摘要:

We have used a two‐dimensional numerical model of mesospheric cloud formation to simulate the effects of internal gravity waves on the formation and appearance of ice clouds at the mesopause. We have used gravity wave parameters (amplitude, wavelength, period, etc.) consistent with measurements of gravity waves in the high‐latitude mesosphere, including observations of band and billow structures in noctilucent clouds (NLCs). We have investigated both the dynamical effects due to wave‐induced winds and the microphysical effects due to wave‐induced temperature perturbations. We find that the structures observed in NLC from the ground are predominantly caused by dynamical processes. Since the time required for ice crystal growth at the mesopause is much longer than the observed wave periods and ice crystal sublimation is much more rapid than deposition growth, the primary effect of wave‐induced temperature perturbations is to decrease the albedo of the clouds. The fact that gravity waves with periods less than a few hours are ubiquitous near the high‐latitude, summertime mesopause imposes an additional constraint on the formation of visible NLC. We find that the mean mesopause temperature must be about 5 K colder for NLC to form when gravity waves are present than when they are not present. Long‐period (>10 hours) waves may temporarily enhance the brightness of existing NLC but will not induce the for

ISSN:0148-0227    

DOI:10.1029/93JD01736

年代:1994

数据来源: WILEY

摘要:

The horizontal distributions of air in the temperature ranges −70°>T>−77°C, −77°>T>−85°C and T−85°C did occur immediately outside both vortices, so separating the effects of peel‐off, type I PSCs and sulfuric acid aerosol processing may be difficult on occasions. What is clear is that all three processes are intimately associated

ISSN:0148-0227    

DOI:10.1029/93JD02054

年代:1994

数据来源: WILEY

摘要:

A study of ozone transfer from the stratosphere to the troposphere has been performed during two phases of the evolution of a cutoff low using both ozone vertical profiles and objective analysis of the European Center for Medium range Weather Forecasting to compute potential vorticity distributions and air mass trajectories. Ozone profiles were measured by a ground‐based lidar system at the Observatoire de Haute Provence (OHP)(43°55N, 5°42E) on November 1990. A stratospheric ozone transport into the troposphere has been observed during a tropopause fold which occurred at the beginning of the cutoff low formation. On a timescale of a few days the correlation is rather good between the potential vorticity and the ozone time evolution in the upper part of the tropopause fold, and the ozone to potential vorticity ratio is of the order of 30–40 ppb/PVu (1 PVu = 10−6K m2s−1kg−1) in the tropopause fold. The amount of stratospheric ozone transferred by the folding process is estimated to be 6.5±3.5 1032molecules d−1, using a trajectory analysis, the potential vorticity distribution along the trajectory path, and temperature radio sounding profiles, in order to estimate the likelihood of the downward transport of ozone. During the erosion phase of the cutoff low, the tropopause definition has been changing, and the stratospheric ozone decrease associated with the cutoff low evolution corresponds to an ozone transport of 4.0±2.0 1032molecules d−1, which is of the same order of magnitude as the impact of the folding process. The mechanism resulting in the downward flux of ozone can be twofold: a tropopause lifting associated with convective clouds and small‐scale turbulent mixing near the jet stream. None of them can be discarded, due to the large uncertainties for their respective influences, when using the data set avail

ISSN:0148-0227    

DOI:10.1029/93JD02551

年代:1994

数据来源: WILEY

摘要:

Trace gas measurements pertinent to understanding the transport and photochemical formation of O3were made at a surface site in rural Georgia as part of the Southern Oxidant Study during the summer of 1991. It was found that there was a strong correlation between O3and the oxidation products of NOx: O3(ppb) = 27 + 11.4 (NOy(ppb) − NOx(ppb)),r2= 0.78. This fit is similar to that observed at other rural sites in eastern North America and indicates a nominal background O3level of 27 ppb; values higher than 27 ppb are due to photochemical production in the recent past, which varied from near zero to ≈50 ppb. The origin of the O3above background was investigated by using a free radical budget equation to calculate an in situ O3production rate in terms of measured concentrations of NO and free radical precursors (O3, HCHO, peroxides, and other carbonyls). A comparison of observed and predicted diurnal trends in O3indicates significant O3production in the afternoon at a time when O3concentration is either steady or decreasing. The afternoon near‐surface layer is thereby a source region for O3which can be exported. In situ production accounts for approximately one half of the morning increase in O3concentration on days with high O3; the remainder is due to entrainment of dirty air aloft by the growing convective boundary layer. Additional evidence for the role of vertical transport in controlling the hour‐to‐hour changes in O3is found in the diurnal cycles of SO2and HNO3which also have rapid increases in the morning. The day‐to‐day variability of O3was investigated using a back trajectory model. NOyconcentration at the measurement site could be reasonably accounted for by considering NOxemission sources located within 1‐day transport distance. In as much as there is a strong correlation between O3and NOy, the coincidence between trajectory location and NOxemission sources appears to be an important factor influencing midday O3concentration. Hydrocarbon measurements are consistent with NOxbeing the limiting factor for

