Non-heating Season Research Articles

Seasonal heat regulation in photovoltaic/thermal collectors with switchable backplate technology: Experiments and simulations

Photovoltaic/thermal (PV/T) collectors can simultaneously generate electrical and thermal energy, but the overheating phenomenon often occurs in hot seasons, leading to poor electrical performance and shortened lifetime in PV modules. In this study, a thermal regulation strategy by switching the backplate is introduced into the conventional PV/T collector, which can adapt to different heat demands in different seasons by opening or closing the backplate, achieving better electrical performance in summer without affecting winter heating. Comparative experiments between the switchable and conventional PV/T collectors are conducted to observe the performance advantage. The results show that the maximum stagnation temperature of the switchable PV/T collector is 18.1 °C lower than the conventional one, which relatively improves electrical efficiency by 8.6%. Subsequently, an annual performance prediction by a validated mathematical model illustrates that the thermal performance of the switchable PV/T collector in the heating season is comparable to the conventional PV/T collector, but the electrical efficiency can be increased by 6.26% in the non-heating season. Moreover, further study shows the proposed strategy has efficient regulation in other types of PV/T collectors similarly, demonstrating the positive role of switchable backplate technology in enhancing electrical efficiency and addressing the seasonal contradiction in PV/T collector operation.

Particulate polycyclic aromatic hydrocarbons in rural households burning solid fuels in Xuanwei County, Southwest China: occurrence, size distribution, and health risks.

The study is about the size distribution and health risks of polycyclic aromatic hydrocarbons (PAHs) in indoor environment of Xuanwei, Southwest China particle samples were collected by Anderson 8-stage impactor which was used to gather particle samples to nine size ranges. Size-segregated samples were collected in indoor from a rural village in Xuanwei during the non-heating and heating seasons. The results showed that the total concentrations of the indoor particulate matter (PM) were 757 ± 60 and 990 ± 78 μg/m3 in non-heating and heating seasons, respectively. The total concentration of indoor PAHs reached to 8.42 ± 0.53 μg/m3 in the heating season, which was considerably greater than the concentration in the non-heating season (2.85 ± 1.72 μg/m3). The size distribution of PAHs showed that PAHs were mainly enriched in PMs with the diameter <1.1 μm. The diagnostic ratios (DR) and principal component analysis (PCA) showed that coal and wood for residential heating and cooking were the main sources of indoor PAHs. The results of the health risk showed that the total deposition concentration (DC) in the alveolar region (AR) was 0.25 and 0.68 μg/m3 in the non-heating and heating seasons respectively. Throughout the entire sampling periods, the lifetime cancer risk (R) based on DC of children and adults varied between 3.53 ×10-5 to 1.79 ×10-4. During the heating season, the potential cancer risk of PAHs in adults was significant, exceeding 10-4, with a rate of 96%.

Control priority based on source-specific DALYs of PM2.5-bound heavy metals by PMF-PSCF-IsoSource model in urban and suburban Beijing

To determine the priority control sources, an approach was proposed to evaluate the source-specific contribution to health risks from inhaling PM2.5-bound heavy metals (PBHMs). A total of 482 daily PM2.5 samples were collected from urban and suburban areas of Beijing, China, between 2018 and 2019. In addition to the PMF-PSCF model, a Pb isotopic IsoSource model was built for more reliable source apportionment. By using the comprehensive indicator of disability-adjusted life years (DALYs), carcinogenic and noncarcinogenic health risks could be compared on a unified scale. The study found that the annual average concentrations of the total PBHMs were significantly higher in suburban areas than in urban areas, with significantly higher concentrations during the heating season than during the nonheating season. Comprehensive dust accounted for the largest contribution to the concentration of PBHMs, while coal combustion contributed the most to the DALYs associated with PBHMs. These results suggest that prioritizing the control of coal combustion could effectively reduce the disease burden associated with PBHMs, leading to notable public health benefits.

