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.

Characteristics of the atmospheric boundary layer height: A perspective on turbulent motion

Turbulent motion is the essential difference between the atmospheric boundary layer and the free atmosphere. Due to the lack of detection methods for vertical turbulent structures, commonly used methods usually focus on material distribution, thermal effects, or dynamic effects, and they fail to reflect the boundary layer objectively in terms of turbulence. Therefore, to date, the acquisition and characteristic analysis of the atmospheric boundary layer height under turbulent angles have not been achieved. This study proposes a method for obtaining the height of the boundary layer based on the power-law exponent of atmospheric turbulence. The proposed method is validated and analyzed with data from radiosondes obtained under sunny, cloudy, and light rain weather conditions, demonstrating its advantages. With the proposed method, the first published acquisition of boundary layer height characteristics based on turbulent motion is achieved, including a statistical analysis of the daily and monthly variation characteristics of the boundary layer height over the Shenzhen area in China. Moreover, different oscillation frequencies of the boundary layer height under different wind directions are revealed. The results of this study break the traditional bottleneck of not being able to obtain the height of the boundary layer based on turbulence, and provide a new perspective for the acquisition and research of the boundary layer height.

Temporal variability, meteorological influences, and long-range transport of atmospheric aerosols over two contrasting environments Agartala and Patiala in India

The present study focused on the temporal variability, meteorological influences, potential sources, and long-range transport of atmospheric aerosols over two contrasting environments during 2011-2013. We have chosen Agartala (AGR) city in Northeast India as one of our sites representing the rural-continental environment and Patiala (PTA) as an urban site in Northwest India. The seasonal averaged equivalent black carbon (eBC) concentration in AGR ranges from 1.55 to 38.11 µg/m3 with an average value of 9.87 ± 8.17 µg/m3, whereas, at an urban location, PTA value ranges from 1.30 to 15.57 µg/m3 with an average value of 7.83 ± 3.51 µg/m3. The annual average eBC concentration over AGR was observed to be ~ 3 times higher than PTA. Two diurnal peaks (morning and evening) in eBC have been observed at both sites but were observed to be more prominent at AGR than at PTA. Spectral aerosol optical depth (AOD) has been observed to be in the range from 0.33 ± 0.09 (post-monsoon) to 0.85 ± 0.22 (winter) at AGR and 0.47 ± 0.04 (pre-monsoon) to 0.74 ± 0.09 (post-monsoon) at PTA. The concentration of eBC and its diurnal and seasonal variation indicates the primary sources of eBC as local sources, synoptic meteorology, planetary boundary layer (PBL) dynamics, and distant transportation of aerosols. The wintertime higher values of eBC at AGR than at PTA are linked with the transportation of eBC from the Indo-Gangetic Plain (IGP). Furthermore, it is evident that eBC aerosols are transported from local and regional sources, which is supported by concentration-weighted trajectory (CWT) analysis results.

The role of boundary layer height in India on transboundary pollutions to the Tibetan Plateau

The aerosols over the Tibetan Plateau (TP) play an important role in radiative budget and hydrologic cycle over Asia even the northern hemisphere. Adjacent to the major emission sources of air pollutants, transboundary pollutions transported to the TP due to the unique geographical location and climatic characteristics, is an important exogenous driver of multi-layer changes over the TP. The influence of boundary layer height (BLH) in India to the transboundary pollution over the TP from 1980 to 2018 was investigated in the study. Results showed that air pollutants transported to the TP is more efficient within the boundary layer compared with free troposphere. The BLH decreases with the rate of 1.8 m/season in these decades. Moreover, it also has a significant correlation with AOD (-0.4). Accompanied with westerly wind and the topographic forcing in the higher boundary layer, dust particles were uplifted from the northern India to the high altitude. Compared with a higher BLH, the lower BLH is difficult for the long transport of pollutants with weaker westerly wind over the TP and its difference of dust concentration with 0.2 μg m^-3 in the upper troposphere. The solar radiation enhancement increases the sensible heat and accelerate the upward of the atmosphere in high BLH events, which uplifts the pollutants accumulated in lower troposphere to higher altitudes and provides thermodynamic conditions for the pollutants transorted to the TP with westerly winds. This study provides confidence for the source, long-term transport of the TP aerosol, and its environmental and climatic impacts on climate systems in the Northern Hemisphere.

