Stratospheric air intrusions promote global-scale new particle formation

New particle formation in the free troposphere is a major source of cloud condensation nuclei globally. The prevailing view is that in the free troposphere, new particles are formed predominantly in convective cloud outflows. We present another mechanism using global observations. We find that during stratospheric air intrusion events, the mixing of descending ozone-rich stratospheric air with more moist free tropospheric background results in elevated hydroxyl radical (OH) concentrations. Such mixing is most prevalent near the tropopause where the sulfur dioxide (SO2) mixing ratios are high. The combination of elevated SO2 and OH levels leads to enhanced sulfuric acid concentrations, promoting particle formation. Such new particle formation occurs frequently and over large geographic regions, representing an important particle source in the midlatitude free troposphere.

Traceability and policy suggestions for ozone pollution in heavy industrial city in Northeast China

In the heavy industrial city of Northeast China, there has been a significant decrease in particulate matter pollution while experiencing a sharp increase in ozone (O3) pollution. However, the main influencing factors and source contributions to O3 remain unclear. Taking the case of Siping as an example, this study analyzed the spatiotemporal characteristics, assessed local source contributions to O3, and revealed regional transmission effects using numeric simulation and statistical methods. Temporally, higher O3 concentrations were observed in summer and the afternoon, with hourly peaks up to 254 µg/m3. Spatially, O3 pollution was mainly contributed by background concentrations (34.52%), external transport (34.50%), and local emissions (30.98%) during the case study period (June 11-18, 2021). Among the local emission sources, biological emissions, the industrial sector, and the traffic sector accounted for 35.30%, 32.09%, and 23.58% of the O3 concentration, respectively. For regional atmospheric transmission, high O3 pollution was accompanied by wind from the southwest directions, and the trajectory of air mass transport suggests that eastern Mongolia, the Korean Peninsula, and its neighboring regions contribute to O3 pollution. Furthermore, sensitivity analysis showed that O3 pollution in Siping is a co-controlled region by anthropogenic volatile organic compounds (AVOCs) and NOX, which implies control in an optimal ratio of VOCs and NOX emissions. Thus, our results highlight the importance of joint prevention and control of O3 pollution in the region, optimization of biogenic landscape ecology, and control of VOCs and NOx in both the industrial and transport sectors.

Does Ozone Pollution Share the Same Formation Mechanisms in the Bay Areas of China?

As important regions of transition between land and sea, the three bay areas of Bohai Bay (BHB), Hangzhou Bay (HZB), and Pearl River Estuary (PRE) in China often suffer from severe photochemical pollution despite scarce anthropogenic emissions. To understand the causes of high ozone (O₃) concentrations, the high O₃ episode days associated with special synoptic systems in the three bays were identified via observations and simulated by the weather research and forecasting coupled with community multiscale air quality (WRF-CMAQ) model. It was revealed that the interaction between synoptic winds and mesoscale breezes resulted in slow wind speeds over the HZB and PRE, where air pollutants transported from upwind cities gained a long residence time and subsequently participated in intensive photochemical reactions. The net O₃ production rates within the bay areas were even comparable to those in surrounding cities. This finding was also applicable to BHB but with lower net O₃ production rates, while high levels of background O₃ and the regional transport from farther upwind BHB partially elevated the O₃ concentrations. Hence, these three bay areas served as O₃ "pools" which caused the accumulation of air pollutants via atmospheric dynamics and subsequent intense photochemical reactions under certain meteorological conditions. The results may be applicable to other similar ecotones around the world.

Remarkable spring increase overwhelmed hard-earned autumn decrease in ozone pollution from 2005 to 2017 at a suburban site in Hong Kong, South China

