Baseline Observatory Research Articles

The decline in tropical land carbon sink drives high atmospheric CO2 growth rate in 2023

Abstract Atmospheric CO2 growth rate (CGR), reflecting the carbon balance between anthropogenic emissions and net uptake from land and ocean, largely determines the magnitude and speed of global warming. CGR at Mauna Loa Baseline Observatory reached record high in 2023. Here we quantified major components of global carbon balance for 2023, by developing a framework which integrated fossil fuel CO2 emissions data, an atmospheric inversion from Global ObservatioN-based system for monitoring Greenhouse GAses (GONGGA) with two artificial intelligence (AI) models derived from dynamic global vegetation models. We attributed the record high CGR increase in 2023 compared to 2022 primarily to the large decline in land carbon sink (1803 ± 197 TgC year−1), with minor contributions from a small reduction in ocean carbon sink (184 TgC year−1) and a slight increase in fossil fuel emissions (24 TgC year−1). At least 78% of the global decline in land carbon sink was contributed by the decline in tropical sink, with GONGGA inversion (1354 TgC year−1) and AI simulations (1578 ± 666 TgC year−1) showing similar declines in the tropics. We further linked this tropical decline to the detrimental impact of El Niño-induced anomalous warming and drying on vegetation productivity in water-limited Sahel and southern Africa. Our successful attribution of CGR increase within the framework combining atmospheric inversion with AI simulations enabled near-real-time tracking of the global carbon budget which had a one-year reporting lag.

Impact of Anthropogenic Aerosol Transport on Cloud Condensation Nuclei Activity During Summertime in Qilian Mountain, in the Northern Tibetan Plateau

AbstractIn this study, we conducted field campaigns at two mountain‐top observatories on the Qilian Mountains (QLM) in the northeastern Tibetan Plateau of China, which plays a vital role in sustaining water resources for the downstream arid and semi‐arid regions. The two observatories, Waliguan Baseline Observatory (WLG) and Laohugou station (LHG), are situated on the eastern and western edge of the QLM, respectively. The objectives of this study were to examine the properties of atmospheric aerosols, CCN concentrations (NCCN) at varying supersaturation levels (SS = 0.2%–1.0%), and the hygroscopic nature of aerosols in the QLM, especially during the formation of orographic cloud. Notably, the average aerosol concentration and NCCN was approximately 2–3 factors higher at WLG compared to LHG. The chemical compositions of aerosols were primarily dominated by sulfate and organic aerosol (OA) at these two sites. A very high hygroscopicity parameter of aerosol calculated using chemical composition was observed at these two sites (0.37 ± 0.03 at LHG vs. 0.30 ± 0.04 at WLG). Enhanced aerosol loading episodes impacted by anthropogenic emission were observed at these two sites. Exploration on high‐loading cases at each site, we found that the CCN activity was dominated by aerosol size, but the chemical processes of aerosol during the formation of orographic cloud could also be important in CCN activity, especially for low SS conditions. These findings collectively underscore the significant impact of anthropogenic air plumes on CCN concentrations in the QLM and their potential influence on precipitation patterns.

Arctic black carbon during PAMARCMiP 2018 and previous aircraft experiments in spring

Abstract. Vertical profiles of the mass concentration of black carbon (BC) were measured at altitudes up to 5 km during the PAMARCMiP (Polar Airborne Measurements and Arctic Regional Climate Model simulation Project) aircraft-based field experiment conducted around the northern Greenland Sea (Fram Strait) during March and April 2018 from operation base Station Nord (81.6∘ N, 16.7∘ W). Median BC mass concentrations in individual altitude ranges were 7–18 ng m−3 at standard temperature and pressure at altitudes below 4.5 km. These concentrations were systematically lower than previous observations in the Arctic in spring, conducted by ARCTAS-A in 2008 and NETCARE in 2015, and similar to those observed during HIPPO3 in 2010. Column amounts of BC for altitudes below 5 km in the Arctic (>66.5∘ N; COLBC), observed during the ARCTAS-A and NETCARE experiments, were higher by factors of 4.2 and 2.7, respectively, than those of the PAMARCMiP experiment. These differences could not be explained solely by the different locations of the experiments. The year-to-year variation of COLBC values generally corresponded to that of biomass burning activities in northern midlatitudes over western and eastern Eurasia. Furthermore, numerical model simulations estimated the year-to-year variation of contributions from anthropogenic sources to be smaller than 30 %–40 %. These results suggest that the year-to-year variation of biomass burning activities likely affected BC amounts in the Arctic troposphere in spring, at least in the years examined in this study. The year-to-year variations in BC mass concentrations were also observed at the surface at high Arctic sites Ny-Ålesund and Utqiaġvik (formerly known as Barrow, the location of Barrow Atmospheric Baseline Observatory), although their magnitudes were slightly lower than those in COLBC. Numerical model simulations in general successfully reproduced the observed COLBC values for PAMARCMiP and HIPPO3 (within a factor of 2), whereas they markedly underestimated the values for ARCTAS-A and NETCARE by factors of 3.7–5.8 and 3.3–5.0, respectively. Because anthropogenic contributions account for nearly all of the COLBC (82 %–98 %) in PAMARCMiP and HIPPO3, the good agreement between the observations and calculations for these two experiments suggests that anthropogenic contributions were generally well reproduced. However, the significant underestimations of COLBC for ARCTAS-A and NETCARE suggest that biomass burning contributions were underestimated. In this study, we also investigated plumes with enhanced BC mass concentrations, which were affected by biomass burning emissions, observed at 5 km altitude. Interestingly, the mass-averaged diameter of BC (core) and the shell-to-core diameter ratio of BC-containing particles in the plumes were generally not very different from those in other air samples, which were considered to be mostly aged anthropogenic BC. These observations provide a useful basis to evaluate numerical model simulations of the BC radiative effect in the Arctic region in spring.