ISSN:0148-0227    

DOI:10.1029/93JD02991

年代:1994

数据来源: WILEY

摘要:

The global distribution of ozone during the October 31, 1978, to May 6, 1993, observing lifetime of the Nimbus 7 total ozone mapping spectrometer (TOMS) is described, with emphasis on the low ozone amounts observed during 1992 and 1993. Ozone amount time series are extended beyond May 6, 1993, to the end of July 1993 using preliminary Meteor 3 TOMS data. Time series for zonally averaged ozone amounts show that there has not been a significant shift in the seasonal patterns of ozone maxima and minima caused by the Mount Pinatubo eruption or by the onset of very low ozone values during 1992 and 1993. There has been a relatively slow, nearly linear decrease in the amount of ozone over the entire globe from 1979 to the end of 1991, with rates ranging from no change at the equator to a 4–6% decrease per decade at midlatitudes and a 10–12% decrease per decade at higher latitudes. After the eruption of Mount Pinatubo during June 1991, the ozone amount decreased in the equatorial latitudes (10°S to 10°N) for about 6 months (−10 Dobson units (DU) between 0° and 10°S and −3 DU between 0° and 10°N). During 1992 and continuing into 1993, the rate of ozone decrease deviated from the previously linear trend with the onset of changes that were large in comparison with the historical range of ozone values from 1979 to 1991. The first of the large decreases in ozone amount occurred earlier, in February 1990 to May 1990, at 50°–70°N. At high northern latitudes, the 1993 decreased ozone amounts were about 12.5% below the envelope of historical values; at midlatitudes they were about 7% lower; and at low latitudes they were about 4% lower. Area‐weighted averages in the northern and southern hemispheres show that most of the 1992–1993 ozone losses have occurred in the northern hemisphere. The 1993 global average (70°S to 70°N) ozone amount is 3% below the 1979 to 1991 minimum, 5% below the historical envelope in the northern hemisphere, and near the lower boundary of the historical envelope in the southern hemisphere. In the 70°–60°S latitude band, the ozone losses between 1979 and 1993 have reduced the annual minimum amount to values below those see

ISSN:0148-0227    

DOI:10.1029/93JD02594

年代:1994

数据来源: WILEY

摘要:

Injection of ethane or propane has been suggested as a means for reducing ozone loss within the Antarctic vortex because alkanes can convert active chlorine radicals into hydrochloric acid. In kinetic models of vortex chemistry including as heterogeneous processes only the hydrolysis and HCl reactions of ClONO2and N2O5, parts per billion by volume levels of the light alkanes counteract ozone depletion by sequestering chlorine atoms. Introduction of the surface reaction of HCl with HOCl causes ethane to deepen baseline ozone holes and generally works to impede any mitigation by hydrocarbons. The increased depletion occurs because HCl + HOCl can be driven by HOxradicals released during organic oxidation. Following initial hydrogen abstraction by chlorine, alkane breakdown leads to a net hydrochloric acid activation as the remaining hydrogen atoms enter the photochemical system. Lowering the rate constant for reactions of organic peroxy radicals with ClO to 10−13cm3molecule−1s−1does not alter results, and the major conclusions are insensitive to the timing of the ethane additions. Ignoring the organic peroxy radical plus ClO reactions entirely restores remediation capabilities by allowing HOxremoval independent of HCl. Remediation also returns if early evaporation of polar stratospheric clouds leaves hydrogen atoms trapped in aldehyde intermediates, but real ozone losses are small in such

ISSN:0148-0227    

DOI:10.1029/93JD03089

年代:1994

数据来源: WILEY

摘要:

