Atmospheric Background Station Research Articles

Atmospheric CO2 in the megacity Hangzhou, China: Urban-suburban differences, sources and impact factors

Limited observation sites and insufficient monitoring of atmospheric CO2 in urban areas restrict our comprehension of urban-suburban disparities. This research endeavored to shed light on the urban-suburban differences of atmospheric CO2 in levels, diurnal and seasonal variations as well as the potential sources and impact factors in the megacity of Hangzhou, China, where the economically most developed region in China is. The observations derived from the existing Hangzhou Atmospheric Composition Monitoring Center Station (HZ) and Lin'an Regional Atmospheric Background Station (LAN) and the newly established high-altitude Daming Mountain Atmospheric Observation Station (DMS), were utilized. From November 2020 to October 2021, the annual averages of HZ, LAN and DMS were 446.52 ± 17.01 ppm, 441.56 ± 15.42 ppm, and 422.02 ± 10.67 ppm. The difference in atmospheric CO2 mole fraction between HZ and LAN was lower compared to the urban-suburban differences observed in other major cities in China, such as Shanghai, Nanjing, and Beijing. Simultaneous CO2 enhancements were observed at HZ and LAN, when using DMS observations as background references. The seasonal variations of CO2 at LAN and DMS exhibited a high negative correlation with the normalized difference vegetation index (NDVI) values, indicating the strong regulatory of vegetation canopy. The variations in boundary layer height had a larger influence on the low-altitude HZ and LAN stations than DMS. Compared to HZ and LAN, the atmospheric CO2 at DMS was influenced by emissions and transmissions over a wider range. The potential source area of DMS in autumn covered most areas of the urban agglomeration in eastern China. DMS measurements could provide a reliable representation of the background level of CO2 emissions in the Yangtze River Delta and a broader region. Conventional understanding of regional CO2 level in the Yangtze River Delta through LAN measurements may overestimate background concentration by approximately 10.92 ppm.

Seasonal changes in water-soluble brown carbon (BrC) at Nanling background station in South China

Brown carbon (BrC) is an important light-absorbing component of organic carbon (OC), causing large uncertainty in aerosol radiative forcing evaluation and being related to health issues as well. Knowledge of BrC in an atmospheric background station is beneficial to understand its role in a changing climate. A year-long sampling campaign was conducted at Nanling background station to get a comprehensive knowledge of WS-BrC, a total of seventy-two PM2.5 samples throughout a year were used. Light absorption and fluorescence spectra of WSOC were analyzed synchronously using a fluorescence spectrophotometer. The low levels of PM2.5, OC, and elemental carbon (EC) conferred a background site. The optical properties of WS-BrC were characterized using excitation-emission matrix (EEM) fluorescence spectroscopy. The WS-BrC made a significant contribution (365 nm, 18% ± 10%) to total carbonaceous aerosol absorption. The mass absorption efficiency (MAE) of WS-BrC is 0.81 ± 0.34 m2 gC–1, and varies among seasons due to the different sources or atmospheric processing. Three EEM fluorescent components were identified by parallel factor (PAFAFAC) analysis, including two humic-like substances (HULIS, C1, C2), and one phenolic-like component. The HULIS components accounted for approximately 70% of the total fluorescence intensities. Primary combustion emissions showed enhanced activity during the winter and spring seasons, but there were no significant influences on WS-BrC in spring. Secondary sources contributed significantly to WS-BrC during winter, summer, and autumn (all exceeding 50%), except for spring. Photooxidation is a significant process in the formation of secondary WS-BrC in winter and autumn, but there may be another formation pathway in summer, i.e., the ammonia pathway. This study contributes to our understanding of BrC in the background atmosphere.

