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Secondary organic aerosol (SOA) makes a sizable contribution to fine-particulate-matter (PM2.5) pollution, especially during high-PM episodes. Past studies of SOA evolution at the episode scale mainly rely on measurements of bulk SOA mass, with few studies probing individual SOA molecular tracers. In this study, we continuously monitored (at a bi-hourly resolution) SOA tracers specific to a few common volatile organic compound (VOC) precursors at a suburban site in Hong Kong for a 4-month period from the end of August to December 2020. The SOA molecules include tracers for SOA derived from biomass burning (BB) emissions, monoaromatics, naphthalene/methylnaphthalenes, and three biogenic VOCs (isoprene, monoterpene, and sesquiterpene). Generally, the SOA tracers showed regional characteristics for both anthropogenic and biogenic SOA as well as for the BB-derived SOA. This work focused on the seasonal variation and evolution characteristics of SOA tracers during 11 city-wide PM2.5 episodes, which are defined as periods with PM2.5 concentrations exceeding 35 µg m−3 at 3 or more of the 15 general air quality monitoring stations cross the city. Mass increment ratios (MIR), calculated as the ratio of the mass concentration prior to an episode to that during an episode, were examined for individual species during each episode. During most episodes, the SOA tracer concentrations were enhanced (i.e. MIR >1), and the maximum MIR values were in the range of 5.5–11.0 for SOA tracers of different precursors. Episodes on summer and fall days showed notably larger MIR values than those falling on winter days, indicating the higher importance of SOA to the formation of summer/fall PM2.5 episodes. Simultaneous monitoring of six tracers for isoprene SOA revealed the dominance of the low-NOx pathway in forming isoprene SOA in our study region. The multiple monoterpene SOA products suggested fresher SOA in winter, evidenced by the increased presence of the early-generation products. Thus, the current study has shown by example the precursor-specific SOA chemical evolution characteristics during PM2.5 episodes in different seasons. This study also suggests the necessity to apply high-time-resolution organic marker measurement at multiple sites in order to fully capture the spatial heterogeneity of haze pollution at the city scale.

As the theoretical upper bound of evapotranspiration (ET) or water use by ecosystems, potential ET (PET) has always been widely used as a variable linking a variety of disciplines, such as climatology, ecology, hydrology, and agronomy. However, substantial uncertainties exist in the current PET methods (e.g., empiric models and single-layer models) and datasets, because of unrealistic configurations of land surface and unreasonable parameterizations. Therefore, this study comprehensively considered interspecific differences in various vegetation-related parameters (e.g., plant stomatal resistance and CO2 effects on stomatal resistance) to calibrate and parametrize the Shuttleworth-Wallace (SW) model for forests, shrubland, grassland and cropland. We derived the parameters using identified daily ET observations with no water stress (i.e., PET) at 96 eddy covariance (EC) sites across the globe. Model validations suggest that the calibrated model could be transferable from known observations to any location. Based on four popular meteorological datasets, relatively realistic canopy height and time-varying land use/land cover and Leaf Area Index, we generated a global 5 km ensemble mean monthly PET dataset that includes two components of potential transpiration (PT) and soil evaporation (PE) for the 1982–2015 time period. Using this new dataset, the climatological characteristics of PET partitioning and the spatio-temporal changes in PET, PE and PT were investigated. The global mean annual PET was 1200 mm with PT/PET of 40 % and PE/PET of 60 %, and moreover controlled by PT and PE over 43 % and 57 % of the globe, respectively. Globally, the annual PET and PT significantly (phttps://doi.org/10.11888/Terre.tpdc.300193 (Sun et al., 2023).

Secondary organic aerosol (SOA) makes a sizable contribution to fine particulate matter (PM2.5) pollution, especially during episodic hours. Past studies of SOA evolution at episode-scale mainly rely on measurements of bulk SOA mass and few studies probe individual SOA molecular tracers. In this study, we continuously monitored at a bihourly resolution SOA tracers specific to a few common volatile organic compound (VOC) precursors at a suburban site in Hong Kong for four-month from the end of Aug. to Dec. 2020. The SOA molecules include tracers for SOA derived from biomass burning emissions, monoaromatics, naphthalene/methylnaphthalenes, and three biogenic VOCs (i.e., isoprene, monoterpene and sesquiterpene). Generally, the SOA tracers showed regional characteristics for both anthropogenic and biogenic SOA, as well as the biomass burning-derived SOA. This work focused on the seasonal variation and evolution characteristics of SOA tracers during eleven city-wide PM2.5 episodes, which are defined to be periods of PM2.5 exceeding 35 μg/m3 at three or more of the 15 general air quality monitoring stations cross the city. Mass increment ratios (MIR), calculated as the ratio of mass concentration between before and during an episode, were examined for individual species in each episode. During most episodes, the SOA tracers were enhanced in their concentrations (i.e., MIR > 1) and maximum MIR values were in the range of 5.5–11.0 for SOA tracers of different precursors. Episodes on summer and fall days showed notably larger MIR values than those falling on winter days, indicating a higher importance of SOA to formation of summer/fall PM2.5 episodes. Simultaneous monitoring of six tracers for isoprene SOA revealed the dominance of the low-NOx pathway in forming isoprene SOA in our study region. The multiple monoterpene SOA products suggested fresher SOA in winter, evidenced by an increased presence of the early generation products. The current study has shown by example the precursor-specific SOA chemical evolution characteristics during PM2.5 episodes in different seasons. This study also suggests the necessity to apply the high time resolution organic marker measurement at multiple sites to fully capture the spatial heterogeneity of the haze pollution at the city scale.

Global warming has a significant impact on the ecosystem of the high-altitude areas of the Tibetan Plateau, such as land degradation and the rapid retreat of glaciers. It is important to understand the effects of global warming occurring over hundreds or thousands of years. However, the long-term reconstruction of temperature is still lacking for the region of Western Sichuan Plateau (WSP). In this study, we reconstructed summer (June–July) temperatures back to 1383 for the WSP region based on a significant positive correlation between the standard chronology and summer temperature. The reconstruction passed statistical tests and explained 55.4% of the climatic variance in the period for which data are available (1962 to 2005). There have been five periods with warm summers and five with cold summers during the past 623 years. Warm periods occurred in 1383–1445, 1525–1590, 1700–1724, 1825–1870, and 1930–1960, while relatively cold periods occurred in 1446–1524, 1591–1675, 1725–1824, 1900–1929, and 1961–1990. A comparison with observed data and regional temperature reconstruction series shows that the reconstruction has a high degree of consistency and is indicative and reliability. The spatial correlation was analyzed with Climatic Research Unit gridded data to detect the spatial representativeness of the reconstruction, confirming that the reconstruction has a strong regional temperature signal for the WSP. Furthermore, ensemble empirical mode decomposition indicates quasi-oscillations of 2.8–3.3 years, 6.4–8.3 years, 11.1–14.2 years, 25–38.4 years, 89 years, and 138 years in the reconstructed temperature time series during the past 623 years. El Niño Southern Oscillation cycles, solar activity, and Atlantic multi-decadal oscillation can potentially be the primary forcing factors. Meanwhile, the coordinated action of temperature-induced drought stress with a late growing season related to delayed snowmelt can potentially be attributed to the divergence problem (observed around 2005).