Julius S. Chang

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.

A Re-Evaluation of Sulfur Budgets, Lifetimes, and Scavenging Ratios for Eastern North America

Lifetimes, scavenging ratios, andbudgets describe the cycling of atmosphericconstituents and are often used in formulating airpollution control strategies. Most previous studiesof sulfur lifetimes, budgets, and scavenging ratioshave been based on limited observational data or datafrom highly simplified models. The Regional AcidDeposition Model (RADM2.61) shows some skill inpredicting atmospheric mixing ratios of acidicmaterials and other related trace constituents andacid deposition patterns in North America, and so,analysis of its established, theoretical, databaseserves as a counterpoint to previous studies of sulfurbudgets, lifetimes, and scavenging ratios. The annualbudget shows that the net transport (outflow minusinflow) of sulfur compounds out of eastern NorthAmerica is equal to the total deposition within thedomain. Of the total deposition, 63% is from wetdeposition and 37% is from dry deposition. Theannual average lifetime of sulfur dioxide (38 hours),estimated by the turnover time, is limited by aqueousconversion, while that for sulfate aerosols (54 hours)is limited by their removal in precipitation. Theannual average lifetime of sulfur in this domain isslightly more than three days. Episodic lifetimes andbudgets, based on particular synoptic situations, showlarge variations around the annual values. Episodicprecipitation scavenging ratios exhibit similarvariability and are used to offer explanations ofseveral potential biases found in the wet sulfurdeposition amounts as predicted by the EMEP sulfurtransport model and other published results.

Theoretical estimates of the dynamic, radiative and chemical effects of clouds on tropospheric trace gases

Abstract Using a subhemispheric-scale tropospheric chemistry model, we examine the influence of cloud-scale processes on regional-scale tropospheric chemistry. Reactions producing sulfuric acid and ozone (O 3 ) are found to be very sensitive to the dynamic, radiative, and aqueous chemical processes occurring in clouds. During a three-day spring-time period over the northeast U.S., aqueous-phase chemical reactions in clouds occupying 1–2% of the tropospheric volume are calculated to oxidize SO 2 to sulfuric acid at a rate comparable to gas-phase reactions occurring throughout the troposphere. During the passage of a midlatitude cyclonic storm system, the model calculates that over 65% of the sulfuric acid in the troposphere over the northeast U.S. is formed in cloud droplets via aqueous-phase chemical reactions. The model calculations show that the vertically integrated O 3 formation rate is enhanced by ∼50% when vertical motions in clouds are allowed to rapidly transport surface-emitted nitrogen oxides and organic compounds into the upper troposphere This results from a rapid dilution and transport of these oxidant precursors out of the planetary boundary layer into a colder and drier chemical environment where photolysis rates are significantly enhanced by cloud albedo. Below-cloud photolysis rates are calculated to be sensitive to the mean cloud-drop radius and cloud liquid-water content, implying that photochemistry during cloudy periods is sensitive to the microphysical structure of overlying clouds. Sulfuric acid and O 3 formation rates are highly correlated with tropospheric cloud amount, implying that an uncertainty in the mesoscale distribution of cloudiness leads to a similar uncertainty in the rates of these tropospheric chemical reactions. At present, it is difficult to evaluate parameterizations of these cloud-scale processes in regional or global-scale tropospheric chemical models. These model results suggest that accurate microphysical, dynamical and mesoscale characterizations of clouds are required in regional or global-scale tropospheric chemical models.

Diagnostic Modeling of PAMS VOC Observation

Although a number of gas-phase chemical mechanisms, such as CBM-IV, RADM2, and SAPRC have been successful in studying gas-phase atmospheric chemical processes, they all used different combinations of lumped organic species to describe the role of organics in gas-phase chemical processes. Photochemical Assessment Monitoring Stations (PAMS) have been in use for over a decade and yet it is not clear how the detailed organic species measured by PAMS compare to the lumped modeled species. By developing a detailed mechanism specifically for the PAMS organics and embedding this diagnostic model within a regional-scale transport and chemistry model, one can then directly compare PAMS observation with regional-scale model simulations. By means of this comparison one can perhaps better evaluate model performance. The Taiwan Air Quality Model (TAQM) was modified by adding a submodel with transport processes and chemical mechanism for interactions of the 56 species observed by PAMS. It is assumed that TAQM can simulate the overall regional-scale environment including time evolution of oxidants and radicals; these results are then used to simulate the evolution of PAMS organics with species-specific source functions, meteorological transport, and chemical interactions. Model simulations of each PAMS organic were compared with PAMS hourly surface measurements. A case study with data collected at three sites in central Taiwan showed that when meteorological simulations were comparable with observations, diurnal patterns of most organics performed well with PAMS data after emissions were corrected. It is found emissions of over half of the PAMS species require correction, some by surprisingly large factors. With such correlation, simulated time evolution of ratios of ethylbenzene/m,p-xylenes and ethane/n-butane showed similar behaviors as shown by observation data. From the results of PAMS organics diurnal variations as well as indicator ratios, one can conclude that PAMS Air Quality Model (PAMS-AQM) has been successfully developed and can be applied to the study of evolution of PAMS organics in regional and urban environments. Further, one finds that an existing VOC emissions estimation procedure heavily dependent on U.S.-data based emissions speciation factors is suspect in application in Taiwan and perhaps in other countries as well. A protocol, using PAMS-AQM for testing consistency between detailed VOC emissions and PAMS observations, has been developed and demonstrated.

