Edwin L. Williams

Environmental persistence of organic compounds estimated from structure-reactivity and linear free-energy relationships. Unsaturated aliphatics

Structure-reactivity relationships (SRR) and linear free-energy relationships (LFER) are presented for environmentally important chemical reactions of unsaturated aliphatic contaminants in air and water. SRR of the form log k(k=rate constant for reaction with O3, OH, NO3 NO2 and Cl) vs ionization potential, torsional frequency and absorption maximum, as well as LFER of the form log k (A) vs log k (B), where A and B = O3, OH, NO3, NO2 or Cl are presented and can be used to estimate reaction rate constants and environmental persistence (in air and water) for many unsaturated compounds for which no data exist. As examples of application, rat constants for reactioons with OH (gas phase), OH (water) and NO3 (gas phase) are estimated for some 150 unsaturated compounds including alkenes (from monosubstituted to tetrasubstituted), cycloalkenes, dienes, terpenes, aromatic olefins, unsaturated carbonyls, heterocyclic and other unsaturated organics.

Ambient Levels of Formaldehyde and Acetaldehyde in Atlanta, Georgia

Ambient levels of formaldehyde and acetaldehyde have been measured at four locations in the Atlanta, Georgia, area during July and August 1992. Location-averaged concentrations were 2.7 - 3.0 ppb for formaldehyde and 2.6 - 3.2 ppb for acetaldehyde (2 hr - samples). The highest concentrations measured were 8.3 ppb for formaldehyde and 8.4 ppb for acetaldehyde. On the average, ambient aldehyde concentrations measured at two elevations (6 m and 30 m) at a near-downtown location were similar; those recorded at a predominantly upwind site were slightly lower, and those recorded at a predominantly downwind site were slightly higher. The formaldehyde/acetaldehyde concentration ratio (ppb/ppb) ranged from 0.2 to 3.3 and averaged 1.04 ± 0.46 (217 samples). A brief discussion of the data is presented and includes method performance evaluation, diurnal variations, examination of the acetaldehyde/formaldehyde concentration ratio, and comparison with literature data for other urban areas.

Atmospheric chemistry of olefins: a product study of the ozone-alkene reaction with cyclohexane added to scavenge hydroxyl radical.

Carbonyl and carboxylic acid products of the ozone-olefin reaction, in which the hydroxyl radical is produced, have been identified and measured for eight alkenes in the presence of excess cyclohexane, i.e., ruder conditions that minimize subsequent reactions of OH with the alkenes and with their carbonyl product. The carbonyls expected to form directly, i.e., alkene+ozone→1,d,3-trioxolane adduct→two carbonyls+two Criegee biradicals, were observed as major product (e.g., formaldehyde and d-butanone from d-methyl-1-butene). Their yields ranged from 9±2% for 1,3-butadiene to 88±4% for 2,3-dimethyl-d-butene. For alkenes that lead to two Criegee biradicals, carbonyl yields were consistent with preferential formation of the more substituted biradical

Ambient levels of the peroxyacyl nitrates PAN, PPN, and MPAN in Atlanta, Georgia

Ambient levels of the peroxyacyl nitrates [RC(O)OONO2] PAN (R = CH3), PPN (R = C2H5-) and MPAN [R = CH2 = C(CH3)-] have been measured in July - August 1992 in downtown Atlanta, Georgia. Ambient levels of PAN reached 2.9 ppb and averaged 0.71 ± 0.53 ppb(n=817). PPN reached 0.37 ppb and averaged 0.14 ±0.14 ppb (n = 119). Diurnal variations of PAN and PPN were similar and included nighttime minimaand late afternoon/early evening maxima. MPAN was observed on only 12 occasions at levels averaging 0.32 ± 0.07 ppb. A brief descriptive analysis of the data is presented together with a documentation of our measurement and calibration protocols and the results of an interlaboratory comparison involving co-located measurements of ambient PAN.

Thermal Decomposition of PAN, PPN and Vinyl-PAN

The gas phase thermal decomposition rates of the C1 and C2-substituted peroxyacyl nitrates (RC(O)OONO2), PAN (R = CH3), PPN (R = C2H5) and vinyl-PAN (R = CH2 = CH-) have been measured at ambient temperature (288 - 299 K) and 1 atm. of air. Our results for PAN (k = A exp (-Ea/RT), log10 (A, s-1) = 16.2 ± 1.6, Ea = 26.9 ± 2.1 kcal / mol, k298 = 3.0 × 10−4S−1) are consistent with literature data. Thermal decomposition rates for PPN and vinyl-PAN are similar to that for PAN, with k298 = 3.0 × 10−4S−1 for PAN, 3.4 × 10−4S−1 for PPN and 3.0 × 10−4S−1 for vinyl-PAN. Implications for the atmospheric persistence of PPN and vinyl-PAN as compared to that of PAN are briefly discussed.

