We recommend ultraviolet and visible absorption spectra to represent particular types of atmospheric organic particles. Spectra of liquids and particles can be compared using the absorption coefficient of bulk material divided by material density. Reported absorption by absorbing organic aerosol from combustion is greater than that of organic material isolated by humic acid extraction. We examine ultraviolet and visible spectra of 200 organic compounds, concluding that visible absorption may be attributable to n → π* electronic transitions in a small fraction of oxygenated compounds. Absorption spectra can be communicated using the band‐gap and Urbach relationships instead of the absorption Angstrom exponent. Water‐soluble atmospheric aerosol has a band‐gap of about 2.5 eV; insoluble aerosol may have a lower band‐gap and higher absorption. Although different types of organic carbon may exhibit a continuum in absorption, there is a sharp distinction between the most‐absorbing organic carbon and black carbon.

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Absorption cross sections of methane and ammonia for electromagnetic radiation in the range from 1300 A to 374 were obtained with a two-meter radius of curvature, grazing incidence vacuum spectrograph. For methane 85 wavelengths were measured. Starting from σ = 14 Mb (1 Mb = 10—18 cm2) at 1300 A, the absorption first increased slowly and then rose more rapidly to a maximum of σ = 56 Mb at 960±278 A corresponding to the first ionization potential of methane. The absorption then decreased gradually to 14 Mv at 374 A. The total f-value pertaining to the transitions of 2p-electrons was found to be 6.1. For ammonia, 100 wavelengths were measured. Band absorption appeared at longer wavelengths. The continuous absorption began near 1210 A and extended to the shorter wavelengths. The structure of the absorption contour suggested the existence of two ionization continua: a weaker one with maximum σ = 21 Mb at 1130±20 A and a larger one with its maximum of 34 Mb at 730±20 A. These two maxima corresponded to the ionization limits of the outer electrons of ammonia. The total f-value was found to be 5.9.

Absorption cross sections of CO2 and CO in the region from 1306 A to 374 A have been measured. The absorption spectrum of CO2 consists of strong resonance bands between 1306 A and 700 A, with one of them at 923 A showing an unusually large cross section of 117×10—18 cm2. A continuum with bands superimposed on it was observed to start at about 860 A or 14.4 ev which corresponded to the first ionization potential of CO2. From 700 A toward shorter wavelengths, the contour of the continuum was better defined and smooth with a broad maximum of 36×10—18 cm2 at 550 A and a total f-value of 4.4. The CO absorption between 1306 A and 955 A was less than 10—18 cm2. Prominent resonance bands appeared at 924 A and extended to 600 A. An absorption continuum could be identified between 876 A and 374 A showing maximum values of σ≃19×10—18 cm2 between 750 A and 550 A with perhaps a small dip of 15×10—18 cm2 at 644 A. The f-value of this continuum was 2.8.