Study on long-term operation characteristics of the medium-deep ground source heat pump system with solar heat storage

Long-term heating operation of the medium-deep ground source heat pump (MGSHP) system will cause the underground temperature around the mid-deep borehole heat exchanger (MBHE) significantly lower than its initial value, which will reduce the operating efficiency of the MGSHP system. In some cases, the temperature of the circulating water in MBHE even is lower than the lowest value required by the heat pump unit, which will lead to operation failure of the unit. Attenuating the ground temperature decrease by storing heat underground during non-heating seasons is an effect way to maintain the heat extraction capacity of the MBHE. In this paper, the influence of storing solar energy underground during non-heating seasons on the long-term operating characteristics of the MGSHP system is studied. The MBHE heat transfer model which considers the heat storage process is developed and embedded in the software of TRNSYS. Based on the numerical simulation model of solar assisted mid-deep ground source heat pump (SAMGSHP) system built by TRNSYS, the long-term characteristics including the variations of the fluid temperature of the MBHE, the ground temperature distribution and the COP of the heat pump unit of the MGSHP system with heat storage during non-heating seasons are analyzed and compared with the case without heat storage. The results show that storing solar energy underground during non-heating seasons can effectively improve the ground temperature, significantly slow down the decline of the circulating water temperature of the MBHE in heating seasons and apparently increase the COP of the heat pump unit. The results can serve as a reference for engineering applications of MGSHP systems, especially those cannot completely meet the original design heating load.

Exploring Heat Demand Forecasting in District Heating Networks Using Random Parameter Linear Regression Model

Abstract Accurate forecasting of heat demand in district heating networks is essential for their efficient and sustainable operation. This paper presents a novel approach using a random parameter linear regression model to forecast heat demand, distinguishing itself from classical linear regression models by its ability to address unobserved heterogeneity among parameters. Through a case study in Estonia and utilizing data from 2018 to 2023 and considering seasonality and consumption patterns, the study investigates determinants of heating demand in district heating networks. Two models were trained for heating and non-heating seasons. Results indicate significant impacts of weather conditions, energy prices, time of day, and network infrastructure on heat supply during the heating season, while only time of day and electricity prices were significant drivers during the non-heating season, with no notable influence of weather conditions. Prediction accuracy was slightly enhanced using the random parameter linear regression model, with a mean absolute percentage error of 9.66 % compared to 9.99 % for the Multi Linear Regression Model on the testing set.

Unintended consequences of English school stock energy-efficient retrofit on cognitive performance of children under climate change

School building stock retrofit forms a key part of UK's commitment to net-zero carbon target by 2050. However, with a changing climate, the retrofit of school buildings may have unintended consequences on classroom thermal environments and cognitive performance of children in non-heating seasons. This paper aims to quantify the impact of school stock retrofit in accordance with increasingly tightening energy efficiency regulatory requirements on cognitive performance of English children, while also exploring the potential adaptation measures under climate change. The results indicate that English schools that undergo envelope insulation exhibit higher Cognitive Performance Loss (CPL) of children compared to their original conditions. Passive climate adaptation measures, especially increased daytime ventilation rate was found to be an effective strategy for mitigating the impact of climate change on cognitive performance. However, the benefits of passive measures will diminish as the climate warms, while air conditioning will be required to maintain cognitive performance loss at relatively low levels. In addition to the reduction of heating load, envelope insulation can provide benefits for English schools in cooler climatic regions from both cognitive performance and energy point of view. The study calls for the CPL to be added as one of the key performance indicators when considering the long-term impact of climate change on schools. This would enable policy makers and relevant stakeholders to make more holistic decisions regarding school stock retrofit while ensuring that classrooms are conducive to learning.