Ozone (O₃) pollution has been a persistent problem in Hong Kong, particularly in autumn when severe O₃ pollution events are often observed. In this study, linear regression analyses of long-term O₃ data in suburban Hong Kong revealed that the variation of autumn O₃ obviously leveled off during 2005-2017, mainly due to the significant decrease of autumn O₃ in 2013-2017 (period II), despite the increase in 2005-2012 (period I). In addition, the rise of O₃ in summer and winter also ceased since 2013. In contrary, O₃ continuously increased throughout the spring of 2005-2017, especially in period II. Consequently, an incessant increase of overall O₃ was observed during 2005-2017. A statistical model combining Kolmogorov-Zurbenko filter with multiple linear regressions, and a photochemical box model incorporating CB05 mechanism were applied to probe the causes of the above trends. In general, O₃ production was controlled by VOC-limited regime throughout 13 years. The meteorological variability and regional transport facilitated the O₃ growth in period Ι. In contrast, the unchanged O₃ level in period II was attributable to the negative impact of meteorological variability and reduction of regional transport effect on O₃ formation and accumulation, as well as the negligible change in locally-produced O₃. In autumn of period II, the inhibitory meteorological variability, reduced regional transport, and alleviated local production were the driving force for the hard-earned decrease of O₃. However, the remarkable rise of spring O₃ was caused by the reduction of NO_x, especially in the spring of period II. The findings of the long-term and seasonal variations of O₃ pollution in Hong Kong are helpful for future O₃ mitigation.

Increased ozone pollution alongside reduced nitrogen dioxide concentrations during Vienna's first COVID-19 lockdown: Significance for air quality management

BACKGROUND

Lockdowns amid the COVID-19 pandemic have offered a real-world opportunity to better understand air quality responses to previously unseen anthropogenic emission reductions.

METHODS AND MAIN OBJECTIVE

This work examines the impact of Vienna's first lockdown on ground-level concentrations of nitrogen dioxide (NO2), ozone (O3) and total oxidant (Ox). The analysis runs over January to September 2020 and considers business as usual scenarios created with machine learning models to provide a baseline for robustly diagnosing lockdown-related air quality changes. Models were also developed to normalise the air pollutant time series, enabling facilitated intervention assessment.

CORE FINDINGS

NO2 concentrations were on average -20.1% [13.7-30.4%] lower during the lockdown. However, this benefit was offset by amplified O3 pollution of +8.5% [3.7-11.0%] in the same period. The consistency in the direction of change indicates that the NO2 reductions and O3 increases were ubiquitous over Vienna. Ox concentrations increased slightly by +4.3% [1.8-6.4%], suggesting that a significant part of the drops in NO2 was compensated by gains in O3. Accordingly, 82% of lockdown days with lowered NO2 were accompanied by 81% of days with amplified O3. The recovery shapes of the pollutant concentrations were depicted and discussed. The business as usual-related outcomes were broadly consistent with the patterns outlined by the normalised time series. These findings allowed to argue further that the detected changes in air quality were of anthropogenic and not of meteorological reason. Pollutant changes on the machine learning baseline revealed that the impact of the lockdown on urban air quality were lower than the raw measurements show. Besides, measured traffic drops in major Austrian roads were more significant for light-duty than for heavy-duty vehicles. It was also noted that the use of mobility reports based on cell phone movement as activity data can overestimate the reduction of emissions for the road transport sector, particularly for heavy-duty vehicles. As heavy-duty vehicles can make up a large fraction of the fleet emissions of nitrogen oxides, the change in the volume of these vehicles on the roads may be the main driver to explain the change in NO2 concentrations.

INTERPRETATION AND IMPLICATIONS

A probable future with emissions of volatile organic compounds (VOCs) dropping slower than emissions of nitrogen oxides could risk worsened urban O3 pollution under a VOC-limited photochemical regime. More holistic policies will be needed to achieve improved air quality levels across different regions and criteria pollutants.

Temporal Variability of Tropospheric Ozone Pollution in the Agricultural Region of Central-Eastern Poland

The aim of the study was to assess the temporal variability of tropospheric ozone pollution. The research was carried out for the agricultural region of central-eastern Poland, an area covering the Lublin Voivodeship. One-hour averages of automatic measurements of tropospheric ozone concentration in 2015–2017 were used for the study. The data were obtained from three measuring stations belonging to the Chief Inspectorate of Environmental Protection in Poland. The stations were located as part of the Air Quality Monitoring System in rural communes in the north-western, central and southern parts of the Lublin Voivodeship. Statistical analysis of the data showed that the tropospheric ozone concentrations were significantly dependent on weather conditions during the years of the study. At each monitoring station, the one-hour average O3 concentrations showed a clear structure over the course of the day: they were higher in the late morning and early afternoon than in the early morning and at night. The highest O3 concentrations were observed at the Florianka measurement station, located in Roztocze National Park. This area had high forest cover and was located at the highest elevation above sea level of the three measuring stations. In the light of climate change and increasing O3 concentrations, further scientific research on atmospheric air pollution is crucial, especially in agricultural areas associated with food production.

