Eulerian Model Evaluation Field Study Research Articles

Measurement and interpretation of cloud effects on the concentrations of hydrogen peroxide and organoperoxides over Ontario, Canada

Measurements of gaseous hydrogen peroxide (H 2O 2) and organoperoxides (ROOH) in clear air and cloudy air were made from a Twin Otter aircraft over central Ontario, Canada, during March and April of 1990 as part of the Eulerian Model Evaluation Field Study (EMEFS). In clear boundary layer air (altitude 500 m to 1750 m), unperturbed by the presence of cloud, the ratio [H 2O 2]/[ROOH] depends on NO 2 mixing ratios, while in the presence of cloud perturbing H 2O 2, the ratio [H 2O 2]/[ROOH] was significantly decreased compared with those in clear air mainly due to the high solubility of H 2O 2. In order to examine the effects of cloud processing on air that was detrained from cloud, we selected measurements made in cloud-free air located between two passes made in air with high liquid water content. Gaseous H 2O 2 concentrations in these detrained air masses were often higher than those in the cloudy air, and sometimes higher than those in the clear air for high NO 2 conditions (> 500 ppt) but lower than clear-sky levels for low NO 2 (< 500 ppt) conditions. This result is in general agreement with that from photochemical modeling, namely, for chemical regimes typical of high-NO 2 (more polluted) conditions, where aqueous-phase reactions of dissolved HO 2 lead to net production of H 2O 2(aq) that can subsequently be detrained from the cloud.

Vertical distributions of sulfur species simulated by large scale atmospheric models in COSAM: Comparison with observations

A comparison of large-scale models simulating atmospheric sulfate aerosols (COSAM) was conducted to increase our understanding of global distributions of sulfate aerosols and precursors. Earlier model comparisons focused on wet deposition measurements and sulfate aerosol concentrations in source regions at the surface. They found that different models simulated the observed sulfate surface concentrations mostly within a factor of two, but that the simulated column burdens and vertical profiles were very different amongst different models. In the COSAM exercise, one aspect is the comparison of sulfate aerosol and precursor gases above the surface. Vertical profiles of SO2, SO2−4, oxidants and cloud properties were measured by aircraft during the North Atlantic Regional Experiment (NARE) experiment in August/September 1993 off the coast of Nova Scotia and during the Second Eulerian Model Evaluation Field Study (EMEFSII), in central Ontario in March/April 1990. While no single model stands out as being best or worst, the general tendency is that those models simulating the full oxidant chemistry tend to agree best with observations although differences in transport and treatment of clouds are important as well.

Vertical profiles and horizontal transport of atmospheric aerosols and trace gases over central Ontario

The Canadian intensive measurement periods of the Eulerian Model Evaluation Field Study (EMEFS) were conducted in the summer of 1988 and the spring of 1990 in central Ontario, Canada. The project involved using instrumented research aircraft to measure vertical profiles of atmospheric aerosols and trace gases over special ground observing sites. Oxidant concentrations were often highest aloft. There were no strong diurnal variations in O3 and H2O2, suggesting that oxidant concentrations reported in this paper, as well as other species, seem to be most influenced by transport mechanisms rather than local chemical transformations. Aerosol and SO2 concentration vertical profiles tended to be similar to those of water vapor mixing ratio, showing a maximum near the surface, which points to a ground level source for these constituents. The inorganic aerosol was dominated by sulphate during the summer measurements, but particulate nitrate was important during the spring. The horizontal transport of these species was calculated for air with back trajectories from the north or south, showing transport to occur primarily from west to east for both cases, with components to the south or north, respectively. The sulphur is transported primarily as SO2 in the spring and about half as SO2 and half as SO4= in the summer. Air from the south is a source of particles, S, H2O, O3, H2O2 and NO2 to this region. Similarly, cloud water concentrations of sulphate and nitrate were higher with southerly trajectories, as were aircraft filter measurements of sulphate, nitrate, ammonium, and nitric acid.

