Rainout Processes Research Articles

Variation of stable isotope signals in precipitation during Typhoon In-Fa (2021), Eastern China

ABSTRACT High-resolution stable isotope precipitation records are crucial for understanding the mechanism of isotope variations during typhoon precipitation events. We present hourly stable isotope time series of precipitation during Typhoon InFa (2021) in Kunshan and Yangzhou, in the lower reaches of the Yangtze River, Eastern China. Precipitation δ18O in Kunshan and Yangzhou ranged from −1.3‰ to −12.3‰ and −2.8‰ to −17.1‰, respectively. Both δ18O time series shifted to more negative values as the typhoon approached. The evolution of precipitation δ18O during the typhoon is likely caused by a combination of Rayleigh distillation and microphysical processes. Precipitation δ18O values in Yangzhou are more negative than those in Kunshan with a lag of ~14 h. This was mainly controlled by further continuous rainout processes during the movement of Typhoon InFa from Kunshan to Yangzhou and higher condensation efficiency. This study improves our understanding of the mechanisms underlying stable isotope changes during typhoon precipitation.

Rocky sub-Neptunes formed by pebble accretion: Rain of rock from polluted envelopes

Sub-Neptune planets formed in the protoplanetary disk accreted hydrogen-helium (H,He) envelopes. Planet formation models of sub-Neptunes formed by pebble accretion result in small rocky cores surrounded by polluted H,He envelopes, where most of the rock (silicate) is in vapor form at the end of the formation phase. This vapor is expected to condense and rain out as the planet cools. In this Letter, we examine the timescale for the rainout and its effect on the thermal evolution. We calculate the thermal and structural evolution of a 10 M⊕ planet formed by pebble accretion, taking into account material redistribution from silicate rainout (condensation and settling) and from convective mixing. We find that the duration of the rainout in sub-Neptunes is on an ∼Gyr timescale and varies with envelope mass: planets with envelopes below ∼0.75 M⊕ rain out into a core-envelope structure in less than 1 Gyr, while planets in excess of 0.75 M⊕ of H,He preserve some of their envelope pollution for billions of years. The energy released by the rainout inflates the radius with respect to planets that start out from a plain core-envelope structure. This inflation would result in estimates of the H,He contents of observed exoplanets based on the standard core-envelope structure to be too high. We identify a number of planets in the exoplanet census where rainout processes may be at work, plausibly resulting in their H,He contents to be overestimated by up to a factor two. Future accurate age measurements by the PLATO mission may allow for the identification of planets formed with polluted envelopes.

Oxygen isotope variability in precipitation, dripwater, and modern calcite responding to ENSO based on 11 years’ monitoring in Yuhua Cave, Southeast China

The scientific explanation of cave δ18O in the East Asian Summer Monsoon (EASM) region is a crucial issue restricting stalagmite-inferred paleoclimate research. The connection between El Niño Southern Oscillation (ENSO) and monsoonal precipitation/cave δ18O and the mechanism involved also remain contentious. Long-term cave monitoring is an effective solution to deal with these issues. Here, we present 11-year-long precipitation, dripwater, and modern calcite δ18O data from Yuhua Cave in Southeast China, located in the frontal zone affected by the EASM. The upstream convection and rainout processes mainly regulate seasonal and interannual variations in precipitation δ18O (δ18Op). The δ18Op series shows a strong response to EASM activities associated with the Intertropical Convergence Zone migrations and the activities of the West Pacific Subtropical High affected by ENSO activities. Due to the influence of the epikarst reservoir mixing effect, the seasonal variation of dripwater δ18O (δ18Od) is irregular, and its amplitude is much smaller than that of δ18Op. The δ18Od can reflect integrated δ18Op changes within the residence time of rainwater in the karst reservoir and can inherit the ENSO signal in δ18Op. The inferred recharge models for the Yuhua drip sites reveal differences in the response to ENSO in terms of both timing and amplitude, which can be attributed to variations in the residence time of rainwater and the mixing ratio of new and old water within the karst aquifer. Higher rainfall would lead to a more sensitive response of δ18Od to ENSO activities, with a shorter time lag and greater amplitude. The modern calcite δ18O (δ18Oc) series show significant seasonal variations controlled by the fractionation coefficient varied with cave temperature. On the inter-annul time scale, the change of δ18Oc is mainly controlled by δ18Od and can record the ENSO-related variations. Our findings imply that high-resolution stalagmite δ18O reconstructions from Yuhua Cave may be able to identify historical ENSO-related variability in the EASM on annual to decade time scales.