Quantification of the enhancement of PM2.5 concentration by the downward transport of ozone from the stratosphere

The downward transport of ozone (O3) stemming from the stratosphere-to-troposphere exchange (STE) can be a significant contributor to background O3. Such enhancement of background O3 may also influence ground-level PM2.5, particularly in polluted regions which have abundant precursor emissions. In this study, we quantified the STE impact on tropospheric O3 and its subsequent influence on surface PM2.5 across the northern hemisphere. The sensitivity analyses was conducted by using a comprehensive hemispheric atmospheric modeling system. Results suggest the surface PM2.5 concentration can be considerably enhanced by the STE in polluted regions including East China, East US, and Europe, mostly in winter and spring. In winter, the PM2.5 concentrations in East China, East US, and Europe are estimated to be enhanced by 1.3%, 3.5% and 5.5% due to the STE. The STE-enhanced PM2.5 concentrations are typically higher on high pollution days suggesting likely increasing contributions in regions with the growing pollution levels. During the heavy polluted days, the PM2.5 concentrations in East China can be enhanced by 2.289 μg/m3 in winter and 2.034 μg/m3 in spring due to the STE. The STE-enhanced PM2.5 also exhibits strong diurnal variations following a pattern similar to the total PM2.5 concentration, with high increasing ratio in the morning and low at afternoon, suggesting that the enhancement is most pronounced during peak pollution events. The STE-enhanced PM2.5 is exclusively contributed by the increase of nitrate, ammonium, and secondary organic aerosol which in-turn are strongly influenced by the atmospheric oxidation capacity.

Variations in Ozone Concentration over the Mid-Latitude Region Revealed by Ozonesonde Observations in Pohang, South Korea

Ozone absorbs harmful UV rays at high elevations but acts as a pollutant gas in the lower atmosphere. It is necessary to monitor both the vertical profile and the total column ozone. In this study, variations in the ozone concentration of Pohang were divided into three vertical layers: the stratospheric layer (STL), the second ozone peak layer (SOPL), and the tropospheric layer (TRL). Our results indicated that the ozone concentration in the STL, SOPL, TRL, and total column ozone increased by 0.45%, 2.64%, 5.26%, and 1.07% decade−1, respectively. The increase in the SOPL during springtime indicates that stratosphere–troposphere exchange is accelerating, while the increase during summertime appears to have been influenced by the lower layers. The growth of tropospheric ozone concentration is the result of both increased ozone precursors from industrialization in East Asia and the influx of stratospheric ozone. Our results reaffirmed the trend of ozone concentration in mid-latitudes of the northern hemisphere from vertical profiles in Pohang and, in particular, suggests that the recent changes of ozone in this region need to be carefully monitored.

Representing Organic Compound Oxidation in Chemical Mechanisms for Policy-Relevant Air Quality Models under Background Troposphere Conditions

This intercomparison has taken thirteen chemical mechanisms and compared how they treat VOC oxidation and degradation and its relationship to the photochemical formation of ozone and hydroxyl radicals. Here, we have looked in some detail at the incremental responses of hydroxyl radicals to incremental additions of a range of organic compounds under conditions appropriate to the background atmosphere. Most of the time, with most organic compounds and most chemical mechanisms, incremental additions of an organic compound led to depletion of hydroxyl radical concentrations. The chemical mechanisms studied demonstrated increasingly negative incremental hydroxyl radical reactivities with increasing carbon numbers for the alkanes ethane, propane and n-butane. Hydroxyl radical incremental reactivities for the simple alkenes, ethylene and propylene, were reasonably consistent across the chemical mechanisms studied. However, this consistent representation did not extend to trans but-2-ene, where reactivity estimates spanned a range of a factor of five. Incremental reactivities were reasonably well-defined for isoprene which was encouraging in view of its importance to background tropospheric chemistry. The most serious discrepancies emerging from this study were found with the aromatics toluene and o-xylene, and with the Master Chemical Mechanism and these are discussed in some detail.