Short-term association of fine particulate matter and its constituents with oxidative stress, symptoms and quality of life in patients with allergic rhinitis: A panel study

BackgroundShort-term exposure to fine particulate matter (PM2.5) and its specific constituents might exacerbate allergic rhinitis (AR) conditions. However, the evidence is still inconclusive. MethodWe conducted a panel study of 49 patients diagnosed with AR > 1 year prior to the study in Taiyuan, China, to investigate associations of individual exposure to PM2.5 and its constituents with oxidative parameters, symptoms, and quality of life among AR patients. All participants underwent repeated assessments of health and PM exposure at 4 time points in both the heating and nonheating seasons from June 2017 to January 2018. AR patients’ oxidative parameters were assessed using nasal lavage, and their subjective symptoms and quality of life were determined through in-person interviews using a structured questionnaire. Short-term personal exposure to PM2.5 and its constituents was estimated using the time-microenvironment-activity pattern and data from the nearest air sampler, respectively. We applied mixed-effects regression models to estimate the short-term effects of PM2.5 and its constituents. ResultsThe results showed that exposure to PM2.5 and its constituents, including BaP, PAHs, SO42−, NH4+, V, Cr, Cu, As, Se, Cd, and Pb, was significantly associated with increased oxidative stress, as indicated by an increase in the malondialdehyde (MDA) index. Exposure to PM2.5 and its components (V, Mn, Fe, Zn, As, and Se) was associated with decreased antioxidant activity, as indicated by a decrease in the superoxide dismutase (SOD) index. Additionally, increased visual analog scale (VAS) and rhinoconjunctivitis quality of life questionnaire (RQLQ) scores indicated that exposure to PM2.5 and its constituents exacerbated inflammatory symptoms and affected quality of life in AR patients. ConclusionExposure to PM2.5 and specific constituents, could exacerbate AR patients' inflammatory symptoms and adversely affect their quality of life in the heavily industrialized city of Taiyuan, China. These findings may have potential biological and policy implications.

Insights the dominant contribution of biomass burning to methanol-soluble PM2.5 bounded oxidation potential based on multilayer perceptron neural network analysis in Xi'an, China

Atmospheric fine particulate matter (PM2.5) is associated with cardiorespiratory morbidity and mortality due to its ability to generate reactive oxygen species (ROS). Ambient PM2.5 samples were collected during heating and nonheating seasons in Xi'an, China, and the ROS-generation potential of PM2.5 was quantified using the dithiothreitol (DTT) assay. Additionally, positive matrix factorization combined with multilayer perceptron was employed to apportion sources contributing to the oxidation potential of PM2.5. Both the mass concentration of PM2.5 and the volume-based DTT activity (DTTv) were higher during the heating season than during the nonheating season. The primary contributors to DTTv were combustion (biomass and coal) sources during the heating season (>52 %), whereas secondary formation dominated DTT activity during the nonheating season (35.7 %). In addition, the secondary reaction process promoted the generation of intrinsic oxidation potential (OP) of sources. Among all the sources investigated (traffic source, industrial emission, mineral dust, biomass burning, secondary formation and coal combustion), the inherent oxidation potential of biomass burning was the highest, whereas that of mineral dust was the lowest. Our study indicates that anthropogenic sources, especially biomass burning, should be prioritized in PM2.5 toxicity control strategies.

Examining the Risk of Summertime Overheating in UK Social Housing Dwellings Retrofitted with Heat Pumps

The UK government has announced its ten-point plan to annually install 600,000 low-carbon heat pumps by 2028. However, there is a lack of evidence showing potential overheating risk in dwellings retrofitted with heat pumps. This paper examines the prevalence and magnitude of summertime overheating across 24 naturally ventilated social housing dwellings retrofitted with ground source heat pumps (GSHPs). The dwellings are located in a socially deprived area in Oxford (UK). The empirical study included longitudinal monitoring of indoor temperatures in the living rooms and bedrooms during the non-heating seasons of 2021 and 2022 (May–September), which included a record-breaking heatwave in July 2022. Indoor temperature and CO2 levels in bedrooms were monitored across a subset of six dwellings alongside the monitoring of window opening state in three bedrooms to understand the effect of natural ventilation in removing excess heat. About 136 thermal comfort surveys were conducted to ascertain the subjective responses of residents. Overheating risk assessment was carried out using CIBSE static and adaptive methods, which revealed that summertime overheating was prevalent across half of the dwellings in the non-heating season of 2022, as compared to 17% overheated dwellings in 2021. Bungalows with upgraded cavity wall insulation and top floor flats facing south and south-west had a propensity to overheat. The variation in indoor temperature and CO2 levels across a small sample also indicated the relationship between overheating and residents’ behaviour. Given that the majority of the dwellings were occupied by retired elderly people with low incomes who are vulnerable to heat and cannot afford active forms of cooling, it is vital to deploy passive design measures, such as appropriate shading devices that are suitable for a heating-dominated climate and enhanced ventilation, as part of home energy retrofits. Implementing reversible heat pumps coupled with solar PVs can provide cooling during heatwaves while delivering low-carbon heat in the winter.

