Diurnal asymmetry in slant column density of NO2, O3, H2O and O4 during CAIPEEX–IGOC over Mahabubnagar, a rural site in Southern Peninsular India

In order to study the column densities of atmospheric trace gases over a rural environment, zenith-sky scattered light observations have been carried out by employing a high-precision, portable UV-V-IR spectrometer (Ocean Optics Model HR2000) at Mahabubnagar (16°42′N, 77°58′E) during the Cloud Aerosol Interaction and Precipitation Enhancement Experiment–Integrated Ground Observational Campaign during October 1, 2011–November 11, 2011. The observed and calculated differential optical density spectra of NO2, O3, H2O and O4 are compared in the spectral range 462–498 nm and found good agreement within a percent deviation up to 1, 1, 0.5 and 0.8 %, respectively. Differential slant column densities (SCDdiff) of NO2, O3, H2O and O4 are retrieved to present the diurnal variation at morning and evening hours between 65° and 95° solar zenith angles (SZAs). The SCDdiff at morning and evening 90° SZA are observed to be 6.4 × 1016 and 9.4 × 1016 mol cm−2 for NO2; 1.03 × 1020 and 1.38 × 1020 mol cm−2 for O3; 1.7 × 1024 and 1.8 × 1024 mol cm−2 for H2O, 1.23 × 1044 and 1.57 × 1044 mol cm−2 for O4, respectively. The diurnal variations of NO2 in twilight period are observed to vary from 36 to 75 %, O3 from 23 to 40 %, H2O from 2 to 20 % and O4 from 23 to 53 % during the study period. The SCDs of NO2, O3 and O4 are observed to be higher in the evening twilight hours compared to the morning twilight hours, which may be due to higher temperature observed at evening as compared to morning between 65° and 95° SZAs.