Cloud Condensation Nuclei concentration (CCN) monthly medians measured at Kennaook Cape Grim 1981 to 2002 using the static thermal diffusion chamber

Brief description

This data set is the record of monthly median number concentration for cloud condensation nuclei active at 1.2%, 0.96%, 0.71%, 0.47% and 0.23% measured at the Kennaook Cape Grim Baseline Air Pollution Monitoring Station (KCGBAPS) between 1981 and 2002.
The KCGBAPS is located on the northwest tip of Tasmania (40.6833° S, 144.6833° E). Commencing operations in 1976, and in its current facilities since 1981, the Baseline Air Pollution Station at Kennaook Cape Grim is Australia’s contribution to international efforts for monitoring the global background atmosphere for trends due to human activities and natural variability.
The data published here are the monthly medians under baseline conditions (i.e. when wind direction measured at 10 m was in the 190°to 280° ‘baseline’ sector. Daily mean spectra were calculated from a minimum of three daily spectra, and the monthly medians are determined from the daily means calculated from manual measurements. The number of baseline spectra included in each monthly medium are also published with the data set. For some months the number of baseline spectra is small; a factor that should be taken into account when using these data. Usually the cause for few spectra is a combination of the manual operation of the CCN counter and a low baseline frequency. In cases where no baseline spectra were recorded in a month the NAN are included in the data set.
Subsets of these data have been described in Ayers and Gras (1991), Ayers et al. (1997), Boers et al., (1994), Gras (1990), Gras (1995) and Gras and Keywood (2017).
See KCGBAPS_CCN_Static_Thermal_Diffusion_Chamber_1981_2002_README.pdf for references

Lineage: Concentration data for CCN that are active at 1.2%, 0.96%, 0.71%, 0.47% and 0.23% supersaturations were measured at KCGBAPS between 1991-2002 using a static thermal diffusion chamber that is described in Bartlett (1984) and Ayers (1981). In this instrument, two circular plates made of thick nickel-plated brass placed 12.5m horizontally apart were held at slightly different temperatures (with the top plate being warmer than the bottom plate) and the temperature gradient across the plates was linear. Both plates were kept wet and were free of horizontal temperature gradients. This was achieved by circles of thin wet black filter paper adhering to the plates with the capillary transfer of water to the top paper from a reservoir in the plate ensuring that the top plate was always wet, while condensation on the cooler bottom plate keeps the bottom plate moist. This resulted in supersaturation occurring between the plates which arose because of the exponential increase in saturation water vapour with pressure between the plates since half-way between the plates the temperature is midway between that of the plates, but the water vapour pressure is greater than the saturation water vapour pressure at that temperature. Particles moving between the plates are subjected to this supersaturation and if they form water droplets are CCN. The water droplets scatter light from an incident beam which is detected by a photocell.

The instrument was calibrated by relating the peak value of scattered light intensity to the number of droplets in the sample volume at the time at which the peak occurred, where the number of droplets were counted in photographs taken at the time of the peak.

This instrument was included in an intercomparison workshop for CCN instruments at NASA in 1980 (Kocmund et al., 1981) and was found to be one of the five reliable static diffusion chambers that agreed within about 15 per cent with one another and with the best four continuous flow devices.

The inlet used between 1982 and September 1981 is described in the Baseline Air Monitoring Report 1976 (https://doi.org/10.25919/5f3ad3c8e98fc). Air was sampled via a 10 cm rigid PVC pipe with a downward-pointing inlet approximately 3 m above the ground. The flow rate down the pipe was approximately 1 m s-1. Rain, drizzle and particles > 20 µm were “deliberately” lost. A manifold at the end of the pipe allowed short connections to the counter.
The inlet used between October 1981 and December 2002 is described in the Baseline Atmospheric Program (Australia) 1981-1982 (https://doi.org/10.25919/5f319936aa48c ). Air was sampled via a 10 m stainless steel pipe 150 mm in diameter, with a rain hood. The flow rate down the pipe was approximately 6-7 m s-1.
See KCGBAPS_CCN_Static_Thermal_Diffusion_Chamber_1981_2002_README.pdf for references