Atmospheric processes control the stable isotopic variability of precipitation in the middle–lower reaches of the Yangtze River Basin, East Asian monsoon region

Paleoclimatic records in the East Asian monsoon region (EAM) depend on precipitation isotopes as proxies for past hydroclimatic variability. However, the potential mechanisms controlling precipitation isotopic variability in this region remain poorly understood. This study collected daily precipitation isotopes in the middle-lower reaches of the Yangtze River Basin from 2017 to 2020 to explore how convective behaviors and moisture sources determining the variability of precipitation stable isotopes. Daily precipitation δ18O presented a weak inverse relationship with local precipitation amount and air temperature. Results of correlation analysis showed that the maximum correlation zones of precipitation δ18O between different cloud levels and outgoing long wave radiation were identified in the upstream regions such as the Indian Peninsula, BoB, and Indo-China Peninsula during the rainy season. Back-trajectory analysis showed that substantial water vapor from maritine sources in the Indian Ocean and the Bay of Bengal (BoB) largely contributed to the lower δ18O values during the rainy season (−7.5 ‰) because of intense rainout processes in the upstream sourced regions, demonstrated by significant regional correlation between precipitation δ18O and regional precipitation amount. The regional correlations were stronger in the upstream regions than the local influences, indicating that precipitation isotopes in this region are more influenced by regional convective processes in the upstream region during the rainy season. Using the modified Stewart model, the mean subcloud evaporation fraction estimated was greater in rainy season (24.1 % vs 36.1 % for HK and HZ, respectively) than that in dry season (15.1 % vs 19.9 % for HK and HZ, respectively). These findings highlight the importance of upstream processes in controlling precipitation isotopes in the EAM region and provide reliable evidence on the interpretation of monsoonal climate variability.

Spatial and Temporal Variations of Stable Isotopes in Precipitation in the Mountainous Region, North Hesse

Patterns of stable isotopes of water (18O and 2H) in precipitation have been used as tracers for analyzing environmental processes which can be changed by factors such as the topography or meteorological variables. In this study, we investigated the isotopic data in precipitation for one year in the low mountain range of North Hesse, Germany, and analyzed mainly for altitude, rainfall amount, and air temperature effects on a regional scale. The results indicate that the isotopic composition expressed an altitude effect with a gradient of −0.14‰/100 m for δ18O, −0.28‰/100 m for δ2H and 0.83‰/100 m for Deuterium excess. Patterns of enrichment during warmer months and depletion during colder months were detected. Seasonal correlations were not consistent because the altitude effect was superimposed by other processes such as amount and temperature effects, vapor origins, orographic rainout processes, moisture recycling, and sub-cloud secondary evaporation. Precipitation was mostly affected by secondary evaporation and mixing processes during the summer while depleted moisture-bearing fronts and condensation were more responsible for isotope depletion during winter. In autumn and spring, the amount effect was more prominent in combination with moisture recycling, and large-scale convective processes. The altitude effect was also detected in surface water. The investigated elevation transect with multiple stations provided unique insights into hydrological and climatic processes of North Hesse on a regional scale. The spatial heterogeneity and mixing of different processes suggest that multiple rainfall stations are required when rainfall isotopes serve as forcing data for hydrological applications such as transit time assessments in complex terrains.