Analysis of atmospheric greenhouse gases in north Xinjiang

Flask air samples were collected weekly at the Akedala (AKD) regional atmosphere station in north Xinjiang, China, and atmospheric carbon dioxide (CO2) and methane (CH4) mole fractions were measured using cavity-ring down spectroscopy. AKD station is under a typical temperate continental semiarid and arid monsoon climate with westerly air flow. This paper presents the long-term trends and seasonal variations in atmospheric CO2 and CH4 concentrations for the first time and discusses the possible influencing mechanisms. The CO2 and CH4 background mixing ratios were 412.1 ± 0.36 ppm and 1968.9 ± 0.95 ppb, respectively, in 2018. The growth rates were 2.62 ppm y−1 and 9.67 ppb y−1 for CO2 and CH4, respectively, during 2010–2018, which were higher than the surface measurements at Waliguan Baseline Observatory at similar latitudes. The averaged CO2 seasonal cycle showed a minimum in summer and a maximum in winter, and the annual amplitude was high at 18.7 ppm. The CH4 seasonal cycle showed a rapid recovery in summer during July and August. Back-trajectory analysis showed that the atmospheric information at AKD station can provide some implications for uptake from Kazakhstan and Russia, as well as information about natural and anthropogenic emissions in northern Xinjiang. This study revealed that the long-term upward trend of the CO2 concentration in AKD might be attributed to emissions from energy consumption of heavy industry. The comprehensive effects of livestock, geological micro seepage and oil and gas production, and vertical transport also enhanced the atmospheric CH4 concentrations. The results of this study may improve our basic understanding of observed atmospheric CO2 and CH4 in China and Central Asia and the underlying fluxes to be used in atmospheric inversion models.

Speciated atmospheric mercury at the Waliguan Global Atmosphere Watch station in the northeastern Tibetan Plateau: implication of dust-related sources for particulate bound mercury

Abstract. To understand the ambient levels and sources of atmospheric mercury (Hg) in the Tibetan Plateau, a full-year continuous measurement of speciated atmospheric mercury was conducted at the Waliguan (WLG) Baseline Observatory (3816 m a.s.l.) from May 2012 to April 2013. Mean concentrations (± 1 SD) of gaseous elemental mercury (GEM), gaseous oxidized mercury (GOM) and particulate bound mercury (PBM) during the whole study period were 1.90±0.80 ng m−3, 12.0±10.6 and 65.4±63.2 pg m−3, respectively. Seasonal variations of GEM were very small, while those of PBM were quite large with mean values being 4 times higher in the cold (102.3±66.7 pg m−3) than warm (22.8±14.6 pg m−3) season. Anthropogenic emissions to the east of the Tibetan Plateau contributed significantly to GEM pollution at WLG, while dust particles originating from desert and Gobi regions in Xinjiang province and the Tibetan Plateau to the west of WLG were responsible for PBM pollution at WLG. This finding is also supported by the significant positive correlation between daily PBM concentration and daily cumulative absorbing aerosol index (AAI) in air masses transported during the preceding 2 d.

