Towards closing the gap between hygroscopic growth and activation for secondary organic aerosol: Part 1 – Evidence from measurements

Secondary Organic Aerosols (SOA) studied in previous laboratory experiments generally showed only slight hygroscopic growth, but a much better activity as a CCN (Cloud Condensation Nucleus) than indicated by the hygroscopic growth. This discrepancy was examined at LACIS (Leipzig Aerosol Cloud Interaction Simulator), using a portable generator that produced SOA particles from the ozonolysis of α-pinene, and adding butanol or butanol and water vapor during some of the experiments. The light scattering signal of dry SOA-particles was measured by the LACIS optical particle spectrometer and was used to derive a refractive index for SOA of 1.45. LACIS also measured the hygroscopic growth of SOA particles up to 99.6% relative humidity (RH), and a CCN counter was used to measure the particle activation. SOA-particles were CCN active with critical diameters of e.g. 100 nm and 55 nm at super-saturations of 0.4% and 1.1%, respectively. But only slight hygroscopic growth with hygroscopic growth factors ≤1.05 was observed at RH98%, the hygroscopic growth increased stronger than would be expected if a constant hygroscopicity parameter for the particle/droplet solution was assumed. An increase of the hygroscopicity parameter by a factor of 4–6 was observed in the RH-range from below 90% to 99.6%, and this increase continued for increasingly diluted particle solutions for activating particles. This explains an observation already made in the past: that the relation between critical super-saturation and dry diameter for activation is steeper than what would be expected for a constant value of the hygroscopicity. Combining measurements of hygroscopic growth and activation, it was found that the surface tension that has to be assumed to interpret the measurements consistently is greater than 55 mN/m, possibly close to that of pure water, depending on the different SOA-types produced, and therefore only in part accounts for the discrepancy between hygroscopic growth and CCN activity observed for SOA particles in the past

Short-term temporal variation in PM2.5 mass and chemical composition during the Atlanta Supersite Expriment, 1999

Measurements in urban Atlanta of transient aerosol events in which PM2.5 mass concentrations rapidly rise and fall over a period of 3-6 hr are reported. The data are based on new measurement techniques demonstrated at the U.S. Environmental Protection Agency (EPA) Atlanta Supersite Experiment in August 1999. These independent instruments for aerosol chemical speciation of NO3-, SO4(2-), NH4+, and organic and elemental carbon (OC and EC), reconstructed the observed hourly dry PM2.5 mass to within 20% or better. Data from the experiment indicated that transient PM2.5 events were ubiquitous in Atlanta and were typically characterized by a sudden increase of EC (soot) and OC in the early morning or SO4(2-) in the late afternoon. The frequent temporal decoupling of these events provides insights into their origins, suggesting mobile sources in metro Atlanta as the main contributor to early morning PM2.5 and more regionally located point SO2 sources for afternoon PM2.5 events. The transient events may also have health implications. New data suggest that short-term PM2.5 exposures may lead to adverse health effects. Standard integrated filter-based techniques used in PM2.5 compliance monitoring networks and in most past PM2.5 epidemiologic studies collect samples over 24-hr periods and thus are unable to capture these transient events. Moreover, health-effects studies that focus on daily PM2.5 mass alone cannot evaluate the health implications of the unique and variable chemical properties of these episodes

Characterisation of aerosol properties and radiative forcing at an anthropogenically perturbed continental site

Measurements of optical, chemical and physical properties of ambient aerosol particles were obtained at a continental anthropogenically pertubed environment. For aerosol sampled at the site from January to December 1995, the mean gravimetric mass concentration for particles with diameter d(p) ≤ 1μm on 24-hr filter samples was 11.0 ± 7.6 μg/m3. Ion chromatographic (IC) analysis chemically identified 50 ± 15% of the total gravimetric mass for fine particles (d(p) ≤ 1 μm). Most of the IC identified material (88 ± 14%) consisted of NH4+ and SO42-. Thermal optical reflectance (TOR) analysis of small number of samples showed that the material not identified with IC consisted of elemental and organic carbon. To examine the implications of the measurements and effect of their variation, the mean annual direct radiative forcing (ΔF(R)) due to aerosol particles at the site was estimated by a simple model using as input measured total light scattering coefficients, hemispheric backscatter ratio and hygroscopic growth factor. For a mean annual height of the mixed layer at 800 m, an assumed single scattering albedo (ω) value of 0.85 and wavelength 550 nm,ΔF(R) as function of relative humidity (RH) ranged between -0.51±0.47 (RH = 40%) and -0.63 ±0.58 (RH = 80%) W/m2. Sensitivity tests pointed to the importance of accurate quantification of ω

Towards closing the gap between hygroscopic growth and activation for secondary organic aerosol - Part 2: Theoretical approaches

We examine the hygroscopic properties of secondary organic aerosol particles generated through the reaction of α-pinene and ozone using a continuous flow reaction chamber. The water activity versus composition relationship is calculated from measurements of growth factors at relative humidities up to 99.6% and from measurements of cloud condensation nuclei activity. The observed relationships are complex, suggesting highly non-ideal behavior for aerosol water contents at relative humidities less than 98%. We present two models that may explain the observed water activity-composition relationship equally well. The first model assumes that the aerosol is a pseudo binary mixture of infinitely water soluble compounds and sparingly soluble compounds that gradually enter the solution as dilution increases. The second model is used to compute the Gibbs free energy of the aerosol-water mixture and shows that the aerosol behaves similarly to what can be expected for single compounds that contain a certain fraction of oxygenated and non-polar functional groups