Role of Iodine-Assisted Aerosol Particle Formation in Antarctica

Abstract

New particle formation via the ion-mediated sulfuric acid and ammonia molecular clustering mechanism remains the most widely observed and experimentally verified pathway. Recent laboratory and molecular level observations indicate iodine-driven nucleation as a potentially important source of new particles, especially in coastal areas. In this study, we assess the role of iodine species in particle formation using the best available molecular thermochemistry data and coupled to a detailed 1-d column model which is run along air mass trajectories over the Southern Ocean and the coast of Antarctica. In the air masses traversing the open ocean, ion-mediated SA-NH₃ clustering appears insufficient to explain the observed particle size distribution, wherein the simulated Aitken mode is lacking. Including the iodine-assisted particle formation improves the modeled Aitken mode representation with an increase in the number of freshly formed particles. This implies that more particles survive and grow to Aitken mode sizes via condensation of gaseous precursors and heterogeneous reactions. Under certain meteorological conditions, iodine-assisted particle formation can increase cloud condensation nuclei concentrations by 20%-100%.

Keywords: Southern Ocean; iodic acid; modeling; new particle formation; secondary aerosols.

Figure 1

Measured particle number size distributions (dN/d log dp) at Aboa (a) and Neumayer (c) used for the BaseCase simulations. The modeled number size distributions are shown in panels (b,d).

Figure 2

Median number size distributions at Aboa (upper panel) and Neumayer (lower panel) for the BaseCase simulations. The shaded areas indicate the 25th and 75th percentile range. The D_p between the green and magenta dashed line (25–100 nm) indicates the Aitken mode range, and the D_p above 100 nm (and <1 μm) represents the accumulation mode.

Figure 3

Gas-phase concentrations at Aboa (upper panel) and Neumayer (lower panel) for the BaseCase simulations. The red dots indicate the measured median values for the selected period. The central coral colored line in the box represents median values, while the whiskers indicate the maximum and minimum values. It should be noted, however, that the measurements at Neumayer are incomplete, with gaps in the data for the selected period.

Figure 4

Simulated formation rates (J, cm^–3) along the trajectory for all simulations and at the stations, Aboa and Neumayer (upper and lower panel), via the SA-NH₃ (a,d), IA-HIO₂ (b,e), and SA-DMA (c,f) and pathways for the BaseCase simulations. The “o” symbol with red edges indicates J values over land, “+” represents the values over ocean, and the diamonds represent the station values. The sizes of dots and stars indicate how far the air mass was from the station, with smaller points indicating air masses closer to the station and larger points indicating air masses further away from the station. Since the simulated J_SA-DMA values over land and sea for Neumayer (panel d–f) are similar in magnitude, the J values over sea are overlapped by the J values over land, i.e., the “+” are underneath the red “o”. Data points with formation rates of less than 10^–6 cm^–3 are excluded from the figure.

Figure 5

Median size distributions at Aboa (upper panel) and Neumayer (lower panel) for the BaseCase and AN-AD simulations. The D_p between the green and magenta dashed line (25–100 nm) indicates the Aitken mode range, and the D_p above 100 nm (<1 μm) represents the accumulation mode.