Background Characteristics and Influence Analysis of Greenhouse Gases at Jinsha Atmospheric Background Station in China

Central China has been acting as a major convergence zone for sources and sinks in China, such that the climate change studies of Central China have taken on critical significance. The Jinsha atmospheric background station refers to the sole background monitoring site in Central China. It is noteworthy that the greenhouse gas attributes of the Jinsha atmospheric background station represent the greenhouse gas conditions of Central China. The seasonal and daily variations in CO2, CH4, and CO in the scope of time between October 2019 to April 2021 at the station were examined in this study. The effect of meteorological conditions on greenhouse gas concentrations at the site was evaluated. Furthermore, the primary transmission origins affecting the station were identified using the backward trajectory through potential source contribution function analysis. As indicated by the results, the background concentrations at the Jinsha station in 2020 for CO2, CH4, and CO reached 424.1 ± 0.1 ppm, 2046.2 ± 0.6 ppb, and 324.1 ± 1.1 ppb, respectively. CO2 varied on a daily basis with higher nighttime levels, which was affected by the boundary layer elevation, photosynthesis, and human activities. In autumn, CH4 levels peaked under the effect of agricultural activities in Central China. However, CO2 and CO concentrations reached the maximum in winter, majorly affected by the transmissions from the Beijing–Tianjin–Hebei region and Hubei. Under China’s comprehensive carbon neutrality, more attention should be paid to the emissions from winter heating and industrial activities in the Beijing–Tianjin–Hebei region, and effects exerted by transport in the monitoring process should be differentiated in depth.

Transport pathways of carbon monoxide from Indonesian fire pollution to a subtropical high-altitude mountain site in the western North Pacific

Abstract. Dry conditions associated with the El Niño–Southern Oscillation (ENSO) and a positive Indian Ocean Dipole (IOD) are known to have caused major fire pollution events and intense carbon emissions over a vast spatial expanse of Indonesia in October 2006 and 2015. During these two events, a substantial increase in the carbon monoxide (CO) mixing ratio was detected by in situ measurements at Lulin Atmospheric Background Station (LABS; 23.47∘ N 120.87∘ E; 2862 ma.s.l.) in Taiwan, which is the only background station in the subtropical western North Pacific region. Compared to the long-term October mean (2006–2021), CO was elevated by ∼ 47.2 ppb (parts per billion; 37.2 %) and ∼ 36.7 ppb (28.9 %) in October 2006 and 2015, respectively. This study delineates plausible pathways for the CO transport from Indonesia to LABS using Measurement of Pollution in the Troposphere (MOPITT) CO observations and Modern-Era Retrospective analysis for Research and Applications, version 2 (MERRA-2) reanalysis products (winds and geopotential height – GpH). Two simultaneously occurring transport pathways were identified, namely (i) horizontal transport in the free troposphere and (ii) vertical transport through the Hadley circulation (HC). The GpH analysis of both events revealed the presence of a high-pressure anticyclone over the northern part of the South China Sea (SCS), which played an important role in the free-tropospheric horizontal transport of CO. In this scenario, CO in the free troposphere is transported on the western edge of the high-pressure system and then driven by subtropical westerlies to LABS. Simultaneously, uplifted CO over Indonesia can enter the HC and be transferred to subtropical locations such as LABS. The vertical cross section of the MOPITT CO and MERRA-2 vertical pressure velocity supported the transport of CO through the HC. Furthermore, the results revealed a distinct HC strength in two events (higher in 2006 compared to 2015) due to the different ENSO conditions. Overall, the present findings can provide some insights into understanding the regional transport of pollution over Southeast Asia and the role of climate conditions on transport pathways.

MAX-DOAS Measurements of Tropospheric NO2 and HCHO Vertical Profiles at the Longfengshan Regional Background Station in Northeastern China