Exploration of the heterogeneous effect of climate change on ozone concentration in an urban environment

Ozone is a significant causative agent of mortality in cities. Urban environments are expressly vulnerable to global warming because of the extensive emission of air pollutants with urban heat island effect enhancing much rapidly the ozone concentration than in the less urbanized regions. This effect previously was not studied in local scale. It was hypothesized that climate change will cause heterogenic increase of ozone concentration in the different parts of the cities. To study this effect, the near-surface ozone concentration of 10 points of a Hungarian city was measured and modeled. At first step, the local correlations between solar radiation, air temperature, relative humidity and the near surface ozone concentrations at 3 m height were determined, specifying the local ozone-producing conditions. Then, based on the scenario of the Intergovernmental Panel on Climate Change 5th assessment report, the future seasonal near-surface ozone concentrations were modeled. Based on the model, it was determined that climate change will result in a heterogenic increase of near-surface ozone concentration.

Spatio-temporal assessment and seasonal variation of tropospheric ozone in Pakistan during the last decade

This study uses the tropospheric ozone data derived from combined observations of Ozone Monitoring Instrument/Microwave Limb Sounder instruments by using the tropospheric ozone residual method. The main objective was to study the spatial distribution and temporal evolution in the troposphere ozone columns over Pakistan during the time period of 2004 to 2014. Results showed an overall increase of 3.2 ± 1.1 DU in tropospheric ozone columns over Pakistan. Spatial distribution showed enhanced ozone columns in the Punjab and southern Sindh consistent to high population, urbanization, and extensive anthropogenic activities, and exhibited statistically significant temporal increase. Seasonal variations in tropospheric ozone columns are driven by various factors such as seasonality in UV-B fluxes, seasonality in ozone precursor gases such as NO_x and volatile organic compounds (caused by temperature dependent biogenic emission) and agricultural fire activities in Pakistan. A strong correlation of 96% (r = 0.96) was found between fire events and tropospheric ozone columns in Pakistan.

Multi-year composite view of ozone enhancements and stratosphere-to-troposphere transport in dry intrusions of northern hemisphere extratropical cyclones

We examine the role of extratropical cyclones in stratosphere-to-troposphere (STT) exchange with cyclone-centric composites of O3 retrievals from the Microwave Limb Sounder (MLS) and the Tropospheric Emission Spectrometer (TES), contrasting them to composites obtained with the Modern-Era Retrospective-analysis for Research and Applications (MERRA and MERRA-2) reanalyses and the GEOS-Chem chemical transport model. We identify 15,978 extratropical cyclones in the northern hemisphere (NH) for 2005-2012. The lowermost stratosphere (261 hPa) and middle troposphere (424 hPa) composites feature a 1,000 km-wide O3 enhancement in the dry intrusion (DI) airstream to the southwest of the cyclone center, coinciding with a lowered tropopause, enhanced potential vorticity, and decreased H2O. MLS composites at 261 hPa show that the DI O3 enhancements reach a 210 ppbv maximum in April. At 424 hPa, TES composites display maximum O3 enhancements of 27 ppbv in May. The magnitude and seasonality of these enhancements are captured by MERRA and MERRA-2, but GEOS-Chem is a factor of two too low. The MERRA-2 composites show that the O3-rich DI forms a vertically aligned structure between 300 and 800 hPa, wrapping cyclonically with the warm conveyor belt. In winter and spring DIs, O3 is enhanced by 100 ppbv or 100-130% at 300 hPa, with significant enhancements below 500 hPa (6-20 ppbv or 15-30%). We estimate that extratropical cyclones result in a STT flux of 119±56 Tg O3 yr-1, accounting for 42±20 % of the NH extratropical O3 STT flux. The STT flux in cyclones displays a strong dependence on westerly 300 hPa wind speeds.

Climate change impacts on human health over Europe through its effect on air quality