The innovative development of atmospheric background stations in China: Thoughts and recommendations

Atmospheric background monitoring refers to the long-term observation of atmospheric composition that is well mixed and not affected by local pollution at a fixed location which is far from the influence of human activities. Atmospheric background monitoring can reflect the long-term changes of atmospheric composition caused by human activities on a global or regional scale. Therefore, it plays a very important role in both socioeconomic development and scientific research. Since the 1950s, some observation stations abroad have carried out background observation of the main atmospheric components such as carbon dioxide, ozone and atmospheric heavy metals. Additionally, the Global Ozone Observing System (GO₃OS) and the Background Air Pollution Monitoring Network (BAPMoN) were established by the World Meteorological Organization (WMO) in 1957 and 1968 to fulfill long-term and continuous observation of the chemical constituents of reactive gases, greenhouse gases, aerosols and acid rain. In 1989, WMO integrated GO₃OS and BAPMoN into the Global Atmosphere Watch (GAW). WMO/GAW has achieved great accomplishments owing to its long-term, systematic and accurate observation of the physical and chemical characteristics of atmospheric composition. During the period of 1981 to 2007, China Meteorological Administration (CMA) has built six representative regional GAW (Global Atmosphere Watch) stations, i.e., Shangdianzi (117.12°E, 40.65°N, <sc>293.3 m</sc> asl), Lin’an (119.75°E, 30.28°N, <sc>138.6 m</sc> asl), Longfengshan (127.60°E, 44. 73°N, <sc>331 m</sc> asl), Akedala (87.93°E, 47.10°N, <sc>562 m</sc> asl), Shangri-La (99.73°E, 28.01°N, <sc>3580 m</sc> asl), and Jinsha (114.2°E, 29.63°N, <sc>750 m</sc> asl), following the construction criteria of the GAW regional background stations. In addition, in accordance with the requirements of GAW global background station site selection, the Chinese government and the UN Global Environmental Facility (GEF) jointly invested and established a global GAW station in China, namely, the China GAW Baseline Observatory (CGAWBO), located at Mt. Waliguan (100.89°E, 36.28°N, <sc>3816 m</sc> asl) in Qinghai Province in 1994. These seven global/regional GAW stations build the current operational network of atmospheric background observation stations in China. Thanks to their long-term continous and precise atmospheric composition monitoring data and special representations, the stations at Mt. Waliguan, Shangdianzi, Lin’an, and Longfengshan were selected as the National Field Scientific Observation and Research Stations by the Ministry of Science and Technology of the People’s Republic of China in 2005. It is well known that the long-time series and excellent data on meteorological and atmospheric composition obtained from these seven global/regional GAW stations have played an unique role in China’s efforts on addressing climate change, climate change diplomacy and negotiation, air pollution control, and scientific research. With China’s socioeconomic development stepping into the new era, however, the deficiencies of the current GAW stations have become increasingly significant. Major problems lie in the unbalanced and insufficient development of these stations, as well as in the lack of innovation and demonstration. In this article, the status and the achievements of these seven stations are reviewed and summarized for the first time, followed by the identification and analysis of major problems of development. At last, it is recommended that future endeavors should be concentrated on enhancing the comprehensive observation capability and international cooperation, as well as further improving the construction and capacities of the national field scientific research stations. Besides, it is also suggested that the development of atmospheric background station shall serve China’s national and local development strategy, as well as strengthen its role in China’s progress of ecological civilization.

The De-Icing Comparison Experiment (D-ICE): a study of broadband radiometric measurements under icing conditions in the Arctic

Abstract. Surface-based measurements of broadband shortwave (solar) and longwave (infrared) radiative fluxes using thermopile radiometers are made regularly around the globe for scientific and operational environmental monitoring. The occurrence of ice on sensor windows in cold environments – whether snow, rime, or frost – is a common problem that is difficult to prevent as well as difficult to correct in post-processing. The Baseline Surface Radiation Network (BSRN) community recognizes radiometer icing as a major outstanding measurement uncertainty. Towards constraining this uncertainty, the De-Icing Comparison Experiment (D-ICE) was carried out at the NOAA Atmospheric Baseline Observatory in Utqiaġvik (formerly Barrow), Alaska, from August 2017 to July 2018. The purpose of D-ICE was to evaluate existing ventilation and heating technologies developed to mitigate radiometer icing. D-ICE consisted of 20 pyranometers and 5 pyrgeometers operating in various ventilator housings alongside operational systems that are part of NOAA's Barrow BSRN station and the US Department of Energy Atmospheric Radiation Measurement (ARM) program North Slope of Alaska and Oliktok Point observatories. To detect icing, radiometers were monitored continuously using cameras, with a total of more than 1 million images of radiometer domes archived. Ventilator and ventilator–heater performance overall was skillful with the average of the systems mitigating ice formation 77 % (many &gt;90 %) of the time during which icing conditions were present. Ventilators without heating elements were also effective and capable of providing heat through roughly equal contributions of waste energy from the ventilator fan and adiabatic heating downstream of the fan. This provided ∼0.6 ∘C of warming, enough to subsaturate the air up to a relative humidity (with respect to ice) of ∼105 %. Because the mitigation technologies performed well, a near complete record of verified ice-free radiometric fluxes was assembled for the duration of the campaign. This well-characterized data set is suitable for model evaluation, in particular for the Year of Polar Prediction (YOPP) first Special Observing Period (SOP1). We used the data set to calculate short- and long-term biases in iced sensors, finding that biases can be up to +60 W m−2 (longwave) and −211 to +188 W m−2 (shortwave). However, because of the frequency of icing, mitigation of ice by ventilators, cloud conditions, and the timing of icing relative to available sunlight, the biases in the monthly means were generally less than the aggregate uncertainty attributed to other conventional sources in both the shortwave and longwave.