Study of aerosol optical properties at two urban areas in the north of Vietnam with the implication for biomass burning impacts

The spatiotemporal distribution and characterization of aerosol optical properties in the north of Vietnam were investigated extensively using the long-term measurements obtained from Moderate Resolution Imaging Spectroradiometer (MODIS) (from 2010 to 2019) and two Aerosol Robotic Network (AERONET) stations (Nghia Do, Hanoi City, from 2010 to 2019 and Son La from 2012 to 2019) located in fast-developing urban areas. This study also analyzed the tendency of AOD over different wavelengths as well as the influences of meteorological parameters and boundary layer height (BLH) on AOD and Ångström exponent (AE). The annual mean AOD₅₀₀ at Nghia Do and Son La stations were 0.81 and 0.78, respectively. Our results revealed the existence of particles emitted from anthropogenic activities in Hanoi and Son La with the dominance of fine particles (more than 90% of the AE_440-870 were larger than 1). Besides, a high percentage of AE_440-870 larger than or approximately 1.5 during the dry and transition months in Son La indicated the existence of biomass-burning aerosol particles due to forest fires and burning of agriculture residuals. The classification results for aerosol types showed the presence of both biomass burning and urban/industrial aerosol types at Nghia Do and Son La sites. Among the investigated meteorological parameters, surface solar radiation expressed a significant correlation with AE_440-870 in all three seasons at the two sites due to the secondary formation of fine particles induced by the high solar radiation condition. The impacts of meteorological parameters and BLH on AOD were not observed simultaneously during three seasons at Nghia Do and Son La stations.

Mass size distributions, composition and dose estimates of particulate matter in Saharan dust outbreaks

The present study highlights the importance of examining the contribution of Saharan dust (SD) sources not only in terms of overall mass contribution but also in terms of composition, size distribution and inhaled dose. The effect of SD intrusions on PM and the respective major and trace metals mass concentrations and size distributions was investigated in a suburban site in Athens, Greece. SD events were associated, on average, with lower boundary layer heights (BLH) compared to the non-Sahara (nSD) dust days. During SD events, PM_1-10 concentrations showed an increasing trend with increasing atmospheric BLH, in contrary to the fine PM (PM₁). Generally, increased PM₁ and CO (i.e. anthropogenic origin) levels were observed for BLH lower than around 500 m. The average contribution of SD to PM₁₀ and PM_2.5 mass concentration was roughly equal to 30.9% and 19.4%, respectively. The mass size distributions of PM and specific major and trace elements (Na, Al, Si, S, Cl, K, Ca, Fe, and Zn) displayed a somewhat different behavior with respect to the mass origin (Algeria-Tunisia vs Libya-Egypt), affecting in turn the regional deposition of inhaled aerosol in the human respiratory tract (HRT). The average PM deposited mass in the upper and lower HRT was 80.1% (Head) and 26.9% (Lung; Tracheobronchial and Pulmonary region) higher for SD days than for nSD days. Higher doses were estimated in the upper and lower HRT for the majority of the elements, when SD intrusions occurred, supporting the increasingly growing interest in exploring the health effects of SD. Only the mass deposition for S, and Na in the lower HRT and Zn in the upper HRT was higher in the case of nSD.

Is the atmospheric boundary layer altitude or the strong thermal inversions that control the vertical extent of aerosols?

It is well known that the atmospheric boundary layer (ABL) plays a significant role in controlling the variability of atmospheric constituents such as aerosols and trace-gases. Hence, significant diurnal and seasonal variation in these will be observed as the ABL altitude does. However, on several occasions, high aerosol concentration in the lidar measurements is observed even above the ABL altitude. This raised a question that up to what extent ABL altitude acts as a capping layer for these pollutants? From the detailed analysis carried out using long-term (2010-2018) lidar observations and simultaneous radiosonde profiles obtained from Gadanki, India, we show that 'there exist thermal inversions (TI), which are stronger than the ABL inversions, that fully control the vertical extent'. The detailed characteristics of TI (inversion strength (IS) and inversion depth (ID)) are also obtained. The results revealed that aerosol concentrations below the TI altitude increases with IS (ID) up to 3-4 K (300-400 m) during winter whereas in pre-monsoon it increases up to 2-3 K (100-200 m). Thus, IS of up to 2-4 K is required to fully trap the aerosol concentrations and this TI coincide with the ABL inversions for 51.7% only, particularly during the winter and pre-monsoon seasons. This analysis is further extended to different geographical locations of India using the aerosol profiles obtained from CALIPSO and a network of 23 radiosonde stations. The observed results provided further evidence that the vertical distribution of aerosols is restricted to the maximum extent by the TI but not the ABL altitude. These observations lead us to propose a hypothesis that 'trapping of aerosols fully occurs up to particular IS and ID only and the ABL altitude is not the deciding factor most of the time for capping the aerosol vertical distribution'. These findings will greatly help in modeling the diffusion and transport of air pollutants in the lower troposphere.

