Causal attribution of accumulated winter haze PM2.5 mass to new-particle-formation-driven growth versus other secondary pathways in Beijing

Chu et al. (2021, Science of the Total Environment 753, 142207) reported that during the winter of 2018 in Beijing, new particle formation (NPF) events and subsequent particle growth coincided with faster PM2.5 accumulation on the path from clean air to haze, using photochemical age (the toluene-to-benzene OH clock) to order the observations, and hypothesized that NPF could have an important contribution to the increase of PM2.5 mass concentration in the winter of Beijing participating in haze formation. Whether nucleation-and-growth causally produces a substantial fraction of haze PM2.5 mass — as opposed to other secondary formation pathways acting on pre-existing particles, to primary emissions, or to advected mass — remains quantitatively unresolved. This problem asks for a quantitative, source-resolved attribution of the accumulated wintertime urban haze PM2.5 mass between the NPF route and the other processes contributing to haze mass, going beyond the correlational observation that PM2.5 accumulates faster on NPF days. Literature to 2026 quantifies NPF's share of haze particle number and the total secondary share of mass separately, and the most recent work instead offers a regime-level modeling framework for NPF's mass contribution while stating that this contribution remains unclear; no published source-resolved split of haze mass by formation pathway exists.

Background

Severe wintertime haze — episodes of high mass concentration of fine particulate matter with aerodynamic diameter below 2.5 μm ($PM_{2.5}$) together with strongly reduced visibility — recurs in Beijing and the North China Plain. A defining feature of this haze is that most particulate mass is secondary, produced in the atmosphere by gas-to-particle conversion rather than emitted directly. Two families of secondary processes compete. In the first, new particle formation (NPF) — nucleation of nanometer-scale particles from gas-phase vapors such as sulfuric acid ($H_{2}SO_{4}$) and oxygenated organics, followed by growth via further condensation — creates entirely new particles that can add mass as they mature into the $PM_{2.5}$ range. In the second, vapors condense onto pre-existing particles or react in their aqueous films, growing the existing population without creating new particles; aqueous-phase and heterogeneous chemistry on existing aerosol is known to drive rapid sulfate and secondary organic aerosol production during Beijing haze.

The source study (Chu et al. 2021) followed a single winter (23 January – 31 March 2018) at an urban Beijing site, ordering observations by photochemical age $t_{a}$ — the integrated hydroxyl-radical (OH) exposure of an air mass, estimated from the declining toluene-to-benzene ratio, since OH removes toluene faster than benzene so the ratio acts as a chemical clock. They reported that NPF probability and intensity peak at intermediate photochemical ages (about 12–24 h) and vanish for ages above about 48 h; that $PM_{2.5}$ accumulates roughly 2.5 times faster on NPF days than on non-NPF days (median about 0.5 versus 0.2 $\mu$g m$^{-3}$ h$^{-1}$); that highly oxygenated organic molecules (HOMs — low-volatility organic molecules formed by autoxidation and multigeneration oxidation of volatile organic compounds, in recent literature often called oxygenated organic molecules, OOMs) decline with photochemical age while gas-phase $H_{2}SO_{4}$ stays approximately constant, consistent with a balance between photochemical production and removal by the condensation sink (CS — the rate at which vapors are lost to the surface area of existing particles); and that the nucleation-and-growth sequence plausibly feeds haze development. The authors explained the faster $PM_{2.5}$ accumulation on NPF days by the increase of the condensation sink with photochemical age on those days, and stated the mass contribution as a hypothesis: NPF "could have an important contribution to the increase of $PM_{2.5}$ mass concentration in the winter of Beijing participating in haze formation". Their supporting evidence is a coincidence between NPF occurrence and the $PM_{2.5}$ accumulation rate, which does not by itself establish how much of the accumulated mass was produced by nucleation-and-growth rather than by condensation onto pre-existing particles, by primary emissions, or by transport of polluted air.

Whether NPF and subsequent growth actually cause a substantial fraction of haze $PM_{2.5}$ mass — as opposed to other secondary pathways acting on pre-existing particles, primary emissions, or advected mass — is the unresolved question. The tension is genuine: haze conditions carry a condensation sink roughly an order of magnitude larger than clean conditions, which should suppress nucleation, yet nucleation is observed during haze periods, and particles nucleated in cleaner precursor air may seed later haze. The literature offers number-dimension attributions (NPF's share of haze particle number) and total-secondary mass shares, but not a source-resolved split of haze mass between the nucleation-and-growth route and the other processes contributing to it.

Problem Statement

Deliver a quantitative, source-resolved attribution of wintertime urban haze in Beijing — specifically of the accumulated $PM_{2.5}$ mass, defined as the net $PM_{2.5}$ mass added between the clean-air state preceding a haze episode and that episode's peak — among (i) the new-particle-formation route, meaning mass produced by nucleation of new particles and their subsequent growth into the $PM_{2.5}$ range, (ii) other secondary formation pathways acting on pre-existing particles (gas-phase condensation, aqueous-phase and heterogeneous chemistry), (iii) primary emissions, and (iv) advected (transported) mass, meaning mass that entered the study domain (urban Beijing) already within the $PM_{2.5}$ size range irrespective of the pathway that formed it upwind. To make these four classes mutually exclusive by construction, cells (i)–(iii) partition the mass formed or emitted within the domain and cell (iv) collects all mass advected in already in the $PM_{2.5}$ range — with growth into the $PM_{2.5}$ range of particles nucleated outside the domain counted under the new-particle-formation route — so that no increment of mass is counted under two categories and the four fractions sum to 100%. The attribution must be expressed as these fractional contributions with quantified uncertainties, and the answer must state whether the new-particle-formation route meets the source study's hypothesis that NPF could have an important contribution to the increase of $PM_{2.5}$ mass concentration in the winter of Beijing — an important contribution being defined for this task as an NPF-route fraction of at least 10% of the accumulated $PM_{2.5}$ mass — going beyond the correlational observation that haze $PM_{2.5}$ mass grows faster on NPF days. The source study prescribes no method for the attribution, so observation-based budget analysis, model-based source tagging, and measurement-constrained simulation are all admissible insofar as they deliver the source-resolved mass fractions. An answer that quantifies only the fraction of particle number (rather than mass) attributable to NPF, or only the total secondary share of mass without splitting it between the NPF route and other secondary pathways, does not close the question.

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