Open AccessRelationship between photolysis frequencies derived from spectroscopic measurements of actinic fluxes and irradiances during the IPMMI campaign
Author(s) -
McKenzie Richard,
Johnston Paul,
Hofzumahaus Andreas,
Kraus Alexander,
Madronich Sasha,
Cantrell Chris,
Calvert Jack,
Shetter Rick
Publication year - 2002
Publication title -
journal of geophysical research: atmospheres
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.67
H-Index - 298
eISSN - 2156-2202
pISSN - 0148-0227
DOI - 10.1029/2001jd000601
Subject(s) - irradiance , photodissociation , zenith , noon , nitrogen dioxide , atmospheric sciences , spectroradiometer , environmental science , solar irradiance , troposphere , solar zenith angle , ozone , radiative transfer , atmospheric chemistry , physics , meteorology , optics , chemistry , photochemistry , reflectivity
The relationship between photolysis frequencies derived fromspectroscopic measurements of actinic fluxes and irradiances was determined during acoordinated measurement campaign (International Photolysis Frequency Measurement andModeling Intercomparison campaign (IPMMI)). When differences in viewing geometriesare taken into account, the measurements are in close agreement. An empiricalrelationship, which is useful for high sun (noon) conditions or for daily integrals,was found to convert irradiance data to photolysis frequencies. For low‐sunconditions (large solar zenith angle), model calculations were shown to improve theaccuracy. However, the input parameters to the model are site specific and theconversion depends on diffuse/direct ratios. During cloudy conditions, significantimprovements in the conversion can be achieved by assuming the radiation field tocomprise entirely diffuse isotropic radiation when the UVA transmission by cloud isless than 0.8. Changing cloud conditions remain the greatest limitation, but theytend to bias the results away from the clear‐sky case in a systematic way.Furthermore, although the cloud effects on the photolysis rates of nitrogen dioxide ( J (NO 2 )) are rather large, they aremuch smaller for ozone photolysis ( J (O 3→ O( 1 D))), which is of prime importance in troposphericchemistry. The study shows the potential for deriving historical and geographicaldifferences in actinic fluxes from the extensive records of ground‐basedmeasurements of spectral irradiance.