Determining key upstream convection and rainout zones affecting δ18O in water vapor and precipitation based on 10-year continuous observations in the East Asian Monsoon region

An increasing number of studies have recognized the crucial impact of upstream convective activities and rainout processes on the stable isotopic composition of precipitation (δ18Op) and water vapor (δ18Ov) at mid and low latitudes. However, it is difficult to precisely identify the upstream convection and rainout zones using the common method based on the spatial distribution of time-lagged temporal correlation. Using a 10-year continuous high-resolution δ18Ov and δ18Op dataset at Nanjing (southeast China), the longest record of this kind, we develop an improved time-lagged correlation method for Key Upstream Convection Zones Identification (KUCZI). Utilizing this method, we find that summer δ18Ov and δ18Op at Nanjing are primarily controlled by the convective activities and rainout processes along the moisture transport pathway from the Maritime Continent (MC), via the Indo-China Peninsula and South China Sea (ICP_SCS), to Southeast China (SEC), particularly over SEC. Contrary to the conclusion of many existing studies, there is a possibility that the Indian Ocean is not a major convection zone affecting δ18Op and δ18Ov at Nanjing. Our results indicate that we may need to reconsider the role of the Indian Ocean on the paleoclimate interpretation of stalagmite δ18O records in the East Asian Monsoon (EAM) region.

Hydrometeorological Processes and Moisture Sources in the Northeastern Tibetan Plateau: Insights from a 7-Yr Study on Precipitation Isotopes

Abstract The northeastern Tibetan Plateau is located in a climatic junction, which is considered an ideal region to explore the interactions between the summer monsoons and the westerly circulation patterns. However, to date, the needed long-term precipitation-based isotopic dataset is too limited to predict the interactions and patterns. This paper presents an evaluation of hydrometeorological processes and climate dynamics in the northeastern Tibetan Plateau based on a 7-yr precipitation isotope dataset covering the summer monsoon periods from 2012 to 2018. Results illustrated remarkable seasonal isotopic variability, characterized by lower δ18O and δ2H values in June with an average of −10‰ and −66.7‰, respectively. Higher δ18O and δ2H values in July averaged −6.7‰ and −39.5‰, respectively. This clear isotopic variability is largely related to seasonal changes of moisture sources and hydrometeorological processes. These precipitation isotopic values were primarily determined by the amount of precipitation, relative humidity, and convective activity, but showed no correlation with air temperature. Backward trajectory model results showed that Xinjiang, northern China, the Arctic, central Asia, and the South China Sea (SCS) were the primary sources of precipitation for the study site with varying seasonal contributions. The maritime moisture source of the SCS primarily resulted in the lowest precipitation δ18O values during the prevailing summer monsoon, which is mainly as a result of the strong convective activity and rainout processes along the air trajectory. The higher average deuterium excess (d-excess) value of precipitation in September indicated continental sources from central Asia (e.g., 75.4%) as land vapor recycling increases d-excess concentration in the atmosphere. These findings provide further insights into the main factors of precipitation isotopic variability related to atmospheric processes along the trajectory and the relevant factors in the monsoon regions. Significance Statement Recently, scientists and policy makers have become aware that Tibetan hydroclimate variability provides evidence of changes in regional and global circulation patterns that may result in the intensification of climate-driven extremes. However, these studies largely depend on crucial paleoclimate records of past precipitation isotopes in monsoon regions, which contain great uncertainties because of the complex relationship between climatic variability and precipitation isotopes. This study first presented a 7-yr isotopic dataset to understand the hydrological processes and climate dynamics controlling the isotopic variability in the northeastern Tibetan Plateau. The findings reveal important factors on the isotopic variability associated with atmospheric processes and their key climatic variables, which can enhance our interpretation of the paleoclimate records in monsoon regions.

South American Summer Monsoon variability over the last millennium in paleoclimate records and isotope-enabled climate models

