Atmospheric chemistry

Quick links: NO_x chemistry Carbon Capture CureAir

ADMS 6 and ADMS-Urban include several methods of estimating the effects of local atmospheric chemistry processes on pollutant emissions. The standard options for modelling chemistry, described in detail in the ADMS Technical Specification documents, include:

• Generic Reaction Set (GRS) chemistry, which is implemented in both ADMS 6 and ADMS-Urban though with some differences (specifically ADMS-Urban includes more NO_x reactions and also a contribution from conversion of gaseous SO₂ to sulphate which is added to generic particulate species PM₁₀ and PM_2.5);
• The Trajectory model, which extends the GRS in ADMS-Urban to include effects on background concentrations of NO_x and O₃ over larger spatial and time-scales in an urban area; and
• NO_x-NO₂ correlation (ADMS-Urban), a simple empirical approach based on analysis of NO_x and NO₂ measurements in London by Derwent and Middleton.

Additional options available via extended licences and/or model versions include:

• Amine chemistry (ADMS 6) for carbon capture and storage, usually run in conjunction with the dilution and entrainment modelling option; and
• CBIV (ADMS-Urban), a simplified version of the Carbon-Bond chemistry scheme using an assumed distribution of VOC species based on measurements in London.

CERC have also investigated the NO_x chemistry modelling options in AERMOD and extended them to include a mechanism which uses similar reactions and an analogous calculation method to the ADMS GRS chemistry.

NO_x chemistry

When nitrogen oxide (NO_x, the sum of NO and NO₂) is emitted from a combustion process via an industrial stack, the NO₂/NO_x in-stack ratios for many source types are typically of the order 5 - 20%. This is much lower than the proportion of NO_x that is NO₂ in the atmosphere well downstream of sources, which typically lies in the range 50 - 90% depending on solar intensity and ambient pollution levels. The chemical reactions dominating the conversion of NO to NO₂ occur over relatively short time and spatial scales and can be described by a simplified coupled system of equations. This set of equations is solved explicitly within the ADMS model. Up until recently, the US EPA AERMOD model included a number of methodologies of varying complexity to approximate this conversion. As part of an ongoing project for the American Petroleum Institute (API), CERC have developed an explicit NO_x chemistry scheme for AERMOD, which has been included as a BETA option in the latest release (version 22112); the associated evaluation of this new scheme has been documented in a comprehensive report.

Datasets obtained from measurement campaigns (for instance in the vicinity of power plants and oil well drill rigs) can be used to evaluate dispersion model performance. CERC has undertaken a number of such studies for clients such as BP and the API. Research outcomes have been presented at conferences (presentations available here and here) and published in scientific journals (see for instance these papers: here and here).

Carbon Capture and Storage modelling

CERC was involved in the ‘CO₂ capture Mongstad’ project, a major venture overseen by Statoil Petroleum AS and Gassnova SF (the Norwegian State enterprise for carbon capture and storage).

CERC took part in a project to improve the understanding and quantification of the dispersion of degradation products of amines in the atmosphere. Amines can react with other species in the exhaust gas and the atmosphere to produce degradation products (nitrosamines, nitramines) that are potentially harmful.

CERC’s contributions to the project included the development of ADMS to incorporate chemical reaction schemes of the amines and their products, and case studies of dispersion modelling of the amines and their degradation products in the atmosphere. Further modelling was carried out for a range of emissions profiles from Statoil’s testing programme, to determine their impact on air quality.

A summary report from the project can be found here, and more details of the amine chemistry modelling can be accessed here.

As a result of the work carried out under this project, ADMS 6 has the capability to model amine chemistry. An optional feature allows assessment of the impact of emissions due to amine-based carbon capture activities.

Coupling Urban and Regional processes: Effects on Air Quality (CureAir)

Under the NERC-funded CureAir project the ADMS-Urban GRS chemistry scheme has been evaluated in comparison with the Master Chemical Mechanism, as part of collaborative work with the University of Leeds. CERC have also extended the ADMS-Urban chemistry scheme to include some photochemical effects of HONO and Isoprene, which contribute to increased concentrations of radical species and thus indirectly affect concentrations of the air quality pollutants NO₂ and O₃. Continuing work involves further evaluation of the effect of the urban heat island temperature predicted by the ADMS-Urban Temperature and Humidity model on chemistry reaction rates and thus pollutant concentrations.