NO2 Mixing Ratios Research Articles

Stratospheric NO measurements: A new balloon‐borne chemiluminescent instrument

A description of a new chemiluminescent balloon sonde for measuring stratospheric nitric oxide is given. First results from three flights at mid‐latitudes are presented, including measurements showing a slow reduction in NO mixing ratio beginning 2½ hours before sunset. A hysteresis in the NO mixing ratio between ascent in the morning and descent in the afternoon was observed. The afternoon values were found to be up to a factor of 2 larger than the morning values.

Direct measurements of atmospheric CH2O, HNO2, O3, NO2, and SO2 by differential optical absorption in the near UV

Measurements of CH2O, HNO2, O3, NO2, and SO2 at four different sites in western Europe are reported. At the maritime sites (Loophead, Ireland, and Dagebüll, Germany) the mixing ratios of the pollution indicators SO2 and NO2 are <0.5–2 ppb, while concentrations of several tenths of a part per billion of CH2O are consistent with photochemical equilibrium. HNO2 was only found at continental sites (Jülich and Deuselbach) with SO2 and NO2 mixing ratios up to 100 ppb. Surprisingly, NO3 was found to be at least 10 times lower than expected in most cases.

A two-dimensional model of the distribution of trace gases in the stratosphere and troposphere

We report the results of computations of the distribution of trace gases in the troposphere and stratosphere with a two-dimensional photochemical transport model. The region covered by the model extends from pole to pole and from 0 to 50 km. Atmospheric transport is included by using seasonal averaged values for mean meridional and vertical winds and eddy diffusion. The compounds transported in the model are N2O, NO x (NO+NO2), NO y (NOx+HNO3), N2O5, H2O, CH4 and H2O2. Odd hydrogen (OH+HO2+H) is assumed in photoequilibrium. A simple scheme is proposed to simulate the condensation of H2O and scavenging of other gases. Rayleigh scattering is included in the calculation of the photodissociation rates. The resulting distributions are compared whenever possible with experimental data. We attempt to discuss the stratospheric budget of water vapour taking into account photochemical sources (e.g. CH4 oxidation) and sinks. It is shown that similarly to other models serious disagreement seems to exist between the calculated and observed mixing ratios of NO x , NO y and the ratios of the different compound within the family. The amount of experimental data is, however, too sparse to draw any definite conclusion.

Measurements of NO, O3, and temperature at 19.8 km during the total solar eclipse of 26 February 1979

Local measurements of stratospheric NO and O3 mixing ratios and air temperature were made during the total solar eclipse of 26 February 1979. The instrumentation was carried aboard a U‐2 aircraft flown at an altitude of 19.8 km in the region near 47° N ‐ 112° W. Eclipse maximum occurred approximately in the middle of the 2‐3/4‐hr measurement period. The NO mixing ratio was reduced at least a factor of 25 at the maximum of the eclipse. The decrease and recovery of NO during the passage of the Moon's shadow over the measurement region follows approximately the predictions of two independent models. No change was observed in either the O3 mixing ratio or the air temperature that could be attributed to the eclipse.

Nitric acid and aerosol nitrate measurements in the equatorial Pacific Region

Surface nitric acid and particulate nitrate measurements were made in the equatorial Pacific region using impregnated filter techniques. The average HNO3 mixing ratio of 38 pptv is nearly ten times the average noonday NO mixing ratio measured simultaneously by McFarland, et al. The highest HNO3 values, 61 and 68 pptv, were measured when rain‐showers were taking place in the vicinity of the sampler. Particulate NO3− concentrations ranged from 68 to 254 pptm.

Nitrous oxide measurements in the eastern tropical Pacific Ocean

In March 1976 the first N2O measurements were made in the sea water and marine air of the eastern tropical Pacific Ocean during a cruise from San Diego, California, to San Martin, Peru. Continuous measurements of N2O in the marine atmosphere were made throughout the duration of the cruise. The tropospheric mixing ratio was 332 ± 9 p.p.b. v/v N2O. The concentration of N2O in the marine air appeared to show a direct relationship to the N2O concentration in the surface sea water, with the highest N2O mixing ratios over regions which were highly supersaturated, off the coasts of Baja California and Peru, and in the equatorial upwelling region. The surface sea water samples were generally supersaturated with N2O, with an average saturation of 145% relative to the marine atmosphere. The average flux of N2O from the surface sea water into the atmosphere was 0.5 x 10-12 g N2O cm-2 s-1. Only one surface sea water sample showed significant undersaturation; the other 28 surface water samples were approximately saturated or supersaturated. Water samples were also collected down to depths of 300 m during seven hydrocast stations. The stations showed two distinct distribution patterns for N2O concentration versus depth for the region between the surface and 300 m. At five of the seven stations the N2O concentration increased rapidly from the surface to a depth of 100 m, and then remained relatively constant down to the maximum depth of 300 m. At the other two stations the N2O concentration showed the same rapid increase between the surface and 75–100 m, but then the N2O concentration decreased in the next 50–75 m until the concentration of N2O at 150 m was lower than that found at the surface. At these two stations the water from 150 m to 300 m was just saturated or undersaturated with N2O with respect to the atmosphere. These two stations were located in the oxygen-deficient region off the coast of Mexico, which suggests that the low N2O concentrations below the thermocline may be due to denitrification. However, stations in the oxygen-deficient region off the coast of Peru showed considerable N2O supersaturation at all depths. Recent results indicate that the role of N2O in the nitrogen cycle of the ocean may be more complex than has previously been suggested.

