Atmospheric Chemistry

HNO3/H2O Binary System

This project was completed while the Beyer research group was located at Wisconsin Luthern College.

Abstract: We have investigated the HNO3/H2O binary liquid/solid phase diagram using a highly sensitive differential scanning calorimeter (DSC) and infrared spectroscopy of thin films. In particular we have sought to investigate the nitric acid trihydrate (NAT) and nitric acid dihydrate (NAD) regions. We report here a detailed stability region for NAD, including comment on the preferential freezing of NAD vs NAT. We have found metastability for NAD and pure HNO3 outside of their normal thermodynamic stability. We have deciphered and more clearly defined the many eutectic transitions in the region of NAT, NAD, and nitric acid monohydrate (NAM). We also report a new solid phase mentioned previously in the literature that undergoes a transition at ~228 K for samples in the range 54-75 wt % HNO3. We have also detected other new phases that we deem either minor or not relevant for polar stratospheric clouds (PSCs). Finally, we have calculated the fraction of each solid that exists in a frozen sample on the basis of our measured enthalpies of fusion for the various solids. We compare our results to previous work on the nitric acid/water phase diagram, including recent work done on binary aerosols. The implications of our results for the freezing of nitric acid hydrates in PSCs are discussed.

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