This review examines the current literature on the effects of future emissions and climate change on particulate matter (PM) and O₃ air quality and on the consequent health impacts, with a focus on Europe. There is considerable literature on the effects of climate change on O₃ but fewer studies on the effects of climate change on PM concentrations. Under the latest Intergovernmental Panel on Climate Change (IPCC) 5th assessment report (AR5) Representative Concentration Pathways (RCPs), background O₃ entering Europe is expected to decrease under most scenarios due to higher water vapour concentrations in a warmer climate. However, under the extreme pathway RCP8.5 higher (more than double) methane (CH₄) abundances lead to increases in background O₃ that offset the O₃ decrease due to climate change especially for the 2100 period. Regionally, in polluted areas with high levels of nitrogen oxides (NO_x), elevated surface temperatures and humidities yield increases in surface O₃ - termed the O₃ climate penalty - especially in southern Europe. The O₃ response is larger for metrics that represent the higher end of the O₃ distribution, such as daily maximum O₃. Future changes in PM concentrations due to climate change are much less certain, although several recent studies also suggest a PM climate penalty due to high temperatures and humidity and reduced precipitation in northern mid-latitude land regions in 2100.A larger number of studies have examined both future climate and emissions changes under the RCP scenarios. Under these pathways the impact of emission changes on air quality out to the 2050s will be larger than that due to climate change, because of large reductions in emissions of O₃ and PM pollutant precursor emissions and the more limited climate change response itself. Climate change will also affect climate extreme events such as heatwaves. Air pollution episodes are associated with stagnation events and sometimes heat waves. Air quality during the 2003 heatwave over Europe has been examined in numerous studies and mechanisms for enhancing O₃ have been identified.There are few studies on health effects associated with climate change impacts alone on air quality, but these report higher O₃-related health burdens in polluted populated regions and greater PM_2.5 health burdens in these emission regions. Studies that examine the combined impacts of climate change and anthropogenic emissions change under the RCP scenarios report reductions in global and European premature O₃-respiratory related and PM mortalities arising from the large decreases in precursor emissions. Under RCP 8.5 the large increase in CH₄ leads to global and European excess O₃-respiratory related mortalities in 2100. For future health effects, besides uncertainty in future O₃ and particularly PM concentrations, there is also uncertainty in risk estimates such as effect modification by temperature on pollutant-response relationships and potential future adaptation that would alter exposure risk.

Temporal variation of (7)Be concentrations in atmosphere for 8y from 2000 at Yamagata, Japan: solar influence on the (7)Be time series

We have been continuously observing the daily (7)Be concentrations in surface air at Yamagata, Japan (38.25 degrees N, 140.35 degrees E) since 2000. The yearly profile of the (7)Be concentration indicates the variation in galactic cosmic rays owing to solar modulation. Over 8y, the (7)Be concentration, cosmic neutrons, and number of sunspots varied by 37.4%, 12.2%, and 92.8%, respectively. The influence of precipitation on the (7)Be variability was approximately 5%. Hence, the yearly (7)Be concentration was mainly varied by the solar modulation of the (7)Be production rates. Based on the production rates found in an EXPACS simulation, the observed variability indicates (7)Be transport from high latitudes. The daily (7)Be concentrations have two significant periodic components of 19d and 36d. The 36-d component implies a relationship between the sun's rotation and the vertical transport of air masses under quiet solar activity.

Background ozone in the southern Europe and Mediterranean area: influence of the transport processes

The troposphere is subject to continuous inputs, production and removal processes of ozone and its precursors from natural processes and human activities acting together within a very complex system. In order to assess the behaviour of background ozone in the Mediterranean area, a description of trends, seasonal and diurnal behaviours of free tropospheric ozone is provided. In the Mediterranean area and southern Europe the background tropospheric ozone concentration appears significantly affected by three main air mass transport processes: (i) transport of polluted air masses on regional and long-range scales, (ii) downward transport of stratospheric air masses, and (iii) transport of mineral dust from the Sahara desert. In this review of the literature of the last two decades, we present an overview of these phenomena, mainly monitored at high baseline mountain stations representative of background atmospheric conditions.

Surface ozone concentrations and ecosystem health: past trends and a guide to future projections

This paper reviews current understanding of the sources and sinks of ozone in the troposphere, recent studies of long-term trends, and the factors which have to be taken into consideration when constructing and interpreting future models of ozone concentration. The factors controlling surface O(3) concentrations are discussed initially to provide a basis for the ensuing discussion, followed by a summary of the evidence for recent trends in ground-level ozone concentrations, i.e. over the past 3 decades, which have shown a significant increase in the annual average in 'background' air typical of the unpolluted northern hemisphere. Closer to precursor sources, although urban winter concentrations have increased, rural peak spring and summer concentrations during ozone 'episodes' have decreased markedly in response to emissions reductions. In order to determine whether such trends are meaningful, the statistical techniques for determining temporal trends are reviewed. The possible causes of long-term trends in ozone are then discussed, with particular reference to the use of chemistry-transport models to interpret past trends. Such models are also used to make predictions of future trends in surface ozone concentrations, but few are comprehensive in integrating future climate changes with changes in land use and in emissions of ozone precursors. Guidance is given on the likely effects of climate/precursor/chemistry interactions so that model predictions can be judged.