A theoretical assessment of pollutant deposition to individual land types during a regional-scale acid deposition episode

A mesoscale model of pollutant transport, transformation and deposition was used to perform a detailed analysis of acidic deposition to the states of New York and Ohio during a 3-day springtime deposition episode. This model can be used to assess the roles of wet and dry deposition to individual land types in the removal of pollutants from the atmosphere. Over two-thirds (67 %, Ohio; 78 %, New York) of the acidic deposition during this rainy period fell as wet deposition, primarily in the form of H2SO4. Dry deposition of SO2 accounted for 70–75 % of the total dry acidic deposition in both areas, and most of the remaining dry deposition occurred as HNO3. Over both deposition areas, particulate sulfate deposition accounted for <1 % of the total acid deposition. Due to the highly surface-specific nature of the dry deposition process, individual land types displayed unique patterns of pollutant uptake. Water surfaces absorbed primarily SO2, while rougher forested areas absorbed a larger proportion of HNO3 vapor. Urban areas, with their associated material surfaces, were found to absorb significantly less acid in the dry form, and during dry periods most of this deposition may occur as HNO3 vapor, although considerable uncertainty exists regarding the treatment of rainfall-wetted surfaces. These model results suggest that dry pollutant fluxes to individual surface types will show significant variability from any ‘averaged’ flux estimates over larger areas encompassing numerous land types.

On the Eulerian source-receptor relationship

A new approach is proposed to delineate the source-receptor relationship in an Eulerian model. A small, unique oscillatory signal is superimposed on each emission source in the airshed. At receptor sites the concentration time series are analyzed by a Fourier transform to give the amplitudefrequency spectrum. Distinct peaks found in the spectrum are identified at the emission frequencies. The amplitude of the spectrum at the source frequency represents the individual contribution from that emitter. Various atmospheric transport and diffusion models are solved to show the effects of different physical and chemical processes on the oscillatory signals. Both analytical and numerical solutions are used to demonstrate the performance of the method.

Air quality impacted by local pollution sources and beyond – Using a prominent petro-industrial complex as a study case

The present study combines high-resolution measurements at various distances from a world-class gigantic petrochemical complex with model simulations to test a method to assess industrial emissions and their effect on local air quality. Due to the complexity in wind conditions which were highly seasonal, the dominant wind flow patterns in the coastal region of interest were classified into three types, namely northeast monsoonal (NEM) flows, southwest monsoonal (SEM) flows and local circulation (LC) based on six years of monitoring data. Sulfur dioxide (SO2) was chosen as an indicative pollutant for prominent industrial emissions. A high-density monitoring network of 12 air-quality stations distributed within a 20-km radius surrounding the petrochemical complex provided hourly measurements of SO2 and wind parameters. The SO2 emissions from major industrial sources registered by the monitoring network were then used to validate model simulations and to illustrate the transport of the SO2 plumes under the three typical wind patterns. It was found that the coupling of observations and modeling was able to successfully explain the transport of the industrial plumes. Although the petrochemical complex was seemingly the only major source to affect local air quality, multiple prominent sources from afar also played a significant role in local air quality. As a result, we found that a more complete and balanced assessment of the local air quality can be achieved only after taking into account the wind characteristics and emission factors of a much larger spatial scale than the initial (20 km by 20 km) study domain.

Numerical study on dry deposition processes in canopy layer

A coupling model between the canopy layer(CL) and atmospheric boundary layer (ABL) for the study of dry deposition velocity is developed. The model consists of six parts: chemical species conservation equation including absorptive factor; the species uptake action including detailed vertical variation of absorptive element in CL; momentum exchange in CL which is represented by a first-order closure momentum equation with an additional larger-scale diffusive term; momentum exchange in ABL which is described by a complete set of the ABL turbulent statistic parameters; absorptivity (or solubility or reflection) at the surface including effects of the physical and chemical characters of the species, land type, seasonal and diurnal variations of the meteorological variables; and deposition velocity derived by distributions of the species with height in CL. Variational rules of the concentration and deposition velocity with both height and time are simulated with the model for both corn and forest canopies. Results predicted with the bulk deposition velocity derived in the paper consist well with experimental data.

Characterization of the Nonlinear Change in Annual Sulfur Deposition for a Change in Emissions

A central question for atmospheric processes research posed by the National Acid Precipitation Assessment Program (NAPAP) of the United States has been: What degree of nonproportionality in the response in sulfur deposition is expected to be associated with a specified change in precursor emissions? Nonlinearity (or nonproportionality) exists where the fractional changes in emissions of a primary pollutant are not matched by a commensurate (proportional) change in the deposition of its primary or secondary product. Nonlinearity is only expected to occur in wet deposition processes. When sulfur dioxide in the aqueous phase is converted to sulfate, the most reactive oxidant, hydrogen peroxide, can become depleted in the cloud (oxidant limited). During the in-cloud-water conversion if there is more sulfur dioxide than is needed to react fully with available hydrogen peroxide, the hydrogen peroxide can be depleted and unreacted sulfur dioxide left over (an excess). Subsequently, if emissions of sulfur dioxide are reduced, the “excess” sulfur dioxide is reduced, leading to less than proportional (nonlinear) reductions in sulfate deposition. One concern is that if emission controls are implemented, deposition may not reduce in proportion to emissions reductions.