On the organization of CONT17

The Continuous Very Long Baseline Interferometry (VLBI) Campaign 2017 (CONT17) was observed from November 28 to December 12, 2017 and featured three independent observing networks. Two legacy S/X networks of nominally 14 stations each observed in parallel for the full 15 days of the campaign. One of them was made possible in large part by the participation of the ten-station very long baseline array network of the long baseline observatory. Furthermore, for the 5-day period from December 4 to 8, 2017, a six-station broadband network continuously recorded VLBI global observing system data. The different networks will help probe the accuracy of the VLBI estimates of the Earth orientation parameters and investigate possible network biases. We describe the organizational efforts undertaken to realize CONT17, including the analysis of resources such as recording media, electronic data transfer, and correlation, the assignment of stations to the three networks, and source selection and schedule writing.

Seasonal Variation of Wet Deposition of Black Carbon in Arctic Alaska

AbstractBlack carbon (BC) aerosol deposited in and onto Arctic snow increases the snow's absorption of sunlight and accelerates snowmelt. Wet removal of BC from the atmosphere plays a key role in determining its abundance in the Arctic atmosphere and in Arctic snow. However, this process is poorly understood, mainly due to the scarcity of relevant measurements. To study wet deposition of BC, we made measurements of mass concentration of BC in snow and rain (CMBC) and of BC in air (MBC) with high accuracy (16% and 10%, respectively) at the Barrow Atmospheric Baseline Observatory, Alaska, from July 2013 to August 2017 and analyzed them along with routinely measured meteorological parameters from Barrow. Monthly mean MBC near the surface and CMBC were poorly correlated from midwinter to early spring, when CMBC was close to the annual median while MBC was at its annual peak. Seasonal variations in the altitude distribution of MBC may lead to these differences in seasonal variation of MBC near the surface and CMBC. About 50% of the annual wet deposition of BC occurred in the 3 months of summer, associated with high values of total precipitation and BC originating from biomass burning. Size distributions of BC in snow and rain were stable throughout the year, suggesting that the size distribution of BC in the lower troposphere was similarly stable. Calculations by two global models reproduced the observed seasonal variations of CMBC and showed that BC from biomass burning dominated CMBC in summer.

Regional Differences of Chemical Composition and Optical Properties of Aerosols in the Tibetan Plateau

AbstractIn this study, PM2.5 filter samples were collected from two high‐elevation sites located in the southern and northern areas of Tibetan Plateau (TP): the Qomolangma Station for Atmospheric Environmental Observation and Research (QOMS) and Waliguan Baseline Observatory (WLG), respectively. Collected samples were analyzed to examine the regional differences in aerosol properties and relate them to potential chemical processes in the TP area. The aerosol mass concentrations inferred from measured chemical components were higher at WLG (11.2 μg m−3) compared to QOMS (6.8 μg m−3). The chemical composition shows higher contribution of organic aerosol at QOMS than that of WLG. The optical properties of water‐soluble organic carbon (WSOC) from the QOMS samples show higher light absorption efficient than those collected at WLG. The light absorption of WSOC at QOMS indicates significant pH dependence with enhanced light absorption at higher pH values, while the light absorption of WSOC from WLG samples show very weak pH dependence. The different pH dependence property suggests the different chemical composition between them. The molecular‐level chemical composition investigated using high‐resolution mass spectrometry (HRMS) assisted with an electrospray ionization (ESI) shows significant differences in WSOC composition representative of two sampling sites. For QOMS samples, CHO and CHON compounds are the major chemical species detected in the negative (−) ESI mode, while CHO and CHOS compounds are the most abundant chemical species detected by the same method in samples collected at WLG. The differences in their molecular composition indicate the different sources and chemical processes in these two regions.

