Hanwant B. Singh

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.

Tropospheric ozone: Concentrations and variabilities in clean remote atmospheres

Abstract Analysis of long-term ozone (O 3 ) data collected at remote sites between latitudes 19°N and 48°N is complemented by aircraft data to support the conclusion that a significant reservoir of ozone is present in the troposphere. Evidence suggests that this O 3 reservoir is predominantly of stratospheric origin and that photochcmical oxidation processes resulting in O 3 production from HCs (including CH 4 ), CO and NO x of natural origin do not contribute significantly to the net O 3 balance in this reservoir. It is found that the predominant source of tropospheric O 3 is due to injections from the stratosphere. The important sinks are O 3 loss at and near the earths surface and to a less certain degree gas phase photolytic destruction. The possibility that natural precursors produce O 3 , at a rate that roughly offsets the gas phase photolytic destruction, can not be ruled out. This tropospheric O 3 shows a distinct seasonal variation, with a maximum in the spring when 1-h O 3 concentrations can approach or exceed 80 parts per billion (ppb). Year to year variations can be important; for example, at Mauna Loa the hourly averaged O 3 concentrations exceeded 80 ppb 1.6% of the days in 1975 and 0% of the days in 1974. No average diurnal variation of O 3 is observed throughout the year within this reservoir. Because O 3 from this reservoir is mixed downward toward the surface under convective atmospheric conditions, the achievement of a yearly 1-h O 3 standard of 80 ppb may be impossible. Analysis of atmospheric data also indicates that the mean residence time of O 3 in the troposphere is long (≈4 months), while that of NO x , is much shorter (0.4–2 days) than the hitherto accepted value of 8–10 days.

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.

Stratospheric ozone in the lower troposphere. II: Assessment of downward flux and ground-level impact

Abstract Aircraft measurements of four stratospheric O3-intrusion events (two during spring and two during fall) are used in conjunction with concurrent meteorological analyses to estimate the downward 03 flux in the upper and lower troposphere. The aircraft measurements used are among those reported earlier in Part I of this paper (Johnson and Viezee, 1981). The calculated upper tropospheric fluxes for the four cases show good agreement with earlier estimates by Danielsen and Mohnen (1977) of O3 fluxes associated with tropopause folding events. The estimated lower-tropospheric O3 fluxes for the two spring events and for one fall event suggest that more than half of the O3 mass injected into the upper troposphere by these stratospheric intrusions is probably mixed and diluted into the troposphere above 700 mb (3 km ASL). Large, direct impacts of stratospheric O3 intrusions at ground-level are thus unlikely. A review and analysis of the limited number of published observations of high O3 in stratospheric intrusions, and of anomalously high O3 at ground level attributed to stratospheric intrusions, also suggests that direct ground-level impacts may be infrequent (less than 1 per cent of the time), and most likely are associated with O3 concentrations (v/v) of 100 ppb or less. Additional observational studies are required to conclusively quantify the ground-level impact of stratospheric O3.

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.

Atmospheric Carbon Tetrachloride: Another Man-Made Pollutant

On the basis of an analysis of historic worldwide emissions and removal mechanisms for carbon tetrachloride, a possible precursor for stratospheric ozone destruction, it has been demonstrated that the present atmospheric loading and distribution of carbon tetrachloride is primarily attributable to man-made emissions and no natural sources need be invoked to explain its presence in the atmosphere.

The impact of stratospheric ozone on tropospheric air quality

A background of ozone (O3), principally of stratospheric origin, is present in the lower free troposphere. Typical mean O3 levels of 50 ppb, 40 ppb, and 30 ppb are encountered here in spring, summer, and fall, respectively. Maximum hourly O3 concentrations which are twice these mean values can be expected. Ozone from the free troposphere is routinely brought down to ground level under turbulent atmospheric conditions. Deep and rapid Intrusions of stratospheric air into the lower troposphere are associated with low-pressure troughs and occur regularly. In the mid troposphere, O3 levels as high as 300 ppb are found within these intrusions. Observational data showing these intrusions, containing high O3 concentrations, to directly reach ground level are currently lacking. Over the United States, an intrusion was present aloft on 8 9% of the days in 1978. The frequency, however, is somewhat reduced in summer and a northward movement is evident. During 1978, no intrusion occurred south of 30°N between June and...

Atmospheric formation of chloroform from trichloroethylene

Abstract Chloroform has been observed as a photochemical product in simulated ambient air containing trichloroethylene, paralleling a previous observation of carbon tetrachloride formation from perchloroethylene. Some portion of the atmospheric budget of these chloromethanes may perhaps be attributed to this process. Toxic products such as phosgene and chloro‐acetylchlorides are formed. This process may well provide another source of stratospheric ozone‐destroying chlorine atoms.