Chris J. Walcek

A Theoretical Method for Computing Vertical Distributions of Acidity and Sulfate Production within Cumulus Clouds

Abstract An aqueous chemistry model has been combined with an entraining cumulus cloud model to predict vertical distributions of pH (=−log10[H+]) within a cloud. The cloud model predicts vertical variations of temperature, pressure, entrainment and liquid water content. The aqueous chemistry model incorporates the effects of soluble aerosols, trace gases and aqueous-phase SO2 oxidation on cloudwater pH levels. In the absence of SO2 oxidation, vertical distributions of pH were found to be highly variable, depending on several chemical and meteorological factors. For clouds forming in an environment where the aqueous composition is determined primarily by dissolved sulfate condensation aerosol, pH was found to increase with height above cloud base primarily due to dilution by increasing water content. If cloudwater acidity levels are predominantly determined by dissolved SO2, HNO3 and formic acid gas, pH variations with height were not so clearly defined, with the diluting effects of entrainment, liquid wa...

SO2, sulfate and HNO3 deposition velocities computed using regional landuse and meteorological data

Deposition velocity fields were generated for SO2, sulfate and HNO3 over eastern United States and southeastern Canada by combining detailed landuse data with meteorological information predicted using a mesoscale meteorology model. When there is significant variation in land type within an averaging area, it was found that subgrid scale meteorological variations can significantly influence area-averaged deposition velocities. The assumption that uu is constant over the averaging area can realistically address the subgrid variations in wind speed and friction velocity. For a 3-day springtime simulation, domain-averaged mid-day SO2, sulfate and HNO3 deposition velocities at a height of approximately 40 m were found to be 0.8 cm s−1, 0.2 cm s−1, and 2.5 cm s−1, respectively. At night, the deposition velocities were approximately 50%, 45% and 70% of the corresponding daytime values for SO2, sulfate and HNO3. Using a simple parameterization to account for rainfall-wetted surfaces increased domain-averaged SO2 deposition velocities by up to a factor of two, indicating that precipitation can significantly enhance dry deposition of SO2.

Minor flux adjustment near mixing ratio extremes for simplified yet highly accurate monotonic calculation of tracer advection

A simplified but very accurate method for calculating advection of mixing ratios in a mass conservative and absolutely monotonic manner in divergent or nondivergent multidimensional flows is presented. This scheme uses a second-order-accurate, upstream approximation with monotone limiters and additionally adjusts fluxes at two cell edges around local extremes of a tracer distribution to significantly improve overall advection calculations. The minor flux adjustment slightly aggregates mass around local peaks in a manner which counters the inherent numerical diffusion associated with most numerical advection algorithms when advecting poorly resolved features. When advecting tracer shapes which are resolved by fewer than 10–20 grid cells, this scheme is significantly more accurate than higher-order algorithms for a wide range of test problems. For well-resolved tracer distributions this algorithm is very accurate and usually preserves local peak and minimum values almost perfectly. The scheme is positive-definite, but negative values can be advected with no modifications. A generalized algorithm and FORTRAN subroutine is presented for advecting mixing ratios or other conservative quantities through variable-spaced grids of one to three dimensions, including deformational flows. One- and two-dimensional tests are presented and compared with other higher-order algorithms. The computational requirements of this algorithm are significantly lower than those of other higher-order and less accurate schemes.

A simple but accurate mass conservative, peak-preserving, mixing ratio bounded advection algorithm with FORTRAN code

Abstract A simplified but very accurate method for calculating advection of mixing ratios in a mass conservative and monotonic manner is presented. This scheme replaces the polynomial approximations of other advection algorithms with dual-linear segments, and employs a special treatment near local maxima and minima to preserve extremes very well. Before updating mixing ratios, fluxes at grid-cell faces are bounded to yield conservative, monotonic, maximum-bounded and minimum-bounded solutions to the advection equation with very little numerical diffusion, even in locally deformational flows. “Bounded” here means that updated mixing ratios never exceed local maxima or fall below local minima. If an initial tracer distribution is everywhere positive, the solution is positive-definite, but negative values can be advected. With a “sharpening” option fully enabled, local maxima or minima are usually perfectly preserved, and features as small as 2–3Δx in width are advected with virtually no numerical diffusion for many Courant numbers and advection distances. Algorithms are presented for advecting mixing ratios through variable-spaced grids in any flow, including strongly deformational flows. One- and two-dimensional tests of the scheme are presented. Relative to other advection algorithms, this scheme yields significantly lower peak, distribution and rms errors, especially when advecting poorly resolved features with sizes 1–3Δx. Therefore this scheme may be useful in applications where peak and minima preservation of small features is desirable. A 41-line FORTRAN advection subroutine which can be readily applied for general advection problems is provided in the appendix.