A three‐dimensional global model investigation of seasonal variations in the atmospheric burden of anthropogenic sulfate aerosols

A global three‐dimensional chemical transport model is used to investigate seasonal variations of anthropogenic sulfur in the troposphere. Particular emphasis is placed on detailed comparisons of the modeled surface sulfur dioxide (SO2) and sulfate (SO4) concentrations and sulfate wet deposition fluxes with measurements from the Eulerian Model Evaluation Field Study (EMEFS) and Cooperative Program for Monitoring and Evaluation of the Long Range Transmission of Air Pollutants in Europe (EMEP) field programs in North America and Europe, respectively. Initial comparisons of model results with measurements reveal a systematic tendency of the model to overestimate SO2 concentrations and underestimate SO4 concentrations while producing a reasonable fit to measured wet deposition fluxes. Through a series of sensitivity tests we find that the addition of a nonphotochemical pathway for converting SO2 to SO4 in the boundary layer with a pseudo first‐order rate of constant of 1–2×10−6 s−1 provides the most reasonable method of bringing the model results into better agreement with the EMEFS and EMEP data sets. We propose that this additional pathway may be related to heterogeneous reactions between SO2 and atmospheric aerosols that typically are not included in models of the atmospheric sulfur cycle. Despite the vastly improved simulation of surface SO2 and SO4 when this hypothetical heterogeneous oxidation pathway is included, the model is unable to simultaneously simulate the large seasonal cycle in surface SO4 observed over eastern North America and the almost total absence of a seasonal cycle in surface SO4 over Europe. The seasonal cycles in model‐predicted column SO4 burdens are similar, but not identical, to those for surface SO4 because of regional differences in transport, free tropospheric oxidation, and in‐cloud removal. We find that the summer‐to‐winter ratio in column SO4 is larger over eastern North America than it is over Europe; however, both are larger than that for eastern Asia, where wintertime column SO4 is predicted to exceed summertime column SO4.

Evaluation of a three-dimensional cloud chemistry model

To evaluate the effectiveness of a three-dimensional dynamical cloud model in simulating cloud water chemistry, the ion concentrations in simulated clouds are compared with the results of chemical analyses of collected cloud water samples. The model is initialized with surface and aircraft measurements of clear air chemistry. The data that are used in the evaluation come from non-precipitating cumulus clouds that were sampled during the Eulerian Model Evaluation Field Study (EMEFS I) that was conducted in central Ontario in the summer of 1988. Three different cases are considered. In general, the simulated ion concentrations agree well with the observed concentrations when the ambient aerosol is assumed to be activated with 1.00% efficiency. In two of the cases the predominant contribution to the cloud water sulphate is activation of the aerosol. In the third case, in which the clear air sulphate concentration is small, oxidation of sulphur dioxide by hydrogen peroxide and ozone is the dominant source of sulphate.

Estimation of Trends in Atmospheric Concentrations of Sulfate in the Northeastern United States.

Daily atmospheric concentrations of sulfate collected at six locations in the northeastern United States are regressed against meteorological factors, ozone, seasonal cycles, and time in order to determine if a significant trend in sulfate can be detected. The data used in this analysis were collected during the Sulfate Regional Experiment (SURE, 1977-1978) and the Eulerian Model Evaluation Field Study (EMEFS, 1988-1989). Ozone, specific humidity, and seasonal terms (reflecting the potential of the atmosphere for oxidation of sulfur dioxide) emerged as important explanatory variables. After accounting for the variability explained by environmental factors, the median estimated change in sulfate concentration from the six locations over the 11-year period is -22% (or -28% if ozone is not used as an explanatory variable). Although there are wide variations among locations, these changes are commensurate with an estimated 25% decline in sulfur emissions in the northeastern U.S. during the same period. These analyses provide insight into methods for detecting reductions in sulfate that may be expected to occur as a result of the Clean Air Act Amendments of 1990. Uncertainties in the estimates, with consideration of serial correlation in the data, imply a minimum detectable reduction of 10% using this modeling procedure with similar data availability.