Southern California air quality study: Peroxyacetyl nitrate

Abstract Information on the spatial and seasonal variations of ambient peroxyacetyl nitrate (PAN) has been obtained from simulataneous measurements made at five, seven or nine Southern California locations in June-September and November-December, 1987. Summertime (smog season) levels of PAN were consistent with photochemical formation during transport, and increased substantially from coastal to inland locations. Daily maxima (up to 30 ppb) coincided with those of ozone at all locations and shifted from midday at coastal sites to late afternoon inland. Elevated levels of PAN, e.g. up to 19 ppb on 3 December 1987, were observed during the fall at all coastal and central locations, where they consistently exceeded summertime levels.

A passive sampler for airborne formaldehyde

A simple, inexpensive passive sampler is described that is capable of reliable measurements of formaldehyde at the parts per billion (ppb) levels relevant to indoor and outdoor air quality. The passive sampler consists of a modified dual filter holder in which the upper stage serves as the diffusion barrier, the lower stage includes a 2,4-dinitrophenylhydrazine (DNPH)-coated filter which collects formaldehyde, and the space between the two stages serve as the diffusion gap. The measured sampling rate, 18.8 ± 1.8 ml min−1, was determined in experiments involving sampling of ppb levels of formaldehyde with the passive sampler and with DNPH-coated C18 cartridges and agrees well with the value of 19.4 ± 2.0 ml min−1 calculated from theory. The measured sampling rate was independent of formaldehyde concentration (16–156 ppb) and sampling duration (1.5–72 h). The precision of the measurements for colocated passive samplers averaged 8.6% in purified and indoor air (office and museums) and 10.2% in photochemically polluted outdoor air. With a 1.2-μm pore size Teflon filter as the diffusion barrier, the detection limit is 32 ppb h, e.g. 4 ppb in an 8-h sample, 1.3 ppb in a 24-h sample, and so on. Perceived advantages and limitations of the sampler are discussed including flexibility, cost effectiveness and possible negative bias at high ambient levels of ozone.

Evolved gas analysis of secondary organic aerosols

Secondary organic aerosols have been characterized by evolved gas analysis (EGA). Hydrocarbons selected as aerosol precursors were representative of anthropogenic emissions (cyclohexene, cyclopentene, 1-decene and 1-dodecene, n-dodecane, o-xylene, and 1,3,5-tri-methylbenzene) and of biogenic emissions (the terpenes α-pinene, β-pinene and d-limonene and the sesquiterpene trans-caryophyllene). Also analyzed by EGA were samples of secondary, primary (highway tunnel), and ambient (urban) aerosols before and after exposure to ozone and other photochemical oxidants. The major features of the EGA thermograms (amount of CO2 evolved as a function of temperature) are described. The usefulness and limitations of EGA data for source apportionment of atmospheric particulate carbon are briefly discussed.

Monitoring ambient ozone with a network of passive samplers: a feasibility study.

Ambient levels of ozone have been measured at 46 mountain forest, desert, Class I Wilderness areas and other remote locations using a network of passive samplers. Typical values were 40-80 ppb (2 week samples) and exhibited temporal variations (studied for up to 1 year) as well as changes with elevation (studied up to 10 500 ft (3 200 m)). The performance of the passive sampler was evaluated with respect to reproducibility, field controls, data capture (>0.95), precision for co-located samples (av. = 11.9%, n = 103), and the role of other atmospheric oxidants as potential interferents (2 locations). Suggestions for additional sampler performance evaluation and network operation are outlined.

Formation and Thermal Decomposition of Butyl-Substituted Peroxyacyl Nitrates: n-C4H9C(O)OONO2 and i-C4H9C(O)OONO2

The butyl-substituted peroxyacyl nitrates n-C 4 H 9 C(O)-OONO 2 and i-C 4 H 9 C(O)OONO 2 have been synthesized in the liquid phase, prepared in-situ in the gas phase by sunlight irradiation of aldehyde-NO mixtures, measured by electron capture gas chromatography, and characterized in a number of gas-phase and liquid-phase tests. Gas-phase yields as a fraction of initial NO were 0.39 for the n-butyl isomer and 0.20 for the isobutyl isomer. The corresponding gas-phase aldehyde oxidation mechanisms are outlined. Thermal decomposition in the presence of excess NO yielded n-butanal and isobutanal as the major carbonyl products.(...)