Abstract. The South American Summer Monsoon (SASM) is the main driver of regional hydroclimate variability across tropical and subtropical South America. It is best recorded on paleoclimatic timescales by stable oxygen isotope proxies, which are more spatially representative of regional hydroclimate than proxies for local precipitation alone. Network studies of proxies that can isolate regional influences lend particular insight into various environmental characteristics that modulate hydroclimate, such as atmospheric circulation variability and changes in the regional energy budget as well as understanding the climate system sensitivity to external forcings. We extract the coherent modes of variability of the SASM over the last millennium (LM) using a Monte Carlo empirical orthogonal function (MCEOF) decomposition of 14 δ18O proxy records and compare them with modes decomposed from isotope-enabled climate model data. The two leading modes reflect the isotopic variability associated with (1) thermodynamic changes driving the upper-tropospheric monsoon circulation (Bolivian High–Nordeste Low waveguide) and (2) the latitudinal displacement of the South Atlantic Convergence Zone (SACZ). The spatial characteristics of these modes appear to be robust features of the LM hydroclimate over South America and are reproduced both in the proxy data and in isotope-enabled climate models, regardless of the nature of the model-imposed external forcing. The proxy data document that the SASM was characterized by considerable temporal variability throughout the LM, with significant departures from the mean state during both the Medieval Climate Anomaly (MCA) and the Little Ice Age (LIA). Model analyses during these periods suggest that the local isotopic composition of precipitation is primarily a reflection of upstream rainout processes associated with monsoon convection. Model and proxy data both point to an intensification of the monsoon during the LIA over the central and western parts of tropical South America and indicate a displacement of the South Atlantic Convergence Zone (SACZ) to the southwest. These centennial-scale changes in monsoon intensity over the LM are underestimated in climate models, complicating the attribution of changes on these timescales to specific forcings and pointing toward areas of important model development.

Moisture Sources and Climatic Controls of Precipitation Stable Isotopes Over the Tibetan Plateau in Water‐Tagging Simulations

AbstractUnderstanding the climate controls of precipitation δ18O in the Tibetan Plateau (TP) is crucial for paleoclimate reconstructions from a wealth of regional archives. We use the outputs of iCAM5 model to quantify the different moisture contribution to local precipitation δ18O and to understand the climate controls of precipitation δ18O in the TP based on water‐tagging. The model shows some deficiencies in simulating the spatial and temporal characteristics of precipitation δ18O and the local climatic controls across the TP. Among all the tagged source regions, South Asia and the Indian Ocean contribute the most to the precipitation δ18O in the monsoon‐controlled domain, followed by the East Asia source region. The westerlies are identified as major moisture sources to the precipitation δ18O in the westerlies‐controlled domain. South Asia and the Indian Ocean also contribute substantially for the westerlies‐controlled domain. On interannual time scales, summer precipitation δ18O in the monsoon‐controlled domain is dominated by rainout processes occurring along the moisture transport pathway, indicating that precipitation δ18O variations here potentially record changes in the regional upstream convection. The δ18O signal can be altered by changes in the moisture source location, which implies that enhanced moisture delivery from remote source regions leads to more negative precipitation δ18O due to an increase in the rainout effect during transport. Our results have implications for the interpretation of past variations of archives with precipitation stable isotopes, such as ice cores, tree rings, lake sediments, and speleothems in the TP and surrounding regions.

ITCZ precipitation and cloud cover excursions control Cedrela nebulosa tree-ring oxygen and carbon isotopes in the northwestern Amazon

The lack of delimitation between the South American Monsoon System (SAMS) and the Intertropical Convergence Zone (ITCZ) has led to problematic calibration of archives in paleoclimate studies, particularly in northern South America. We show for the first time recorded in a paleoclimate archive that the ITCZ is the primary controller of oxygen (δ18OTR) and carbon (δ13CTR) isotopes in Cedrela nebulosa tree-rings (1864–2018). In contrast, a monsoonal pattern is not observed at this latitude. Spatial correlations revealed that δ18OTR better reflects months of higher precipitation (Mar–Jun) in the western Amazon than local rainout processes at decadal time scale, owing to the strong convection in the basin at this time of the year. Similarly, this study identified cloud cover as a vital controller in sunshine duration, which influences the phenology of Cedrela nebulosa. We interpret the variability of the δ13CTR as an enhancement in the photosynthetic rate during light-increased months (Jul–Sep), strongly regulated by cloudiness reduction when the ITCZ rain band retreats to northwestern South America in austral winter. Overall, these results reveal that Cedrela nebulosa is well-adapted to wet environments, and its cellulose-based stable isotopic signals reflect the direct influence of the ITCZ excursions from austral autumn to winter (Mar–Sep) with minimal SAMS control.

