This spectrometer uses high-power light emitting diode as the light source and enhances molecular absorption or particle light extinction through high-reflectivity mirrors which sharply increase light pathlengh to 2 — 10 km, to achieve ultrahigh measurement sensitivity. By adjusting the light source, mirrors and broadband light detector, it is possible to measure trace gases such as HCHO, glyoxal (CHOCHO), NO, NO₂, NO₃, N₂O₅, O₃, HONO, IO, I₂ and Br₂ directly through their molecular absorption, and aerosol extinction at high accuracy. This technique has high sensitivity (1 ppt — 1 ppb within 1 s sampling interval), high selectivity (spectrum retrieval based on characteristic molecular absorption "fingerprints") and does not require any reference gas (the so-called "absolute" measurements), making it well-suited for gas detection where high accuracy, high sensitivity and unambiguous identification are required.