Urban Green Spaces Under Climate Warming: Controlling the Spread of Allergenic Pollution Through Residential Area Spatial Layout Optimization

Abstract

In response to the demands of climate change and urban sustainability, urban green space construction in China has rapidly expanded, while simultaneously giving rise to allergenic pollen pollution. Focusing on the central urban area of Xi’an, Shaanxi Province, China, this study utilizes urban surveys, field measurements, and pollen particle microscopy to analyze the seasonal variation in allergenic pollen pollution concentrations and the physical dispersion characteristics of allergenic pollen particles in residential areas. The study also examines the impact of urban residential area spatial layout on regulating allergenic pollen pollution. The results show that (1) allergenic pollen pollution in Xi’an’s residential areas exhibits significant seasonal characteristics, with spring, summer, and autumn being the primary seasons. The highest concentrations occur in spring, dominated by tree pollen, followed by summer and autumn with a predominance of herbaceous pollen. (2) Pollution concentrations in residential areas are affected by the diurnal temperature variation, with higher concentrations observed in public green spaces compared to residential green spaces and roadside green spaces. (3) Allergenic pollen pollution shows a layered characteristic in the vertical direction, with concentrations concentrated around 13 m above ground due to the effects of diurnal temperature variation and local microclimate. (4) Urban pollen pollution concentrations are positively correlated with high temperatures and negatively correlated with high humidity, while local circulations influence pollen dispersion concentrations in residential areas. (5) Design indicators such as plot ratio and building stagger affect the dispersion concentrations of allergenic pollen pollution in residential areas. The findings provide a scientific basis for optimizing residential area spatial design to mitigate allergenic pollen pollution and offer strategic guidance for improving the health and livability of urban environments.

1. Introduction

The accelerating pace of global warming has been further intensified by rapid urbanization. The urban heat island (UHI) effect—a phenomenon where metropolitan areas exhibit significantly higher temperatures than surrounding rural regions—has emerged as a critical environmental concern worldwide. This temperature escalation stems from multiple interacting factors, including high-density infrastructure, complex road networks, and exponential growth in energy consumption [1]. Concurrently, while municipalities worldwide implement urban greening strategies to counteract UHI effects, an underrecognized secondary consequence—allergenic pollen pollution—is gaining scientific attention. Epidemiological data reveal a sustained global rise in pollen allergy prevalence (10–40%), with specific urban clusters exhibiting acute impacts: emergency visits for seasonal allergies in northeastern U.S. cities increased by 37% due to concentrated Ulmus and Betula plantings [2], while central Tokyo records 1.6–2.1 times higher pollen concentrations than its suburbs [3]. China demonstrates comparable patterns, with national allergy rates reaching 17.6%, while specific northern cities report alarming rates up to 52.9%, positioning this as a pressing public health challenge [4]. Notably, joint reports from the World Health Organization (WHO) and the International Association of Allergy and Clinical Immunology emphasize that urban populations experience 1.8–2.4 times higher pollen allergy incidence compared to rural counterparts. This urban–rural disparity is attributed to synergistic interactions between atmospheric pollutants, climate-driven phenological changes in plants, and the modified allergenicity of urban pollen sources.

Under the dual drivers of global climate change and urbanization, the urban heat island (UHI) effect not only amplifies thermal stress but also critically modifies the dispersion patterns of allergenic pollen. Mounting evidence indicates that temperature elevation directly modulates pollen production cycles, alters atmospheric transport dynamics, and ultimately exacerbates allergenic pollen pollution in residential zones [5]. Internationally, research has been conducted on reducing urban pollen concentrations. For instance, some countries have established green corridors exceeding 50 m in width or implemented “low-allergenicity vegetation zoning”, both of which have significantly reduced pollen concentration gradients [6]. But current research on the relationship between rising temperatures and pollen dispersion patterns remains inadequate, particularly regarding how residential spatial configurations modulate pollen dispersal processes and the potential for mitigating allergenic pollution through optimized urban form design. These critical knowledge gaps necessitate further systematic investigation. Based on this, this study aims to quantitatively analyze the effects of different spatial patterns of residential areas on the dispersion of allergenic pollen, with a view to providing theoretical support for the development of scientific and effective pollen pollution prevention and control measures, the improvement of urban air quality, and the reduction in the risk of allergic diseases, as well as providing a reference basis for the construction of a healthy and livable urban environment.