Temporal Variations of Stable Isotopes in Precipitation from Yungui Plateau: Insights from Moisture Source and Rainout Effect

Abstract Long-term continuous monitoring of precipitation isotopes has great potential to advance our understanding of hydrometeorological processes that determine stable isotope variability in the monsoon regions. This study presents 4-yr daily precipitation isotopes from Yungui Plateau in southwestern China that are influenced by Indian summer monsoon and East Asian monsoon. The local meteoric water line (LMWL; δ2H = 8.12δ18O + 11.2) was first established at the Tengchong (TC) site, which was close to the global meteoric water line (GMWL; δ2H = 8δ18O + 10), indicating little secondary subcloud evaporation in the falling rain. Precipitation δ18O values exhibited significant inverse relationships with precipitation amount (r = −0.42), air temperature (r = −0.43), and relative humidity (r = −0.41) with lower correlation coefficients throughout the entire period, which indicated that precipitation isotopic variability in TC could not be well explained by the local meteorological factors but influenced by other combined factors of regional precipitation amount and upstream rainout. Precipitation δ18O values showed a clear V-shaped trend throughout the observation period, characterized by higher δ18O values during the premonsoon period whereas lower values during the postmonsoon period. This seasonal variation of precipitation δ18O values was associated with the seasonal movement of the intertropical convergence zone and seasonal changes in moisture transport. Combined with backward trajectory analysis, precipitation δ18O values were estimated by a Rayleigh distillation model showing that upstream rainout processes from the Bay of Bengal (BoB) toward land (Myanmar) and recycling moisture over land were key factors affecting the isotopic compositions of the TC precipitation. These findings could enhance our understanding of atmospheric dynamics and moisture source in the monsoon regions and will potentially facilitate the interpretation of numerous isotopic proxy records from this region. SIGNIFICANCE STATEMENT The variability of the summer monsoon and its onset, duration, and failure directly determine the strong rainfall and drought in a given region and have great impacts on regional societies and agriculture. To better understand this variability, this study presented a 4-yr daily dataset of precipitation isotopes on the Yungui Plateau of southwestern China to explore atmospheric processes and moisture sources that drive isotopic variability in this region. Precipitation δ18O exhibited remarkably seasonal variability, with higher values in premonsoon period and lower values in the postmonsoon period. During the Indian summer monsoon period, moisture sources primarily originated from the BoB toward the TC site, experiencing rainout processes and local moisture recycling over land using a Rayleigh fractionation model. These findings shed new light on the temporal variations of precipitation stable isotopes and facilitate our understanding of hydrological cycle in the monsoon regions.

Uncertainty in Aerosol Rainout Processes through the Case of the Radioactive Materials Emitted by the Fukushima Dai-ichi Nuclear Power Plant in March 2011

The process of an aerosol rainout in wet deposition induces large uncertainties among atmospheric aerosol simulations, especially for particles in the fine mode. In this study, we performed an intercomparison study of four different rainout schemes on the model (the nonhydrostatic icosahedral atmospheric model or NICAM) to simulate particulate Cs-137 in the emission scenario of the March 2011 accident at the Fukushima Dai-ichi Nuclear Power Plant. The schemes include global climate models (GCMs) approach with a simple tuning parameter to determine the scavenging coefficient, and another optimized for cloud resolving models (CRMs) to account for prognostic precipitation and realistic vertical transport. The third approach was the conventional method under the assumption of a pseudo-first-order approximation based on the surface precipitation flux. The fourth approach was involved in offline chemical transport models (CTMs) with a simplified parametric analysis approach to clouds and precipitation flux. In most experiments, statistical metrics of the Cs-137 concentrations using in-situ measurements were calculated to be within ±30 % (bias), 0.6–0.9 (correlation), 67–112 Bq m−3 (uncertainty), and < 40 % (precision within a factor of 10). The CRM-type method yielded the best results but required a lower limit of tuning parameters to compensate for the results. Both the GCM-type and the conventional methods were also useful by setting proper tuning parameters. The CTM-type yielded better correlation and lower uncertainty but larger negative bias. These analyses suggest the overestimation of the conversion rate from cloud droplets into raindrops by the NICAM. However, this cannot be resolved by simply interchanging cloud microphysics schemes. It was found that the sensitivity of the rainout scheme has a stronger influence on the Cs-137 concentration than the different treatments of cloud microphysics. Thus, to replicate the observed Cs-137 distribution, it is essential to have a better meteorological field as well as a proper rainout scheme.