The vertical profiles of nitrogen dioxide (NO2) and formaldehyde (HCHO) in the troposphere at the Longfengshan (LFS) regional atmospheric background station (127°36' E, 44°44' N, 330.5 m above sea level) from 24 October 2020 to 13 October 2021 were retrieved from solar scattering spectra by multi-axis differential optical absorption spectroscopy (MAX-DOAS). We analyzed the temporal variations of NO2 and HCHO as well as the sensitivity of ozone (O3) production to the concentration ratio of HCHO to NO2. The largest NO2 volume mixing ratios (VMRs) occur in the near-surface layer for each month, with high values concentrated in the morning and evening. HCHO has an elevated layer around the altitude of 1.4 km consistently. The means ± standard deviations of vertical column densities (VCDs) and near-surface VMRs were 4.69 ± 3.72 ×1015 molecule·cm-2 and 1.22 ± 1.09 ppb for NO2, and they were 1.19 ± 8.35 × 1016 molecule·cm-2 and 2.41 ± 3.26 ppb for HCHO. The VCDs and near-surface VMRs for NO2 were high in the cold months and low in the warm months, while HCHO presented the opposite. The larger near-surface NO2 VMRs appeared in the condition associated with lower temperature and higher humidity, but this relationship was not found between HCHO and temperature. We also found the O3 production at the Longfengshan station was mainly in the NOx-limited regime. This is the first study presenting the vertical distributions of NO2 and HCHO in the regional background atmosphere of northeastern China, which are significant to enhancing the understanding of background atmospheric chemistry and regional ozone pollution processes.

Composition Characteristics and Sources of Non-polar Organic Compounds in PM2.5 in the Background Atmosphere of Yangtze River Delta

In order to explore the composition and source characteristics of non-polar organic compounds (NPOCs) in atmospheric fine particulate matter in the Yangtze River Delta region, 129 PM2.5 samples were collected at the Regional Atmospheric background station in Lin'an from December 2019 to November 2020. Including polycyclic aromatic hydrocarbons (PAHs), n-alkanes, and hopanes, the main sources of organic aerosols were investigated using molecular tracers, eigen ratios, and orthogonal matrix factorization models. The results showed that the average annual mass concentration of PM2.5 in Lin'an was approximately (32.36±20.44) μg·m-3, and the average annual mass concentration of NPOCs was approximately (59.05±40.39) ng·m-3, showing the seasonal characteristics of being high in winter and low in summer. n-alkanes mainly came from fossil fuels and biomass (grass, wood, etc.) burning, followed by cuticle wax emission from higher plants. PAHs mainly came from the mixed contribution of non-fossil sources such as coal burning motor vehicle emissions and biomass combustion. Hopanes were mainly derived from motor vehicle emissions, which were also affected by coal burning in winter. Backward trajectory cluster analysis and potential source analysis showed that Lin'an was mainly affected by external air mass transport. Combined with the orthogonal matrix-factor decomposition model, NPOCs observed during the sampling period were analyzed, and non-fossil sources such as coal burning sources, transportation emission sources, and biomass combustion were obtained. In winter, transportation sources were the main source, accounting for 59%. In spring and summer, coal burning was the main source, accounting for 58% and 57%, respectively. In autumn, biomass combustion and other non-fossil sources dominated, accounting for 64%.

Temporal Variation and Source Analysis of Atmospheric CH4 at Different Altitudes in the Background Area of Yangtze River Delta

Through an analysis of CH4 data observed at different altitudes at the atmospheric background station in Lin’an from 2016 to 2020, in combination with back-trajectory and distribution characteristics of potential source areas, the CH4 concentration variations at higher and lower altitudes and their relationships with sources and sinks were studied. The results showed that the CH4 concentration was characterized by notable diurnal variations. The largest concentration difference occurred between 5 and 7 a.m.; the concentration difference in summer was higher than that in the other three seasons. Background filtering of the hourly CH4 concentration was carried out using a numerical method. The results showed that the difference in the CH4 background concentration between the two altitudes was 4.6 ppb (SD = 7.9). The CH4 background concentrations at the two altitudes had the same seasonal variation: double peaks and valleys. The peaks appeared in May and December, and the valleys appeared in March and July. In spring and summer, the potential CH4 source areas were mainly distributed in the rice planting and wetland discharge regions. In autumn, they were mainly distributed in regions affected by fugitive emissions from rice planting and coal mining. In winter, they were mainly distributed in livestock and poultry management regions.