Visible absorption spectroscopy was employed for stratospheric measurements at McMurdo Station, Antarctica, during the summer and fall seasons in 1990, 1991, 1992, and 1993. Observed column amounts of NO2were as much as 50% smaller in 1992 and 1993 than in 1990 and 1991. The measured decreases in NO2are believed to be due to the hydrolysis of N2O5on the enhanced volcanic aerosols injected by the massive eruption of Mount Pinatubo in mid‐1991. Such decreases in NO2are broadly consistent with observations at other latitudes and with model calculations. However, substantially reduced NO2was also observed during the polar day period in summer under conditions of continuous local solar illumination. The formation of the key N2O5intermediate and reduction of NO2at this time of year probably occurs via two mechanisms: (1) excursions of air parcels to lower latitudes where nighttime chemistry can produce N2O5and (2) conversion of a small fraction of the NO3formed during the day to N2O5. Model calculations show that the latter mechanism can be important in spite of the fast daytime photolysis of NO

ISSN:0148-0227    

DOI:10.1029/93JD03088

年代:1994

数据来源: WILEY

摘要:

We have measured the rate of reaction of N2O5with H2O on monodisperse, submicrometer H2SO4particles in a low‐temperature flow reactor. Measurements were carried out at temperatures between 225 K and 293 K on aerosol particles with sizes and compositions comparable to those found in the stratosphere. At 273 K, the reaction probability was found to be 0.103±0.006, independent of H2SO4composition from 64 to 81 wt %. At 230 K, the reaction probability increased from 0.077 for compositions near 60% H2S04to 0.146 for compositions near 70% H2SO4. Intermediate conditions gave intermediate results except for low reaction probabilities of about 0.045 at 260 K on aerosols with about 78% H2SO4. The reaction probability did not depend on particle size. These results imply that the reaction occurs essentially at the surface of the particle. A simple model for this type of reaction that reproduces the general trends observed is presented. The presence of formaldehyde did not affect the reaction ra

ISSN:0148-0227    

DOI:10.1029/93JD01907

年代:1994

数据来源: WILEY

摘要:

For areas in the United States not meeting the federal air quality standard for ozone, an issue of continuing controversy is the emphasis to be placed on controlling nitrogen oxides (NOx) in addition to emissions of reactive organic gases (ROG). To assess conditions under which ROG or NOxcontrols would be most effective, we have analyzed predictions from four studies that represent different locations and meteorological conditions, distinct chemical inputs, e.g., with or without significant biogenic emissions, and different air quality models. A consistent association is found between the sensitivity of ozone to reductions in ROG versus NOxemissions and the simulated total reactive nitrogen (NOy) at the time and place of peak ozone. In the studies examined, ozone was predicted to be reduced most effectively by ROG controls at locations where NOyconcentrations exceeded a threshhold value falling in the range of 10 to 25 ppb, whereas NOxcontrols were predicted to be more effective where NOyconcentrations were below that threshhold. The NOylevel explains much of the difference in ozone sensitivity at different locations and provides a basis for comparison of predicted sensitivity from different models. In contrast, the morning concentration ratio of ROG to NOxthat has been used in the past is a less reliable indicator of O3sensitivity. Measurement of NOyconcentrations along with ozone would assist in empirical testing of model predictions of responses to emission reductions.

ISSN:0148-0227    

DOI:10.1029/93JD03224

年代:1994

数据来源: WILEY

摘要:

The hydroxyl radical (OH) is the primary oxidant in the atmosphere, responsible for many photochemical reactions that affect both regional air quality and global climate change. Because of its high reactivity, abundances of OH in the troposphere are less than 1 part per trillion by volume (pptv) and thus difficult to measure accurately. This paper describes an instrument for the sensitive detection of OH in the troposphere using low‐pressure laser‐induced fluorescence. Ambient air is expanded into a low‐pressure detection chamber, and OH is both excited and detected using theA2∑+(v′ = 0) →X2Π(v″ = 0) transition near 308 nm. An injector upstream of the detection axis allows for the addition of reagent NO to convert ambient HO2to OH using the fast reaction HO2+ NO → OH + NO2. Using recent advances in laser and detector technologies, this prototype instrument is able to detect less than 1×105molecules cm−3(0.004 pptv) of OH with an integration time of 30 s with negli

ISSN:0148-0227    

DOI:10.1029/93JD03342

年代:1994

数据来源: WILEY