Variability and Controls on δ18O, d‐excess, and ∆′17O in Southern Peruvian Precipitation

AbstractThe isotopic composition of precipitation is used to trace water cycling and climate change, but interpretations of the environmental information recorded in central Andean precipitation isotope ratios are hindered by a lack of multi‐year records, poor spatial distribution of observations, and a predominant focus on Rayleigh distillation. To better understand isotopic variability in central Andean precipitation, we present a three‐year record of semimonthly δ18Op and δ2Hp values from 15 stations in southern Peru and triple oxygen isotope data, expressed as ∆′17Op, from 32 precipitation samples. Consistent with previous work, we find that elevation correlates negatively with δ18Op and that seasonal δ18Op variations are related to upstream rainout and local convection. Spatial δ18Op variations and atmospheric back trajectories show that both eastern‐ and western‐derived air masses bring precipitation to southern Peru. Seasonal d‐excessp cycles record moisture recycling and relative humidity at remote moisture sources, and both d‐excessp and ∆′17Op clearly differentiate evaporated and non‐evaporated samples. These results begin to establish the natural range of unevaporated ∆′17Op values in the central Andes and set the foundation for future paleoclimate and paleoaltimetry studies in the region. This study highlights the hydrologic understanding that comes from a combination of δ18Op, d‐excessp, and ∆′17Op data and helps identify the evaporation, recycling, and rainout processes that drive water cycling in the central Andes.

Distinguishing in-cloud and below-cloud short and distal N-sources from high-temporal resolution seasonal nitrate and ammonium deposition in Vienna, Austria

Reactive nitrogen (Nr: nitrate and ammonium) washout in Vienna (Austria) precipitation events were investigated in 2019. A total of 958 samples from 61 rain events representing >90% of annual precipitation were collected at 5–30 min intervals for nitrate (NO3−) and ammonium (NH4+) analyses and meteorological information. The data revealed systematic seasonal concentration variations for all Nr-species and a clear influence of rush-hour traffic on the kinetics of N-scavenging processes. The monthly nitrate and ammonium deposition was 0.69 ± 0.21 kg ha−1 month−1 and 1.02 ± 0.30 kg ha−1 month−1, respectively. Around 30% of nitrate and 20% of ammonium was dry deposition, and ∼30% of each N-species was from distal sources associated with rainout processes. The half-life of below-cloud N-species were similar in the warmer seasons (1.7 ± 0.2 h and 2.3 ± 0.4 h for nitrate and ammonium). In winter, the ammonium half-life was significantly lower (1.4 h). Nr removal by wet-only in-cloud scavenging was slower than predicted by empirical models. HYSPLIT trajectory analysis revealed that Nr rainout from distal sources in spring had no prevailing direction, but higher Nr contributions were from N and W. In summer and winter, air masses from W, SW and SE were related to intense, medium, and low Nr contributions, respectively. The origin and path of these trajectories coincided with known NOx hotspots in Europe.

Stable isotopes of atmospheric water vapour and precipitation in the northeast Qinghai‐Tibetan Plateau

