DIURNAL VARIATIONS OF CO2 MIXING RATIO IN THE LOWER ATMOSPHERE BY THREE WAVELENGTH DIAL

We have conducted the measurement of high accurate CO2 mixing ratio profiles by measuring the temperature profiles simultaneously using the three wavelength CO2 DIAL. The measurements of CO2 diurnal variation in the lower atmosphere were carried out on sunny and cloudy days respectively. We find out that increasing of the CO2 mixing ratio occurs over the altitude of about 2 km from the surface during nighttime. On the other hand, the CO2 mixing ratio decreases over the lower atmosphere during daytime. In particular, the CO2 mixing ratio decreases earlier on sunny days than on cloudy days after sunrise. This result suggests that CO2 absorption by photosynthesis greatly contributes to the strength of the solar radiation.


INTRODUCTION
Validating and improving the global atmospheric transport model requires precise measurement of the carbon dioxide (CO2) profile. In particular, knowledge of CO2 diurnal variation in the lower atmosphere is important for the elucidation of sinks and sources. We have developed a ground based direct detection three ZDYHOHQJWK ȝP DIAL to achieve measurements of vertical CO2 concentration and temperature profiles in the atmosphere 1-3 . In the DIAL observation, the temperature dependence of the CO2 mixing ratio due to the temperature dependence of the absorption cross section is 1.6 ppm/K near the ground and the air density in the atmosphere. This means that the difference between the model and the actual temperature profile greatly influences the accuracy of the CO2 mixing ratio. We carried out measurements of CO2 diurnal variations on the sunny and cloudy day. In this paper, we report on the details of CO2 diurnal variations and the differences in CO2 profiles due to weather.

CO2 DIURNAL VARIATIONS
The CO2 DIAL consists of an optical parametric generator (OPG) transmitter that excited by the LD pumped Nd:YAG laser. The on-line DFB seed laser onto the line center (1572.992 nm) by the wavelength control unit including CO2 reference cell. The off-line DFB seed laser (1573.137 nm) is operated by the free-run mode. The third DFB seed laser (1573.040 nm) for the temperature measurement is operated by the offset lock system. These seed lasers were connected to an optical fiber switch, and the switching speed is 2.5 ms. The atmospheric backscatters were collected by a 250 mm telescope and was connected into the nearinfrared PMT module operated with an analog mode. We realized the CO2 concentration measurement for daytime by using the narrowband interference filter with 1.0 nm FWHM.
Atmospheric CO2 measurements were conducted from 0.39 to 2.5 km by the three wavelength DIAL at the Hino campus of Tokyo Metropolitan University (35.7N, 139.4E). Figure 1 shows the time-height cross section of the CO2 mixing ratio with an integration time of 30 minutes. Japan was covered with high pressure during the observation period. It was fine and the wind was weak. A CO2 gas sensor (LI-COR, LI-7500) was installed at the our lidar laboratory. CO2 mixing ratio between the ground and 0.39 km altitude were interpolated. The sunrise was 5:00LT and the sunset was 18:23LT.
CO2 mixing ratios increased from 22:00 LT on 21 April to 07:00 LT on 22 April at the altitude of 1.5 km or less. On the other hand, CO2 mixing ratios decreased during daytime (especially in the afternoon). Furthermore, the increase or decrease in CO2 mixing ratios occurs earlier in the lower layer than in the upper layer. Diurnal variations are attributable primarily to the natural photosynthetic and respiration cycle of vegetation and soils and to anthropogenic sources in urban areas.
EPJ Web Conferences 237, 03011 (2020) ILRC 29 https://doi.org/10.1051/epjconf/202023703011 2 shows the time-height cross section of the CO2 mixing ratio on 30 -31 October, 2018. It was cloudy during DIAL measurement and the clouds were about 5 km altitude and the wind was weak. The sunrise was 6:01LT. After 10:00LT, background level increased and we could not continue the DIAL measurement. As in the case of , the CO2 mixing ratio at night time increased to an altitude of 1.5 km, and the value was 410 ppm or more. However, even after 2 hours from sunrise, the decrease in CO2 mixing ratio as shown in Fig.  1 was not measured. We consider that the weaker photosynthetic activity on cloudy days compared to sunny days is due to the difference in the decrease of the CO2 mixing ratio after sunrise.

CONCLUSION
:H KDYH GHYHORSHG WKH WKUHH ZDYHOHQJWKV ȝP DIAL system for measurements of the CO2 mixing ratio and the atmospheric temperature profiles in the troposphere. The CO2 DIAL conducted measurements of a diurnal variation of CO2 mixing ratio and temperature profile on 21-22 April 2018 and 30-31 October 2018 from 0.39 to 2.5 km altitude. We can see that the increasing of CO2 mixing ratios occurs over the altitude of about 2 km from surface during nighttime. On a sunny day (April), the CO2 mixing ratio decreased significantly for two hours after sunrise. On the other hand, on a cloudy day (October), the CO2 mixing ratio did not decrease significantly even after two hours from sunrise. However, we could not continue the measurement except for four hours after sunrise due to the deterioration of the signalto-noise ratio of increase of background radiation arising from clouds over the lidar site, so we could not know the behavior of the CO2 mixing ratio over the entire daytime. Diurnal variations are attributable to the natural photosynthetic and respiration cycle of vegetation and soils and to anthropogenic sources in urban areas. However, we found out that CO2 mixing ratio in the lower atmosphere attributes primarily to the photosynthesis of plants under the environment around our lidar site.