A. C. Delany

Thunderstorms: An Important Mechanism in the Transport of Air Pollutants

Acid deposition and photochemical smog are urban air pollution problems, and they remain localized as long as the sulfur, nitrogen, and hydrocarbon pollutants are confined to the lower troposphere (below about 1-kilometer altitude) where they are short-lived. If, however, the contaminants are rapidly transported to the upper troposphere, then their atmospheric residence times grow and their range of influence expands dramatically. Although this vertical transport ameliorates some of the effects of acid rain by diluting atmospheric acids, it exacerbates global tropospheric ozone production by redistributing the necessary nitrogen catalysts. Results of recent computer simulations suggest that thunderstorms are one means of rapid vertical transport. To test this hypothesis, several research aircraft near a midwestern thunderstrom measured carbon monoxide, hydrocarbons, ozone, and reactive nitrogen compounds. Their concentrations were much greater in the outflow region of the storm, up to 11 kilometers in altitude, than in surrounding air. Trace gas measurements can thus be used to track the motion of air in and around a cloud. Thunderstorms may transform local air pollution problems into regional or global atmospheric chemistry problems.

Atmosphere-surface exchange measurements

The exchange of various trace species and energy at the earths surface plays an important role in climate, ecology, and human health and welfare. Surface exchange measurements can be difficult to obtain yet are important to understand physical processes, assess environmental and global change impacts, and develop robust parameterizations of atmospheric processes. The physics and turbulent structure of the atmospheric boundary layer are reviewed as they contribute to dry surface exchange rates (fluxes). Micrometeorological, budget, and enclosure techniques used to measure or estimate surface fluxes are described, along with their respective advantages and limitations. Various measurement issues (such as site characteristics, sampling considerations, sensor attributes, and flow distortion) impact on the ability to obtain representative surface-based and airborne flux data.

Modification of a Commercial Gas Filter Correlation CO Detector for Enhanced Sensitivity

Abstract Carbon monoxide is important in tropospheric chemsitry and useful in studies of cloud dynamics, yet measurements of this gas, especially from aircraft, are too few to characterize fully the atmospheric sources, sinks and distribution of CO. This article describes how a commercial infrared gas filter correlation analyzer (GFC) can be modified to provide sufficient sensitivity and response time for clean air CO measurements. Modifications include improved IR detection, a chemical zero, and sample gas preparation to eliminate interferences effectively, including a minor interference from ozone. The modified instrument demonstrates a detection limit of ∼24 ppb (signal-to-noise ratio 2:1 at the ±1σ noise level, with a 60 s time constant), a broad linear dynamic range, temperature and pressure independence, calibration stability, and a minimum response time of ∼30 s. We present examples of surface and airborne measurements including results from 12 flights over New Mexico showing rapid vertical transpo...

Direct measurements of nitrogen oxides and ozone fluxes over grassland

Using the eddy correlation method, fluxes of nitric oxide, nitrogen dioxide, ozone, water, and sensible heat were measured at a site 20 km north of Denver, Colorado over mature crested wheat grass, 0.75 m high in late June and early July. During this period the weather was fair with no synoptic disturbances. In the early morning a well-mixed diluted urban pollution plume traversed the site, by late morning aged pollution had mixed downward into the local boundary layer, and by afternoon the air came from a relatively unpolluted area of the high plains. The mean trace gas concentrations reflect this repeated pattern of local air flow. The fluxes of the trace gases were influenced both by the variation of the means and by other factors including temperature and biological activity. Ozone fluxes were found to be always negative and proportional to the mean, with an average deposition velocity for this case of about 0.006 m s-1. For the oxides of nitrogen this simple treatment was not appropriate. Both deposition and emission were observed, generally deposition predominated in the morning and emission in the afternoon with observed variations in the fluxes of NOx=NO+NO2 from −0.3 to +0.2 ppbv m s-1.

Chemical Sensor Resolution Required for Measuring Surface Fluxes by Three Common Micrometeorological Techniques

We define the chemical or compositional resolution required to measure the surface emission/deposition flux of trace constituents under different meteorological conditions by means of the eddy correlation, gradient, and conditional sampling techniques. These chemical resolutions are defined for the full range of different atmospheric conditions and are reported in terms of commonly measured micrometeorological parameters.

Modification of the relaxed eddy accumulation technique to maximize measured scalar mixing ratio differences in updrafts and downdrafts

A modification to the relaxed eddy accumulation (REA) flux measurement technique is proposed which maximizes the scalar mixing ratio difference in updrafts and downdrafts. This technique was developed with the goal of measuring the stable isotope ( 13 C/ 12 C and 18 O/ 16 O) ratios of updraft and downdraft air and thus the net fluxes of 13 C 16 O2 and 12 C 18 O 16 O. Current mass spectrometer precision is small relative to measured isotopic gradients in CO2 in the Earths boundary layer, and the conventional REA approach is likely to be ineffective. The new technique, which we refer to as hyperbolic relaxed eddy accumulation (HREA), uses the conditional sampling concept of hyperbolic hole analysis to control sampling of air during only those turbulent events which contribute most strongly to the flux. Instead of basing updraft/downdraft sampling decisions strictly on vertical wind velocity, CO2 mixing ratio ((CO2)) fluctuations or those of another scalar are also used. Simulations using 10-Hz data show that a wind-based/ scalar-based sampling threshold can achieve a factor of 2.7 increase in scalar updraft/downdraft (CO2) differences over simple REA. During midday periods with strong photosynthetic fluxes, up/down (CO2) differences with HREA of 8 -10 ppm are possible, compared with 3-5 ppm for the best conventional REA case. Corresponding isotopic differences can likely be resolved with current mass spectrometers using this approach.