AbstractStable water isotopes (δ18O and δ2H) are an important source signature for understanding the hydrological cycle and altered climate regimes. However, the mechanisms underlying atmospheric water vapour isotopes in the northeast Qinghai‐Tibetan Plateau of central Asia remain poorly understood. This study initially investigated water vapour isotopic composition and its controls during the premonsoon and monsoon seasons. Isotopic compositions of water vapour and precipitation exhibited high variability across seasons, with the most negative average δ18O values of precipitation and the most positive δ18O values of water vapour found during the premonsoon periods. Temperature effect was significant during the premonsoon period but not the monsoon period. Both a higher slope and intercept of the local meteoric water line were found during the monsoon period as compared with in the premonsoon period, suggesting that raindrops have been experienced a greater kinetic fractionation process such as reevaporation below the cloud during the premonsoon periods. The δ2H and δ18O signatures in atmospheric water vapour tended to be depleted with the occurrence of precipitation events especially during the monsoon period and probably as a result of rainout processes. The monthly average contribution of evaporation from the lake to local precipitation was 35.2%. High d‐excess values of water vapour were influenced by the high proportion of local moisture mixing, as indicated by the gradually increasing relative humidity along westerly and Asian monsoon trajectories. The daily observation (observed ε) showed deviations from the equilibrium fractionation factors (calculated ε), implying that raindrops experienced substantial evaporative enrichment during their descent. The average fraction of raindrops reevaporation was estimated to be 16.4± 12.9%. These findings provide useful insights for understanding the interaction between water vapour and precipitation, moisture sources, and help in reconstructing the paleoclimate in the alpine regions.

Variation of δ18O in precipitation and its response to upstream atmospheric convection and rainout: A case study of Changsha station, south-central China

The results of analyses of the stable isotopes of oxygen in precipitation (δ18Op) are presented for every rainfall event from January 2010 to December 2017 in Changsha, south-central China. Our aims were to elucidate the variations of δ18Op on different timescales and to identify the main meteorological drivers of variations in the oxygen isotopic composition of precipitation. Results showed that there were no statistically significant and consistent negative correlations between δ18Op and local precipitation amount on either daily or monthly timescale; however, changes in δ18Op in Changsha responded sensitively to the variation of precipitation in the key upstream area along air mass trajectories. Year-to-year, the strongest negative lagged correlations (r’) between δ18Op and the preceding average precipitation amount varied from −0.79 to −0.63 (all significant at the 0.001 level) in the warm half-year (from April to September) of 2010–2017. However, in the cold half-year (from October to the following March), corresponding r’ values varied from −0.79 to −0.38 that were all significant at the 0.001 level, except for the year 2012. These findings suggest that the amplitude of the isotopic signal was closely linked with the position and intensity of upstream rainout activity. This was supported by strong relationships between precipitation-weighted mean δ18O (δ18Ow) and average precipitation amount in the key upstream area at the monthly scale; correlation coefficients were − 0.76 and − 0.57 between 2010 and 2017 in the warm half-year and cold half-year, respectively (both significant at the 0.001 level). Results advance our understanding of the temporal variation of the stable oxygen isotopic composition of precipitation, and demonstrate that local isotopic proxy records may be influenced by upstream rainout processes.

Stability of aqueous formaldehyde under γ irradiation: prebiotic relevance

AbstractFormaldehyde is a precursor of sugars, which are compounds essential in all forms of life and a necessary molecule for prebiotic processes. This work focuses on evaluating the stability of formaldehyde exposed to a high radiation field simulating prebiotic conditions on primitive Earth, such as the ocean or shallow waters. Formaldehyde may have been formed from reactions in the atmosphere and from rainout processes reached water bodies. In our experiments, we employed γ radiation and found that formaldehyde was labile towards radiation and decomposed even at low irradiation doses due to the fact that aldehyde/hydrate groups present in formaldehyde structure are very reactive under irradiation. However, after exposing this molecule to several doses of irradiation, we detected the formation of formic acid and glycolaldehyde – both of which are of prebiotic interest. We also observed formaldehyde regeneration by one of its radiolytic products: formic acid.