A spectroscopy pipeline for the Canary island long baseline observatory meteor detection system

We demonstrated the capability of the updated Canary Island Long Baseline Observatory (CILBO) meteor detection system to measure relative elements intensities of meteors. Meteor spectra provide valuable information on the chemical properties of individual meteoroids. In some cases, this may be the only information on the chemical composition of the parent bodies, and on transforming processes that occur during the meteoroid’s journey from its source to Earth. The CILBO spectroscopic program has been created with the intention of carrying out regular systematic spectroscopic observations. At the same time, the meteoroid trajectory and pre-atmospheric orbit are independently measured from data collected by the other cameras in the network. We presented the meteor spectroscopy pipeline developed by the Meteor Research Group of the European Space Agency, and it’s application to the spectroscopic survey of Geminid meteor shower observed by CILBO.

Chemical characterization and sources of submicron aerosols in the northeastern Qinghai–Tibet Plateau: insights from high-resolution mass spectrometry

Abstract. An Aerodyne high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS) was deployed along with other online instruments to study the highly time resolved chemistry and sources of submicron aerosols (PM1) at Waliguan (WLG) Baseline Observatory, a high-altitude (3816 m a.s.l.) background station located at the northeast edge of the Qinghai–Tibet Plateau (QTP), during 1–31 July 2017. The average PM1 mass concentration during this study was 9.1 µg m−3 (ranging from 0.3 to 28.1 µg m−3), which was distinctly higher than those (2.0–5.7 µg m−3) measured with the Aerodyne AMS at other high-elevation sites in the southern or central QTP. Sulfate showed a dominant contribution (38.1 %) to PM1 at WLG followed by organics (34.5 %), ammonium (15.2 %), nitrate (8.1 %), BC (3.0 %) and chloride (1.1 %). Accordingly, bulk aerosols appeared to be slightly acidic throughout this study, mainly related to the enhanced sulfate contribution. All chemical species peaked at the accumulation mode, indicating the well-mixed and highly aged aerosol particles at WLG from long-range transport. Positive matrix factorization (PMF) on the high-resolution organic mass spectra resolved four distinct organic aerosol (OA) components, including a traffic-related hydrocarbon-like OA (HOA), a relatively fresh biomass burning OA (BBOA), an aged biomass burning OA (agBBOA) and a more-oxidized oxygenated OA (OOA). On average, the two relatively oxidized OAs, OOA and agBBOA, contributed 34.4 % and 40.4 % of organics, respectively, while the rest were 18.4 % for BBOA and 6.8 % for HOA. Source analysis for air masses showed that higher mass concentrations of PM1 and enhanced contributions of sulfate and biomass-burning-related OA components (agBBOA + BBOA) were from the northeast of the WLG with shorter transport distance, whereas lower PM1 mass concentrations with enhanced OOA contribution were from the west after long-range transport, suggesting their distinct aerosol sources and significant impacts of regional transport on aerosol mass loadings and chemistry at WLG.

Source sector and region contributions to black carbon and PM&amp;lt;sub&amp;gt;2.5&amp;lt;/sub&amp;gt; in the Arctic

Abstract. The impacts of black carbon (BC) and particulate matter with aerodynamic diameters less than 2.5 µm (PM2.5) emissions from different source sectors (e.g., transportation, power, industry, residential, and biomass burning) and geographic source regions (e.g., Europe, North America, China, Russia, central Asia, south Asia, and the Middle East) to Arctic BC and PM2.5 concentrations are investigated through a series of annual sensitivity simulations using the Weather Research and Forecasting – sulfur transport and deposition model (WRF-STEM) modeling framework. The simulations are validated using observations at two Arctic sites (Alert and Barrow Atmospheric Baseline Observatory), the Interagency Monitoring of Protected Visual Environments (IMPROVE) surface sites over the US, and aircraft observations over the Arctic during spring and summer 2008. Emissions from power, industrial, and biomass burning sectors are found to be the main contributors to the Arctic PM2.5 surface concentration, with contributions of ∼ 30 %, ∼ 25 %, and ∼ 20 %, respectively. In contrast, the residential and transportation sectors are identified as the major contributors to Arctic BC, with contributions of ∼ 38 % and ∼ 30 %. Anthropogenic emissions are the most dominant contributors (∼ 88 %) to the BC surface concentration over the Arctic annually; however, the contribution from biomass burning is significant over the summer (up to ∼ 50 %). Among all geographical regions, Europe and China have the highest contributions to the BC surface concentrations, with contributions of ∼ 46 % and ∼ 25 %, respectively. Industrial and power emissions had the highest contributions to the Arctic sulfate (SO4) surface concentration, with annual contributions of ∼ 43 % and ∼ 41 %, respectively. Further sensitivity runs show that, among various economic sectors of all geographic regions, European and Chinese residential sectors contribute to ∼ 25 % and ∼ 14 % of the Arctic average surface BC concentration. Emissions from the Chinese industry sector and European power sector contribute ∼ 12 % and ∼ 18 % of the Arctic surface sulfate concentration. For Arctic PM2.5, the anthropogenic emissions contribute &gt; ∼ 75 % at the surface annually, with contributions of ∼ 25 % from Europe and ∼ 20 % from China; however, the contributions of biomass burning emissions are significant in particular during spring and summer. The contributions of each geographical region to the Arctic PM2.5 and BC vary significantly with altitude. The simulations show that the BC from China is transported to the Arctic in the midtroposphere, while BC from European emission sources are transported near the surface under 5 km, especially during winter.

