Boundary layer new particle formation over East Antarctic sea ice – possible Hg-driven nucleation?

Abstract

Abstract. Aerosol observations above the Southern Ocean and Antarctic sea ice are scarce. Measurements of aerosols and atmospheric composition were made in East Antarctic pack ice on board the Australian icebreaker Aurora Australis during the spring of 2012. One particle formation event was observed during the 32 days of observations. This event occurred on the only day to exhibit extended periods of global irradiance in excess of 600 W m −2 . Within the single air mass influencing the measurements, number concentrations of particles larger than 3 nm (CN 3 ) reached almost 7700 cm −3 within a few hours of clouds clearing, and grew at rates of 5.6 nm h −1 . Formation rates of 3 nm particles were in the range of those measured at other Antarctic locations at 0.2-1.1 ± 0.1 cm −3 s −1 . Our investigations into the nucleation chemistry found that there were insufficient precursor concentrations for known halogen or organic chemistry to explain the nucleation event. Modelling studies utilising known sulfuric acid nucleation schemes could not simultaneously reproduce both particle formation or growth rates. Surprising correlations with total gaseous mercury (TGM) were found that, together with other data, suggest a mercurydriven photochemical nucleation mechanism may be responsible for aerosol nucleation. Given the very low vapour pressures of the mercury species involved, this nucleation chemistry is likely only possible where pre-existing aerosol concentrations are low and both TGM concentrations and solar radiation levels are relatively high (∼ 1.5 ng m −3 and ≥ 600 W m −2 , respectively), such as those observed in the Antarctic sea ice boundary layer in this study or in the global free troposphere, particularly in the Northern Hemisphere.

Unexpectedly high ultrafine aerosol concentrations above East Antarctic sea ice

“…A local new particle formation event occurred on 18 October and has been excluded from this analysis. This event is described in detail in Humphries et al (2015). ingly, the well-defined downward flow is apparent in both scenarios, however the depth of its penetration into the upper troposphere differs significantly.…”

“…Full details of the sampling system and data filtering are given in Humphries et al (2015), however it is important to note that a significant fraction (up to 70 % for 3 nm particles) of CN 3−10 was lost in the 21.7 m long sample tube (used for logistical reasons) despite the high flow rates utilised. Appendix Fig.…”

“…These were used for data filtering (e.g. ozone; and other aspects of the project and details have been included in Humphries et al (2015).…”

“…The difference between CN 3 and CN 10 can be used to characterise the number concentration of nanoparticles (3-10 nm; CN 3−10 ) which have recently formed from gas-to-particle conversion. Appropriate data filtering utilising in situ wind and ozone data (see Humphries et al, 2015, for details) was performed to remove the influence of ship exhaust and to obtain a data set that reflected the background ambient atmosphere. Operational and logistical constraints meant that a more comprehensive aerosol measurement suite was unable to be deployed during this voyage.…”

“…In contrast, measurements within the polar cell lasted 26 days, and resulted in median CN 3 concentrations of 816 cm −3 (x = 1130 cm −3 ). Except for one new particle formation event which resulted in measured CN 3 concentrations over 7500 cm −3 (described in detail in Humphries et al, 2015), background aerosol CN 3 concentrations generally ranged between 500 and 2000 cm −3 , with not infrequent periods above 2600 cm −3 . It is important to note that due to the inability to accurately characterise inlet efficiencies for aerosol diameters below 10 nm (as mentioned earlier), the values reported here represent only lower bounds for ambient concentrations and could be up to 3 times higher.…”