Investigation of washout and rainout processes in sequential rain samples

In this study rainfall events were sampled sequentially between April and June 2014 (one event in April, two in May and one in June 2014) in the city center of Bolu province located in the western Black Sea Region of Turkey. Particle size distributions, pHs, major ion compositions and fractional distributions, neutralization factors and washout, rainout identifications were investigated by evaluating the data obtained from the sequences of the rain events. Local wind directions and upper atmospheric three dimensional back trajectory calculations were also used to identify the source regions affecting the receptor site. The primary purposes of this study were to present the effectiveness of washout and rainout processes and to identify the contributions of local and distant source regions to the rainfall events affecting the north western part of Turkey. The average pH values for the subsamples were determined as 5.20 ± 0.47, 5.51 ± 0.21, 6.97 ± 0.23 (6.66–7.20) and 6.42 ± 0.16 (6.17–6.66), respectively for the four rain events sampled. Ninety percent of the particles were smaller than 1135 μm for the first fraction, 444,5 μm for the second fraction, and 0.79 μm for the third fraction, respectively. Except for the fourth fraction, sequences of the rain event showed a linear decrease in particle sizes. The highest rainout to washout ratios were observed for nitrate ion (72.4%) and followed by ammonium ion (63.5%), chloride (48.4%), sulfate (46.3%), magnesium ion (41.5%), and potassium ion (31.2%). Strong correlations of nitrate and sulfate with Ca2+, Mg2+ and Na+ suggested that the rain droplets were neutralized by these alkaline ions.

Stable isotope composition of precipitation at different elevations in the monsoon marginal zone

The Qilian Mountains in western China are an important ecological and security barrier and are the primary water source for inland river basins. In this study, we collected precipitation samples and meteorological data from 4 sampling sites at different elevations on the northern slope of the Qilian Mountains from October 2016 to October 2017. The purpose was to analyse spatial and temporal variations of the isotope composition of precipitation and to evaluate the influence of temperature, elevation, monsoon circulation and continuous rainout processes on the measured values. The results show that the isotopic composition exhibit obvious seasonal variations; the higher δ18O values occur in summer and autumn, and the lower values occur in winter and spring, while the lower d-excess values occur in spring and summer and the higher d-excess values occur in autumn and winter. Stable isotope values varied with elevation. δ18O and δD values decreased with increasing elevation, and d-excess increased with increasing elevation. The average annual elevation gradient for δ18O was −0.26‰/100 m, δD was −1.77‰/100 m, and d-excess was 3‰/100 m, and the elevation gradient in summer was higher than in winter. The slope and intercept of LMWL increased with increasing elevation. The elevation gradients for slopes and intercepts are 0.07/100 m and 1/100 m, respectively. The temperature effect is most significant below 0 °C. Local moisture recycling and sub-cloud evaporation also play an important role. Moisture sources are mainly controlled by westerly circulation. Influenced by moisture from the Asian monsoon, the δ18O and δD values of precipitation show a weak “precipitation effect” in July and August, and the values gradually become more negative with continued rainout. This study improves knowledge of the isotope evolution of precipitation in the Qilian mountains, and lays the foundation for further research on isotope hydrology in cold and arid regions.

Effects of transport patterns on chemical composition of sequential rain samples: trajectory clustering and principal component analysis approach

The chemical composition and long-range transportation (LRT) of rain events were assessed in this study. For this purpose, a fully automated wet-only sequential sampler was used to differentiate between washout and rainout processes. The chemical composition of elements (Al, As, Ca, Cd, Co, Cr, Cu, Fe, K, Mg, Mn, Na, Ni, Pb, V, and Zn) and ions (F−, Cl−, NO3 −, SO4 −2, and NH4 +) were quantified in 172 rainwater samples. Cluster analysis (CA) statistical approach was used to classify the back trajectories of the rain events. The CA revealed a seven-cluster solution which provided better explanations for the effects of possible source regions on the receptor site. Consequently, principal component analysis (PCA) was conducted on the normalized cluster-based mean concentrations of the chemical species in order to statistically identify the similarities among the clusters. In conclusion, there were four main sources which strongly affected the chemical composition of precipitation in the study area namely: (i) anthropogenic pollutants from Southwestern and Eastern Europe, (ii) Saharan dust intrusion from Northern Africa, (iii) resuspension of crustal material from nearby regions, and (iv) marine aerosols from Mediterranean and the Black Sea. The proposed methodology combining trajectory cluster analysis, chemical analysis, and principal component analysis was satisfactory to identify the source regions of the trajectories carrying the observed pollutants to the study area.