Characteristics and source analysis of greenhouse gas concentration changes at Akedala Station in Central Asia

Mole fractions of atmospheric carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), and sulfur hexafluoride (SF6) have been continuously measured since September 2009 at the Akedala Station (47°06′N, 87°58′E, 563.3 masl) in China. The station is located in the Central Asia and northwest of China, and it is the only station in that region with background conditions for long-term greenhouse gas observations. Characteristics of the mole fractions, growth rates, and influence of long-distance transport were studied considering data from September 2009 to December 2019. The greenhouse gases concentrations at Akedala Station show a trend of year-on-year growth, with CO2 concentrations ranging from 389.80 × 10−6 to 408.79 × 10−6 (molar fraction of substances, same below), CH4 concentrations ranging from 1890.07 × 10−9 to 1976.32 × 10−9, N2O concentrations ranging from 321.26 × 10−9 to 332.03 × 10−9, and SF6 concentrations ranging from 7.04 × 10−12 to 10.07 × 10−12, the growth rate of which is similar to the decadal average growth rate in the Northern Hemisphere. There exist obvious seasonal variations, with CO2 concentrations showing high in winter and low in summer and CH4 showing a distinct “W”-shaped trend while N2O and SF6 showing little difference between the four seasons. A relatively strong correlation and homology exist among the four greenhouse gases except in summer, and the analysis based on backward trajectories model shows that the Akedala Station is influenced by the airflow from northwest or southwest throughout the year. The Akedala Station is an important atmospheric background station in Central Asia, and its greenhouse gas concentration levels and variation characteristics are significantly different from those of the background stations in the monsoon region. Its degree changes are closely related to local source emissions, monsoon transport, and atmospheric photochemical processes.

Analysis of Atmospheric CO2 and CO at Akedala Atmospheric Background Observation Station, a Regional Station in Northwestern China.

Air samples were collected by flasks and analyzed via a Picarro G2401 gas analyzer for carbon dioxide (CO2) and carbon monoxide (CO) at the Akedala Atmospheric Background Station in Xinjiang, China, from September 2009 to December 2019, to analyze the changes in the characteristics of atmospheric CO2 and CO and determine the sources. The results show that the annual average CO2 concentration showed an increasing trend (growth rate: 1.90 ppm year−1), ranging from 389.80 to 410.43 ppm, and the annual average CO concentration also showed an increasing trend (growth rate: 1.78 ppb year−1), ranging from 136.30 to 189.82 ppb. The CO2 concentration and growth rate were the highest in winter, followed by autumn, spring, and summer. The CO concentration and growth rate were also the highest in winter due to anthropogenic emissions, ecosystem effects, and diffusion conditions. The main trajectories of CO2 and CO determined by the Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) model were parallel to the Irtysh River valley and then passed through the Old Wind Pass. Furthermore, the main source regions of CO2 and CO at the Akedala Station were eastern Kazakhstan, southern Russia, western Mongolia, and the Xinjiang Tianshan North Slope Economic Zone of China. This study reflects the characteristics of long-term changes in CO2 and CO concentrations at the Akedala station and provides fundamental data for the studies on environmental changes and climate change in Central Asia.

Insights into aerosol chemical composition and optical properties at Lulin Atmospheric Background Station (2862 m asl) during two contrasting seasons

Continental outflows from peninsular Southeast Asia and East Asia dominate the widespread dispersal of air pollutants over subtropical western North Pacific during spring and autumn, respectively. This study analyses the chemical composition and optical properties of PM10 aerosols during autumn and spring at a representative high-altitude site, viz., Lulin Atmospheric Background Station (23.47°N, 120.87°E; 2862 m a.s.l.), Taiwan. PM10 mass was reconstructed and the contributions of major chemical components were also delineated. Aerosol scattering (σsp) and absorption (σap) coefficients were regressed on mass densities of major chemical components by assuming external mixing between them, and the site-specific mass scattering efficiency (MSE) and mass absorption efficiency (MAE) of individual components for dry conditions were determined. NH4NO3 exhibited the highest MSE among all components during both seasons (8.40 and 12.58 m2 g−1 at 550 nm in autumn and spring, respectively). (NH4)2SO4 and organic matter (OM) accounted for the highest σsp during autumn (51%) and spring (50%), respectively. Mean MAE (mean contribution to σap) of elemental carbon (EC) at 550 nm was 2.51 m2 g−1 (36%) and 7.30 m2 g−1 (61%) in autumn and spring, respectively. Likewise, the mean MAE (mean contribution to σap) of organic carbon (OC) at 550 nm was 0.84 m2 g−1 (64%) and 0.83 m2 g−1 (39%) in autumn and spring, respectively. However, a classification matrix, based on scattering Ångström exponent, absorption Ångström exponent, and single scattering albedo (ω), demonstrated that the composite absorbing aerosols were EC-dominated (with weak absorption; ω = 0.91–0.95) in autumn and a combination of EC-dominated and EC/OC mixture (with moderate absorption; ω = 0.85–0.92) in spring. This study demonstrates a strong link between chemical composition and optical properties of aerosol and provides essential information for model simulations to assess the imbalance in regional radiation budget with better accuracy over the western North Pacific.

