Wendy S. Goliff

Diurnal and weekday variations in the source contributions of ozone precursors in California's South Coast Air Basin.

Abstract For at least 30 years, ozone (O3) levels on weekends in parts of California’s South Coast (Los Angeles) Air Basin (SoCAB) have been as high as or higher than on weekdays, even though ambient levels of O3 precursors are lower on weekends than on weekdays. A field study was conducted in the Los Angeles area during fall 2000 to test whether proposed relationships between emission sources and ambient nonmethane hydrocarbon (NMHC) and oxides of nitrogen (NOx) levels can account for observed diurnal and day-of-week variations in the concentration and proportions of precursor pollutants that may affect the efficiency and rate of O3 formation. The contributions to ambient NMHC by motor vehicle exhaust and evaporative emissions, estimated using chemical mass balance (CMB) receptor modeling, ranged from 65 to 85% with minimal day-of-week variation. Ratios of ambient NOx associated with black carbon (BC) to NOx associated with carbon monoxide (CO) were approximately 1.25 ± 0.22 during weekdays and 0.76 ± 0.07 and 0.52 ± 0.07 on Saturday and Sunday, respectively. These results demonstrate that lower NOx emissions from diesel exhaust can be a major factor causing lower NOx mixing ratios and higher NMHC/NOx ratios on weekends. Nonmobile sources showed no significant day-of-week variations in their contributions to NMHC. Greater amounts of gasoline emissions are carried over on Friday and Saturday evenings but are, at most, a minor factor contributing to higher NMHC/NOx ratios on weekend mornings.

Methyl chloride formation by gas phase thermal chlorine atom reaction with methyl iodide and methyl bromide

Thermal chlorine atoms react with gaseous CH3I and CH3Br at 295K to produce CH3Cl with yields of 8.6% and 0.6%, respectively. The studies utilized radioactive 38Cl formed by the 37Cl (n,γ) 38Cl nuclear reaction in gaseous CClF3 at pressures from 760 to 4,000 torr. The initially energetic 38Cl atoms are thermalized by multiple inelastic collisions with CClF3 prior to reaction with the methyl halide. The reaction rate constant for thermal chlorine attack on CH3Br to form CH3Cl at 295K and 1–5 atmospheres pressure is (2.0±0.5) × 10−15 cm³ molecule−1 s−1. The substitution of Cl for Br/I in a methyl halide molecule cannot be a significant source of atmospheric CH3Cl because of its abundance from other sources. However, the replacement of I by Cl in other iodohalocarbons might in some cases provide a marker for the presence of atomic chlorine.

Ambient measurements of 2,2,4-trimethyl, 1,3-pentanediol monoisobutyrate in Southern California.

2,2,4-trimethyl, 1,3-pentanediol monoisobutyrate (TPM) is a widely used solvent found in water-based coatings. Ambient measurements of TPM are reported here for the first time. Although this compound has been previously measured in indoor air, this study illustrates successful detection and quantification of TPM in ambient air at three locations in Southern California: Pico Rivera, Azusa, and Riverside. TPM was detected in every sample collected, with concentrations ranging from 0.7 to 49.5 parts per trillion (ppt). Collections took place during summer 2009, fall 2009, winter 2009/2010, and spring 2010, for 5–7 days during each season. The highest mean concentrations were observed during the summer months for each city, when coating activities are typically at their highest. Implications 2,2,4-Trimethyl, 1,3-pentanediol monoisobutyrate (TPM) is a widely used solvent found in water-based coatings. Ambient measurements of TPM are reported here for the first time. The highest mean concentrations were observed during the summer months for each city, when coating activities are typically at their highest. Unreacted TPM constitutes approximately 0.01% of the total nonmethane hydrocarbon (NMHC) concentrations in the Los Angeles air basin and given its slow reactivity rate in forming ozone, this would be an approximate upper limit for the fraction of ozone that it is responsible for forming.

Chapter 4 Analyte losses during storage

4.6 Summary The US EPA recommendations for storage conditions for environmental sampling media are more stringent than those based on experimental data. In the real world, the EPA requirements are hard for most analytical laboratories to fulfil. In these cases, the authors recommend alternative storage conditions listed in Table 4.1 (with US EPA recommendations in parentheses) based on the available literature.