Low-pressure and nascent yields of stabilized Criegee intermediates CH2OO and CH3CHOO in ozonolysis of propene.

The yields of stabilized Criegee intermediates (sCIs), both CH2OO and CH3CHOO, produced from ozonolysis of propene at low pressures (7-16 Torr) were measured indirectly using cavity ringdown spectroscopy (CRDS) and chemical titration with an excess amount of sulfur dioxide (SO2). The method of monitoring the consumption of SO2 as a scavenger and the production of secondary formaldehyde (HCHO) allows characterization of the total sCI and the stabilized CH2OO yields at low pressure and in a short residence time. Both the total sCI and the stabilized CH2OO yields in the propene ozonolysis were found to decrease with decreasing pressure. By extrapolating the 7-16 Torr measurements to the zero-pressure limit, the nascent yield of the total sCIs was determined to be 25 ± 2%. The ranges of nascent yields of stabilized CH2OO and stabilized CH3CHOO were estimated to be 20-25% and 0-5%, respectively. The branching ratios of the stabilized and high-energy CH2OO* and CH3CHOO* were also determined.

Low-Pressure and Nascent Yields of Thermalized Criegee Intermediate in Ozonolysis of Ethene.

The yields of thermalized formaldehyde oxide (CH2OO, the simplest Criegee intermediate) produced from ozonolysis of ethene at low pressures were measured indirectly using cavity ringdown spectroscopy (CRDS) and chemical titration with an excess amount of sulfur dioxide (SO2). The method of monitoring the consumption of SO2 as a scavenger allows better characterization of the CH2OO at low pressure and short residence time. The yield of thermalized CH2OO from ethene ozonolysis was found to decrease with decreasing pressure. The nascent yield of thermalized CH2OO was determined to be 20.1 ± 2.5% by extrapolation of the 7-19 Torr measurements to the zero-pressure limit. Kinetic models enable better evaluation and understanding of the different measurement methods of thermalized Criegee intermediates. The information on the low-pressure yields from this work serves as a benchmark for theoretical calculations and facilitates a better understanding of the alkene ozonolysis reaction mechanisms.

Atmospheric observation-based estimation of fossil fuel CO2 emissions from regions of central and southern California.

Combustion of fossil fuel is the dominant source of greenhouse gas emissions to the atmosphere in California. Here, we describe radiocarbon (14CO2) measurements and atmospheric inverse modeling to estimate fossil fuel CO2 (ffCO2) emissions for 2009-2012 from a site in central California, and for June 2013-May 2014 from two sites in southern California. A priori predicted ffCO2 mixing ratios are computed based on regional atmospheric transport model (WRF-STILT) footprints and an hourly ffCO2 prior emission map (Vulcan 2.2). Regional inversions using observations from the central California site suggest that emissions from the San Francisco Bay Area (SFBA) are higher in winter and lower in summer. Taking all years together, the average of a total of fifteen 3-month inversions from 2009 to 2012 suggests ffCO2 emissions from SFBA were within 6 ±â€¯35% of the a priori estimate for that region, where posterior emission uncertainties are reported as 95% confidence intervals. Results for four 3-month inversions using measurements in Los Angeles South Coast Air Basin (SoCAB) during June 2013-May 2014 suggest that emissions in SoCAB are within 13 ±â€¯28% of the a priori estimate for that region, with marginal detection of any seasonality. While emissions from the SFBA and SoCAB urban regions (containing ~50% of prior emissions from California) are constrained by the observations, emissions from the remaining regions are less constrained, suggesting that additional observations will be valuable to more accurately estimate total ffCO2 emissions from California as a whole.