Opinion: Challenges and needs of tropospheric chemical mechanism development

Abstract

Abstract. Chemical mechanisms form the core of atmospheric models to describe degradation pathways of pollutants and ultimately inform air quality and climate policy makers and other stakeholders. The accuracy of chemical mechanisms relies on the quality of their input data, which originate from experimental (laboratory, field, chamber) and theoretical (quantum chemistry, theoretical kinetics, machine learning) studies. The development of robust mechanisms requires rigorous and transparent procedures for data collection, mechanism construction and evaluation, and creation of reduced or operationally defined mechanisms. Developments in analytical techniques have led to a large number of identified chemical species in the atmospheric multiphase system which have proved invaluable for our understanding of atmospheric chemistry. At the same time, advances in software and machine learning tools have enabled automated mechanism generation. We discuss strategies for mechanism development, applying empirical or mechanistic approaches. We show the general workflows, how either approach can lead to robust mechanisms and that the two approaches complement each other to result in reliable predictions. Current challenges are discussed related to global change, including shifts in emission scenarios that result in new chemical regimes (e.g. low NO scenarios, wildfires, mega/gigacities) and require the development of new or expanded gas- and aqueous-phase mechanisms. In addition, new mechanisms should be developed to also target oxidation capacity, and aerosol chemistry impacting climate, human and ecosystem health.

Perspective on Theoretical and Experimental Advances in Atmospheric Photochemistry

“…The question therefore arises of how these computational results can be best used to improve current understanding of the chemistry of the Earth’s atmosphere. Drawing from the best practice developed for translation of laboratory experimental data, the most effective use of new results from computational chemistry is by their incorporation into the chemical schemes used in global, regional, and local models of atmospheric chemistry. , The computational results can supplement existing experimental data by filling gaps, serve as validation of some experimental results, replace values of parameters currently included in models as estimates or educated guesses, guide the development of SARs, and expand the range of photochemical processes included in the existing models. , …”

“…One way to achieve this might be a one-stop website with user-friendly interfaces for integrated quantum chemistry codes that can compute absorption spectra and photochemical pathways for a molecule of interest. Another could be software that can reliably simulate photochemical reactions in a gaseous mixture, drawing inspiration from computational reactors for ground-state chemistry such as the ab initio Nanoreactor of Martínez and co-workers or the automated reaction mechanism generation methods of Zádor and co-workers, Martínez-Núñez and co-workers, and the GECKO-A , and SAPRC-22 atmospheric chemical models. Progress in automated discovery of photochemical reactions is less developed, but efforts are exemplified by the recently reported nonadiabatic Nanoreactor …”

“…Incorporation of comprehensive chemical schemes into computer models of atmospheric chemistry can make them prohibitively expensive to run, so reduced chemical schemes are often preferred . Meanwhile, the number of possible photochemical processes and chemical reactions that need to be studied experimentally is beyond the capacity of research laboratories worldwide. , Laboratory measurements should determine not just reaction rate coefficients ( k ) or wavelength-dependent absorption cross-sections (σ(λ)) and photochemical quantum yields (Φ(λ)), but also their dependence on pressure and temperature . To illustrate the challenge, the Master Chemical Mechanism (MCM v3.3.1) currently includes 5832 species and 17224 reactions .…”