Impact of 21 June 2020 Annular Solar Eclipse on Meteorological Parameters, O3 and CO at a High Mountain Site in Taiwan

The annular solar eclipse observed on 21 June 2020 over Taiwan provided a rare opportunity to follow the responses of various meteorological parameters and trace gases to the solar eclipse. For the first time, the impact of the solar eclipse on solar radiation, air temperature (T), relative humidity (RH), atmospheric pressure (P), wind speed (WS), and trace gases (ozone and carbon monoxide) is delineated at the Lulin Atmospheric Background Station (LABS; 23.47°N, 120.87°E; 2,862 m MSL) in Taiwan. Over the Taiwan region, the solar eclipse began at 14:49 local time (LT; UTC+8), reached maximum obscuration at 16:13 LT, and finished at 17:24 LT. Compared to the control period (average of 13–20 June 2020), the weather parameters and trace gases show pronounced changes on the day of the eclipse (21 June 2020). A significant decrease in UV-B and solar irradiance at LABS was observed during the peak phase of the solar eclipse due to the occultation of the Sun by the Moon. On the eclipse day, the T decreased significantly (~4°C) after 17 minutes of maximum solar darkening. Due to the cloudiness and low temperatures on the day of the eclipse, 100% RH was apparent during the period of the eclipse at LABS. We also noticed a steady decline in WS just after the onset of the eclipse at LABS. Interestingly, a marked decline in ozone and carbon monoxide was seen during the eclipse day at LABS. Ozone was reduced by about 10 ppb (40%), with a delay of about 2 hours from the peak phase of the eclipse. Overall, our results show the impact of the solar eclipse on high altitude measurements for the first time, and further this work provides a useful new contribution to the literature examining solar eclipse-induced atmospheric changes.

Humic-like substances (HULIS) in springtime aerosols at a high-altitude background station in the western North Pacific: Source attribution, abundance, and light-absorption

Atmospheric humic-like substances (HULIS) are important components of biomass-burning (BB) emissions and highly associated with light-absorbing organic aerosols (often referred to as brown carbon). This study highlights the importance of BB-emitted HULIS aerosols in peninsular Southeast Asian outflow to the subtropical western North Pacific. We determined various key light-absorbing characteristics of HULIS i.e. mass absorption cross-section (MACHULIS), absorbing component of the refractive index (kHULIS), and absorption Ångström exponent (AAEHULIS) based on ground-based aerosol light absorption measurements along with HULIS concentrations in springtime aerosols at Lulin Atmospheric Background Station (LABS; 2862 m above mean sea level), which is a representative high-altitude remote site in the western North Pacific. Daily variations of HULIS (0.58–12.92 μg m−3) at LABS were mostly linked with the influence from incoming air-masses, while correlations with BB tracers and secondary aerosols indicated the attribution of primary and secondary sources. Stronger light absorption capability of HULIS was clearly evident from MACHULIS and kHULIS values at 370 nm, which were about ~1.5 times higher during BB-dominated days (1.16 ± 0.75 m2 g−1 and 0.05 ± 0.03, respectively) than that during non-BB days (0.77 ± 0.89 m2 g−1 and 0.03 ± 0.04, respectively). Estimates from a simple radiative transfer model showed that HULIS absorption can add as much as 15.13 W g−1 to atmospheric warming, and ~46% more during BB-dominated than non-BB period, highlighting that HULIS light absorption may significantly affect the Earth–atmosphere system and tropospheric photochemistry over the western North Pacific.

