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The formation and fate of stratospheric N2O5 are key processes for controlling stratospheric total reactive nitrogen (NOy). Heterogeneous (gas / particle) processing converts N2O5 into HNO3 (a long-lived NOx reservoir), which in turn strongly influences stratospheric ozone (O3) destruction. Even so, the uptake coefficient of N2O5, γ(N2O5), and its dependencies have yet to be determined by in situ stratospheric observations. The NOAA Stratospheric Aerosol Processes, Budget, and Radiative Effects (SABRE) campaign achieved wintertime in situ stratospheric N2O5 observations between ~30 – 80° N and altitudes between 10 – 17 km enabling empirical determination of γ(N2O5) in great detail. While the average γ(N2O5) is 0.061, values show systematic variability related to the N2O "average age of air" tracer. Stratospheric air masses < 0.75 yr old exhibit γ(N2O5) that vary by more than a factor of 100. In contrast, γ(N2O5) for air masses >1 yr old show a log-linear positive correlation with N2O (R2 = 0.55). The variability is connected to particle composition. Biomass burning and other tropospheric-influenced aerosol exhibited small and variable γ(N2O5): 0.0030 – 0.054, 9th – 91st %, respectively. Background stratospheric particles and those influenced by the Hunga Tonga volcanic eruption exhibited moderate values (0.033 – 0.096) and particles formed on meteoric dust exhibited the greatest values (0.053 – 0.10). Strong correlations between γ(N2O5) and observed particle properties (R2 >0.9) suggest N2O5 hydrolysis proceeds by two competing mechanisms depending on the particle composition: 1) an organic mass fraction dependent, bulk-solvation-limited process for tropospheric-influenced particles and 2) an acid-catalyzed, near-surface mass accommodation limited process for sulfuric acid particles. The strong correlations enable determination of physio- and thermo-chemical properties (e.g. reacto-diffusive length, enthalpy and entropy of accommodation, critical cluster size), which provide wide corroboration of laboratory studies by field observations. The new constraints on γ(N2O5) are used with the Community Earth System Model 2 (CESM2) with WACCM6, to understand the influence on O3 under various scenarios such as volcanic eruption and stratospheric aerosol injection.
Dr. Zachary Decker is a research scientist at CIRES and NOAA Chemical Sciences Laboratory. Zach's research interests include wildfire smoke and aircraft exhaust impacts on air quality and climate as well as stratospheric processes and how they may impact potential future climate intervention. Most of his research is conducted in the field by measurements on aircraft or on the ground.
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