Hiroaki Minoura

Temperature dependence of secondary organic aerosol formation by photo-oxidation of hydrocarbons

Photo-oxidation experiments on hydrocarbons were performed with a temperature-controlled smog chamber to study the temperature dependence of secondary organic aerosol (SOA) formation. A higher SOA yield was obtained at lower temperature and with a higher concentration of SOA generated. The relationship of SOA yield to temperature and SOA concentration is expressed by a gas/particle partitioning absorption model considered with temperature dependence. Under the condition of the same SOA concentration, the SOA yield at 283 K was approximately twice that at 303 K. It has been clarified experimentally that temperature is one of the most important factors in SOA formation. The experiments were performed not only with three aromatic hydrocarbons (toluene, m-xylene and 1,2,4-trimethylbenzene) and one biogenic alkene (α-pinene), but also with one alkane (n-undecane) on which few experiments for SOA formation have been performed. n-Undecane indicates a lower SOA yield than any other hydrocarbon investigated in this study.

Some characteristics of surface ozone concentration observed in an urban atmosphere

Abstract Some characteristics of surface ozone concentration were obtained from continuous observation, with time resolution of 1 s. These are compared with the concentrations of NO and NO2 and meteorological conditions which were observed simultaneously. The observed concentrations of O3 and NO showed a hyperbolic relationship approximately described as [O3]×[NO]=21.6 ppb2. This is consistent with a proposal that the photochemical reaction in the O3–NOx system was dominant in controlling the O3 concentration near the surface layer. An inverse linear relationship of [O3]+[NO2]=30 ppb is also recognized, and the background O3 concentration in the Nagakute area is estimated to be about 30 ppb from this relation. The yearly average background O3 concentration calculated from the equation for transportation by wind approximates the value obtained above. The positive dependency of O3 production on solar radiation and the negative relationships of NO2 concentration due to the NO2 photolysis process were recognized and both the dependencies on the solar radiation became very close during the winter season. The summer maximum cannot be explained by seasonal variations in the background O3 concentration the sensitivity to solar radiation intensity, and the photochemical reaction only in the O3–NOx system.

Rapid change in nitrate and sulfate concentrations observed in early stage of precipitation and their deposition processes

The concentrations of H+, nitrate (NO3-), and sulfate (SO42-) in rainwater and their temporal changes were analyzed on the basis of continuous observation from 1 July 1991 to 30 June 1992 at a suburb of Nagoya, Japan. The yearly average for pH was 4.4. In general, an increasing pH with increase in precipitation amount was observed for rain events. Relatively high pH rainwater was sometimes observed at the beginning of rainfall, even though high concentrations of NO3- and SO42- were involved. The high pH values were considered to be caused by the neutralization process with particulate matter containing cations. The yearly averaged ratio of equivalent concentration of nitrate to sulfate (N/S) in rainwater was 0.58. In the early stage of rain, the N/S value was usually more than 1.0 due to the difference of scavenging process between NO3- and SO42-. High values of N/S ranging from 5 to 10 were found under the atmospheric conditions of calm winds and low humidity, during which it is possible that atmospheric particles float for a long time in the air before a rain event. The adsorption of NO3- in the early stage of rainfall by particulate matter was suggested from the difference in scavenging processes of NO3- and SO42-. A possible scavenging process, called limb cloud scavenging, is presented to explain the interaction of particles and nitrate ions at the early stage of rain. In limb cloud scavenging, the repeated migration of cloud particles or raindrops between the inside and outside of clouds increases the absorption of ions to a highly condensed level, thus increasing the N/S value of rainwater. The influence of global scale seasonal phenomena with large amounts of particulates, such as typhoons or Asian dust storms, was also studied.

Ion concentration changes observed in drizzling rains

Abstract Ion concentrations of NO 3 − and SO 4 2− in individual rains were observed at Nagagute in the suburbs of Nagoya, Japan, from July 1, 1991 to June 30, 1992. Decreasing concentrations of NO 3 − and SO 4 2− in rainwater collected during precipitation were frequently observed and this decreasing pattern is expressed by exponential functions. The ion concentration decrease is more influenced by the ion scavenging loss in the precipitation area than the ion supply due to transportation and oxidation processes. Under calm atmospheric conditions, the empirical ion scavenging coefficients were derived from changes in ion concentration in a drizzling rain with a rain intensity of below 15 mm h −1 . These coefficients were linearly proportional to the rain intensity. The average empirical scavenging coefficients of NO 3 − and SO 4 2− were 5.2 × 10 −4 and 2.2 × 10 −4 s −1 , respectively. The coefficient of N0 3 − is 2.5 times more sensitive to the rain intensity than that of SO 4 2− . The NO 3 − and SO 4 2− concentrations and pH were well simulated in a numerical model containing the obtained empirical scavenging coefficients.