An evaluation of three methods for measuring black carbon in Alert, Canada

Abstract. Absorption of sunlight by black carbon (BC) warms the atmosphere, which may be important for Arctic climate. The measurement of BC is complicated by the lack of a simple definition of BC and the absence of techniques that are uniquely sensitive to BC (e.g., Petzold et al., 2013). At the Global Atmosphere Watch baseline observatory in Alert, Nunavut (82.5° N), BC mass is estimated in three ways, none of which fully represent BC: conversion of light absorption measured with an Aethalometer to give equivalent black carbon (EBC), thermal desorption of elemental carbon (EC) from weekly integrated filter samples to give EC, and measurement of incandescence from the refractory black carbon (rBC) component of individual particles using a single particle soot photometer (SP2). Based on measurements between March 2011 and December 2013, EBC and EC are 2.7 and 3.1 times higher than rBC, respectively. The EBC and EC measurements are influenced by factors other than just BC, and higher estimates of BC are expected from these techniques. Some bias in the rBC measurement may result from calibration uncertainties that are difficult to estimate here. Considering a number of factors, our best estimate of BC mass in Alert, which may be useful for evaluation of chemical transport models, is an average of the rBC and EC measurements with a range bounded by the rBC and EC combined with the respective measurement uncertainties. Winter-, spring-, summer-, and fall-averaged (± atmospheric variability) estimates of BC mass in Alert for this study period are 49 ± 28, 30 ± 26, 22 ± 13, and 29 ± 9 ng m−3, respectively. Average coating thicknesses estimated from the SP2 are 25 to 40 % of the 160–180 nm diameter rBC core sizes. For particles of approximately 200–400 nm optical diameter, the fraction containing rBC cores is estimated to be between 10 and 16 %, but the possibility of smaller undetectable rBC cores in some of the particles cannot be excluded. Mass absorption coefficients (MACs) ± uncertainty at 550 nm wavelength, calculated from light absorption measurements divided by the best estimates of the BC mass concentrations, are 8.0 ± 4.0, 8.0 ± 4.0, 5.0 ± 2.5 and 9.0 ± 4.5 m2 g−1, for winter, spring, summer, and fall, respectively. Adjusted to better estimate absorption by BC only, the winter and spring values of MACs are 7.6 ± 3.8 and 7.7 ± 3.8 m2 g−1. There is evidence that the MAC values increase with coating thickness.

Observations of high level of ozone at Qinghai Lake basin in the northeastern Qinghai-Tibetan Plateau, western China

Measurements of surface ozone (O3), nitrogen oxides (NOx = NO + NO2), carbon monoxide (CO), and dew point were made at Qinghai Lake (QHL), China, a basin in the remote Tibetan Plateau area, in October 2010 and October 2011. The O3 mixing ratio was found to be high with average of 41 ± 9 ppb in October 2010 and 57 ± 10 ppb in October 2011. The observed diurnal pattern of O3 mixing ratio was characterized by a minimum between 07:00 and 10:00 local standard time (LST) increasing ~20 ppb to a broad peak occurring between 13:00 and 18:00 LST. This diurnal pattern differs substantially from that observed at WMO’s GAW Baseline Observatory located above the basin on Mount Waliguan, ~130 km southeast of QHL. The elevated O3 mixing ratios observed in the afternoon are attributed to in situ photochemical production in the air trapped in the QHL basin by surrounding mountains. The low O3 mixing ratios observed in the morning are most likely due to surface removal in a shallow nocturnal boundary layer. The data indicate substantial impacts of pollution on air quality even in this remote area. The high O3 values observed in 2011 may cause observable damage to the vegetation, adding stress to an ecosystem ready under the threat of desertification.

