Analysis of cloud condensation nuclei composition and growth kinetics using a pumped counterflow virtual impactor and aerosol mass spectrometer

Abstract

Abstract. We present a new method of determining the size and composition of CCN-active aerosol particles. Method utility is illustrated through a series of ambient measurements. A continuous-flow thermal-gradient diffusion chamber (TGDC), pumped counterflow virtual impactor (PCVI), and Aerodyne time-of-flight mass spectrometer (AMS) are operated in series. Ambient particles are sampled into the TGDC, where a constant supersaturation is maintained, and CCN-active particles grow to ∼2.5 ± 0.5 µm. The output flow from the TGDC is directed into the PCVI, where a counterflow of dry N 2 gas opposes the particle-laden flow, creating a region of zero axial velocity. This stagnation plane can only be traversed by particles with sufficient momentum, which depends on their size. Particles that have activated in the TGDC cross the stagnation plane and are entrained in the PCVI output flow, while the unactivated particles are diverted to a pump. Because the input gas is replaced by the counterflow gas with better than 99 % efficiency at the stagnation plane, the output flow consists almost entirely of dry N 2 and water evaporates from the activated particles. In this way, the system yields an ensemble of CCN-active particles whose chemical composition and size are analyzed using the AMS. Measurements of urban aerosol in downtown Toronto identified an external mixture of CCN-active particles consisting almost entirely of ammonium nitrate and ammonium sulfate, with CCN-inactive particles of the same size consisting of a mixture of ammonium nitrate, ammonium sulfate, and organics. We also discuss results from the first field deployment of the TGDC-PCVI-AMS system, conducted from

Contact freezing: a review of experimental studies

“…The collection rates could be enhanced if the aerosol concentration and the residence time are increased. The aerosol concentration can be increased with the help of an aerosol particle concentrator and/or a counter flow virtual impactor (it is commonly used to separate cloud droplets or ice crystals from interstitial aerosols but it is also possible to use it as an aerosol concentrator; Slowik et al, 2011). Field measurements will allow us to validate the laboratory experiments and their data will be very valuable for validation of numerical models (cloud resolving, regional and global climate models).…”

Mass spectrometric approaches for chemical characterisation of atmospheric aerosols: critical review of the most recent advances

“…[160] The possible positive correlation of k org with O/C ratio measured by AMS was also suggested. [162] Slowik et al [163] and Zelenyuk et al [164] respectively used AMS and SPMS instruments with the inlets connected to counterflow virtual impactors that segregate dry residuals of cloud and fog droplets from inactivated background aerosol. In their studies, cloud droplet residuals were probed directly by online MS techniques, yielding explicit data on the chemical composition of CCN active particles present in ambient air.…”

A Phase Separation Inlet for Droplets, Ice Residuals, and Interstitial Aerosols

“…One technique capable of capturing ice and droplet residuals is the Counterflow Virtual Impactor (CVI) and its laboratory counterpart, the Pumped-Counterflow Virtual Impactor (PCVI). These methods use the idea that activated droplets or ice crystals are significantly larger than unactivated aerosol (Slowik et al, 2011). By separating based on mass, researchers can study differences between activated and interstitial aerosol.…”

“…The 50% cut size or "D50" describes the smallest particle that is transmitted through the PCVI at 50% transmission efficiency. To adjust the D50, the AF-to-IF ratio can be adjusted, reducing or increasing the inertial barrier to particle passage (Kulkarni et al, 2011;Slowik et al, 2011).…”

“…Since the original characterizations by Boulter et al (2006) and Kulkarni et al (2011), the PCVI has been used in many studies and adapted for use in several cloud-sampling inlets (Baustian et al, 2012;Slowik et al, 2011). The most recent advancement is the ability to build a PCVI using three-dimensional (3D) stereolithography (SLA) printing (Koolik, 2017).…”