No abstract

[1] Although secondary organic aerosol formation is well studied, the extent to which oxidation products of biogenic volatile organic compounds condense onto primary aerosols and modify their cloud-nucleating properties remains highly uncertain. Here we show that water-soluble organic acids produced from the reaction between α-pinene and ozone rapidly accumulate onto preexisting particles forming coatings of organic materials that reach a mass fraction of 80-90% within a time period of 30 to 60 min for the reactant conditions of 7 to 37 ppbv α-pinene and 20 ppbv ozone. Cloud condensation nuclei (CCN) measurements reveal that the initially hydrophobic aerosols are rapidly converted to efficient CCN at a supersaturation of 0.22%. Our results imply that changes in the activation potential of a significant fraction of the atmospheric aerosol population are controlled by the formation and composition of coatings formed during the aging process, rather than by the original particle size or composition. Citation:

Measurements of cloud particle properties from aircraft by optical and impact techniques are subject to artifacts following particle breakup prior to detection. The impact kinetic energy to surface energy ratio (ℒ) provides a breakup criterion at ℒ ≥ 7 for water and ice with major fragmentation for ℒ > 100. This applies to optical imaging probes for particle concentration, size, and projected area spectra measurement. Uncertainty arises should impacting particles shatter and disperse, defeating the intent of the original measurements. Particle shatter is demonstrated in Formvar replicas (University of North Dakota Citation) and video records of particle approach and impact on the Cloudscope (NCAR C-130, NASA DC-8) at airspeeds of 130 and 200 m s−1. Sufficient impact kinetic energy results in drop splash and ice shatter, with conversion to surface energy and ultimately thermal energy through viscous dissipation and ice defect production occurring down to the molecular scale. The problem is minimized in design by reducing the regions responsible for particle breakup to a minimum and locating sensors in regions inaccessible to shatter fragments.

Spatial characterization and probability distribution of particles in stratiform mixed-phase clouds in the temperature range 5 to −45 • C were investigated based on 81 hours of flight on the NCAR C-130 during the Alliance Icing Research Study II. Cloud particles were video-recorded impacting on the optical window of a cloudscope, and Liquid Water Content (LWC) and Ice Water Content (IWC) derived from measurements of the power to evaporate particles of different size, shape and density collected by the T probe constant temperature sensors. Data were recorded at 1 Hz for a resolution of 130 m. Transitions between liquid, glaciated and mixed-phase regions were sharp; frequent supercooled liquid regions lasted less than 2 seconds (<260 m), while less frequent mixed-phase and glaciated regions persisted between 2 and 300 seconds (0.26 to 39 km) along the aircraft track. LWC, IWC and Total Water Content (TWC) were measured in concentrations up to 1.25, 0.45 and 1.25 g m −3 with an uncertainty of ∼0.02 g m −3 . The IWC/TWC ratio showed two maxima at mostly liquid (<0.1) and mostly ice (>0.9); mixed-phase corresponded to about 40% of the measurements. The large T probe showed mixed-phase clouds with ice fraction less than half at temperatures below −20 • C, while the small T probe measured higher ice proportion, suggesting that ice was present in the form of preferentially collected small particles. At high temperatures (>−15 • C) ice and water were measured in concentrations up to 0.2 and 0.6 g m −3 respectively; at temperatures below −20 • C they were both measured in concentrations up to 0.3 g m −3 .