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Understanding the formation and long-timescale evolution of atmosphericplasmas produced by ultrashort, long wavelength IR (LWIR) pulses is animportant but partially understood problem. Of particular interest are plasmasproduced in air with a peak laser intensity $\sim$10$^{12}$ W/cm$^2$, theso-called clamping intensity observed in LWIR atmospheric guiding experimentswhere tunneling and multi-photon ionization operative at near-IR or shorterwavelengths are inoperative. We find that avalanche breakdown on the surface ofaerosol (dust) particles can act to seed the breakdown of air observed abovethe 200 GW/cm$^2$ threshold when a train of 3 ps 10.6 $\mu$m laser pulsesseparated by 18 ps are used. The breakdown first appears at the best focus butpropagates backwards towards the focusing optic as the plasma densityapproaches critical density and makes forward propagation impossible. Thevelocity of the backward propagating breakdown can be as high as 10$^9$ cm/s,an order of magnitude greater than measured with ns pulse-produced breakdownand can be explained rather well by the so-called breakdown wave mechanism.Transverse plasma expansion with a similar velocity is assisted by UVphotoionization and is observed as a secondary longitudinal breakdown mechanismin roughly 10 percent of the shots. When a cm size, TW power beam ispropagated, interception of aerosol particles is guaranteed and several (40cm$^{-3}$) breakdown sites appear, each initially producing a near criticaldensity plasma. On a 10 ns-1 $\mu$s timescale, shockwaves from each site expandradially and coalesce to produce a large hot gas channel. The radial velocityof the expansion agrees well with the prediction of the blast wave theorydeveloped for ultrafast atmospheric detonations.

Observation of breakdown wave mechanism in avalanche ionization produced atmospheric plasma generated by a picosecond CO$_2$ laser