Short-term variations of atmospheric CO2 and dominant causes in summer and winter: Analysis of 14-year continuous observational data at Waliguan, China

Using a 14-year revised dataset of atmospheric CO2 mixing ratios continuously measured at Mount Waliguan Baseline Observatory of western China during 1995–2008, the short-term variations of ambient CO2 and the dominant causes were studied. A comparison and evaluation of the revised and original data is included. Ambient CO2 in summer was usually elevated during nighttime and declined in daytime with peak-to-peak diurnal amplitude of 2.4 ± 0.3 ppm (year-to-year fluctuations). In winter, there was often a small increase during daytime with amplitude of 0.8 ± 0.2 ppm. Analysis of the local horizontal winds shows that the diurnal cycles of ambient CO2 are the combined result of changes of nearby sources/sinks and local meteorological circulation. Only in winter do we find a statistically increase trends of 0.03 ppm yr−1 for CO2 diurnal peak-to-peak amplitudes, reflecting local changes in sources and sinks of CO2 during 1995–2008 under relatively consistent meteorological conditions. In summer, atmospheric CO2 were depleted when prevailing winds came from the populated northeastern regions due to the enhanced vegetation photosynthesis which will result in low CO2; on the contrary to summer, elevated CO2 were closely associated with air parcels from populated north/northeastern or northwestern regions in winter, when the terrestrial exchanges become weak and anthropogenic emissions dominate ambient CO2. ΔCO2 and ΔCO showed significant positive correlation (r > 0.9, p < 0.01) in winter, reflecting common sources under certain synoptic meteorological conditions. But they didn't exhibit any correlation in summer. The intercepts for the linear fit of ΔCO2 and ΔCO were close to zero in winter, indicating less influence from non-CO related CO2 sources or sinks (e.g. respiration and photosynthesis), whereas the intercepts in summer were much more negative (as low as −3.7 ppm in July), reflecting enhanced biospheric CO2 uptake. The ratio of ΔCO2/ΔCO was 25–35 ppm ppm−1 in winter, which is approximately 30%–42% higher than that observed in Beijing and derived from emission inventories, reflecting less impact from human activities due to the remote location of Waliguan.

Temporal variation of atmospheric CH4 and the potential source regions at Waliguan, China

We used a revised dataset of the atmospheric CH4 mixing ratios that were continuously measured at Mount Waliguan Baseline Observatory of western China during 2002–2006 to study the temporal variations and the potential sources regions of ambient CH4. Approximately 58% observed values were filtered as background data by applying a robust local regression mathematic procedure. The median, 10% and 90% percentiles of CH4 background mixing ratios were 1831.8, 1820.7 and 1843.5 ppb (10−9 mol mol−1), respectively; the atmospheric CH4 time series showed large variations with a fluctuation of 200 ppb (maximum minus minimum), but the background CH4 exhibited a much smaller variation (about 38 ppb), indicating strong impacts from local or regional sources and sinks. Ambient CH4 displayed significant diurnal variations in different seasons during 2002–2006, as was the combined result of changes of nearby sources/sinks and local meteorological circulation. The averaged CH4 seasonal cycle showed minimum in spring and winter, maximum in summer (from June to August), as is completely opposite to that observed at Mauna Loa and Niwot Ridge. The incongruous variation should be attributed to enhancing regional/local sources (e.g., herd) around the site as well as the dominant air flow from the polluted north/northeastern region (e.g., Xining and Lanzhou city) in summer. Another possible reason is, compared to Mauna Loa and Niwot Ridge, the photochemical reaction at Waliguan is weaker. An analysis of 5 d back-trajectory (500 hPa) showed that the episode of high CH4 mixing ratios was associated with advection from the heavily populated regions east or southeast (e.g., Xining and Lanzhou) of Waliguan and northwest of Qinghai via Ge’ermu urban area where growing industrial activities as well as crops residue burning provide large sources of CH4, suggesting a large source area; the low values were observed most frequently when air masses originated from the sparsely populated Tibet and south of Qinghai and Xinjiang Uygur Autonomous Region (XUAR). Additionally, atmospheric CH4 would be enhanced when air masses originated from Ningxia and northwest of Gansu areas (mainly growing rice) along the Yellow river region, especially in summer, suggesting agriculture source of CH4. Our study helps to accurately estimate sources and sinks of CH4, and thus is significant for understanding greenhouse effect over a regional scale and predicting global change in future.