Measurements of N2O, CF2Cl2, CFCI3, CH3CCl3, CCl4, and CH3Cl in the troposphere and lower stratosphere over North Amrica.

Samples of tropospheric and stratospheric air were collected on aircraft flights between October 4 and October 13, 1976. The region of the atmosphere surveyed was over the North American continent (approximately 18°N to 65°N latitude). Samples were obtained at heights ranging from about 3, 000ft. to 45, 000ft. Measurements of N2O, CF2Cl2, CFCl3, CH3CCl3, and CCl4 were made by GC/EC techniques, and of CH3Cl by GC/MS. The latitudinal and height profiles of these trace gases are reported in this paper. Calculations based on these measurements yield the following average tropospheric concentrations: [N2O]=(330±2) ppb v, [CF2Cl2]=(236±7) ppt v, [CFCl3]=(140±5) ppt v, [CH3CCl3]=(145±25) ppt v, and [CCl4]=(134±9) ppt v (where the ± values are the standard deviations). The N2O mixing ratio decreases only slightly in the lower stratosphere. CF2Cl2 and CFCl3 mixing ratios decrease by about 10% and 15% respectively, within 4km above the tropopause. CCl4 mixing ratio declines by about 50% within 4km above the tropopause. CH3CCl3 mixing ratios also show a sharp decline above the tropopause, similar to that of CCl4. CH3Cl on the other hand displays almost no change in mixing ratio between the free troposphere and the lower stratosphere.

Tropospheric NO measurements up to 7 km

Mixing ratios of nitric oxide were measured by the chemiluminescence method as part of the 1977 and 1978 Global Atmospheric Measurement Experiment of Tropospheric Aerosols and Gases (Gametag) missions. Since the method is not highly specific it can provide only upper limits for the NO mixing ratio. Values for maritime air were ≤30–40 parts per trillion by volume (pptv) and showed little variation with latitude or whether or not the measurements were taken in or above the boundary layer. Over continental North America mixing ratios in unpolluted air were ≤40–60 pptv, independent of altitude. Values in excess of 1 ppbv were obtained in industrial regions, over forest fires, and even over small Eskimo villages in northern Canada. In polluted air the values decreased with an increase in altitude.

Atmospheric nitrous oxide in the mid‐latitudes of the southern hemisphere

Results from the first continuous program for studying atmospheric nitrous oxide (N2O) in the southern hemisphere are presented. Daily measurements at Aspendale, Australia (38.1°S, 145.2°E), yield an average N2O mixing ratio of 340 ppb in dry air for the period October 1976 to May 1978. No significant long‐term trend has been detected. Analysis of air collected at several clean air (marine and rural) locations reveals a significantly lower mixing ratio of 337–338 ppb. The difference is attributed to an urban source of N2O. Results from an extensive air sampling network illustrate the homogeneous distribution of N2O in the troposphere and reveal no variations in mixing ratio which could be attributable to seasonal or synoptic variations. The N2O mixing ratio decreases above the tropopause. Preliminary results of the direct measurement of N2O exhalation from soil are reported. Both the constancy of the N2O mixing ratio in the troposphere and the low exhalation rates from soil suggest a long lifetime for N2O in the atmosphere. This implies small sources and sinks and makes the atmospheric N2O cycle susceptible to anthropogenic inputs. All measurements were made by electron capture gas chromatography with a precision of approximately 1%. The uncertainty in the absolute value of the N2O mixing ratio is 10–15%. The need for accurate and reproducible gas standards and for regular interlaboratory comparisons of standards is discussed.

Nitric oxide measurements in the equatorial Pacific region

Surface nitric oxide mixing ratios were measured in the equatorial Pacific region using a chemiluminescent detector. The average NO mixing ratio was about 4 pptv during noontime conditions for nine days of measurements. These NO mixing ratios, which are much lower than any previously reported, imply that the O3 production rate due to CO and CH4 oxidation may be less than previously believed. They also imply smaller OH/HO ratios as a consequence of the lower HO2 to OH² conversion by NO.

A preliminary report on the numerical simulation of the three‐dimensional structure and variability of atmospheric N2O

A numerical simulation of atmospheric N2O using the GFDL 3‐D tracer model with a small uniform surface source (15 Mton yr−1) and stratospheric destruction (150 yr lifetime) has been run to a state near transport and chemical statistical equilibrium. The resulting N2O tropospheric distribution is relatively uniform with a slight excess in the Southern Hemisphere. In the model stratosphere there is a sharp poleward decrease in N2O mixing ratio away from high values in the tropics with pronounced winter minimums at 50°S and 60°N. Even with the small uniform surface source, relative standard deviations of N2O in the surface layer range from 0.1% to 0.8%, well within the range of recent measurements. Additional experiments suggest that motions acting upon N2O accumulation in the source region boundary layer and upon the mixing ratio gradient between the troposphere and lower stratosphere are the major sources of tropospheric N2O variability.