A study of temperature variability on admissions and deaths for cardiovascular diseases in Northwestern China

ObjectiveTo explore the effect of temperature variability (TV) on admissions and deaths for cardiovascular diseases (CVDs).MethodThe admissions data of CVDs were collected in 4 general hospitals in Jinchang City, Gansu Province from 2013 to 2016. The monitoring data of death for CVDs from 2013 to 2017 were collected through the Jinchang City Center for Disease Control and Prevention. Distributed lag nonlinear model (DLNM) was combined to analyze the effects of TV (daily temperature variability (DTV) and hourly temperature variability (HTV)) on the admissions and deaths for CVDs after adjusting confounding effects. Stratified analysis was conducted by age and gender. Then the attribution risk of TV was evaluated.ResultsThere was a broadly linear correlation between TV and the admissions and deaths for CVDs, but only the association between TV and outpatient and emergency room (O&ER) visits for CVDs have statistically significant. DTV and HTV have similar lag effect. Every 1 ℃ increase in DTV and HTV was associated with a 3.61% (95% CI: 1.19% ~ 6.08%), 3.03% (95% CI: 0.27% ~ 5.86%) increase in O&ER visits for CVDs, respectively. There were 22.75% and 14.15% O&ER visits for CVDs can attribute to DTV and HTV exposure during 2013–2016. Males and the elderly may be more sensitive to the changes of TV. Greater effect of TV was observed in non-heating season than in heating season.ConclusionTV was an independent risk factor for the increase of O&ER visits for CVDs, suggesting effective guidance such as strengthening the timely prevention for vulnerable groups before or after exposure, which has important implications for risk management of CVDs.

Research on multi-objective optimization of control strategies and equipment parameters for a combined heating system of geothermal and solar energy in cold and arid regions based on TRNSYS

Focusing on the multiple objectives of maximum solar heating capacity, maximum COP of heat pump and minimum soil heating capacity, a model of one typical building and two heating strategies was established in the TRNSYS. Non-dominated Sorting Genetic Algorithm (NSGA) and Technique for Order Preference by Similarity to Ideal Solution (TOPSIS) based on entropy weight are used to optimize operation parameters. Optimized control strategies and equipment parameters of a combined geothermal and solar system in cold and arid regions were taken as the research objects. The final optimized strategy and the original strategy of the single ground source heat pump heating system were compared. Research results show that the optimized control strategy of the geothermal solar heating system transforms solar hot water from the storage tank to heating terminals in the heating season and transforms solar hot water to the ground heat exchanger in the non-heating season. Compared to GSHP, the optimizing strategy saves 16.8 MWh annually, and the energy saving rate is 20.59%. Soil temperature drop increases from 2.74 °C to 0 °C. The annual electricity saving cost intensity is about 4 yuan/m2, and the CO2 emission reduction intensity is 4.3 kg CO2/m2. This study provides a solution for soil temperature reduction in one year and low-cost operation in cold and arid regions during ground source heat pump applications.

