3.1.1. Seasonal Variations
 Figure 1 presents the time series of the hourly observations of δD, δ18O, and d of atmospheric water vapor and event-based precipitation data. Table 1 summarizes monthly mean values of precipitation and water vapor δD, δ18O, and d and standard deviation of vapor values. There was considerably less variability in the vapor δD, δ18O, and d in the summer season (June–August) than in the rest of the year. In the cold season, the day-to-day variations could exceed 200‰, 25‰, and 50‰ for δD, δ18O, and d, respectively. In the summer, the variations were usually less than 100‰, 10‰, and 25‰ for δD, δ18O, and d, respectively. The months of June–August mark the peak activity of the monsoon in eastern China [Wang and Gaffen, 2001]. In comparison, in the humid continental climate in New England, where precipitation is evenly distributed throughout the year, equal day-to-day variability is seen in both the cold and warm seasons [Lee et al., 2006].
Figure 1. Hourly values of (a) δD, (b) δ18O, and (c) deuterium excess (d) of atmospheric water vapor (dots) and precipitation (circles) from December 2006 to December 2007 in Beijing, China.
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Table 1. Summary of Monthly Mean Values of Data Used in This Studya
|Date||w (mmol mol−1)||T (°C)||RH (%)||P (mm)||Vapor||Precipitation (‰)||Vapor in Equilibrium (‰)|
|δD (‰)||δ18O (‰)||d (‰)||rw, D||rw, 18O||rRH, d||δD||δ18O||d||δD||δ18O||d|
 The seasonal variations of δD and δ18O in precipitation were positively correlated with the vapor values. The event-based d of precipitation was highly variable, ranging from −42‰ to 24‰, and reached its lowest value on day of year (DOY) 128–129 (8–9 May). On these days, the δ18O in precipitation reached its highest value. The δD in precipitation reached its highest value on DOY 131 and decreased slowly through the summer season, a pattern similar to the results of Yamanaka et al.  for the North China Plain.
 The hourly maximum and minimum were approximately −51‰ and −370‰ for the vapor δD and −3‰ and −52‰ for the vapor δ18O. In comparison, the observed maximum and minimum hourly δ18O values of atmospheric water vapor in New Haven, Connecticut, are approximately −10‰ and −38‰, respectively [Lee et al., 2006]. The hourly d of atmospheric vapor generally fluctuated between −40‰ and 89‰, mostly in the winter months. The maximum vapor δD and δ18O occurred on DOY 131 and DOY 128, respectively. These maximum values were in approximate equilibrium with precipitation water. On DOY 128–129, a short rain shower produced 0.5 mm rain with the highest δ18O of 6.9‰ of the year. On DOY 131, a short rain shower produced 1.8 mm rain with the highest δD of 28‰ of the year. Positive δD and δ18O values in precipitation have also been reported by a number of studies in China and tend to occur in small rain showers [e.g., Yamanaka et al., 2004; Yu et al., 2006; Liu et al., 2007; Tian et al., 2008]. They are likely caused by the evaporation from falling raindrops under the cloud.
 According to Table 1, in general, δD and δ18O of atmospheric water vapor were higher in the warm season than in the cold season. The highest monthly mean values of the vapor δD (−106‰) and δ18O (−14.0‰) occurred in June 2007, and the lowest values (−224‰ and −30.4‰) occurred in January 2007. In comparison, the highest monthly mean value of the vapor δ18O in southern New England occurs in May (−15.1%), and the lowest value occurs in January (−29.4‰) [Lee et al., 2006]. The seasonal course of the monthly mean vapor d was not in phase with those of δD and δ18O. The highest monthly mean value (19.6‰) occurred in March 2007, and the lowest value (5.7‰) occurred in June 2007.
 The annual mean values of the vapor δD, δ18O, and d were −154‰, −20.7‰, and 11.8‰, respectively. Our δD and δ18O values were lower than the 7 year average of −140‰ and −18.9‰ reported for Heidelberg, Germany [Jacob and Sonntag, 1991]. Our d value was almost identical to the 7 year average of 11.5‰ of the same data set. Our δ18O value was similar to the annual mean of −20.8‰ for New Haven [Lee et al., 2006]. Weighted by precipitation amount, the annual mean values of δD, δ18O, and d in precipitation were −58‰, −8.0‰, and 6.2‰, respectively. Our annual δ18O value in precipitation was higher than another annual mean of −8.8‰ in Beijing, China, in 1979–1980 [Wei and Lin, 1994] and was in the range of −20.4‰ to −5.4‰ over the entire country of China based on a 55-site data set [Liu et al., 2008]. That the d in precipitation was significantly lower than the vapor d may be indicative of partial evaporation of raindrops beneath the cloud base level, which decreases the d in precipitation and increases the vapor d [Jacob and Sonntag, 1991].
3.1.2. Diurnal Variations
 Figure 2 shows the 24 h ensemble average values of the vapor δD, δ18O, and d. In Figure 2, the full data set was broken into the winter (December–February), spring (March–May), summer (June–August), and autumn (September–November) seasons. The peak-to-peak variation of the vapor δD was 23‰, 14‰, 4‰, and 16‰ in the winter, spring, summer, and autumn seasons, respectively. The peak-to-peak variation of the vapor δ18O was 3.4‰, 2.5‰, 1.0‰, and 2.8‰, respectively. The minimum δD and δ18O occurred in the early afternoon hours (1200 to 1600 CST), and the maximum δD and δ18O occurred around midnight. These diurnal variations seemed to be in phase with the variations in the vapor mixing ratio. Welp et al. [2008a] suggested that lower δ18O values in midday than at midnight may be related to the entrainment of the lighter vapor from the free atmosphere into the convective boundary layer. In the present study, the small diurnal amplitude in the summer may indicate a weak entrainment flux at the top of the boundary layer in the peak monsoon season.
Figure 2. Twenty-four hour ensemble average values of (a) δD, (b) δ18O, (c) d, and (d) water vapor mixing ratio (w) of atmospheric water vapor for winter (December–February; diamonds), spring (March–May; squares), summer (June–August; circles), and autumn (September–November; triangles).
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 To our best knowledge, Figure 2c represents the first data showing the diurnal change in the vapor d. Its peak-to-peak variation was 9.4‰, 7.3‰, 3.5‰, and 7.7‰ in the winter, spring, summer, and autumn seasons, respectively. The diurnal pattern of d was in opposite phase with those of the vapor δD and δ18O. The mixing of air between the free atmosphere and the boundary layer may have played a role in the diurnal variations in d, although to date, no researcher has made comparative measurement of the vapor d in these two air layers.