Temporal trend and sources of speciated atmospheric mercury at Waliguan GAW station, Northwestern China

Abstract. Measurements of speciated atmospheric mercury were conducted at a remote mountain-top station (Waliguan (WLG) Baseline Observatory) at the edge of northeastern part of the Qinghai-Xizang Plateau, Western China. Mean concentrations of total gaseous mercury (TGM), particulate bound mercury (PBM), and gaseous oxidized mercury (GOM) during the whole sampling campaign were 1.98±0.98 ng m−3, 19.4±18.1 pg m−3, and 7.4±4.8 pg m−3, respectively. Levels of speciated Hg at WLG were slightly higher than those reported from remote areas of North America and Europe. Both regional sources and long-rang transport played important roles in the distribution of atmospheric TGM and PBM at WLG, whereas GOM showed major links to the regional sources, likely as well as the in-situ productions via photochemical processes. Regional sources for speciated Hg were mostly located to the east of WLG, which is the most industrial and urbanized areas of Qinghai province. Potential source contribution function (PSCF) results showed that eastern Gansu, western Ningxia and Shanxi Province were likely the potential source regions of WLG, with good accordance with locations of urban areas and industrial centers. Moreover, we found that Northern India may be a significant source region for WLG during the sampling campaign, and this is the first published evidence suggesting long-range transport of atmospheric Hg from India to the Northeastern Tibetan Plateau. Seasonal and diurnal variations of TGM were in contrast to most of the previous studies in China, with relatively higher levels in warm seasons and night, respectively. The temporal trend of TGM also highlighted the impact of long-range transport on the distribution of TGM in ambient air at WLG.

Decreasing anthropogenic methane emissions in Europe and Siberia inferred from continuous carbon dioxide and methane observations at Alert, Canada

The rate of increase in global atmospheric methane (CH4) abundance has steadily declined since the late 1980s with near zero increase from 1999 through 2006. At the Canadian Baseline Observatory at Alert, Canada (82°28′N, 62°30′W), continuous measurements of methane (CH4) and carbon dioxide (CO2) have been made since 1987. During winter, both gases are frequently highly correlated during well‐defined episodes lasting from 2 to 5 days. We observe a gradual decrease in the ratios of CH4/CO2 during these episodes from ∼16 ppb CH4 (ppm CO2)−1 to ∼12 ppb CH4 (ppm of CO2)−1 over the entire record. An atmospheric transport model with prescribed CO2 and CH4 source distributions is used to partition simulated CH4 events into contributions by region. We show that anthropogenic emissions from Europe and Siberia account for more than 85% of the CO2 and CH4 enhancements simulated at Alert, but without a change in CH4 emissions, modeled CH4/CO2 ratios remain constant. To reproduce the observed trend in the ratio of CH4/CO2, the model requires a reduction in emissions of CH4 on the order of 30 Tg (13.6 to 33.4 Tg) in Europe and Siberia over the observational period. This is about twice the drop reported by the Emission Database for Global Atmospheric Research (EDGAR) emissions inventory and large enough to account for the leveling off of the global atmospheric CH4 burden observed over the past 20 years.

Organochlorine pesticides, polybrominated biphenyl ethers and lead isotopes during the spring time at the Waliguan Baseline Observatory, northwest China: Implication for long-range atmospheric transport

Located in the center of the Asian continent, the Waliguan Baseline Observatory (WBO) is the highest and the most inland Global Atmospheric Watch (GAW) station in the world. In the present study, organochlorine pesticides (OCPs) and polybrominated biphenyl (PBDEs) were analyzed in the daily collected samples at the WBO from 2 April to 23 May 2005, in an attempt to investigate the long-range atmospheric transport (LRAT) of persistent organic pollutants (POPs) in northwest China. The mean concentrations of OCPs at WBO were slightly higher than the Arctic regions, and the concentrations of total PBDEs (8.3±4.0 pg m −3) was comparable with other remote areas. The low temperature dependence of the POP concentrations in the air, and the absence of local use of these compounds implied LRAT to WBO. As suggested by backward air trajectory analysis, the high concentrations of γ-HCH, DDTs and PBDEs, during the sampled spring period, were mainly related to air mass passing over the neighboring countries (e.g., Russia, Kazakhstan). Additional evidence was also obtained by the lead isotopic composition analysis, which showed similar 206Pb/ 207Pb and 208Pb/ 207Pb ratios as those of ores in these countries. The present results indicate that the WBO may serve as an excellent GAW station for monitoring the LRAT of POPs in the Asian continent.