Kyung-Hwan Kwak

Relationship between rooftop and on-road concentrations of traffic-related pollutants in a busy street canyon: Ambient wind effects

Rooftop and on-road measurements of O3, NO2, NOx, and CO concentrations were conducted to investigate the relationship between rooftop and on-road concentrations in a busy and shallow street canyon with an aspect ratio of ∼0.3 in Seoul, Republic of Korea, from 15 April to 1 May 2014. The median road-to-roof concentration ratios, correlation coefficients between rooftop and on-road concentrations, and temporal variations of rooftop and on-road concentrations are analyzed according to the rooftop wind directions which are two cross-canyon and two along-canyon directions. The analysis results indicate that the relationship is strong when the rooftop is situated on the downwind side rather than on the upwind side. Relative to the cross-canyon wind directions, one of the along-canyon wind directions can more enhance the relationship. A conceptual framework is proposed to explain the effect of ambient wind direction on the relationship between rooftop and on-road concentrations in a street canyon.

Temporal and Spatial Variations of NOx and Ozone Concentrations in Seoul during the Solar Eclipse of 22 July 2009

Abstract The temporal and spatial variations of NO, NO2, and O3 concentrations in Seoul, South Korea, during the solar eclipse of 22 July 2009 are investigated by analyzing data measured at 25 environmental monitoring stations. The NO2 concentration increases and the NO and O3 concentrations decrease because the efficiency of NO2 photolysis decreases during the solar eclipse. About an hour after the maximum obscuration, the reduction in the average O3 concentration over Seoul is estimated to be 45%. The maximum reduction in the O3 concentration downwind of the NOx source area is higher and occurs later than that in the downtown region. Deviations from the NO–NO2–O3 photostationary state in the downwind region are larger than those in the downtown region. This result implies that, in addition to the photochemical effect, the effect of transport by winds increases the reduction potential of the O3 concentration in the downwind region during the solar eclipse.

Sensitivity of Ozone to NO x and VOCs in a Street Canyon

The sensitivity of ozone to and volatile organic compounds (VOCs) emission rates under different ventilation rates and emission ratios in a street canyon is investigated using a chemistry box model. The carbon bond mechanism IV (CBM-IV) with 36 gaseous species and 93 chemical reactions is incorporated. and VOCs emission rates considered range from 0.01 to with intervals of . Three different ventilation rates and three different emission ratios are considered. The simulation results show that the ozone concentration decreases with increasing emission rate but increases with increasing VOCs emission rate. When the emission ratio of VOCs to is smaller than about 4, the ozone concentration is lower in the street canyon than in the background. On average, the magnitude of the sensitivity of ozone to emission rate is significantly larger than that to VOCs emission rate. As the emission rate increases, the magnitude of the sensitivity of ozone to and VOCs emission rates decreases. Because the ozone concentration is lower in the street canyon than in the background, the increased ventilation rate enhances ozone inflow from the background. Therefore, the increase in ventilation rate results in the increase in ozone concentration and the decrease in the magnitude of the sensitivity of ozone to and VOCs emission rates when the emission ratio of VOCs to is smaller than about 4. On the other hand, the increase in emission ratio results in the increase in ozone concentration because the chemical ozone production due to the photolysis is enhanced. In the present experimental setup, the contribution of the change in emission ratio to the change in the sensitivity of ozone to emission rate is larger than that of the change in ventilation rate.

Diurnal Variations of O 3 and NO 2 Concentrations in an Urban Park in Summer: Effects of Air Temperature and Wind Speed

The diurnal variations of O3 and NO2 in an urban park and the effects of air temperature and wind speed on the diurnal variations are investigated. O3 and NO2 concentrations were observed at a site in an urban park of Seoul from 27 July 2015 to 9 August 2015. The O3 and NO2 concentrations observed in the urban park are compared to those observed at the Gangnam air quality monitoring station (AQMS). The O3 concentration is higher in the urban park than at the Gangnam AQMS in the daytime because the amount of O3 dissociated by NO is smaller as well as partly because the amount of O3 produced in the oxidation process of biogenic volatile organic compounds (VOCs) is larger in the urban park than at the Gangnam AQMS. The NO2 concentration is lower in the urban park than at the Gangnam AQMS during day and night because the observation site in the urban park is relatively far from roads where NOx is freshly emitted from vehicles. The difference in NO2 concentration is larger in the daytime than in the nighttime. To examine the effects of air temperature and wind speed on the diurnal variations of O3 and NO2, the observed O3 and NO2 concentrations are classified into high or low air temperature and high or low wind speed days. The high O3 and NO2 concentrations in the daytime appear for the high air temperature and low wind speed days. This is because the daytime photochemical processes are favorable when the air temperature is high and the wind speed is low. The scatter plots of the daytime maximum O3 and minimum NO2 concentrations versus the daytime averages of air temperature and wind speed show that the daytime maximum O3 and minimum NO2 concentrations tend to increase as the air temperature increases or the wind speed decreases. The daytime maximum O3 concentration is more sensitive to the changes in air temperature and wind speed in the urban park than at the Gangnam AQMS.