Investigation of Particle Formation and Ozone Impacts of Methyl Iodide (CH3I)
Abstract
Iodine containing species, such as molecular iodine (I2) and iodocarbons (e.g., CH2I2, CH3I), have been reported to contribute to new particle formation especially in coastal areas. In addition, iodine chemistry impacts HOx and NOx reaction through depletion of O3. Previously, Jimenez et al. (2003, JGR) evaluated new particle formation from photooxidation of CH2I2 in the Caltech environmental chamber. This study extends the CH2I2 chemistry into another major iodocarbon, CH3I, and evaluates particle formation and ozone chemistry based on chamber experiments and modeling. Ozone chemistry and PM formation is modeled by the SAPRC-07 mechanism with the reactions of CH3I and iodine species added. The reaction mechanism was evaluated by varying the experimental conditions (O3, NOx, and/or CO). Rapid particle nucleation was observed following CH3I photolysis. Chemical composition of particles formed from CH3I was measured by HR-ToF-AMS (Aerodyne High Resolution Time-of-Flight Aerosol Mass Spectrometer). Mass spectra agreed with that of particles from CH2I2 reported by Jimenez et al. (2003), confirming the same particle formation from iodine radical from the photolysis of iodocarbons. The highly fractal-like structure of the secondary particles from CH3I oxidation hinders determination of particle mass based on volume measurement by a commonly used SMPS (Scanning Mobility Particle Sizer). Real-time density measurement of particles by an APM-SMPS (Aerosol Particle Mass analyzer - SMPS) indicated that the effective density increased from ~1.5 to 2.0 g/cm3 during CH3I oxidation for 6 hours. Based on the density measurement, secondary particle formation yield (particle mass produced / CH3I mass reacted) was determined to be 0.3~0.7.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2011
- Bibcode:
- 2011AGUFM.A13F0444N
- Keywords:
-
- 0305 ATMOSPHERIC COMPOSITION AND STRUCTURE / Aerosols and particles;
- Tropospheric Ozone