2. Current Situation of the Study Area and Research Methods

2.1. Xi’an City Location and Current Situation Analysis

Xi’an (34°17′ N, 108°54′ E), situated in the Guanzhong Plain of China’s Loess Plateau in central Shaanxi Province, represents a quintessential high-density urban center in Northwest China (Figure 1). As a city with profound historical significance, its compact urban structure accommodates approximately 13 million residents within a built-up area of 3538 km² (35% of the total 10,108 km² administrative territory). The municipal jurisdiction comprises 11 districts, two counties, seven national/provincial development zones, and one national-level new district. Characterized by a warm temperate semi-humid continental monsoon climate, Xi’an exhibits distinct seasonal variations in temperature and humidity. This climatic seasonality, coupled with its high-density urban configuration, creates marked spatiotemporal heterogeneity in allergenic pollen composition and dispersion intensity. These conditions establish Xi’an as an optimal study area for investigating urban aeroallergen dynamics, enabling mechanistic insights into pollen dispersion patterns under high-density urbanization while informing evidence-based strategies for urban green space management and respiratory health protection—particularly critical given rising pollen allergy prevalence.

2.1.1. Analysis of Urban Warming and the Heat Island Effect in Xi’an

Based on the 2023 land surface temperature (LST) data, an analysis was conducted to retrieve and examine the LST of Xi’an. Residential areas in both the city center and its suburban regions were selected for comparison. This study analyzed the relationship between temperature and the quantity of allergenic pollen released by the same plant species. The data indicate that the concentration of allergenic pollen increases as temperature rises. Additionally, residential areas in the city center consistently exhibit higher temperatures than those in the suburban areas, resulting in a greater amount of allergenic pollen. These findings underscore the importance of studying the impact of urban warming on allergenic pollen particles (Figure 2).

2.1.2. Impact of Allergenic Pollen in Xi’an and the Current Status of Allergenic Green Spaces in Residential Areas

(1)

Current Status of Allergenic Pollen Impact in Xi’an

With the advancement of urban greening initiatives, pollen-induced allergies have progressively emerged as a critical public health concern in metropolitan areas. The Xi’an Municipal Health Commission, in collaboration with local meteorological authorities, has established a pollen monitoring and early warning system, providing daily pollen concentration reports with preventive guidelines for susceptible populations. Utilizing validated monitoring data (N = 197 valid samples) from the Xi’an Public Meteorological Service Center spanning 1 March to 16 October, 2023, this study constructs an interannual variation profile of allergenic pollen concentrations (Figure 3).

Statistical analysis reveals pronounced seasonal variations in allergenic pollen concentrations across Xi’an: spring (March–May) exhibited the highest mean concentration (1602.3 grains/1000 mm²), significantly surpassing autumn (September–October: 25.6 grains/1000 mm²) and summer (June–August: 128.9 grains/1000 mm²). The spring high-concentration phase (>1000 grains/1000 mm²) persisted for 56 days, peaking at 10,011 grains/1000 mm² in April. Comparatively, daily maxima decreased to 708 grains in autumn and 161 grains in summer. Notably, a bimodal distribution pattern emerged, with the primary peak in April (cumulative 69,322 grains) and secondary peak in October, contrasting with the annual minimum in July (470 grains). Throughout the monitoring period, allergenic pollen exhibited an extended activity window from February to October, with elevated-risk phases concentrated between March and September. This prolonged exposure duration underscores Xi’an’s distinctive challenge in pollen pollution management.

(2)

Classification of Allergenic Green Space Types in Residential Areas

In accordance with China’s Urban Residential Area Planning and Design Code (GB 50180-1993) [7], residential green spaces are defined as comprising public green areas, residential greening zones, service facility-adjacent greenspaces, and road verge plantings—including qualified subterranean/semi-subterranean roof greening meeting local soil depth requirements, while excluding non-conforming rooftop or terrace plantings. Our investigation of 217 residential communities in Xi’an’s urban core revealed that the combination of public green spaces + residential greening zones + road verge plantings constitutes the predominant configuration (Figure 4).

(3)

Checklist of Allergenic Plants in Residential Areas of Xi’an

Based on data collected from sources such as the “Pollen Survey of Northern China Plants” and field surveys in urban areas, a total of 45 species of allergenic pollen plants were identified in the Xi’an region, belonging to 28 families and 38 genera [8]. After organizing the data on the 45 common allergenic pollen plants in Xi’an, a distribution chart of allergenic pollen plant types in residential areas was created (Figure 4). The main sources of allergenic pollen in Xi’an’s residential areas are trees and herbaceous plants, with a smaller contribution from shrubs and climbing plants.