Eulerian model evaluation field study (EMEFS): A summary of surface network measurements and data quality

From June 1988 to May 1990, a Joint Canada-United States measurement program known as EMEFS made daily air and precipitation measurements across eastern North America. Surface data were collected by five participating networks at approximately 130 sites for use in assessing regional air pollutant and deposition model performance. This paper describes the measurement program and focuses on its associated quality assurance program. The concentration measurements were found to have overall uncertainties of ± 2.3% for SO 4 2− and NO 3 − for precipitation and, for the air concentrations, ± 6.4% for SO 2, ± 5.4% for SO 4 2−, and ± 10.3% for NO 3 −. These uncertainty levels generally satisfied the program data quality objectives for use in model evaluation studies and represent the quality of contemporary air precipitation chemistry networks.

Airborne measurements of aqueous and gaseous hydrogen peroxide during spring and summer in Ontario, Canada

Measurements of H2O2, both in the gas phase and in cloudwater samples, were made over central Ontario during the summer 1988 and spring 1990 intensive measuring periods of the Eulerian Model Evaluation Field Study from a Twin Otter aircraft. Cloudwater concentrations range from less than 1.5 μM to greater than 100 μM; corresponding gaseous concentrations range from less than 0.1 parts per billion by volume (ppbv) to approximately 2.5 ppbv. Gas phase H2O2 data are corrected for instrument response and the interphase partitioning of H2O2 is examined. Ratios of the measured to predicted partitioning based on Henry's law equilibrium range from approximately 0.2 to 1. When the data correction is applied, most samples do not differ from equilibrium by more than 20%. Lower ratios are associated with samples from supercooled clouds or samples taken when interstitial SO2 is high. The measured aqueous fraction for the supercooled samples is approximately 20% of that expected by Henry's law equilibrium. The ratios are independent of liquid water content but show some dependence on pH.

Development of emissions inventories for evaluation of eulerian acid deposition models

AbstractDuring the past decade, acid deposition has been a primary environmental problem of concern for the governments of the United States and Canada. In order to better evaluate and study the problem, these governments and other organizations have developed and used Eulerian mathematical models to bring together the sciences of chemistry, physics, and meteorology. Since these models use emissions of acid rain precursors as input, it is important that these emissions estimates be as accurate as possible. A paper, entitled “The Eulerian Model Evaluation Field Study” (EMEFS) [1], gives an overview of the activities of a binational (U.S./Canadian) group organized to conduct an extensive field study to evaluate two Eulerian models. That group appointed and directs the activities of four teams: diagnostic measurements, operational (surface) measurements, model evaluation, and emissions inventory. This paper describes the emissions inventory that was developed by the emissions inventory team (EIT) and is being used as input to the Eulerian models discussed in the above referenced paper.

Vertical profiles of ozone during the EMEFS I experiment in Southern Ontario

Vertical profiles of ozone from the surface to over 15 km altitude were obtained with a combination of free-flying and tethered ozonesondes and differential absorption lidar. During the Eulerian Model Evaluation Field Study (EMEFS I) of July–August 1988, these profiles were compared with the output of a sophisticated regional-scale three-dimensional Eulerian dispersion and chemistry model, the Acid Deposition and Oxidant Model (ADOM). The results show that ADOM predicts the ozone concentration profiles with accuracy only in the lower troposphere and particularly well during simple synoptic situations. In the upper troposphere, the model does not predict the intrusions of ozone from the stratosphere which appear to be extremely important in determining the ozone concentration profile.

Evaluation of a comprehensive Eulerian air quality model with multiple chemical species measurements using principal component analysis

