Chemical Process Overview
Light can excite airborne molecules and transform them into other molecules.
Photolysis provides the energy required for many chemical reactions to occur in the atmosphere.
- Multiphase Chemistry
Heterogeneous reactions in CMAQ involve both the gas and aerosol phase. It is well-known that some reactions, which happen to be very slow in the gas-phase alone, are sped up by the presence of a surface that stabilizes the reacting molecules.
- Aerosol Microphysics
The major goal of CMAQ’s aerosol module is to comprehensively account for particles' introduction to and removal from the atmosphere as well as the impacts atmospheric processing has on their properties along the way.
- Aqueous Chemistry in Clouds
Clouds can help to “clean” the atmosphere by taking up pollutants from the air and raining them out.
- Cloud Formation on Particles
Forming on small particles in the air, the water droplets that make up clouds and fogs offer a medium for gases in the atmosphere to dissolve and react with each other
Chemical Processes Overview
CMAQ simulates the chemistry of airborne pollutants with a semi-integrated approach. Chemical reactions may occur entirely in the gas phase, entirely in the particle phase or involving gas-phase compounds on particle surfaces – the net impacts of all of these reaction types is computed simultaneously in CMAQ. At the same time, CMAQ computes the effects of condensation and evaporation of compounds between the gas-phase and any existing particles. The role of clouds on chemistry is also calculated in parallel, since oxidationoxidationIn a chemical reaction, molecules that are oxidized lose electrons. The atmosphere is an oxidizing environment. in cloudy air parcelair parcelAn imaginary body of air.s can produce large amounts of particle mass, especially from compounds with sulfur or organic components.
- Multiphase chemistry
- Airborne particle microphysics
- Cloud-borne chemical processes
Photolysis provides the energy required for many chemical reactions to occur in the atmosphere. Light can excite airborne molecules and transform them into other molecules. In order to realistically predict the composition of the atmosphere and how it evolves over time, CMAQ must use accurate estimates of photolysis rates. Methods from several scientific disciplines inform this calculation including physics, optics, meteorology and chemistry.
CMAQ must consider two important qualities of photolysis, which are that 1) the interactions of light with airborne molecules is wavelength-dependent, and 2) the products that are formed from these interactions are dependent on ambient conditions and properties of the molecule.
- Wavelength-dependence of light intensity:
- Sources of light extinction and scattering in the atmosphere must be known, and the propagation of light through the atmosphere must be accounted for.
- Sources of wavelength-dependent extinction and scattering include clouds, gases, aerosols and surface albedoalbedoThe proportion of the incident light or radiation that is reflected by a surface, like a forest, desert, city, or ocean. It can also refer to the light reflected by a cloud., which all depend on location and time.
- The spread of light is determined by solving a differential equation called the radiative transfer equation.
- Factors determining the products that are formed from light interactions:
- Molecular or atomic structure determines the absorption of photons.
- The energy required to break the chemical bonds within the compounds determines whether an absorption of photons will cause a reaction.
- Atmospheric temperature and pressure affect a given structure and determine whether an excited compound disperses the energy from absorbed light by colliding with the other compounds, or if the absorbed energy causes the compound to rearrange or break.
The data used to inform gas-phase and heterogeneous reactions come from thoughtfully-designed laboratory experiments and are constrained with real-world observations. Gas-phase chemical reactions can be very fast, sometimes occuring in hundredths to thousandths of a second. These reactions are responsible for much of the tropospheric ozone formation that can have negative human health impacts, as well as the formation of compounds that can condense to airborne particles and increase the burden of PM2.5 or PM10.
Heterogeneous reactionsHeterogeneous reactionsHeterogeneous reactions occur between reactants in two or more phases (e.g., solid and gas, solid and liquid) or in with participation of an interface (e.g., on the surface of a solid catalyst). in CMAQ involve both the gas and aerosol phase. It is well-known that some reactions, which happen to be very slow or impossible in the gas-phase alone, are sped up by the presence of a surface that stabilizes the reacting molecules or by the chemical environment available in a liquid phase. In the atmosphere, this surface or liquid is provided by airborne particles. Thus, dust storms and high urban particulate matter events are compelling examples of when heterogeneous chemistry is important to consider. Concentrations of gaseous pollutants like ozone can be dramatically affected by these complex pathways.