2.2. Research Methodology

2.2.1. Scope of the Study

Urban allergenic risk demonstrates significant spatial coupling with population agglomeration patterns in high-density built-up areas. This study integrates Xi’an’s 2022 built-environment density data with two-decade demographic distributions (2003–2022) through geospatial kernel density estimation (KDE) analysis using ArcGIS. The analysis reveals a distinct center–periphery diffusion pattern in population hotspots, with core clusters concentrated in Beilin District, Lianhu District, and Yanta District (Figure 5). Concurrently, greenspace distribution analysis in built-up areas reveals higher vegetation coverage within southern and northern zones between the Second and Third Ring Roads, predominantly comprising parklands, road verge plantings, and residential ancillary greenspaces. Notably, significant spatial congruence exists between high-population-density clusters and concentrated green infrastructure. This geographic overlap establishes six central districts—Beilin, Lianhu, Yanta, Xincheng, Weiyang, and Baqiao—as high-risk zones for pollen allergies. And it was defined as the core scope of this study for subsequent field research and statistical analysis of urban pollen-sensitized plants.

2.2.2. Selection of Planning and Design Criteria for Residential Areas

(1)

Residential Planning Indicators

Residential area planning and design serve as critical determinants of spatial layouts and green space allocation. As stipulated in China’s Urban Planning Formulation Method (2006) [9], four regulatory control indicators—floor area ratio (FAR), green space ratio (GSR), building height, and spatial configuration—are defined as both mandatory planning parameters and key metrics in residential zone design. These indicators holistically reflect the impact of spatial layouts on pollen concentrations while offering practical and actionable guidance for improving air quality and mitigating allergenic pollen dispersion through evidence-based urban design strategies.

The green space ratio (GSR), a critical metric for assessing greening levels in residential areas, quantifies the proportion of vegetated areas within urban developments. Empirical studies indicate that elevated GSR values typically correlate with increased plant species diversity and heightened pollen source availability. However, indiscriminate expansion of green infrastructure may paradoxically elevate pollen concentrations, particularly when dominated by high-allergenicity species such as Platanus and Cupressaceae. Strategic selection of low-pollen alternative species (e.g., Magnolia and Hibiscus) demonstrates dual benefits—enhancing ecological connectivity while reducing allergenic risks [10]. Again, the floor area ratio (FAR), a key indicator of building density, exerts substantial influence on pollen dispersion dynamics. High-FAR zones characterized by dense vertical developments and constrained green spaces amplify pollen accumulation through wind shadow effects, where clustered high-rises obstruct atmospheric circulation, increasing local pollen residence time. In contrast, low-FAR configurations facilitate superior aerodynamic penetration, reducing peak pollen concentrations through enhanced advection [11]. Furthermore, the staggered arrangement and spatial openness of building layouts critically influence pollen dispersion patterns. Staggered configurations with permeable design enhance aerodynamic connectivity, effectively mitigating airflow stagnation zones that trap allergenic particles. Localized accumulation of pollen can be reduced by this optimized ventilation [12] (

Table 1. Classification table of building floors in residential areas of Xi’an.

Average Number of Floors of Residential Buildings	Number of Layers
Low level	1st to 3rd floor
Multi-layer Class I	4st to 6rd floor
Multi-layer Class II	7st to 9rd floor
High-rise Class I	10st to 18rd floor
High-rise Class II	19st to 26rd floor

).

(2)

Microclimate factor indicators

Climatic factors significantly influence pollen concentrations, as most allergenic pollen originates from wind-pollinated plants that exhibit heightened sensitivity to meteorological conditions. Key parameters such as temperature, relative humidity, and wind speed/direction collectively govern pollen dispersion dynamics.

Temperature serves as the predominant determinant of pollen concentration dynamics. Experimental evidence demonstrates that elevated temperatures accelerate anther dehydration processes, thereby triggering premature pollen release and subsequent atmospheric loading. Notably, peak pollen concentrations typically occur within the 15–20 °C thermal range, beyond which further temperature increases paradoxically suppress pollen emission [13]. Concurrently, relative humidity exerts biphasic control over pollen dispersion. Moderate humidity levels optimize both pollen viability and aerodynamic buoyancy, whereas hyperhumid conditions induce hygroscopic particle mass gain, critically impairing suspension capacity and reducing effective dispersion radii through accelerated gravitational sedimentation [14].