On the scavenging of SO2 by cloud and raindrops. II: An experimental study of SO2 absorption and desorption for water drops in air

An extention of our previous theory for trace gas absorption into freely-falling cloud and raindrops is presented. This theory describes the convective diffusion of a trace gas through air and into a water drop with internal circulation, the drop falling at its terminal velocity. Using flow fields for the circulating water inside and for the moving air outside the drop, obtained by numerical solutions to the Navier—Stokes equation of motion, we numerically solved the convective diffusion equation to determine the uptake of SO2 by water drops of various sizes, time exposure to the gas phase, and concentration of SO2 in the gas phase. It was found that for drops of radius larger than 1 mm and relatively low gas concentrations (≲10 ppb (v)), resistance to gas diffusion lies mainly in the gas phase; while for drops of radius less than 500 μm and gas concentrations larger than those found in the atmosphere (≳1% (v)), the resistance to diffusion lies primarily in the liquid phase. With drop sizes and gas concentrations between these limits, the rate of SO2 uptake is controlled by a coupled resistance to diffusion inside and outside the drop. In addition to our general model, a simplified version was formulated which allows considerable savings in computer time for evaluation and improved ease of handling without significant loss of accuracy. A comparison between our simplified model and that of Barrie (1978) shows that the boundary-layer approach of Barrie may be a useful alternate approach to estimating trace gas absorption by water drops, provided appropriate values are chosen for the thickness of the boundary layers involved.

The influence of aqueous-phase chemical reactions on ozone formation in polluted and nonpolluted clouds

Abstract Aqueous-phase reactions among dissolved radicals and trace metals have been incorporated into a comprehensive gas-phase chemical reaction mechanism in order to quantify the influence of heterogeneous chemical processes on ozone (O3) formation under a wide range of NOx and hydrocarbon concentrations. In-cloud reactions of dissolved HO2 with itself, the reaction of dissolved O3 and HO2, and when trace metals are present, the reactions of dissolved HO2 and copper dramatically reduce total HO2 and other free-radical concentrations in clouds, thereby reducing the rate at which O3 is produced from anthropogenic NOx and hydrocarbon pollutants. Under typical urban or moderately polluted conditions, local ozone formation rates are reduced by 30–90% when aqueous-phase radical reactions are occurring in the atmosphere. However, when NOx concentrations are less than about 200 ppt, O3 is slowly destroyed, and in-cloud reactions reducing HO2 concentrations decrease the rate at which ozone and other reactive NOx and non-methane hydrocarbons (NMHC) are destroyed, resulting in longer atmospheric chemical lifetimes of O3, NOx, and NMHC. These results suggest that in-cloud reactions strongly influence local O3 production in polluted areas, but longer-term impacts of clouds on O3 formation would be much smaller due to compensating chemical processes in regions remote from NOx emissions. The effects of heterogeneous chemistry are highly dependent on the concentrations of NOx and hydrocarbons. In polluted clouds, aqueous reactions of dissolved copper and iron could be the dominant reactions influencing O3 formation, suggesting the need for further measurements of trace metals in the atmosphere.

Cloud cover and its relationship to relative humidity during a springtime midlatitude cyclone

Abstract Vertical distributions of fractional cloud coverage derived from the U.S. Air Force 3DNEPH satellite, aircraft, and surface-based analysis are compared with related standard meteorological observations over the eastern United States. Cloud cover and related observations are interpolated onto the identical three-dimensional grid consisting of 15 tropospheric levels at various horizontal resolutions ranging from (80 km)2 to (800 km)2 for five local noon periods during a springtime midlatitude cyclone. During the period analyzed, cloud cover maximizes near 900 mb at 35% cloud cover and decreases to near-zero cover at the surface. Above 900 mb, fractional cloudiness gradually decreases to 10%–20% cover at 200 mb. Cloud cover is positively correlated with relative humidity and large-scale vertical velocity, and negatively correlated with wind shear and temperature lapse rate, except in the lowest 100 mb, where cloud cover is weakly correlated with relative humidity, vertical velocity, wind shear, and ...