Geochemical characteristics and socioeconomic associations of carbonaceous aerosols in coal-fueled cities with significant seasonal pollution pattern

Carbonaceous aerosols, comprising organic carbon (OC) and elemental carbon (EC), are critical component of fine particulate matter (PM2.5), with diverse impacts on air quality and human health. This study investigated the concentrations and seasonal patterns of carbonaceous species in PM2.5 during both the heating season (January 2021) and non-heating season (July 2021) in three coal-fueled cities in northern China, as well as the differences in carbonaceous aerosols and their associations with socioeconomic parameters in cities situated on either side of the “Hu Line” in China. The results showed that, owing to intensified coal combustion and unfavorable meteorological conditions, levels of OC, EC, and OC/EC ratios were higher in winter compared to summer. Moreover, the presence of dust (DU) and light pollution (LP) days resulted in elevated OC levels but decreased EC levels. The Char-EC/Soot-EC ratios were highest during LP, followed by CL and DU. A source apportionment analysis demonstrated that coal burning, vehicle exhaust, road dust, and biomass burning were the primary contributors to carbonaceous aerosols, as confirmed by diagnostic ratios, Char-EC/Soot-EC ratios, and PCA analysis. Furthermore, our study found that carbonaceous aerosols concentrations and source apportionment primarily varied with diurnal and seasonal trends and different pollution types. Additionally, at the national scale, population density and urban green space exhibited a positive correlation with OC/EC ratios (p < 0.05), while energy consumption per unit of GDP showed a negative correlation (p < 0.05). The observation that OC/EC ratios were lower in coal-fueled cities than in economy-based cities suggests a more severe pollution scenario. These findings highlight the importance of comprehending of the seasonal variation and chemical characteristics of carbonaceous aerosol for understanding air pollution sources and characteristics, which is essential for both air quality management and human health.

Mechanical and permeability tests under the influence of geomembrane temperature

Aquatic solar energy cross-season heat-storage technology can store and transfer abundant solar energy from the non-heating season to the heating season, which is one of the important technical routes for renewable energy to be used for clean building heating. Anti-seepage components are an important part of the thermal storage of the water body. As a common anti-seepage material for the thermal storage of the water body, a geomembrane is laid on the pool body and the top cover. In this paper, the mechanical and permeability changes of two kinds of geomembranes are analyzed based on the tests under different temperature environments. The results show that GM1 has better thermal stability. The elongation at break of GM1 increases by 33% and GM2 by 189% when the temperature rises by 40 °C. The osmotic coefficient of GM1 is reduced by 45% and GM2 by 58%.

Molecular Characteristics, Sources, and Health Risk Assessment of Gaseous Carbonyl Compounds in Residential Indoor and Outdoor Environments in a Megacity of Northwest China

Carbonyl compounds (CCs) in indoor air pose a significant threat to residents’ health and have garnered considerable attention in recent years. However, most studies have focused on low-molecular-weight carbonyl compounds (LMW-CCs) and have underestimated the impact of high-molecular-weight ones (HMW-CCs), causing a failure to comprehensively understand their effects on health. In this study, we analyzed twenty carbonyls in the indoor and outdoor air at typical residential communities in a megacity in Northwest China by using high-performance liquid chromatography (HPLC) coupled with a photodiode array detector (DAD). The total concentration of indoor carbonyls was 1.4-3.4 times that of outdoor carbonyls. In addition, the concentration of indoor carbonyls was much higher during the heating season than that during the nonheating season. Conversely, the concentration of outdoor carbonyls was higher during the nonheating season than that during the heating season. The principal component analysis (PCA) revealed that indoor carbonyl pollution was primarily influenced by building materials, cooking fume, and wooden furniture. Formaldehyde exposure in indoor environments posed a greater health risk to children than acetaldehyde exposure. HMW-CCs were the primary contributors to indoor odor pollution, which was considered a significant cause of sick building syndrome (SBS). Our findings underscore the crucial role of HMW-CCs in indoor environments in exerting adverse impacts on health.

Performance prediction of a curved-type solar balcony combined with the flexible PV/T system during the non-heating season

Curved surfaces are vital elements for well-designed buildings, while related system design is scarce in the building-integrated photovoltaic/thermal (BIPV/T) field. This study proposes a curved-type solar balcony combined with the flexible PV/T system, aiming to realize the curved PV/T façade application, which could generate electricity and provide hot water. The theoretical model to describe the electrical and thermal behavior of the curved balcony system is put forward and verified. Furthermore, the performance of the curved-type system and the flat-type system with the same absorb collector area is investigated based on the weather data in Hefei, China. The main results are: (1) The curved-type system could enhance the electrical yield and heat gain by the PV/T balcony according to the analysis based on a sunny day and a cloudy day in summer. The enhancement would be more obvious when the proportion of beam radiation is higher. (2) The performance of the comparative systems during the non-heating season demonstrates that the curved system could improve the electrical yield, heat gain, and total exergy. (3) The parametric analysis indicates that the curved system with a bigger central angle and facing south performs better.

