Moisture Content Change of Korean Red Pine Logs During Air Drying: I. Effective Air Drying Days in Major Regions in Korea

The study used the data from Korea Meteorological Administration in order to collect meteorological factors like temperature, relative humidity, and wind speed, to examine their effects on MC level in air drying wood. Korea Meteorological Administration provides an overall statistical data on temperature, precipitation, wind speed, wind direction, humidity, and the amount of solar radiation from Korea’s main regions. In this study, in order to parallelly compare the EADD study result from the 1986 (Jung and Park, 1986) with the recent changes in the 30-years of EADD of main regions in Korea, average monthly temperature, relative humidity, wind speed were collected from the same 24 regions in Korea (Sokcho, Chuncheon, Gangneung, Seoul, Incheon, Ulleung-do, Suwon, Seosan, Cheongju, Daejeon, Chupungnyeong, Pohang, Gunsan, Daegu, Jeonju, Ulsan, Gwangju, Busan, Tongyeong, Mokpo, Yeosu, Jeju, Seogwipo, Jinju) which were used in the precedent study.

EADD was calculated using the meteorological data provided from Korean Meteorological Administration. The EADD calculation was obtained using the method provided by USDA Forest Service Forest Products Laboratory (Reitz, 1972). An appropriate monthly air drying days were hypothesized as 30 days, and two days were deducted from it when the average summer (June, July, August) monthly relative humidity is 5% or more higher than the yearly average and the average summer monthly wind speed was 1.8 m/s or more less than the yearly average. Five days are deducted when the average temperature of spring (March, April, May), fall (September, October, and November), and winter (December, January, February), is 5.5°C less than those of the three summer months; two days were deducted when the average relative humidity is 5% or more higher than the yearly average relative humidity, or when the average wind speed is 1.8 m/s or more less than the yearly average wind speed. When the data from both relative humidity and wind speed are satisfying the above conditions, total of four days are deducted (Jung and Park, 1986).

EADD zones in Korea can be categorized into 4 zones based on the number of EADD: the first zone is the zone with the EADD of below 250 days; the second zone is the EADD range from 250 – 259 days; the third zone is the EADD range from 260 – 269 days; and the fourth zone is the EADD of 270 days or above (Jung and Park, 1986). This study categorizes EADD zones using the EADD data from the 24 regions in Korea based on the recent 30 years of meteorological data, and compares this data with the past 30 years of EADD zone.

In order to measure the change in MC for 16 samples of air air drying Koren red pine log containing the initial MC range from 28 – 83% and the diameter range from 310 – 510 mm, provisional air drying site is installed on June 2016 in National Institute of Forest Science located in Dongdaemungu, Seoul. Upon installation of the air drying site, equilibrirum MC was calculated based on the average monthly temperature, relative humidity, and wind speed from 2016 to 2018. The equation (1) was used to calculate the data, based on the Hailwood-Horrobin’s adsorption model (Simpson, 1973).

where, EMC = equilibrium moisture content (%); h = relative humidity expressed in decimal form (%/100); W, K, K₁, K₂ = temperature dependent parameters.

On Table 1, EADD, standard deviation, air drying calendars of 24 regions in Korea is exhibited. The data was calculated on 1986 utilizing meteorological factors like average monthly temperature, relative humidity, and wind speed (Jung and Park, 1986). This data was calculated using the monthly average of regional meteorological factors range from 18 – 30 period of years, and each year of data measurement is stated below Table 1. Among 24 regions of Korea, the average EADD was 260 days, the maximum was 288 days (Seogwipo), the minimum EADD was 237 days (Chuncheon). EADD was affected the most by average temperature. Although relative humidity periodically influenced on September, wind speed had no impact on the EADD.

On Table 2, EADD of 24 regions in Korea from 1988 – 2017, calculated from average monthly meteorological factors, is displayed. Because of the recent increase on the average temperature due to the global warming, the average of EADD is calculated to be 265 days, which showed 5 days increased compared to the data from 30 years ago shown on Table 1; the maximum EADD was 291 days (Seogwipo), the minimum was 239 days (Chuncheon). By region, EADD of Daegu was increased by 10 days, from 252 to 262, EADD of Jinju was also increased by 1 day, from 259 to 260. The regions showed more than 6 days increase in EADD are Suwon (6 days), Cheongju (8 days), Daejeon (8 days), Chupungnyeong (7 days), Pohang (6 days), Daegu (10 days), Jeonju (6 days), and Ulsan (6 days). The factors affecting EADD are in the order of temperature, relative humidity, and wind speed, and the deduction of EADD due to the wind speed was occurred once in each Chupungnyeong on June 2001 and Ulleung-do on June 2013. The difference among monthly EADD can explain the seasonal change in air drying speed, and by utilizing each region’s air drying calendar, it is possible to predict the days require to air drying woods (McMillen, 1978; Jung and Park, 1986).

Table 3 shows the categorized regions in Korea based on the four EADD zones using the 30 years of meteorological data from 1988 to 2017. Basis on the data from 1986, comparing the EADD zones from 1955 to 1984, Seoul, Suwon, Daejeon changed their zone from the 1^st zone to the 2^nd zone. Daegu, Gwangju, Jinju which previously categorized as the 2^nd zone, shifted their zone to 3^rd zone, and Sokcho, Pohang, Ulsan shifted their zone from the 3^rd to 4^th. As we previously observed from the average EADD increased by 5 days, EADD zones were also shifted northward in latitude, which seemed to be affected by the recent increase of average temperature due to the global warming. Unlike Korea’s usual climate classification which is distinguished by the temperature and precipitation, EADD zones is distinguished by temperature and relative humidity. Therefore, south and east coastal region were categorized under the 4^th zone, the south province and Gangneung were categorized under the 3^rd zone, and the inland area was categorized in 2^nd and 1^st zone.

Meteorologically speaking, although dividing one calendar year to four seasons, three months in each season, is reflected upon the solar zenith angle and the length of day and night which brings the universality based on the latitude, another meteorological method of defining four zones is defined based on the average daily temperature as following (Kwon et al., 2007): I) The cold period has an average daily temperature of below 0°C and abundant of snow; II) The cool period has an average daily temperature range from 0 - 10°C, and frost is observed occasionally; III) The warm period has an average daily temperature range from 10 - 20°C, and there is no frost observed; IV) The hot period has the average daily temperature above 21°C. Korea has a repetitive II-III-IV-I climate condition, total of four seasonal changes are observed in a calendar year.

In order to examine the changes in EADD and equilibrium MC, Seoul region’s monthly average temperature, relative humidity, wind speed and each corresponding equilibrium MC are stated in Table 4.

Based on recent 3 years of average monthly temperature in Seoul, January and February are categorized as (I), March and December are in category (II), April, May, October, November are in category (III), and June, July, August, September are in category (IV). Equlibrium MC was affected more by relative humidity than the temperature, and it had a tendency of decreasing less than 10% during the winter but gradually increased to have equilibrium MC of 12.4% to 14.9% during summer.

Seoul region’s meteorological factors including average monthly temperature, relative humidity, and wind speed stated in this study will be used as a basic data to predict changes in MC distribution of Korean red pine logs during air drying. Decrease in MC in the process of air drying will be considered as a dependent variable, initial MC and diameter of the logs, temperature, relative humidity and wind speed will be used as independent variables when they are applied to a multiple regression analysis. Moreover, the data will be used to determine 2-dimensional mass transfer coefficients, moisture diffusion coefficient and surface emission coefficient, in order to to predict the MC distribution of Korean red pine logs.