Long-range transport of La Soufrière volcanic plume to the western North Pacific: Influence on atmospheric mercury and aerosol properties

Volcanic eruptions are important natural sources of gases and aerosols to the environment. However, their impact on the global cycle of trace elements, including mercury (Hg), remains poorly understood. On 9 April 2021, La Soufrière volcano (13.33°N, 61.18°W) erupted in the Caribbean Sea. Ozone Monitoring Instrument (OMI) measurements of sulfur dioxide (SO2) tracked the transport of volcanic SO2 in the troposphere over long distances from the source, including to the western North Pacific ∼10 day after eruption. A spike in gaseous and particle-bound mercury concentrations along with a peak in aerosol back-scattering coefficient at Lulin Atmospheric Background Station (LABS, 23.47°N, 120.87°E; 2862 m AMSL) coincided with the volcanic plume transport on 19 April. The slope of gaseous elemental mercury vs. carbon monoxide concentrations during the event was 0.027, which is distinctly higher than for Southeast Asia biomass burning (slope = 0.005) and East Asia outflow (0.011) pollution sources. Arrival of the plume high above Taiwan was confirmed by significant changes in signature aerosol properties. Thus, with critical observational evidence, our study reported for the first time enhanced mercury levels ∼18000 km downwind from a volcano source, providing an important check on volcanic Hg emission strength.

Detection of stratospheric intrusion events and their role in ozone enhancement at a mountain background site in sub-tropical East Asia

In this study, we investigated the influences of stratospheric intrusion (SI) on the tropospheric ozone (O3) at Lulin Atmospheric Background Station (LABS, 2862 m MSL), a representative high-altitude site in East Asia. Thresholds for surface O3, carbon monoxide (CO), and relative humidity (RH) were set to identify SI events at LABS. Accordingly, 3.0% of the observation period was impacted by SI leading to a net O3 enhancement of 14.6 ± 9.6 ppb at LABS. Factors influencing the two most common SI event types affecting LABS, i.e. tropopause folds (major) and tropical cyclones (minor), were discussed. We utilized ERA5 reanalysis products (meteorology and O3) to trace and analyze the pathway and mechanism of these SI types over sub-tropical East Asia. With a conceptual model, we illustrate two driving circulations for SI air at ∼200 hPa that affect subtropical East Asia. SI air generated due to Rossby wave breaks are transported equatorward and downward into the troposphere. The southern westerlies passing south of the Tibetan Plateau can further push the SI air eastward. A SI event in 2007 with a significant O3 enhancement of 43.4 ppb caused by typhoon Nari was detected and discussed to depict the second SI type.

Relationships between atmospheric mercury and optical properties of spring outflow aerosols from Southeast Asia

Southeast Asia is a major biomass burning (BB) source area and BB has been identified as an important source of mercury (Hg) and aerosols to the atmosphere. Previous studies found correlations between atmospheric Hg and Delta-C (BC370–BC880) during the BB episodes in low-elevation BB source areas; however, whether this relationship persists during long-range transport remains unknown. Lulin Atmospheric Background Station (LABS), a high mountain background site in central Taiwan, has monitored the impact of BB long-range transports from Southeast Asia on atmospheric Hg and aerosols in downwind areas since 2006 and 2008, respectively. As such, these data provide an opportunity to investigate the relationship between atmospheric Hg (PBM and GOM) and aerosol optical properties, such as Delta-C and absorption Ångström exponent (AAE370-880). In this study, three long-range transport BB events were observed at LABS in the springtime during 2014–2016. Significant positive correlations with PBM/GOM were observed for Delta-C and (R2 = 0.34–0.51) and AAE370-880 (R2 = 0.41–0.54) during the BB events, while poor correlations were shown under non-BB episodes. Additionally, the positive correlation between PBM and AAE370-880 suggests that the Hg(II) may be preferably absorbed onto organic aerosols or originated from the same emission sources. This study demonstrated that AAE370-880 can be used to identify the source of Hg(II) from BB long-range transport. In addition, our results also indicated the influences of aerosol chemical compositions on the partitioning of Hg(II), which is important to further the understanding of atmospheric Hg cycling.