Vertical distribution of NO2 in the stratosphere as determined from balloon measurements of solar spectra in the 4500Å region

The stratospheric NO2 mixing ratio profile in the 20‐40 km altitude range is derived from balloon‐borne observations of the solar spectrum in the visible region. By comparisons of high sun and low sun spectra at float altitude (∼40 km), a number of NO2 features are identified. The resulting NO2 profile shows a gradual increase above 20 km to a peak value of 13 ppb(v) near 35 km, followed by a gradual decrease to 10.5 ppb(v) at 40 km.

Tropospheric N2O mixing ratio measurements

Seventy‐five ground level and six parachute descent grab samples of tropospheric air have been made at a variety of locations throughout the world over a 6‐month period. The N2O mixing ratios in the samples have been determined by electron capture gas chromatography to an absolute accuracy of about 5%, an estimate that is based on the results of several interlaboratory comparisons of calibration standards. The mean ground level N2O mixing ratio from these measurements was 326 ppbv (parts per billion by volume), referenced to ‘dry air,’ and was found to be remarkably constant. The residual standard deviation was 5.3 ppbv. This worldwide mean is consistent with other recent measurements but is considerably larger than many of the earlier measurements. The present data indicate that the N2O residence time in the troposphere is very likely to be greater than 11 years. Furthermore, the data separately show that the possibility of a current annual increase in the N2O mixing ratio by more than 4% seems to be excluded. The parachute descent data, taken just below the tropopause, have a slightly smaller mean, 318 ppbv, but support the contention that the present tropospheric N2O mixing ratio is well above 300 ppbv.

Measurements of NO2Using the Dobson Spectrophotometer

Abstract A method has been developed of measuring atmospheric NO2 using the Dobson spectrophotometer. The intensifies of wavelengths λ4377 and 4448, which have strong differential absorption by NO2, are measured by suitably changing the inclination of the Q-quartz plates to the direction of the light ray. Using appropriate absorption coefficients of N02, an average NO2 mixing ratio was obtained which is of the order of 10minus9 for Aspendale. The errors and the uncertainties involved in the method are discussed. It is believed that the method is useful in determining the average background value of NO2 in spite of the uncertainties involved which could approach 100%.

Simultaneous measurements of No and No 2 in the stratosphere

Simultaneous measurements of NO and NO 2 in the stratosphere leading to an NO x determination have been performed by means of i.r. absorption spectrometry using the Sun as a source in the 5·2 μm band of NO and in the 6·2 μm band of NO 2. The observed abundance of NO P peaks at 26 km where it is equal to (4·2 ± 1) × 10 9 cm −3. The volume mixing ratio of NO p was observed to vary from 1·3 × 10 −9 at 20 km to 1·3 × 10 −8 at 34 km.

Measurement of nitric oxide in the stratosphere between 17·4 and 22·9 km

On 16 March 1973, the York U./U.S.U. chemiluminescent nitric oxide instrument was flown successfully from Hollomon Air Force Base, New Mexico, 32° 50·1′ N lat. The instrument was operated between 17:40 and 22:05UT. Most of the measurements were carried out during three ascent, float and descent cycles between 17·4 and 22·9 km. Within the uncertainty of the measurements no differences were seen among results obtained during ascent, float and descent mode, which indicated that there were no major sampling problems resulting from contamination of the atmosphere. The NO mixing ratio was found to be 0·1 ppb v/v with an accuracy of 60 per cent. No change was observed within this accuracy over the small altitude range investigated.

Chain branching and flame propagation

In the present work, the laminar burning velocities of the NH3+N2O+air flames were measured using the heat flux method at 1 atm and 298 K, with varied equivalence ratios and N2O mixing ratios. For the mixing ratio N2O/(N2O+air) = 0.5, a full range of equivalence ratios was covered. Moreover, at three equivalence ratios an extended range of mixing ratios was investigated. The laminar burning velocity has an approximately linear relationship against the fraction of nitrous oxide in the oxidizer mixture, regardless of the tested equivalence ratios. Several recently published NH3 mechanisms were compared with these new experimental data; among them the models of Nakamura et al. and Stagni et al. show the best performance for NH3+N2O+air flames over the entire range of the mixing ratios. The H/N/O kinetic mechanism of the authors was analyzed and updated focusing on the rate constants of reactions most sensitive in ammonia flame propagation and self-ignition of NH3+O2 and H2+N2O mixtures. The choice of the new rate constants is outlined, however, no modification (adjustment or tuning) of the rate parameters to accommodate experimental results was attempted. The updated mechanism demonstrates significantly improved agreement with all measurements used for the model development and with other experimental data from the literature for ammonia flames and self-ignition. A comparative reaction path analysis for NH3+N2O+air and NH3+air flames revealed that an almost linear increase of the laminar burning velocity with an increased fraction of N2O in the oxidizer originates from the rate controlling reaction N2O+H = N2+OH, which produces OH radicals dominating ammonia oxidation.