Wind speed and direction constitute critical aerodynamic determinants of localized pollen distribution patterns. Empirical analyses identify an optimal wind speed range of 1.5–3.0 m/s for efficient pollen dispersion, where aerodynamic lift forces sustain particle suspension while advective transport maximizes spatial spread [15]. Suboptimal wind speeds (<1.5 m/s) constrain pollen dispersion ranges by weakening aerodynamic lift forces, resulting in localized particle accumulation. Conversely, elevated wind velocities (>3.0 m/s) induce turbulent scavenging mechanisms that transport pollen beyond immediate zones, reducing ambient concentrations. Furthermore, persistent unidirectional airflow facilitates sectoral pollen aggregation, with peak concentrations typically occurring at 20 m downwind from emission sources under stable atmospheric stratification.

2.2.3. Measurement Tools and Methods

Pollen monitoring encompasses four sequential procedures (Figure 6): collection, identification, concentration estimation, and data analysis. The gravitational collection method utilizes slides exposed to ambient air for 24 h to capture naturally settling pollen grains, which are subsequently analyzed in laboratory settings. Identification is performed through microscopic examination of pollen morphology (shape, size, and surface structures), cross-referenced with standardized pollen databases for taxonomic classification. Concentration quantification employs staining techniques, where dyed pollen grains are microscopically enumerated to calculate density per unit area, a methodology validated by institutions such as the Allergy Center of Beijing Shijitan Hospital [16]. Comprehensive metadata—including collection timestamps, geolocations, temperature, and humidity—are systematically recorded to analyze correlations between pollen characteristics and meteorological variables, with particular focus on urban warming impacts on pollen seasonality and concentration patterns, thereby establishing critical datasets for environmental health research.

2.2.4. Data Processing Methods

The data were initially processed using Excel 2019, followed by correlation analysis of pollen concentration with air temperature, relative humidity, and wind speed using SPSS 19.0 software to identify the most influential microclimatic factors. Subsequently, the effects of different green space ratios and floor area ratios on allergenic pollen in residential areas were analyzed, yielding the optimal range of green space and floor area ratios for mitigating allergenic pollen dispersion. Finally, ArcGIS10.8 software was employed for interpolation analysis to assess the influence of different types of green spaces on the spatial and temporal distribution of allergenic pollen and its underlying causes, providing recommendations for the optimal green space layout in residential areas.

3. Study on the Physical Characteristics of Plant-Originated Particulate Pollution in Residential Areas

3.1. Analysis of Factors Influencing the Dispersion Rate of Plant-Originated Particulate Pollution

Wind-mediated pollen dispersal involves the release of particulate matter from the stamens of flowers, which spreads through urban spaces driven by wind mechanisms. The primary consideration is how pollen particles are carried by wind and air currents, allowing them to travel to distant locations. The mode of dispersal is determined by both the dispersal medium and the characteristics of the dispersed particles. Based on distance, plant pollen dispersal can be classified as long-distance or short-distance. Wind-mediated dispersal typically falls under short-distance dispersal [17]. Thus, in measuring pollen dispersal rates, wind speed becomes a significant influencing factor. By artificially controlling wind speed to create the required experimental conditions, the dispersion rate of allergenic pollen particles can be measured.

3.1.1. Field Measurement Plan

(1)

Vertical Velocity Experiment

The diffusion of pollen particles can be divided into several stages: pollen release, deposition, dispersion, and secondary transmission. This experiment focuses on the deposition process of the seeds, using Pinus pollen as the study object. After air-drying, the samples were stored in a desiccator for the experiment. Following the method of Gravuer et al. [18], the settling time of the diffusion units was measured in still air. As shown in Figure 7, a 100 cm glass tube was prepared and placed vertically on a table to create an experimental environment free from lateral airflow interference. A colored glass slide was placed at the lower end of the tube to facilitate the identification of the seeds through color contrast. A stopwatch was used to measure the time it took for the seeds to fall from the top to the bottom of the 100 cm cylindrical glass tube, determining the settling velocity in still air. This process was repeated five times, and the average of the five trials was taken to obtain the settling rate of Pinus pollen particles over a 100 cm distance.

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Horizontal Velocity Experiment

As shown in Figure 8, a 100 cm glass tube was placed horizontally on a table. A colored glass slide was positioned at one end of the tube for observation, while Pinus pollen particles were placed at the other end. Using a miniature fan, the wind speed was measured at 1.9 m/s with an anemometer to facilitate forward diffusion of the pollen. A stopwatch was used to measure the diffusion time from one end of the tube to the other, in order to determine the horizontal diffusion velocity. This process was repeated five times, and the average diffusion time was calculated to obtain the horizontal diffusion rate of Pinus pollen particles over a distance of 100 cm.