The Met Office Hadley Centre climate modelling capability: the competing requirements for improved resolution, complexity and dealing with uncertainty

Predictions of future climate change require complex computer models of the climate system to represent the full range of processes and interactions that influence climate. The Met Office Hadley Centre uses 'families' of models as part of the Met Office Unified Model Framework to address different classes of problems. The HadGEM family is a suite of state-of-the-art global environment models that are used to reduce uncertainty and represent and predict complex feedbacks. The HadCM3 family is a suite of well established but cheaper models that are used for multiple simulations, for example, to quantify uncertainty or to test the impact of multiple emissions scenarios.

Climate/chemistry feedbacks and biogenic emissions

The oxidizing capacity of the atmosphere is affected by anthropogenic emissions and is projected to change in the future. Model calculations indicate that the change in surface ozone at some locations could be large and have significant implications for human health. The calculations depend on the precise scenarios used for the anthropogenic emissions and on the details of the feedback processes included in the model. One important factor is how natural biogenic emissions will change in the future. We carry out a sensitivity calculation to address the possible increase in isoprene emissions consequent on increased surface temperature in a future climate. The changes in ozone are significant but depend crucially on the background chemical regime. In these calculations, we find that increased isoprene will increase ozone in the Northern Hemisphere but decrease ozone in the tropics. We also consider the role of bromine compounds in tropospheric chemistry and consider cases where, in a future climate, the impact of bromine could change.

Impacts of climate change and variability on tropospheric ozone and its precursors

Two coupled climate-chemistry model experiments for the period 1990-2030 were conducted: one with a fixed climate and the other with a varying climate forced by the is92a scenario. By comparing results from these experiments we have attempted to identify changes and variations in physical climate that may have important influences upon tropospheric chemical composition. Climate variables considered include: temperature, humidity, convective mass fluxes, precipitation, and the large-scale circulation. Increases in humidity, directly related to increases in temperature, exert a major influence on the budgets of ozone and the hydroxyl radical: decreasing 03 and increasing OH. Warming enhances decomposition of PAN, releasing NOx, and increases the rate of methane oxidation. Surface warming enhances vegetation emissions of isoprene, an important ozone precursor. In the changed climate, tropical convection generally reduces, but penetrates to higher levels. Over northern continents, convection tends to increase. These changes in convection affect both vertical mixing and lightning NOx emissions. We find no global trend in lightning emissions, but significant changes in its distribution. Changes in precipitation and the large-scale circulation are less important for composition, at least in these experiments. Higher levels of the oxidants OH and H202 lead to increases in aerosol formation and concentrations. These results indicate that climate-chemistry feedbacks are dominantly negative (less 03, a shorter CH4 lifetime, and more aerosol). The major mode of inter-annual variability in the is92a climate experiment is ENSO. This strongly modulates isoprene emissions from vegetation via tropical land surface temperatures. ENSO is also clearly the dominant source of variability in tropical column ozone, mainly through changes in the distribution of convection. The magnitude of inter-annual variability in ozone is comparable to the changes brought about by emissions and climate changes between the 1990s and 2020s, suggesting that it will be difficult to disentangle the different components of near-future changes.

Plutonium from global fallout recorded in an ice core from the Belukha glacier, Siberian Altai

Ice cores from glaciers situated near anthropogenic sources of air pollution provide important archives of the emissions of species with short atmospheric lifetimes. Here we present the history of atmospheric Pu fallout reconstructed from an ice core from the Belukha glacier in the Siberian Altai. Fourteen ice core samples covering the time period 1941-1986 were selected for Pu analysis, chemically processed, and measured using accelerator mass spectrometry. The Pu concentration peaks in 1963, coinciding with the maximum of the nuclear weapons tests and in concordance with the 3H activity concentration peak. The shapes of the 239Pu and 3H profiles reflect two main periods of atmospheric nuclear test activity: premoratorium testing before 1958 and postmoratorium testing in 1961 and 1962. Premoratorium tests contribute about 45% of the integrated Pu inventory. The average 240Pu/239Pu isotopic ratio is 0.18 +/- 0.05, indicating that a large majority of the Pu in the Belukha glacier originates from global stratospheric fallout rather than from direct tropospheric input.