Brown carbon (BrC) — the light-absorbing fraction of organic aerosol — is often treated as a fixed property, but its optical and health-relevant behaviour depends strongly on what is measured and how the sample is taken. This talk makes that case from two datasets.For optics, real-time and filter measurements of the same winter BrC in Seoul give opposite source rankings, because BrC absorption evolves through the day and each method sees a different part of it — a measurement-window effect rather than a property of the atmosphere. For health, combustion particles show that BrC also shapes oxidative-potential assessment: in brown, methoxyphenol-rich samples the hydroxyl-radical (•OH) signal is lowered, consistent with phenolic •OH scavenging. Crucially, a lower •OH signal does not mean the particle is harmless — in cells, BrC-poor combustion PM is the most cytotoxic, the opposite ranking, so the endpoint decides the verdict.Taken together, brown carbon matters for both climate and health, and in both domains the conclusion depends on measurement choices — timescale, method, assay and endpoint.
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