[A-AS11] Atmospheric Chemistry

[AAS11-20] Analysis of long-term variation of isoprene column amount measured with high-resolution FTIR at Rikubetsu, Japan

*Tomoo Nagahama¹, Isao Murata², Isamu Morino³ (1.Institute for Space-Earth Environmental Research, Nagoya University, 2.Graduate School of Environmental Studies, Tohoku University, 3.National Institute for Environmental Studies)

Keywords:Isoprene, Troposhere, Long-term trend

Hydrocarbons in the atmosphere during the daytime produce tropospheric ozone, an air pollutant, through chemical reaction processes with nitrogen oxides (NOx). From the aspect of air quality preservation, it is important to control the emission of hydrocarbons into the atmosphere. Most of the hydrocarbons in the atmosphere are emitted as volatile organic compounds (VOCs), and approximately one-third of these are isoprene. Therefore, it is essential to assess the dynamics of isoprene in the atmosphere to understand air pollution and to monitor its long-term variation. Most of the observations of isoprene are conducted by in-situ sampling observations near the surface, and monitoring observations covering the entire troposphere have rarely been made. In this study, we analyzed the tropospheric column amount of isoprene from solar absorption spectra taken with high-resolution spectroscopy technique. We used data in the 11 μm wavelength band of solar absorption spectra obtained from a ground-based high-resolution Fourier Transform InfraRed (FTIR) spectrometer operated in Rikubetsu, Hokkaido, Japan, together with the National Institute for Environmental Studies (NIES). Using spectral data in the wavenumber range of 885-900 cm-1 acquired since 1995, the vertical distribution of CO₂ and water vapor was preliminarily analyzed using the analysis software SFIT4 (version 1.0.14), and then the vertical distribution of isoprene was determined simultaneously with CFC-12, HCFC-142b, NH₃, and HNO₃ to obtain the column amount. The initial guesses of isoprene and other constituents were based on the mean altitude distribution over land of the WACCM simulations from 1980 to 2040. The spectroscopic parameters of isoprene were adopted from pseudo-spectroscopic parameters by NASA/JPL group. The vertical distribution of temperature and pressure was based on the NCEP Reanalysis daily mean data on the day of observation.
The analysis showed that the isoprene column amount was estimated as (2-8)×10¹⁴ cm^-2, and the value in winter was smaller than that in other seasons. The observed column amounts varied widely and showed no significant long-term trend. In this presentation, we report on the observed temporal variations in isoprene column amount, as well as an improved method for analyzing column amount.