D. D. Davis

Bio: D. D. Davis is an academic researcher. The author has contributed to research in topics: Radiosonde & Latitude. The author has an hindex of 1, co-authored 1 publications receiving 34 citations.

Free tropospheric/boundary‐layer airborne measurements of H2O over the latitude range of 58°S to 70°N: Comparison with simultaneous ozone and carbon monoxide measurements

Abstract: Presented in this paper are the H2O data collected during the 1977 and the 1978 Gametag (Global Atmospheric Measurements Experiment of Tropospheric Aerosols and Gases) field experiments. These data primarily reflect sampling occurring during the horizontal flight legs, involving altitudes of 5–6.5 km (free troposphere) and those in the atmospheric boundary layer at altitudes of ≤2 km. The latitude range covered was 70°N to 58°S. Approximately two thirds of the data were recorded over the open Pacific Ocean with the remaining one third being recorded over continental areas (e.g., Canada and Alaska). The H2O results show that very large variations in the level of this trace gas can occur within the free troposphere over very small geographical distances. They also indicate that over some regions of the Pacific Ocean very low levels of H2O can be found even at tropical and subtropical latitudes. The comparison of Gametag data with existing global dew point compilations suggests that some of the radiosonde middle tropospheric dew point profiles may be significantly in error, the radiosonde results being too high. Comparisons of the Gametag O3 and dew point/H2O data have shown that for the middle troposphere these two variables very frequently are negatively correlated. Comparisons between O3 and CO, when carried out within a given hemisphere, do not show a significant negative or positive correlation. On the basis of our findings, we have concluded that stratospheric/tropospheric dynamic exchange processes, directly and/or indirectly, play a controlling role in defining the tropospheric O3 source strength.

Tropospheric chemistry: A global perspective

Abstract: A model for the photochemistry of the global troposphere constrained by observed concentrations of H2O, O3, CO, CH4, NO, NO2, and HNO3 is presented. Data for NO and NO2 are insufficient to define the global distribution of these gases but are nonetheless useful in limiting several of the more uncertain parameters of the model. Concentrations of OH, HO2, H2O2, NO, NO2, NO3, N2O5, HNO2, HO2NO2, CH3O2, CH3OOH, CH2O, and CH3CCl3 are calculated as functions of altitude, latitude, and season. Results imply that the source for nitrogen oxides in the remote troposphere is geographically dispersed and surprisingly small, less than 107 tons N yr−1. Global sources for CO and CH4 are 1.5 × 109 tons C yr−1 and 4.5 × 108 tons C yr−1, respectively. Carbon monoxide is derived from combustion of fossil fuel (15%) and oxidation of atmospheric CH4 (25%), with the balance from burning of vegetation and oxidation of biospheric hydrocarbons. Production of CO in the northern hemisphere exceeds that in the southern hemisphere by about a factor of 2. Industrial and agricultural activities provide approximately half the global source of CO. Oxidation of CO and CH4 provides sources of tropospheric O3 similar in magnitude to loss by in situ photochemistry. Observations of CH3CCl3 could offer an important check of the tropospheric model and results shown here suggest that computed concentrations of OH should be reliable within a factor of 2. A more definitive test requires better definition of release rates for CH3CCl3 and improved measurements for its distribution in the atmosphere.

On the origin of tropospheric ozone

Abstract: The effects of NOx (NO + NO2) intrusion from the stratosphere on photochemical ozone production in the upper troposphere are investigated. Using the currently accepted reaction rate coefficients, we find that this upper tropospheric ozone source may be significantly larger than the direct injection of ozone from the stratosphere. Many features of the observed tropospheric temporal and spatial ozone distributions appear to be better explained by this upper tropospheric ozone source hypothesis than by either the classical ‘dynamical control’ or ‘photochemical control’ hypotheses. In addition, we find that NOx emissions from high flying subsonic aircraft in the northern hemisphere may cause an ozone increase in the troposphere. The calculated tropospheric ozone increase due to these NOx emissions is not inconsistent with the increases observed by the northern hemispheric ozonesonde stations.

