reduction effects of SOA solution droplets. The surface tension may be important for models inferring the CCN potential from measured hygroscopic properties at sub-saturated conditions. It has been shown that organic material collected from Po Valley fog samples may lower the surface tension in bulk solution and it was inferred that they may similarly affect surface tension in growing cloud droplets [Facchini et al., 1999].[3] Experiments were performed at the PSI chamber [Paulsen et al., 2005] using varying initial concentrations of 183 to 10 ppbv a-pinene, a frequently used aerosol precursor representative of biogenic emissions (comparable results were found when using the anthropogenic organic aerosol precursor 1,3,5-trimethylbenzene). The chemical nature of the SOA particles was investigated using the Aerodyne aerosol mass spectrometer (AMS) [Jayne et al., 2000] (see...

Intercomparison study of six HTDMAs: results and recommendations

“…For dew point sensors, it is recommended to perform also a saturation test. The measured dew points can periodically be checked by feeding air saturated with water vapour into the dew point sensors at a well defined temperature (the same temperature as in the DMA2) and by measuring the resulting dew point and DMA2 temperature (Duplissy et al, 2008;Weingartner et al, 2002). This allows an internal calibration of the dew point sensors by adjusting the measured dew point temperatures to the measured DMA2 temperature.…”

“…The first HTDMA systems used passive temperature control methods (McMurry and Stolzenburg, 1989) ideally keeping their systems in air-conditioned rooms or insulating them from their surroundings (Virkkula et al, 1999), before active temperature control was introduced (Brechtel et al, 2000;Prenni et al, 2001). Present HTDMA instruments use water baths (Cubison et al, 2005;Hennig et al, 2005;, temperature controlled cabinet (Cocker III et al, 2001;Duplissy et al, 2008;Prenni et al, 2001;Villani et al, 2008) or passive, insulated regions (Johnson et al, 2008;Virkkula et al, 1999). HTDMA1 and 6 of this study had at least DMA2 operated in a temperature controlled cabinet; HTDMA3 and 4 had at least DMA2 submersed in a water bath; the DMA2 of HTDMA2 and 5 were placed in a thermally isolated environment (see Fig.…”

“…α-Pinene (Aldrich, 98%) was evaporated in a heated glass sampling bulb and flushed into the humidified chamber with pure air to generate a mixing ratio of 20 (±2) ppb. Rather low, atmospherically relevant precursor concentrations were chosen because it had been shown that high precursor concentrations reduce the hygroscopicity of photo-oxidized SOA due to gas-to-particle partitioning effects of the aerosol (Duplissy et al, 2008). Four xenon arc lamps (4 kW each) were used to simulate the solar light spectrum and initiate the photochemical reaction.…”

Subarctic atmospheric aerosol composition: 3. Measured and modeled properties of cloud condensation nuclei

“…This approach aims at testing the potential bias of CCN predictions if neither measurements of the chemical composition nor a suitable proxy is available. k = 0.1 and k = 0.6 represent a lower (pure organic aerosol [Duplissy et al, 2008, Gysel et al, 2004) and upper limit (pure ammonium sulfate aerosol [Topping et al, 2005]), respectively, for the hygroscopicity of most atmospheric aerosols [Swietlicki et al, 2008], thereby excluding com- pletely insoluble and pure sea salt aerosols. k % 0.3 ± 0.1 has been recommended by Andreae and Rosenfeld [2008] for continental aerosols.…”

What do we learn from long-term cloud condensation nuclei number concentration, particle number size distribution, and chemical composition measurements at regionally representative observatories?

“…CC BY 4.0 License. Duplissy et al, 2008;Frosch et al, 2011;Jimenez et al, 2009;Massoli et al, 2010;Wong et al, 2011). In addition, several studies have investigated the effect of organic surfactants.…”