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.

Effects of temperature and relative humidity on the partitioning of atmospheric oxidized mercury at a high-altitude mountain background site in Taiwan

Gas-particle partitioning of oxidized mercury (Hg) plays an important role in governing the speciation, transport and deposition of atmospheric Hg. Although studies on gas-particle partitioning of oxidized Hg have been conducted at some urban sites, comparable studies at remote mountain sites are still limited. This study analyzes multi-year (2014–2016) data of speciated atmospheric Hg concentrations from Lulin Atmospheric Background Station (LABS, 2862 m above sea level), Taiwan, to explore the factors that influence the gas-particle partitioning of atmospheric oxidized Hg. Mean concentrations (±S.D.) of gaseous elemental mercury (GEM), gaseous oxidized mercury (GOM), and particulate-bound mercury (PBM) were 1.54 ± 0.34 ng m−3, 14.5 ± 26.5 pg m−3, and 5.0 ± 12.0 pg m−3, respectively. In addition, our results indicated that the partitioning of Hg(II) toward particles was favored in the upper free troposphere and/or lower stratosphere. Both temperature (T) and relative humidity (RH) were found to strongly affect the gas-particle partitioning of oxidized Hg. Significant negative correlations between the partitioning coefficient (Kp) and T were obtained for all seasons, but peaked in summer. When RH<30%, Kp decreased with increasing RH. However, Kp and RH were positively correlated when RH>30%. Two empirical Kp-T and Kp-T-RH regression equations: log(1/Kp) = 15.0 – 3887.6(1/T) and log(1/Kp) = 17.92 – 4390.0(1/T) – 0.016RH were developed for free tropospheric air downwind of continental East Asia, which could be implemented in a chemical transport model to improve our understanding of the Hg biogeochemical cycle.

Real-time measurements of PM2.5 water-soluble inorganic ions at a high-altitude mountain site in the western North Pacific: Impact of upslope wind and long-range transported biomass-burning smoke

Real-time measurement of PM2.5 (particulate matter with aerodynamic diameter ≤ 2.5 μm) water-soluble inorganic ions (WSIIs) was conducted at Lulin Atmospheric Background Station (LABS; 2862 m above mean sea level) in the western North Pacific (WNP) during the autumn 2015 and spring 2016. During autumn, the mountain-valley (M-V) circulation is effective in increasing the mass concentrations of PM2.5, WSIIs, and carbon monoxide (CO) at LABS from 12:00 to 18:00 local time every day. In sharp contrast to autumn, five high aerosol-loading events were recorded during the spring, with each event persisting for a few days. These events were harmonized with the long-range transport of biomass-burning (BB) smoke emissions from northern peninsular Southeast Asia (PSEA), as disclosed from the fire count map and backward trajectories. The plumes appear to mask their characteristic diurnal features driven by the local M-V circulation. During both seasons, the prominent pathway of SO2 to SO42− conversion was aqueous phase oxidation as revealed from the relationship between sulfur oxidation ratio (SOR) and aerosol liquid-water content. Furthermore, NO3− was found to be produced primarily via homogeneous reactions during the spring season. The optical measurements were also made along with the semi-continuous measurements of WSIIs in this study. The magnitude of aerosol scattering and absorption coefficients exhibited high values when LABS was either under the influence of M-V circulation or long-range BB transport, although the latter event was apparently greater. Large surface cooling (up to −30.8 W m−2) and atmospheric warming (up to +21.3 W m−2) at LABS attributed to absorption enhancement by BB-induced aerosols can potentially influence the regional radiation budget.

