Louis J. Salas

Atmospheric halocarbons, hydrocarbons, and sulfur hexafluoride: global distributions, sources, and sinks

The global distribution of fluorocarbon-12 and fluorocarbon-11 is used to establish a relatively fast interhemispheric exchange rate of 1 to 1.2 years. Atmospheric residence times of 65 to 70 years for fluorocarbon-12 and 40 to 45 years for fluorocarbon-l1 best fit the observational data. These residence times rule out the possibility of any significant missing sinks that may prevent these fluorocarbons from entering the stratosphere. Atmospheric measurements of methyl chloroform support an 8-to 10-year residence time and suggest global average hydroxyl radical (HO) concentrations of 3 x 105 to 4 x 105 molecules per cubic centimeter. These are a factor of 5 lower than predicted by models. Additionally, methyl chloroform global distribution supports Southern Hemispheric HO levels that are a factor of 1.5 or more larger than the Northern Hemispheric values. The long residence time and the rapid growth of methyl chloroform cause it to be a potentially significant depleter of stratospheric ozone. The oceanic sink for atmospheric carbon tetrachloride is about half as important as the stratospheric sink. A major source of methyl chloride (3 x 1012grams per year), sufficient to account for nearly all the atmospheric methyl chloride, has been identified in the ocean.

Distribution of aromatic hydrocarbons in the ambient air

Aromatic hydrocarbons were measured in twelve United States cities during 1979–1984 with the help of an instrumented mobile laboratory. Approximately 100 measurements were made at each site over a 1–2 week period on a round-the-clock basis. Measurements at three sites were repeated to obtain seasonal differences. Although variabilities exist in these measurements, the average distribution of aromatic hydrocarbons in urban air is benzene 21%, toluene 36%, ethylbenzene 9%, m/p-xylene 15%, o-xylene 7%, 34-ethyl toluene 4%,1,2,4-trimethylbenzene 6 % and 1,3,5-lrimethylbenzene 2 %. Average concentrations in the range of 1–9 ppb benzene, 1–17 ppb toluene, 1–5 ppb ethylbenzene, 0.6–10 ppb m/p-xylene, 0.3–4 ppb o-xylene, 0.2–3ppb 34-ethyltoluene, 0.4–4ppb 1,2,4 trimethylbenzene and 0.1–2ppb 1,3,4-trimethylbenzene have been measured. Maximum concentrations at each site are typically less than 10 times the mean value. For both chemical and meteorological reasons, concentrations of aromatic hydrocarbons are at their highest during night and early morning hours with minimum occurring during late morning and early afternoon. Relative diurnal behavior of aromatic hydrocarbons shows that most are depleted at a rate 5–50 times faster than benzene. Based on data in southern California (site 13), it is estimated that a mean OH concentration of at least 2.6 (±0.6) × 106 moleccm−3 must prevail during 0730–1330h even in February. A long-term examination of benzene and toluene data from southern California air suggests that their levels may have declined by a factor of 5–10 over the last two decades. In remote atmospheres benzene is present at a concentration of 0.1–0.2 ppb, and is 1–3 times more abundant than toluene. It is estimated that such air masses are only 2–7 days away from their urban source.

Distribution of selected gaseous organic mutagens and suspect carcinogens in ambient air.

W An on-site field data collection program, based on short-term studies, was conducted in seven U.S. cities. Atmospheric concentrations, variabilities, and diurnal behaviors of 20 gaseous organic bacterial mutagens or suspect carcinogens are described. Except for benzene and formaldehyde, average concentration levels for all chemicals measured were in the 0-1-ppb range. Benzene and formaldehyde average levels were in the 14 and 10-20-ppb range, respectively. Typical diurnal profiles show highest concentrations during nighttime or early morning hours, with minimum concentrations in the afternoon hours; chemistry plays only a nominal role in defining this diurnal behavior in most cases. It is concluded that organic mutagens have always existed in the atmosphere (and the ocean), although at relatively low background concentrations. Our measurements for this group of 20 chemicals show that in the cleanest environments the present exposure is more than twice the natural background, whereas in the U.S. cities we studied exposure may be 15-30 times greater.

Measurements of peroxyacetyl nitrate (pan) and peroxypropionyl nitrate (ppn) at selected urban, rural and remote sites

PAN and PPN were measured in a series of eight field studies performed at urban, rural and remote locations in the contiguous U.S. during 1983–1985. Seven of the eight studies were performed in the winter/spring period, a period of sparsely available data. Nearly 2000 air samples were analyzed during these studies. Mean PAN and PPN levels in the range of 45–1600 ppt (max. 7.9 ppb) and 5–230 ppt (max. 0.9 ppb), respectively were measured. Despite a great deal of observed variability, PAN and PPN showed virtually identical behavior at all sites and in all seasons, supporting the view that these nitrogenous compounds are produced and destroyed by very similar mechanisms. On the average PPN concentrations were about 8% (range 3–14%) of PAN values. It is inferred that PPN/PAN ratio is highest in urban areas and declines as polluted air masses are transported over long distances.