Factors influencing the boundary layer height and their relationship with air quality in the Sichuan Basin, China

We investigated the factors influencing the daily maximum boundary layer height (h_max) and their relationship with air quality in the Sichuan Basin, China. We analyzed the factors influencing h_max on cloudy and sunny days in winter using five years of observational data and a reanalysis dataset and investigated the relationship between h_max and air quality. The inversion layer in the lower troposphere has a critical impact on h_max on cloudy days. By contrast, the sensible heat flux and wind shear are the main influencing factors on sunny days, although the contribution of the sensible heat flux to h_max is less than that of the wind shear. This is because the turbulence is mainly affected by mechanical mixing induced by the topographic effect of the Tibetan Plateau to the west of the Sichuan Basin. The secondary circulation over the Sichuan Basin is weaker on cloudy days than on sunny days. These results are important for understanding the dispersion of air pollutants over the Sichuan Basin.

Impacts of meteorological conditions on wintertime PM2.5 pollution in Taiyuan, North China

Taiyuan frequently experiences heavy PM_2.5 pollution in winter under unfavorable meteorological conditions. To understand how the meteorological factors influence the pollution in Taiyuan, this study involved a systematic analysis for a continuous period from November 2016 to January 2017, using near-surface meteorological observations, radiosonde soundings, PM_2.5 measurements, and three-dimension numerical simulation, in combination with backward trajectory calculations. The results show that PM_2.5 concentration positively correlates with surface temperature and relative humidity and anti-correlates with near-surface wind speed and boundary layer height (BLH). The low BLH is often associated with a strong thermal inversion layer capping over. In addition to the high local emissions, it is found that under certain synoptic conditions, the southwesterly and southerly winds could bring pollutants from Linfen to Taiyuan, leading to a near-surface PM_2.5 concentration higher than 200 μg m^-3. Another pollution enhancing issue is due to the semi-closed basin of Taiyuan affecting the planetary boundary layer (PBL): the surrounding mountains favor the formation of a cold air pool in the basin, which inhibits vertical exchanges of heat, flux, and momentum between PBL and the free troposphere, resulting in stagnant conditions and poor air quality in Taiyuan. These findings can be utilized to improve the understanding of PM_2.5 pollution in Taiyuan, to enhance the accuracy of forecasting pollution, and to provide scientific support for policy makers to mitigate the pollution.

Diurnal and seasonal variations in surface methane at a tropical coastal station: Role of mesoscale meteorology

In view of the large uncertainties in the methane (CH₄) emission estimates and the large spatial gaps in its measurements, studies on near-surface CH₄ on regional basis become highly relevant. This paper presents the first time observational results of a study on the impacts of mesoscale meteorology on the temporal variations of near-surface CH₄ at a tropical coastal station, in India. It is based on the in-situ measurements conducted during January 2014 to August 2016, using an on-line CH₄ analyzer working on the principle of gas chromatography. The diurnal variation shows a daytime low (1898-1925ppbv) and nighttime high (1936-2022ppbv) extending till early morning hours. These changes are closely associated with the mesoscale circulations, namely Sea Breeze (SB) and Land Breeze (LB), as obtained through the meteorological observations, WRF simulations of the circulations and the diurnal variation of boundary layer height as observed by the Microwave Radiometer Profiler. The diurnal enhancement always coincides with the onset of LB. Several cases of different onset timings of LB were examined and results presented. The CH₄ mixing ratio also exhibits significant seasonal patterns being maximum in winter and minimum in pre-monsoon/monsoon with significant inter-annual variations, which is also reflected in diurnal patterns, and are associated with changing synoptic meteorology. This paper also presents an analysis of in-situ measured near-surface CH₄, column averaged and upper tropospheric CH₄ retrieved by Atmospheric Infrared Sounder (AIRS) onboard Earth Observing System (EOS)/Aqua which gives insight into the vertical distribution of the CH₄ over the location. An attempt is also made to estimate the instantaneous radiative forcing for the measured CH₄ mixing ratio.