UV-VIS nadir spectroscopy

UV-VIS (ultraviolet and visible) nadir spectroscopy retrieves atmospheric columns of nitrogen dioxide (NO2), sulphur dioxide (SO2), ozone and other trace gases from sunlight scattered back to a satellite looking straight down.^[1] It uses differential optical absorption spectroscopy (DOAS), which inverts the differential absorption structure left after smooth scattering and continuum components are removed.^[2] It requires contiguous spectra at roughly 0.2 to 1 nm resolution, typically across about 300 to 500 nm, and works in daylight only. Cloud and coarse pixels limit it.

Requirements. Instruments that also retrieve ozone profiles extend the range to 270 nm or below; TROPOMI (TROPOspheric Monitoring Instrument) uses 270 to 329 nm for its profile.^[3] OMI (Ozone Monitoring Instrument) covers 270 to 500 nm at 0.42 to 0.63 nm,^[4] GEMS (Geostationary Environment Monitoring Spectrometer) covers 300 to 500 nm at under 0.6 nm,^[5] and the OMPS (Ozone Mapping and Profiler Suite) nadir mapper covers 300 to 380 nm.^[6] TROPOMI derives NO2 from a fit between 405 and 465 nm.^[7] Filter imagers such as EPIC (Earth Polychromatic Imaging Camera), with ten discrete filters, lack contiguous sampling and form a different method.^[8]

Pixels. Pixel size varies: 3.6 by 5.6 km for TROPOMI NO2,^[7] 7.5 km for Sentinel-5,^[9] 13 by 24 km for OMI,^[4] 13 by 48 km for EMI (Environment Monitoring Instrument),^[10] 40 by 40 or 40 by 80 km for GOME-2 (Global Ozone Monitoring Experiment-2)^[11] and under 8 km for geostationary Sentinel-4.^[12]

Oxygen A band. Instruments that also reach the O2 A band near 760 nm can add cloud and aerosol layer height: TROPOMI,^[13] Sentinel-4,^[14] Sentinel-5,^[9] GOME-2,^[11] EMI^[10] and TANSO-3.^[15] TROPOMI's aerosol layer height works on cloud-free scenes only,^[16] and GOME-2 provides cloud fraction and pressure.^[17] OMI and GEMS stop at 500 nm.^[4][5] The shortwave-infrared modules of TROPOMI and Sentinel-5 belong to shortwave-infrared absorption spectroscopy, not to this method.^[18][9]

Ozone and plumes. Total ozone columns are a core product;^[19] nadir viewing gives only coarse profile information, and limb sounding gives finer vertical resolution.^[19] TROPOMI produces SO2 columns, and its aerosol layer height covers elevated layers such as ash.^[1][16] These signal plumes, not geology.

Carbon dioxide. The method does not measure CO2. NO2 serves as a co-emitted plume tracer: a Berlin simulation for the Copernicus anthropogenic CO2 monitoring mission found an NO2 instrument detecting about 35 of 50 observable plumes a year, against 6 and 16 for CO2.^[20]

Aerosol. The UV aerosol index is a residual from Rayleigh-only reflectance at 340/380 nm and 354/388 nm.^[21] Near-UV retrievals add single-scattering albedo: OMI's OMAERUV assumes one of three aerosol types, each with a fixed bimodal spherical size distribution and an assumed refractive index,^[22] and GEMS fits three types.^[23] Both prescribe fixed aerosol models, so they indicate absorption, not size distribution or refractive index.

Status as of 21 October 2026. TROPOMI operational products include NO2, SO2, ozone, aerosol index and aerosol layer height.^[1] GOSAT-GW, carrying TANSO-3 with a band near 0.45 micrometres for NO2, launched on 29 June 2025; the NO2 Level 2 product is planned, not yet demonstrated.^[15][24]