The fuel of atmospheric chemistry: Toward a complete description of reactive organic carbon

Abstract

The Earth's atmosphere contains a multitude of emitted (primary) and chemically formed (secondary) gases and particles that degrade air quality and modulate the climate. Reactive organic carbon (ROC) species are the fuel of the chemistry of the atmosphere, dominating short-lived emissions, reactivity, and the secondary production of key species such as ozone, particulate matter, and carbon dioxide. Despite the central importance of ROC, the diversity and complexity of this class of species has been a longstanding obstacle to developing a comprehensive understanding of how the composition of our atmosphere, and the associated environmental implications, will evolve. Here, we characterize the role of ROC in atmospheric chemistry and the challenges inherent in measuring and modeling ROC, and highlight recent progress toward achieving mass closure for the complete description of atmospheric ROC.

Fig. 1. The sources of key reactive emissions into the atmosphere that lead to secondary products of interest for air quality and climate.

Bottom: Emission sectors generate a range of reactive chemical species: sulfur dioxide (SO₂), dimethyl sulfide (DMS), nitrogen oxides (NO_x), ammonia (NH₃), reactive organic carbon (ROC), carbon monoxide (CO), and methane (CH₄). (Given that DMS is a major contributor to the sulfur budget, but a minor component of the ROC budget, we treat it separately here.) Relative global emissions of each are shown on the right (a). Middle (b): The relative contribution of each component to total atmospheric oxidation, estimated from OH reactivity. Top (c): Relative contribution of each species to the global atmospheric burden of secondary PM_2.5, tropospheric O₃, and secondary CO₂. The gray arrow denotes that tropospheric ozone production is catalyzed by NO_x. Note that the secondary CO₂ source shown here is equivalent to ~10% of the global CO₂ emissions from fossil fuels. In all cases—emissions, oxidation, and formation of secondary species—ROC (shown in green throughout) is a major, if not dominant, contributor, highlighting its central importance in tropospheric chemistry. See the Supplementary Materials for a detailed description of methods and sources (31, 58, 68, 69) used to estimate these values.

Fig. 2. Simplified atmospheric life cycle of ROC.

ROC is emitted into the atmosphere from both biogenic and anthropogenic sources, typically as complex mixtures of reduced molecules. Atmospheric oxidation leads to the formation of a large number of secondary ROC species (some of which keep the carbon skeleton intact; others break it apart). This increase in chemical complexity is likely magnified as oxidation continues over multiple generations.

Fig. 3. Illustrative timeline of the advances in the online (real-time or near–real-time) measurement of ambient ROC mass over the last decades.

Colored bars are qualitative estimates of measurable carbon (aerosol in green, gases in blue) as a function of volatility; dashed lines denote the amount of ROC in each bin that is measurable using current (2019) instrumentation. Solid outlines indicate the fraction of carbon that is measured as individual species rather than as unresolved mixtures. Dates correspond to the approximate timing that a given measurement approach was adopted for measuring atmospheric ROC. The major developments in analytical techniques depicted in this timeline (, –, –83) are given in the Supplementary Materials; see (33) for a comprehensive review of modern analytical approaches.