1. INTRODUCTION
Air drying defines a method of drying green wood by stacking it in a air drying yard while exposing it naturally to an open-atmosphere (Jung et al., 2005). Compared to the artificial drying, air drying has advantages of barely having any initial facility fees and energy costs, while air drying has disadvantages of being highly impacted by its location and the climate, requiring larger drying yards, and being unable to dry below the atmospheric moisture level. Because air drying should be considered factors like species, density, dimensions of woods, geographical location and meteorological factors of the drying site, it is difficult to predict air drying time required for logs and lumbers with different moisture content (MC). Location like Korea where there are four distinct seasons, time required for air drying is varied in the season initiating air drying due to the temperature difference between summer and winter (Simpson and Hart, 2001).
Effective air drying days (EADD) is numerical days per a month appropriate for air drying based on the meteorological factor (Rietz, 1972), and when applied, air drying calendar can be presented for different regions. Korean and international studies related to air drying and EADD is subjected to the softwood and hardwood samples with the thickness of 50 mm or below. In Korea, there are studies on analyzing the time required for taun lumbers to reach 20% MC and obtaining air drying calendar (Jung, 1985), calculating EADD among 24 regions in Korea and categorizing 4 zones based on the result (Jung and Park, 1986), calculating and comparing moisture indexes for climate classification using the meteorological data from 82 regions in Korea (Ra, 2018), and suggesting a regression equation for monitoring the MC in Korean red pine and pitch pine wood (Jung et al., 1986). In U.S., there are studies on measuring air drying time for American beech with a thickness of 25 mm – 50 mm (Peck, 1954), ponderosa pine (Peck et al. 1956), sugar maple (Peck, 1957), northern red oak (Peck, 1959), and Douglar fir and yellow poplar (Denig and Wengert, 1982), based on thickness of lumbers, timing of stacking, locations of air drying yards, and meteorological factor. Moreover, the study on categorizing U.S. regions based on the average of ‘growing degree days’ suggested by USDA in 1975 and measuring the required time for air drying from each region was performed (Rietz, 1972; McMillen and Wengert, 1978).
This study is a pilot study for analyzing changes in MC for Korean red pine logs during air drying, and the EADD for each region was calculated using the 30-years of meteorological data provided by Korea Meteorological Administration (http://data.kma.go.kr) such as temperature, relative humidity, and wind velocity from 24 different regions in Korea. The result was compared with the study from 1986 (Jung and Park, 1986) to examine the effect of climate change in EADD.
2. MATERIALS and METHODS
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.
3. RESULTS and DISCUSSION
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 1st zone to the 2nd zone. Daegu, Gwangju, Jinju which previously categorized as the 2nd zone, shifted their zone to 3rd zone, and Sokcho, Pohang, Ulsan shifted their zone from the 3rd to 4th. 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 4th zone, the south province and Gangneung were categorized under the 3rd zone, and the inland area was categorized in 2nd and 1st 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.
4. CONCLUSION
In order to establish a pilot study for analyzing MC level for Korean red pine logs during air drying, the study provides the calculated EADD using the meteorological data from 24 regions in Korea such as average monthly temperature, relative humidity, wind speed, and based on the result, categorizes the EADD zone. The obtained conclusions are followings:
The average EADD was calculated to be 265 days with a range of 239 – 291 days, based on the 30-years of average monthly meteorological data from 1988 to 2017 in 24 regions in Korea. This result showed a total of 5 days increase, compared to 260 days which is the average of the study in 1986. The meteorological factors affecting EADD were in the order of temperature, relative humidity, and wind speed, and EADD deduction resulted from the relative humidity and wind speed was minimal.
As a result of dividing Korea into 4 zones of EADD, the zones of EADD were shifted northward compared to previous study due to global warming.
The difference in monthly EADD can be used to explain the change in seasonal air drying speed, and if utilized with air drying calendar of each region, it is expected to predict the air drying days for woods.