An evaluation of the regional acid deposition model surface module for ozone uptake at three sites in the San Joaquin Valley of California

Plants and soils act as major sinks for the destruction of tropospheric ozone, especially during daylight hours when plant stomata open and are thought to provide the dominant pathway for the uptake of ozone. The present study, part of the California Ozone Deposition Experiment, compares predictions of the regional acid deposition model ozone surface conductance module with surface conductance data derived from eddy covariance measurements of ozone flux taken at a grape, a cotton, and a grassland site in the San Joaquin Valley of California during the summer of 1991. Results indicate that the model (which was developed to provide long-term large-area estimates for the eastern United States) significantly overpredicts the surface conductance at all times of the day for at least two important types of plant cover of the San Joaquin Valley and that it incorrectly partitions the ozone flux between transpiring and nontranspiring components of the surface at the third site. Consequently, the model either overpredicts or inaccurately represents the observed deposition velocities. Other results indicate that the presence of dew does not reduce the rate of ozone deposition, contradicting to model assumptions, and that model assumptions involving the dependency of stomata upon environmental temperature are unnecessary. The effects of measurement errors and biases, arising from the presence of the roughness sublayer and possible photochemical reactions, are also discussed. A simpler model for ozone surface deposition (at least for the San Joaquin Valley) is proposed and evaluated.

Further modification of a commercial NOx detector for high sensitivity

We describe modifications of a commercial NOx detector for very high sensitivity and ease of computer assisted data acquisition. These changes include a larger gold‐coated reaction chamber, increased ozone flow, a faster vacuum pump, analog outputs to define mode and range status, and enhanced cooling for the PMT. The detection limit, defined as a signal‐to‐noise ratio of 1:1 at the ±2 σ level is about 10 ppt with a 1/e response time of about 20 s or about 45 ppt for 1 s. Detection of other reactive nitrogen compounds is also discussed.

California ozone deposition experiment: Methods, results, and opportunities

Abstract The California Ozone Deposition Experiment (CODE) is a program of observations and modeling to improve estimates of the rate of removal of tropospheric ozone at the earths surface used in grid-based photochemical models of ozone production, transport, and removal. The purpose of CODE is to test, diagnose and improve treatment of dry deposition of ozone and other gaseous species. CODE supports a larger air quality measurement and modeling effort comprised of the San Joaquin Valley Air Quality Study (SJVAQS) and Atmospheric-Utilities Signatures: Predictions and Experiments (AUSPEX) joined as SJVAQS/AUSPEX Regional Model Adaptation Project (SARMAP). However, the CODE data are also applicable to a variety of boundary layer and turbulence problems. This paper describes the field methods and data collected during summer (10 July through 6 August) of 1991 in the San Joaquin Valley (SJV) of California and introduces several related papers. General comparisons and conclusions from all the participants are summarized. The core elements of the CODE field effort consisted of a research aircraft for spatial coverage and three ground sites located in a cotton field, grape vineyard, and very dry (senescent) annual grassland. A major portion of the SJV is represented by these three vegetation types. The eddy covariance method is used to compute the vertical fluxes of ozone, carbon dioxide, water vapor, sensible heat and momentum. For the first half of the study period, flights were made mainly for comparison with tower-based fluxes. Subsequent flights were over other vegetation types and to conduct special studies. In addition to the vertical fluxes, the ground-site data include individual leaf measurements of stomatal conductance, radiative leaf temperature, wetness of surrogate leaves, soil temperature profiles and heat flux, soil composition and water content, mean nitrogen oxide and ozone concentrations, solar and net radiation, photosynthetically active radiation, and vertical profiles of wind, temperature, ozone and water vapor. Aircraft data also include reflected short-wave radiation, surface greenness index and radiative surface temperature. Several factors simplify analyses: a nearly constant synoptic situation, lack of cloud cover, low-level (30 m) flights and land use characterized by extensive homogeneous areas with well defined interfaces. Repeated five-km aircraft runs, necessary for a representative flux calculation, were commonly made over a single crop type. In addition, a partial (60%) solar eclipse on 11 July provides an opportunity to examine the influence of light intensity upon the plant-atmosphere exchange of carbon dioxide and ozone via stomatal activity.

An analytic formulation for NO and NO2 flux profiles in the atmospheric surface layer

The chemical reactivity of NO and NO2 is so rapid that their fluxes and concentrations can be considerably modified from that expected for conserved variables in the atmospheric surface layer, even as low as a meter above the surface. Fitzjarrald and Lenschow (1983) have calculated flux and mean concentration profiles for NO, NO2 and O3 in the surface layer using numerical techniques. However, their solutions do not approach the photostationary state at large heights. Here we solve a simpler set of equations analytically (i.e. we assume a constant O3 concentration and neutral hydrodynamic stability), and are able to show how the flux profiles behave at large heights assuming that the concentrations approach their photostationary values. We find, for example, that at large heights the ratio of the flux of NO to that of NO2 is equal to the ratio of their concentrations. These results are relevant to estimating surface fluxes of NO and NO2, and are most applicable to nonurban environments where NO and NO2 concentrations are usually much less than O3 concentration.