Session F03: Atmospheric Chemistry, Physics and Aerosols II

Multiphase chemical kinetics and cloud formation by organic aerosol

Organic aerosol (OA) is ubiquitous in the atmosphere. It is well-known that OA particles can undergo amorphous phase transition; displaying a continuous change from liquid to solid phase state in response to temperature and humidity changes. OA particles can react with atmospheric oxidants, take up water, and act as ice-nucleating particles (INPs). We determined the reactive uptake coefficients of OH radicals by amorphous OA surrogate film substrates for temperatures as low as 213 K using a custom-built chemical ionization mass spectrometer coupled to a low-temperature flow-reactor. The phase state and viscosity of applied substrates were estimated using a poke-flow experiment. The OH reactivity of the organic substrate is interpreted using the resistor model accounting for viscosity changes in the condensed phase. Application of a temperature-controlled flow reactor, we observed that the phase state of OA modulates the resulting particle hygroscopicity after oxidation by OH. Oxidation of solid OA yielded greater hygroscopicity compared to when OA is in a liquid phase state. Lastly, we discuss the potential of laboratory and field-collected OA particles to act as INPs by combining ice nucleation experiments with single-particle micro-spectroscopic analytical techniques such as X-ray microscopy and scanning electron microscopy.   

Application of chemical ionization mass spectrometry in the new particle formation studies.

Chemical ionization mass spectrometry (CIMS) has been demonstrated to be a highly sensitive and fast-response analytical technique. Unlike the traditional, highly-energetic electron impact mass spectrometry (EI-MS), CIMS utilizes soft-ionization methods and thus can generate mass spectra with less fragmentation, which can facilitate the interpretation of molecular information of the analytes. In addition, the ion chemistry implemented by CIMS can be highly versatile, which can be designed specifically for a certain analyte. These advantages of CIMS make it suitable for new particle formation (NPF) studies. For instance, CIMS is the only available technique for in-situ measurements of H$_{\mathrm{2}}$SO$_{\mathrm{4}}$ and amines, which are the key NPF precursors and their ambient concentrations are usually on the order of \textasciitilde 10$^{\mathrm{6}}$ molecules cm$^{\mathrm{-3}}$ or much less. In this work, the ion chemistry and ion sources utilized by CIMS during NPF studies are introduced, including the measurement principles and calibration procedures. Field NPF study results using CIMS at various environments are demonstrated.   

Mid-infrared sensing of atmospheric ammonia: linking farms, cities, and ecosystems

Atmospheric ammonia - the most important base in the atmosphere - degrades the environment through the formation of unhealthy particulate, nitrogen deposition in sensitive ecosystems, and scattering of visible light for climate and visibility impacts. Despite its global importance, ammonia is a challenging measurement to make. Ammonia readily sticks to instrument surfaces such as inlets or sample cells, its atmospheric mole fraction is low (parts per trillion to parts per billion by volume), and it can readily partition between gas phase ammonia and particulate phase ammonium. I will discuss field measurements with open-path, quantum cascade laser-based sensors that have been deployed in field campaigns across the world to understand the spatiotemporal variabilities of its atmosphere-surface fluxes. The open-path configuration - where the sampled air is passively sampled between mirrors of a Herriott cell without actively going through tubing, inlets, or sample manifolds - allows for fast (25 Hz), sensitive (30 pptv), and accurate (20{\%}) measurements to deduce rapid changes in fluxes on platforms such as mobile laboratories, tall towers, and aircraft. The sensor consumers 45 W of power for ease of deployment in power-constrained environments such as in remote settings of agricultural fields or natural ecosystems. Vehicle emissions of ammonia from real-world driving measurements across 5 cities in the United States are twice as high as the EPA National Emissions Inventory, suggesting an important urban emission source that is co-located with emissions of nitrogen and sulfur oxides. Agricultural sources in both Colorado and California show large spatiotemporal variabilities including diurnal, seasonal, and farm-to-farm differences. Agricultural emissions, when combined with new satellite ammonia measurements, in-situ measurements, and ecosystem flux measurements, show significant effects on downstream air quality in cities and nitrogen deposition in remote ecosystems.   

Application of the SOFDI wind data for assessing the performance of models

Second-generation, Optimized, Fabry-Perot Doppler Imager (SOFDI), a triple-etalon Fabry-Perot interferometer, is designed to measure both day and nighttime thermospheric winds from OI 630-nm emission. The SOFDI provides coverage of the ionosphere/thermosphere in the low-latitude region and is currently operating at the Huancayo, Peru near the geomagnetic equator. We present a comparative analysis of the observed SOFDI wind data and several model results, including, but not limited to, Horizontal Wind Model 2014 (HWM-14), Coupled Thermosphere Ionosphere Plasmasphere Electrodynamics (CTIPe) model, Whole Atmosphere Model (WAM), Magnetic mEridional NeuTrAl Thermospheric (MENTAT) model, and SAMI3 model. Here, the performances of these models are examined by comparing their outputs to the direct-measured thermospheric winds using the SOFDI. Finally, we investigate and discuss the sources, drivers, and effects of the wind variability in the low-latitude thermosphere from observational and modeling perspectives.$\backslash $f1 -/abstract-$\backslash $pard$\