Study on the influence of borehole heat exchanger structure and soil thermal property on soil heat storage characteristics of the HST-BHE system

To effectively solve the problem of soil thermal imbalance in cold regions, a new soil heat storage system that absorbs heat from ambient air through heat source tower (HST) and injects the heat into soil through borehole heat exchanger (BHE) in the non-heating season has been proposed in author’s previous research. However, the influence of structure parameters of the BHE and soil thermal properties on soil heat storage characteristics is not deeply studied. In this paper, heat and mass transfer mathematical model of the integrated system of HST and BHE (named as HST-BHE system) was built based on the developed models of BHE and HST firstly. Then, the influence of BHE structural parameters and soil thermal properties on soil heat storage characteristics was studied. The results show that the soil heat storage capacity of the HST-BHE system and temperature difference between the inlet and outlet of BHE increase with the increases of borehole length, buried pipe spacing or soil thermal conductivity, and decrease with the increases of borehole radius under specific conditions. While the borehole wall temperature of the BHE increases with the increase of buried pipe spacing and decreases with the increase of borehole depth, borehole radius or soil thermal conductivity. Unusually, increasing the water flow velocity in BHE will decrease the temperature difference but increase the soil heat storage capacity. These soil heat storage characteristics are controlled by the heat transfer thermal resistances between water and soil in the BHE as well as water and air in the HST, and all of the impact parameters should be optimized in actual engineering applications when considering the economy of the HST-BHE system.

Indoor air quality in Hungarian Passive Houses

The indoor air quality (IAQ) was investigated in 15 Passive Houses in the heating and non-heating seasons between 2019 and 2021 in Hungary. The concentrations of volatile organic compounds (VOCs), aldehydes, ozone, nitrogen dioxide, PM2.5 mass, carbon dioxide, bacteria, fungi, and pollen were measured together with the monitoring of temperature, relative humidity and air change rate (ACR). The IAQ varied considerably among the investigated buildings. Significant seasonal differences were obtained for all physical parameters (temperature, relative humidity, ACR), certain VOCs (benzene, α-pinene, limonene), acetaldehyde, and airborne fungi. Considerable health concern was associated with the indoor concentrations of PM2.5 mass and nitrogen dioxide in many cases based on the evaluation of IAQ in relation to potential adverse health effects, while the peak concentrations of other pollutants (e.g. trichloroethylene, α-pinene, certain aldehydes, fungi) were also of concern in a couple of cases. The median indoor/outdoor concentration ratios of benzene, PM2.5 mass, nitrogen dioxide, ozone and fungi indicated that these pollutants are mainly of outdoor origin, while the other VOCs, aldehydes and bacteria showed higher concentration indoors. Overheating, the lack of proper particle filters in the mechanical ventilation system, and low ACR and relative humidity were identified as frequent problems related to the building characteristics. The emissions from building materials and furniture, the proximity of construction works and unpaved roads, and the allergenic vegetation might considerably influence IAQ. The results highlight that risk reduction measures are needed to create healthier indoor environment in the Passive Houses.

Organosulfur compounds in ambient fine particulate matter in an urban region: Findings of a nontargeted approach