CMAQ includes several descriptions of gas-phase atmospheric chemistry (i.e. mechanisms). Hundreds of chemical compounds and about 1000 chemical reactions are included, depending on which mechanism is chosen. These mechanisms describe important phenomena like catalytic cycling of NOxNOxGases consisting of one molecule of nitrogen and varying numbers of oxygen molecules. Nitrogen oxides are produced in the emissions of vehicle exhausts and from power stations. In the atmosphere, nitrogen oxides can contribute to formation of photochemical ozone (smog), can impair visibility, and have health consequences; they are thus considered pollutants. and volatile organic compound (VOCVOCsOrganic chemicals that have a high vapor pressure (i.e. extremely low boiling point) at ordinary room temperature. VOCs include human-made and naturally occurring chemical compounds. Some VOCs are dangerous to human health or cause harm to the environment. Harmful VOCs typically are not acutely toxic, but continued exposure to them may have long-term health effects.) gases to form and destroy O3O3Ozone (O3) is a colorless gas with a pungent odor. It is found in two layers of the atmosphere, the stratosphere and the troposphere. In the stratosphere, ozone provides a protective layer shielding the Earth from ultraviolet radiation's potentially harmful health effects. At ground level (the troposphere), ozone is a pollutant that affects human health and the environment, and contributes to the formation of smog. and VOC oxidation with hydroxyl radicalsradicalsA radical molecule contains at least one unpaired electron. With few exceptions, these unpaired electrons make free radicals highly chemically reactive towards other substances or themselves., O3, and nitrate radicals among other applications. Because of computing constraints, mechanisms rely on lumping chemical speciesspeciesAn individual molecule or chemical compound. together into “surrogates,” which approximately represent the mass and reactivity of many species, but without having to calculate each of them separately.
- The mechanisms used to represent gas-phase chemistry in CMAQ rely on different strategies to decide how this lumping should occur.
- They are all evaluated with highly detailed measurements of chemical reactions in smog chambers and in the ambient atmosphere.
- Even though they often give very similar predictions for ozone concentrations, important differences occur during certain times of the year, during individual case studies, or for specific chemical compounds.
- For some organic systems, like oxidation products of isoprene, CMAQ includes a semi-mechanistic description that treats many compounds individually and avoids the problems of excessively lumped techniques.
Heterogeneous chemistry is often represented using an uptake coefficient, gamma, which can be converted to a rate constant. The reaction rate of a species participating in a heterogeneous reaction may be influenced by diffusion, reaction, and solubility at the surface or in bulk. Because CMAQ explicitly predicts the amount of particle surface area and volume (see next section) in each grid cell, the model can use that information to compute the rate of heterogeneous reactions of various important compounds. Products of heterogeneous reactions may be gas- or aerosol- phase species.
Airborne Particle Microphysics
Atmospheric particles vary greatly in size, from diameters of around one nanometer (nm) to over 20 micrometers. This enormous range makes it challenging to represent all the processes affecting particles because the importance of any process changes with the size of the particles. For example, small particles less than 100 nm in diameter, coagulate with each other and with larger particles very quickly. This process decreases the number of small particles in the system. On the other hand, large particles are more susceptible to either falling out of the sky due to gravity or acting as seeds for cloud drop growth and being rained out.
Many atmospherically relevant compounds exist, to some degree, in the form of particles. The particles we focus on most in CMAQ applications are so small that they avoid falling from the sky and stay airborne for days to weeks. Some chemical compounds like soot, sea salt, and soil fragments are wholly in particles, while other compounds like ammonia, nitrate, and many organic compounds are semivolatile, or split between the particulate and gas phases. The portion of these semivolatile compounds that is in the particle phase depends on environmental conditions, like temperature and relative humidity, but also on the presence of the trace chemical compounds they interact with.
The major goal of CMAQ’s aerosol module is to comprehensively account for particles' introduction to and removal from the atmosphere, as well as the impacts atmospheric processing has on their properties along the way. Understanding these phenomena is critical for quantifying the impacts of particles on human health and the environment.
