The concentration of nitrogen oxides (NO x) plays a central role in controlling air quality. On a global scale, the primary sink of NO x is oxidation to form HNO 3. Gas-phase HNO 3 photolyses slowly with a lifetime in the troposphere of 10 days or more. However, several recent studies examining HONO chemistry have proposed that particle-phase HNO 3 undergoes photolysis 10-300 times more rapidly than gas-phase HNO 3. We present here constraints on the rate of particle-phase HNO 3 photolysis based on observations of NO x and HNO 3 collected over the Yellow Sea during the KORUS-AQ study in summer 2016. The fastest proposed photolysis rates are inconsistent with the observed NO x to HNO 3 ratios. Negligible to moderate enhancements of the HNO 3 photolysis rate in particles, 1-30 times faster than in the gas phase, are most consistent with the observations. Small or moderate enhancement of particle-phase HNO 3 photolysis would not significantly affect the HNO 3 budget, but could help explain observations of HONO and NO x in highly aged air.

The objective of the ISTP program was to study the origin of solar variability and activity, the transport of manifestations of that activity to the Earth via plasma processes, and the cause-and-effect relationships between that time varying energy transport and the near-earth environment.

Shock parameters at Earth’s bow shock in rare instances can approach the Mach numbers predicted at supernova remnants. We present our analysis of a high Alfvén Mach number (M A = 27) shock utilizing multipoint measurements from the Magnetospheric Multiscale spacecraft during a crossing of Earth’s quasi-perpendicular bow shock. We find that the shock dynamics are mostly driven by reflected ions, perturbations that they generate, and nonlinear amplification of the perturbations. Our analyses show that reflected ions create modest magnetic enhancements upstream of the shock, which evolve in a nonlinear manner as they traverse the shock foot. They can transform into proto-shocks that propagate at small angles to the magnetic field and toward the bow shock. The nonstationary bow shock shows signatures of both reformation and surface ripples. Our observations indicate that although shock reformation occurs, the main shock layer never disappears. These observations are at high plasma β, a parameter regime that has not been well explored by numerical models.