Organosulfur compounds (OSCs) are important components of fine particulate matter (PM2.5); however, little information is available on OSCs in urban regions due to their chemical complexity, especially for novel species such as aromatic sulfonates. To supplement the detection technique and systematically identify OSCs, in this study we developed a nontargeted approach based on gas chromatography and high-resolution mass spectrometry (GC-HRMS) to screen OSCs in PM2.5 of urban Beijing and provide field evidence for their source and formation mechanism. 76 OSCs were found through mass difference of sulfur isotopes and characteristic sulfur-containing fragments. 6 species were confirmed as aromatic sulfonates by authentic standards. 32 OSCs showed higher levels in the heating season, presumably because of the intensive emission, especially from coal combustion. While certain species, with 2-sulfobenzoic acid as the representative, were 2.6-times higher in the non-heating season than in the heating season. Such species were significantly correlated with ozone and aerosol liquid water content (r = 0.2–0.8, p < 0.05), implying an oxidation-involved aqueous-phase formation in the atmosphere. In addition, with an average proportion of ∼95 % of the total sulfobenzoic acids, the predominance of the 2-substitution product over its isomers of 3- or 4-sulfobenzoic acid suggests a more plausible mechanism of radical-initiated reaction of phthalic acid followed by sulfonation, with atmospheric reactivity indicated by ozone and temperature as the determining factor. This study provided not only a nontargeted approach for OSCs in ambient PM2.5, but also field evidence on their secondary formation proposed in previous simulation studies.

Pollution Characteristics and Factors Influencing the Reduction in Ambient PM2.5 in Beijing from 2018 to 2020

Air quality data from 33 environment sites and five regional sites from 2018 to 2020, as well as meteorological data, were used to research PM2.5 variation,spatial and temporal change, diurnal variation, and heavy pollutions in Beijing. The annual average mass concentrations of PM2.5 in Beijing were 51, 42, and 38 μg·m-3, which showed great progress in air quality improvement. However, the PM2.5 concentration in 2020 was still 8.6% above the national limit value despite a 30.9% decline since 2017. The PM2.5 south-north gradient in Beijing remained throughout the three years, but this pattern showed a less significant trend. The highest monthly mean PM2.5 concentrations in Beijing tended to occur in January-March, with the lowest in August-September. NOx, CO, and PM2.5 concentrations were significantly higher in the heating season than in the non-heating season by 58.4%, 52.9%, and 27.5%, respectively. Diurnal variation showed that greater PM2.5 concentrations were observed at nighttime during the heating season and, conversely, at noontime during the non-heating season. Sixteen pollution episodes occurred in Beijing over the last three years, resulting in 25 heavy pollution days distributed in autumn-winter of 2018-2020. The regional heavy pollution characteristics of PM2.5 in Beijing were significant. Through analysis, a continuous pollution reduction was still the most important reason for the yearly decrease in PM2.5. The concentrations of organic matter, elemental carbon, and crustal matter in the PM2.5 in Beijing decreased by 43.3%, 53.2%, and 51.5% since 2017, respectively, and nitrate, sulfate, and ammonium decreased by 34.2%, 52.2%, and 43.7%.The results showed that the control effect of PM2.5 in Beijing was obvious.

Developing PM2.5 and PM10 prediction models on a national and regional scale using open-source remote sensing data

Clean air is the precursor to a healthy life. Air quality is an issue that has been getting under its well-deserved spotlight in the last few years. From a remote sensing point of view, the first Copernicus mission with the main purpose of monitoring the atmosphere and tracking air pollutants, the Sentinel-5P TROPOMI mission, has been widely used worldwide. Particulate matter of a diameter smaller than 2.5 and 10 μm (PM2.5 and PM10) significantly determines air quality. Still, there are no available satellite sensors that allow us to track them remotely with high accuracy, but only using ground stations. This research aims to estimate PM2.5 and PM10 using Sentinel-5P and other open-source remote sensing data available on the Google Earth Engine (GEE) platform for heating (December 2021, January, and February 2022) and non-heating seasons (June, July, and August 2021) on the territory of the Republic of Croatia. Ground stations of the National Network for Continuous Air Quality Monitoring were used as a starting point and as ground truth data. Raw hourly data were matched to remote sensing data, and seasonal models were trained at the national and regional scale using machine learning. The proposed approach uses a random forest algorithm with a percentage split of 70% and gives moderate to high accuracy regarding the temporal frame of the data. The mapping gives us visual insight between the ground and remote sensing data and shows the seasonal variations of PM2.5 and PM10. The results showed that the proposed approach and models could efficiently estimate air quality.