Measurement report: Long-term variations in carbon monoxide at a background station in China's Yangtze River Delta region

Abstract. This study analyzed the long-term variations in carbon monoxide (CO) mixing ratios from January 2006 to December 2017 at the Lin'an regional atmospheric background station (LAN; 30.3∘ N, 119.73∘ E, 138 m a.s.l.) in China's Yangtze River Delta (YRD) region. The CO mixing ratios were at their highest (0.69 ± 0.08 ppm) and lowest (0.54 ± 0.06 ppm) in winter and summer, respectively. The average daily variation in CO exhibited a double-peaked pattern, with peaks in the morning and evening and a valley in the afternoon. A significant downward trend of −11.3 ppb yr−1 of CO was observed from 2006 to 2017 at the LAN station, which was in accordance with the negative trends of the average CO mixing ratios and total column retrieved from the satellite data (Measurements of Pollution in the Troposphere, MOPITT) over the YRD region during the same period. The average annual CO mixing ratio at the LAN station in 2017 was 0.51 ± 0.04 ppm, which was significantly lower than that (0.71 ± 0.12 ppm) in 2006. The decrease in CO levels was largest in autumn (−15.7 ppb yr−1), followed by summer (−11.1 ppb yr−1), spring (−10.8 ppb yr−1), and winter (−9.7 ppb yr−1). Moreover, the CO levels under relatively polluted conditions (the annual 95th percentiles) declined even more rapidly (−22.4 ppb yr−1, r=-0.68, p&lt;0.05) from 2006 (0.91 ppm) to 2017 (0.58 ppm), and the CO levels under clean conditions (the annual 5th percentiles) showed decreasing evidence but not statistically significant (r=-0.41, p=0.19) throughout the years. The long-term decline and short-term variations in the CO mixing ratios at the LAN station were mainly attributed to the implementation of the anthropogenic pollution control measures in the YRD region and to events like the Shanghai Expo in 2010 and Hangzhou G20 in 2016. The decreased CO level may influence atmospheric chemistry over the region. The average OH reactivity of CO at the LAN station is estimated to significantly drop from 4.1 ± 0.7 s−1 in 2006 to 3.0 ± 0.3 s−1 in 2017.

Quantitative analysis of accuracy of concentration inversion under different temperature and pressure

The CO&lt;sub&gt;2&lt;/sub&gt;, CH&lt;sub&gt;4&lt;/sub&gt; and other greenhouse gases are measured by using a home-made Fourier transform infrared spectrometer at the Longfengshan atmospheric background station. Compared with the measurement results of the instrument from the background station which meets the standards of the World Meteorological Organization, the correlation coefficient and the root mean square error of the CO&lt;sub&gt;2&lt;/sub&gt; concentration value are 0.9576 and 18.6015, so the measurement results from the home-made instrument are reliable. In the home-made instrument the calibration spectrum of standard temperature and the calibration spectrum of stand pressure are used to invert the concentration. With the temperature changing, the temperature of the measured gas will vary, thus resulting in error. The research of environmental variable factors can improve the accuracy of concentration inversion. For example, compared with CO&lt;sub&gt;2&lt;/sub&gt; absorption spectrum under 296 K, the CO&lt;sub&gt;2&lt;/sub&gt; absorption spectrum under 297 K will have 1.8% spectrum deviation and its inversion concentration error is 0.41%. This is the main cause of inversion concentration error. Based on the above analysis, the absorption cross section is calculated by using the high-resolution transmission molecular absorption database parameters. Combining with the instrument line shape, the calibration spectra at different temperatures and pressures can be obtained. The calibration spectra at different temperatures and pressures are used to calibrate the concentration inversion. After calibration, compared with the measurement results of the background station instrument, the correlation coefficient and the root mean square error of the CO&lt;sub&gt;2&lt;/sub&gt; concentration value are 0.9637 and 6.7803. The correlation coefficient of CO&lt;sub&gt;2&lt;/sub&gt; concentration value measured by self-developed instrument is improved and root mean square error is reduced. The result shows that the calibration algorithm enhances the accuracy of the measurement results to a certain extent. The above results illustrate the reliability of the home-made FTIR instrument and this experiment provides important data, which lay the foundation forstudying the home-made Fourier transform infrared spectrometer. Of course, improvement can be made in the following areas. Other minor factors may affect the effect of the inversion algorithm. The concentration inversion will have subtle differences at different bands of calibration spectra. So in order to improve the measurement accuracy, we need to choice more reasonable band inversion and more precise parameters from the high-resolution transmission molecular absorption database.