The moment-based algorithm of Binkowski and Roselle (2003) is used to estimate the size-dependence of aerosol concentrations. Each population of particles is assumed to follow a log-normal shape. CMAQ tracks the 0th (number), 2nd (surface area), and 3rd (volume) moments of three distinct populations, which are named the Aitken, Accumulation and Coarse modes.
Mass transfer of compounds between the gas and particle phases as well as particle microphysical processes, like coagulation, and new particle formation are all treated at once, while holding the composition of the gas and particle phases constant for 1-5 mins.
The direct emissions of particles from sources like fires, vehicles, power plants, dust storms and sea spray, are classified as primary emissions and contribute particularly strongly near the source of the emission. Secondary particles come from the condensation of compounds that are either emitted as vapors or are formed in the atmosphere by the chemical reaction of a parent compound that was emitted by some process. For example, sulfur dioxide (SO2) can be emitted by power plants and then oxidized in the atmosphere to form sulfuric acid (H2SO4), which condenses quickly to the surface of any preexisting particles. Generally, secondary particulate mass contributes to pollution much farther from its original emission source than primary emissions.
Cloud-borne Chemical Processes
Clouds and fogs can significantly impact the amount, composition, and location of air pollution through chemistry and precipitation. Forming on small particles in the air, the water droplets that make up clouds and fogs offer a medium for gases in the atmosphere to dissolve and react with each other. Clouds can lead to higher concentrations of certain atmospheric compounds through chemical reactions occurring in and on their droplets. Clouds can also help to “clean” the atmosphere by taking up pollutants from the air and raining them out. Understanding these processes is an important part of understanding how chemicals emitted by man-made and natural processes evolve in the atmosphere, and how they might ultimately impact human health, ecosystems and climate.
Clouds affect trace atmospheric species through a number of physical and chemical processes, including:
- vertical transport (convective updrafts and downdrafts)
- the scavenging of atmospheric aerosols and gases, and subsequent chemical reactions leading to the formation of secondary species
- wet deposition (removal from the atmosphere through rainout or washout)
- altering radiative transfer and optics
In areas where clouds and fogs are present, cloud chemistry can lead to increases in particulate mass, especially in the case of SO2 conversion to sulfate and the formation of secondary organic aerosol (SOA) from the aqueous oxidation of soluble organic compounds (Ervens, 2015; Carlton et al., 2008). Sulfate and organic aerosol are two major components of fine particulate matter, a known contributor to adverse human health effects, and often comprise more than half of the total fine particulate matter (PM) mass (Philip et al., 2014; Hansen et al., 2003; Brewer and Adlhoch, 2005). Cloud droplets form on atmospheric aerosol (activation) and “scavenge” additional gas and aerosol species from the air. These droplets can combine to form larger rain drops which then scavenge additional species as they fall. Scavenging by precipitation and subsequent (wet) deposition to the surface can be an important removal mechanism for aerosols and soluble gas phase species.
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Brewer, P.F. & Adlhoch, J.P. (2005). Trends in speciated fine particulate matter and visibility across monitoring networks in the southeastern United States Journal of Air & Waste Management, 55, 1663–1674. doi: 10.1080/10473289.2005.10464755
Carlton, A.G., Turpin, B.J., Altieri, K.E., Seitzinger, S.P., Mathur, R., Roselle, S.J., & Weber, R.J. (2008). CMAQ model performance enhanced when in-cloud secondary organic aerosol is included: comparisons of organic carbon predictions with measurements. Environmental Science & Technology, 42, 8798–8802. doi: 10.1021/es801192n
Ervens, B. (2015). Modeling the processing of aerosol and trace gases in clouds and fogs. Chemical Reviews, 115(10), 4157–4198. doi: 10.1021/cr5005887
Hansen, D.A., Edgerton, E.S., Hartsell, B.E., Jansen, J.J., Kandasamy, N., Hidy, G.M., & Blanchard, C.L. (2003). The southeastern aerosol research and characterization study: Part 1 – overview. Journal of Air & Waste Management, 53, 1460–1471. doi: 10.1080/10473289.2003.10466318
Philip, S., Martin, R.V., van Donkelaar, A., Lo, J.W.-H., Wang, Y., ... , & Macdonald, D.J. (2014). Global chemical composition of ambient fine particulate matter for exposure assessment, Environmental Science & Technology, 48(22), 13060–13068. doi: 10.1021/es502965b