Measurements of some potentially hazardous organic chemicals in urban environments

Three field studies were conducted in Los Angeles, California; Phoenix, Arizona; and Oakland, California, to better characterize the atmospheric abundance, fate and human exposure of selected organic chemicals that may be potentially hazardous. During field data collection, in-situ analysis using an instrumented mobile laboratory was performed for a total of 33 organics; a dozen of these are suspected carcinogens. The concentrations, variabilities and average daily dosages from exposure to these pollutants were determined. The diurnal behavior and the atmospheric fate of both primary and secondary pollutants were studied. Residence times for a typical polluted atmosphere were estimated. The atmospheric distributions and abundances of many species have been defined for the first time. Average daily-dose levels of all three sites for exposure to halomethanes (excluding fluorocarbons), haloethanes, chloroethylenes, chloroaromatics, aromatic hydrocarbons and secondary organics were determined to be 298, 142, 203, 21, 1880 and 257 μg d−1 respectively. Exposure levels in Los Angeles were typically the highest and those in Oakland the lowest.

Methodology for the analysis of Peroxyacetyl nitrate (PAN) in the unpolluted atmosphere

Abstract A light weight electron capture, gas chromatograph has been laboratory- and field-tested to conduct surface and airborne PAN measurements in the unpolluted troposphere. A dynamic calibration system based on CH3CHO/NO2/Cl2 photolysis studies by Gay et al. (1976) was constructed and successfully tested. PAN was cryogenically preconcentrated prior to analysis. A sensitivity of 5 parts per trillion (ppt) and an overall accuracy of ± 20 % is estimated. It is shown that gas phase coulometry (GPC) is unsuited for absolute PAN analysis—principally, because a significant fraction of PAN is destroyed prior to coulometric detection. The kinetics of this destruction process are nonlinear. PAN measurements at a marine Pacific site, and aboard an aircraft, show that PAN is always present at a concentration range of 10–100 ppt, although concentrations as high as 400 ppt were measured at an altitude of 4.6 km over the Pacific Ocean. Surface PAN measurements at a Pacific marine site indicate a distinct diurnal behavior, tentatively attributed to photochemistry involving alkenes, alkanes and NOx. There was no evidence of PAN diurnal variation in the free troposphere, but the data are currently too sparse. Measurements in the global atmosphere are needed to accurately describe the distribution and the role of PAN in the chemistry of the natural atmosphere.

Measurements of formaldehyde and acetaldehyde in the urban ambient air

Abstract Acetaldehyde and formaldehyde were measured in urban ambient air by analyzing their 2,4-dinitrophenylhydrazine derivatives with reverse-phase, high-performance liquid chromatography (HPLC). A series of nine short term field experiments were performed in eight cities. Concurrent formaldehyde measurements using the chromotropic-acid procedure show reasonable agreement (±30 %) between the two methods. Average summertime ambient urban formaldehyde (HCHO) concentrations of 10–20 ppb (10−9v/v) are significantly higher than the average acetaldehyde (CH3CHO) concentrations of 1–2 ppb. There is evidence of much reduced formaldehyde levels in winter months. Exceptionally high, absolute (8.5 ppb av.) and relative ( HCHO CH 3 CHO ~ 2 ) acetaldehyde concentrations are measured in the South Coast Air Basin of California.

Measurement of volatile organic chemicals at selected sites in California

Abstract Urban air concentrations of 24 selected volatile organic chemicals that may be potentially hazardous to human health and environment were measured during field experiments conducted at two California locations, at Houston, TX, and at Denver, CO. Chemicals measured included chlorofluorocarbons, halomethanes, haloethanes, halopropanes, chloroethylenes and aromatic hydrocarbons. With emphasis on California sites, data from these studies are analysed and interpreted with respect to variabilities in ambient air concentrations, diurnal changes, relation to prevailing meteorology, sources and trends. Except in a few instances, mean concentrations are typically between 0 and 5 ppb. Significant variabilities in atmospheric concentrations associated with intense sources and adverse meteorological conditions are shown to exist. In addition to short-term variability, there is evidence of systematic diurnal and seasonal trends. In some instances it is possible to detect declining trends (e.g. ethylene dibromide and chloroethylenes) resulting from the effectiveness of control strategies.

Tracer techniques for estimating emissions from inaccessible ground level sources

Abstract A methodology is described that can be used to estimate emission rates from manufacturing plants, or other source areas, when access is not possible. An inert tracer is released at a known rate from a vehicle traveling back and forth on a cross-wind road outside the emission area. Samples are collected downwind of the source area (at a distance of 1–5 km), and analyzed for the tracer and emitted materials of interest. The paper describes how the emission rates of the materials of interest can be determined from the measured downwind concentrations. The method is generally insensitive to meteorological conditions if applied at night or under overcast, neutrally stable, daytime conditions; it is suitable for estimating fugitive emissions from sources within 10 or 15 m of ground level. However, it must be applied with discretion if interfering sources are present in the area. Results obtained in a test at a halogenated hydrocarbon manufacturing plant gave promising results and demonstrated the feasibility of the methodology. Suggestions for improvements are given.