Direct measurements of ozone and nitrogen dioxide photolysis rates in the troposphere

Abstract

Direct measurements of the rate of O3 photolysis to O2(1Δg) and O(1D) and of NO2 photolysis to NO and O(3P) are reported as photolysis frequencies j(O3) and j(NO2). The effects of solar zenith angle, total ozone column, cloud cover, aerosol loading, temperature, pressure, and altitude are examined. For a clear sky, zero albedo, and zenith angle between 0° and 65°, the expression j (NO2) = 1.67 × 10−2 exp (−0.575 sec θ) (s−1) gives NO2 photolysis frequencies (photolysis rates per unit reactant) to within about 10% of the measured values, where θ is the solar zenith angle. Pressure has no measurable effect on j(NO2) between 0.15 and 1.2 bars. Temperature has only a small effect between 230 and 400 K, which can be described in terms of an effective activation energy of 0.48±0.05 kJ mol−1. Frequencies of ozone photolysis to O(1D) depend strongly on overhead ozone column and temperature as well as zenith angle; for 300 K, a clear sky, zero albedo, 45° zenith angle, 0.345‐cm ozone column, and altitude between 1.6 and 6 km, j(O3) = 1.6(±0.25) × 10−5 s−1. From 300 to 273 K, j(O3) drops by 40±3% for a 65° zenith angle and a 0.306‐cm ozone column; this value is not valid for different effective ozone columns. Measured photolysis frequencies show only weak dependence on altitude and aerosol loading. These frequencies were measured with instruments sensitive to radiation from the upward 2π sr only. Clouds dramatically reduce both photolysis frequencies, but they reduce the total radiation by a significantly larger factor. Airborne UV radiometers, calibrated against direct measurements of j(NO2), were used to measure albedos of many surfaces. While natural surfaces such as vegetation and oceans have albedos near zero, dense clouds and snow have an albedo near unity. With increasing altitude, molecular and particulate scattering often increase the effective albedo with respect to photolysis frequencies.

Measurement of the photodissociation coefficient of NO2 in the atmosphere: I. Method and surface measurements

“…Table III shows that within the uncertainty of the measurements there is no difference between a single and a double walled cell as expected on the basis of the reflection/refraction analysis discussed above. Similar fmdings were reported by Dickerson et al (1982).…”

“…Reactions with excited species are estimated to increase the photolysis rate in the cell by less than 1.5%. These experiments also indicate that J M is not pressure dependent, in agreement with the fmdings of Dickerson et al (1982).…”

“…Previous methods used to measure JNOs in the troposphere (Jackson et al, 1975;Zafonte et al, 1972;Harvey et al, 1977;Babe et al, 1980;Dickerson et al, 1982) require knowledge of the kinetics of a number of reactions or the absolute concentrations of several species. The novel method to be described in this paper requires only measurements of the pressure in the photolysis cell.…”

Actinic flux and O<sup>1</sup>D photolysis frequencies retrieved from spectral measurements of irradiance at Thessaloniki, Greece

“…Photolysis frequencies were measured in the past by means of chemical actinometry (Bahe et al, 1979;Dickerson et al, 1982;Junkermann et al, 1989;Blackburn et al, 1992;Shetter et al, 1996) as well as filter radiometry (Junkermann et al, 1989;Dickerson et al, 1979;Hofzumahaus et al, 1992;Shetter et al, 1996;Balis et al, 2002). Measurements of spectral actinic flux have started to develop since the 1990's (Müller at al., 1995;Hofzumahaus et al, 1999;Shetter and Müller, 1999;Kraus and Hofzumahaus, 1998), and continued in various campaigns such as ADMIRA (Webb et al, 2002) and IPMMI .…”

A new actinic flux 4<font face="Symbol">p</font>-spectroradiometer: instrument design and application to clear sky and broken cloud conditions

“…The techniques mostly used for measuring photolysis frequencies are filter radiometry (Junkermann et al, 1989;Brauers and Hofzumahaus, 1992;van Weele et al, 1995;Crawford et al, 1999) and chemical actinometry (Bahe et al, 1979;Dickerson et al, 1982;Parrish et al, 1983;Barai and Stedman, 1992;Shetter et al, 1992;Kelley et al, 1995). Chemical actinometers consist of a quartz glass tube containing a gas mixture which is exposed to solar radiation.…”