Tropospheric nitrogen: A three‐dimensional study of sources, distributions, and deposition

Abstract: We simulate the global cycle of reactive nitrogen in a three-dimensional model of chemistry, transport, and deposition. Our model is based on the Lagrangian tracer model described by Walton et al. [1988] and uses winds and precipitation fields calculated by the Livermore version of the NCAR Community Climate Model. The model includes the basic chemical reactions of NO, NO2, and HNO3. For this study, we use prescribed OH and O3 concentrations and calculate the concentrations of NO, NO2, and HNO3 for a perpetual January and a perpetual July. The sources of reactive nitrogen due to fossil-fuel combustion (22 Mt N/yr), lightning discharges (3 Mt N/yr), soil microbial activity (10 Mt N/yr), biomass burning (6 Mt N/yr), and the oxidation of N2O in the stratosphere (1 Mt N/yr) are included. Model-predicted concentrations of NO, NO2, and HNO3 are compared to available measurements. In general, we find reasonable agreement between model predictions and measurements except for concentrations of HNO3 in the remote Pacific. At these latter locations, we require a larger source of reactive nitrogen to fit the observations. This may be supplied by lightning discharges, although increasing this source degrades our agreement with measured HNO3 abundances in the free troposphere. Alternatively, a local marine source could contribute to the measured abundances. Predictions for nitrate deposition by precipitation are within a factor of 2 of measured deposition rates in the northern hemisphere in the summer and in both seasons at remote locations. The model underpredicts nitrate deposition in winter in Europe, due primarily to the excessively strong winds generated by the general circulation model. Model simulations for NOx and HNO3 surface mixing ratios from calculations including only the fossil-fuel source, only natural sources, and all sources acting together, are compared. Anthropogenic sources have substantially increased the concentrations of NOx and HNO3 throughout all continents during both January and July. Fossil-fuel sources are responsible for most of this increase in the northern hemisphere, while both biomass burning and fossil-fuel combustion contribute in the southern hemisphere.

The distribution of carbon monoxide and ozone in the free troposphere

Abstract: The two-dimensional distributions of CO and O3 in the free troposphere during July and August, 1974, are discussed. The data confirm the previous findings that both of these gases are considerably more abundant in the northern hemisphere, but the degree of the asymmetry is somewhat different from what had been reported previously, especially for CO. When examined with respect to other available data sets, the conclusion is drawn that a pronounced seasonal cycle exists for CO in both hemispheres which may be driven by the likely seasonal cycle of the OH radical. The data also indicate that CO concentrations exhibit significant variability with height in the northern hemisphere, whereas southern hemispheric concentrations are quite constant with altitude except in cases where interhemispheric exchange of air may be occurring. A discussion on the vertical and horizontal transport processes inferred from the CO and O3 measurements is presented. The possible interdependence of the photochemical cycles of these two trace gases is also discussed.

Correlative nature of ozone and carbon monoxide in the troposphere - Implications for the tropospheric ozone budget

Abstract: The small-scale vertical variability of troposheric O3 and CO is examined using a set of simultaneous measurements obtained in July and August 1974 between 55 deg S and 67 deg N. From this set of vertical profiles, it is found that many of the fluctuations are coincident in both species, and a method is presented that quantifies the correlation between the observed O3 and CO variability. A two-dimensional depiction of the distribution of these O3-CO correlations reveals that there are regions in the troposphere where these trace gases are positively correlated and that, at the same time, there are preferred locations where these two species are primarily anticorrelated. The regions of anticorrelation are found to be consistent with the traditional picture of the tropospheric ozone cycle, suggesting that this gas is chemically unreactive in the troposphere. On the other hand, the location and magnitude of the region in which these two species are positively correlated indicates that there is considerable in situ production of tropospheric ozone.