6:15 PM - 7:30 PM
[AAS21-P18] Direct measurements of photochemical ozone production rate at a forest area in Japan during summer of 2014
Keywords:photochemical ozone production rate, oxidant, forest area
The P-L(Ox) measurement system has “reaction” and “reference” chambers. The reaction and reference chambers (17.1-cm inner diameter and 50-cm length) are made of quartz and Pyrex, respectively. Inner walls of both the chambers are coated with clear Teflon films to avoid wall loss of O3. An outer wall of the reference chamber is coated with a UV-cut film (50% cutoff wave length of 405 nm). Both the chambers were put in an outdoor location to be exposed directly to sunlight. Ambient air is introduced into both the chambers. In the reaction chamber, photochemical reactions proceed to generate Ox. On the other hand, Ox is not generated in the reference chamber. The difference of Ox concentrations (ΔOx) in air from the two chambers is the Ox produced by photochemical reactions in the reaction chamber. The P-L(Ox) is obtained by dividing ΔOx by a mean residence time of air in the reaction chamber. Ox concentrations were obtained as follows. O3 in Ox is converted into NO2 by the reaction of O3 with large excess of NO, and then the NO2 concentration is measured by a laser-induced fluorescence technique.
The field campaign was conducted at Field Science Education and Research Center, Kyoto University, Wakayama Forest Research Station, in Wakayama Prefecture, Japan. Observation site is in forest area and anthropogenic sources of air pollutants are very low. Observations were conducted from 28 July to 8 August. Most periods of the campaign, O3 concentrations were approximately 10 ppbv in the daytime. NOx (= NO + NO2) concentrations were less than about 1 ppbv throughout the campaign. BVOCs (Biogenic Volatile Organic Compounds) concentrations were high. Results of P-L(Ox) and Ox concentration on 6 August were reported in this abstract. A diurnal variations were observed for P-L(Ox) and Ox concentration, and the maximum rate and concentration were observed around noon. Ox concentration increased in the early morning, while P-L(Ox) was still 0 ppbv h-1 and then increased after a few hours. This result shows that O3 concentration is increased by non-photochemical factors in the early morning. Vertical mixing of air is week at night and O3 concentration near the surface of the ground decreases by deposition, reactions with olefin and so on. Surface of the ground is gradually warmed by sunlight in the morning and vertical mixing of air is activated. O3 concentration would increase because of O3 influx from above. P-L(Ox) achieved a peak value around noon and photochemical O3 production was active in the daytime. Ox concentration did not increase around noon, however. This suggests that O3 increase by photochemical production competes against O3 decrease by non-photochemical factors such as deposition, advection, and reactions of O3 with olefin.