Calculated influence of temperature-related factors on ozone formation rates in the lower troposphere

Abstract Using an atmospheric chemical reaction mechanism applied to air parcels near the earths surface, the sensitivities ozone (O3) formation rates are quantified for changes in four meteorologically controlled parameters: temperature, sunlight intensity, water vapor mixing ratio, and isoprene concentration. Over a wide range of NOx and anthropogenic hydrocarbon concentrations, enhanced photolysis rates and elevated isoprene concentrations are calculated to be the most important factors contributing to increased O3 formation rates on warmer days. These results suggest that the most uncertain yet important meteorological factor controlling regional-scale O3 formation is fractional cloudiness and its impact on photolysis rates.

A theoretical estimate of O3 and H2O2 dry deposition over the northeast United States

Abstract Estimates of short-term, regional-scale spatial distributions of ozone (O 3 ) and hydrogen peroxide (H 2 O 2 ) dry deposition over the northeast U.S. are presented. Dry deposition fluxes to surfaces are computed using a regional tropospheric chemistry model with deposition velocities which vary with local meteorology, land type, insolation, seasonal factors and surface wetness. A compilation of O 3 surface resistances is presented based on a survey of O 3 dry deposition measurements. The surface resistance for H 2 O 2 is assumed to be small under most conditions, causing H 2 O 2 to dry deposit at a rate which is frequently limited by surface-layer turbulence. Regional patterns of dry deposition velocities for these oxidants over the northeast U.S. are computed using landuse data and meteorological information predicted using a mesoscale meteorology model. Domain-averaged O 3 deposition velocities during a spring period reach a mid-day peak of 0.7–0.8 cm s −1 and drop to 0.1–0.2 cm s −1 at night. Domain-averaged H 2 O 2 deposition velocities at a height of approximately 80 m are predicted to reach a mid-day peak of 1.6–2.0cm s −1 , and fall to 0.6–0.9 cm s −1 at night. Time-averaged surface-layer H 2 O 2 concentrations show a latitude dependence, with higher concentrations in the south. H 2 O 2 concentrations are significantly reduced due to efficient wet removal and chemical destruction during the passage of a cyclonic frontal system. In contrast, O 3 concentrations are predicted to rise during the passage of a frontal system due to efficient vertical exchange of midtropospheric air into the boundary layer during convective conditions, followed by synoptic-scale subsidence occurring in the high pressure airmass following a cyclone. Maximum O 3 deposition during this 3-day springtime period occurs in polluted agricultural areas. In contrast, H 2 O 2 dry deposition exhibits a latitude dependence with maximum 3-day accumulations occurring in the south. Domain-averaged mid-day deposition rates for O 3 and H 2 O 2 were 45–50 μmol m −2 h −1 and 4–5 μmol m −2 h −1 . At night, deposition rates were approximately 5–10 μmol m −2 h −1 and 1.5–2.5 μmol m −2 h −1 for O 3 and H 2 O 2 . These model results show that regional patterns of oxidant dry deposition are strongly influenced by oxidant concentrations, atmospheric stability, surface roughness and numerous other surface and meteorological factors. Each of these factors must be well-characterized before regional patterns of biological damage associated with oxidant dry deposition can be quantified.

An Experimental Test of a Theoretical Model to Determine the Rate at which Freely Falling Water Drops Scavenge SO2 in Air

Abstract An experimental method involving the UCLA Rain Shaft is described. This method allows determining the rate at which SO2 is scavenged from air by freely falling water drops. In the present experiment water drops of radii near 300 μm were allowed to pass through a chamber filled with SO2 whose partial pressure was determined by an infrared spectrometer. By varying the length of the gas compartment, the drops could be exposed to SO2 for different intervals of time. An electrochemical method verified by three quantitative chemical methods was used to determine the total amount of sulfur taken up by the drops falling through the gas compartment. The present experimental results were compared with the results from our theoretical model (Baboolal et at., 1981), which was evaluated for the present experimental conditions. Satisfactory agreement between experiment and theory was found.