Using a principal component analysis technique and data on atmospheric gases and aerosols at a rural site in Ontario, Canada from the Eulerian model evaluation field study (EMEFS), the Eulerian acid deposition and oxidant model (ADOM) is evaluated. Seventy-nine and 76% of the variances in the data and model output, respectively, are explained by three principal components. They are a chemically aged/ transported component, a diurnal cycle component, and an area emission component, all characterized by their ratios of gases and temporal variation patterns. The ADOM component contributions to sulphur species are in general agreement with the EMEFS components, but with notable differences for key photochemical species including O 3. The temporal variations of the ADOM components are close to those of the EMEFS components. The EMEFS chemically aged/transported component shows a high degree of photochemical processing, with the ratios [NO x ]/[TNO y ]=0.3 and [O 3]/([TNO y ]−[NO x ])=9±1. The corresponding ADOM component predicts lower G[NO x ]/[TNO y ] and [NO 3]/([TNO y ]−[NO x ]) ratios, probably caused by a chemical mechanism in the model that is too fast, and lower contributions to O 3, NO 2, TNO 3, PAN, TNO y , and HCHO, probably caused by model grid dilution or lower model emissions. The EMEFS diurnal component owes its variance to the daily photochemistry and nighttime dry deposition of the chemical species. In comparison, the matching ADOM component underpredicts the ratio [O 3]/([TNO y ]−[NO x ]) and the NO 2 consumption and O 3 production but overpredicts the contributions to the other species. The EMEFS emission component represents emissions from local/regional area sources. The corresponding ADOM component underpredicts TNO y by 44% and the fraction of TNO y as NO x compared to the EMEFS component, suggesting that the model has lower emissions of NO x and a photochemical mechanism that converts NO x faster than indicated by the EMEFS results.

Correcting RADM's sulfate underprediction: Discovery and correction of model errors and testing the corrections through comparisons against field data

Abstract A serious underprediction of ambient sulfate (SO42−) by two comprehensive, Eulerian models of acid deposition, the Regional Acid Deposition Model (RADM) and the Acid Deposition and Oxidant Model (ADOM), was found in the National Acid Precipitation Assessment Program phase of the Eulerian Model Evaluation Field Study (EMEFS) model evaluation. Two hypotheses were proposed to explain the cause of the underprediction in RADM: insufficient SO42− production by nonprecipitating convective clouds and insufficient primary SO42− emissions. Modifications of the RADM cloud and scavenging module to simulate nonprecipitating cumulus clouds better are described in detail. Three contrasting pairs of tests using data from the EMEFS were applied to these hypotheses: source vs downwind regions, mid-summer vs late summer seasons and sunny-dry vs cloudy-wet synoptic types. The SO42− emissions hypothesis, tested by artificially boosting SO42− emissions, fared better than expected but was rejected because of its poor performance on the regional and seasonal contrast tests. The RADM nonprecipitating cumulus modification successfully captured the seasonal and the late summer synoptic contrasts but improvement is still needed for the regional and mid-summer synoptic contrasts.

Evaluation of the Eulerian acid deposition and oxidant model (ADOM) with summer 1988 aircraft data

Abstract Aircraft measurements from the Eulerian Model Evaluation Field Study (EMEFS) summer of 1988 intensive are compared with predictions of the Acid Deposition and Oxidant Model (ADOM). Aircraft measurements have been compared to surface measurements. Concentrations of H 2 O 2 from surface measurements were less than those at the base of the profile but O 3 , SO 2 and NO 2 agreed well. The horizontal variability in concentration over the approximate area of one ADOM grid square was examined and regionally representative values were defined for comparison with model-calculated values. ADOM simulates O 3 relatively well, although it does not predict the full concentration range. Underpredictions of sulphur, NO 2 and H 2 O 2 are seen aloft.

A case study of ozone production in a rural area of central ontario

An O 3 episode observed at Dorset, a rural site in central Ontario, during a stagnant high pressure period of the intensive Eulerian Model Evaluation Field Study (EMEFS), in the summer of 1988, is simulated using a photochemical box model with a two-layer treatment. In the model analysis, natural hydrocarbon chemistry is simulated based on an isoprene-only scenario. Sensitivity tests indicate that local isoprene emissions are an important contributor to local O 3 production, relative to anthropogenic hydrocarbons (AHCs), during the event. The model calculated isoprene contribution to the local O 3 production, defined as the ratio of the O 3 amount formed in the absence of AHCs to that in the presence of AHCs, is characterized by a strong NO x dependence. A minimum value (∼50%) of the contribution was found at a NO x level of ∼ 6 ppbv for the representative hydrocarbon composition during the episode. At this NO x level, O 3 production was strongly influenced by the presence of AHCs. At significantly higher or lower NO x levels, isoprene is more important than AHCs in the local O 3 production.