Emergent conservation in atmospheric chemical mechanisms

Abstract

Conservation laws are time-invariant properties that constrain many physical systems. For systems of chemical reactions, the law of mass conservation constrains how atoms flow between chemical species. Chemical reaction networks can display emergent conservation not explained by mass conservation: these hidden symmetries arise instead from coupled kinetics. Kinetic invariants emerge when branching reactions with proportional rates cause species concentrations to evolve in lockstep. We detect emergent conservation in a simplified atmospheric chemical mechanism of ozone formation through a data-driven analysis of simulated concentrations, a result matching the theoretical kinetic explanation. Surveying 35 widely used atmospheric chemical mechanisms spanning five orders of magnitude in complexity, we discover emergent conservation in 15 mechanisms. Kinetic invariants constrain the intrinsic dimensionality of chemical systems: mechanisms with emergent conservation evolve in lower-dimensional spaces than their size suggests. Identifying emergent conservation can provide theoretical bounds for exact mechanism reduction and uncover kinetic symmetries in atmospheric chemistry.