3.1.2. Horizontal/Vertical Diffusion Rates of Plant-Derived Pollutant Particles

The settling velocity was calculated using the free fall equation, and the results are presented in

Table 2. Results of five vertical experiments.

Experimental Group	Settling Time (s)	Sedimentation Rate (cm/s)
1	2.25	22.05
2	1.58	15.48
3	2.81	27.54
4	2.56	25.09
5	2.02	19.80

:

$$V_{\left(\mathrm{V}\mathrm{e}\mathrm{r}\mathrm{t}\mathrm{i}\mathrm{c}\mathrm{a}\mathrm{l}\right)}=gt,$$

(1)

where g represents the acceleration due to gravity (taken as 9.8 m/s²), and t is the falling time.

The horizontal velocity was calculated using the acceleration formula, and the results are presented in

Table 3. Results of five horizontal experiments.

Experimental Group	Diffusion Time (s)	Diffusion Rate (cm/s)
1	5.41	5.36
2	3.25	5.09
3	5.60	5.37
4	2.58	5.33
5	2.16	4.43

:

$$V_{\left(\mathrm{H}\mathrm{o}\mathrm{r}\mathrm{i}\mathrm{z}\mathrm{o}\mathrm{n}\mathrm{t}\mathrm{a}\mathrm{l}\right)}=\left\{\begin{array}{c}V=V_0+at\\ V^2-V_0^2=2ax\end{array},\right.$$

(2)

where V₀ is the initial wind speed of 1.9 m/s; a is the acceleration; and t is the diffusion time.

The final results indicate that the vertical settling velocity of pine pollen grains is 21.992 cm/s, while the horizontal diffusion rate is 5.116 cm/s. Exploring the diffusion capabilities of allergenic pollen grains contributes to understanding their diffusion distance during the dispersal process and provides support for subsequent particle diffusion simulations.

3.2. Study on the Impact of Vegetation-Derived Pollutant Particle Dispersion in Residential Areas

3.2.1. Field Measurement Protocol

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Field Measurement Protocol for Planar Diffusion

The intensification of urban heat island (UHI) effects under rising urban temperatures has significantly altered allergenic pollen characteristics, including concentration levels, allergenicity, and seasonal phenology. Empirical studies confirm the critical role of UHI intensity in modulating pollen dispersion dynamics. In Xi’an, UHI intensity exhibits a concentric attenuation pattern, peaking in the central Beilin, Lianhu, and Xincheng districts—areas characterized by high-density built environments and impervious surface coverage that collectively shape complex urban microclimates (Figure 9). To quantitatively assess these interactions, this study selects green space ratio (GSR) and floor area ratio (FAR) as primary indicators of residential spatial configurations. Four representative communities—Jiujintai, Zhongtie Binfei, Xingbi Chuanshuo, and Tianlang Daxingjun Jincheng—were systematically sampled based on their contrasting GSR and FAR profiles, which typify high-UHI zones (

Table 4. Comparison of sample residential area data.

Sample Residential Area	Division	Greening Rate (%)	Floor Area Ratio
Jiujintai Community	Beilin District	36.9	3.6
Tianlangjunjincheng Community	Lianhu District	45	3.8
Zhongtiebinfen Community	Baqiao District	40	3.2
Xingbichuanshuo Community	Xincheng District	40	6.0

).

Residential green space vegetation constitutes a primary source of allergenic pollen in urban communities. Elevated urban temperatures alter plant phenology and pollination behavior, subsequently modifying atmospheric pollen concentration profiles and dispersion patterns. Therefore, the field measurements were conducted through 24 h fixed-point collection and data recording from 7 to 8 June, 2023. Given the variations in vegetation composition and underlying surface characteristics across different green space types, sampling points were systematically classified based on the current conditions of each residential area. This stratification enables targeted analysis of the impacts of distinct green space typologies (e.g., public parks, roadside plantings, residential courtyards) on allergenic pollen concentrations and their dispersion patterns within urban communities (Figure 10).

Pollen sampling across diverse green space typologies within residential areas was conducted using a Hirst-type volumetric spore trap (Burkard Manufacturing is located in London, UK), with synchronous meteorological data collection (temperature, relative humidity, and wind speed) via an AZ-8825 thermo-hygrometer (AZ Instrument Corp is located in Tokyo, Japan) and S-1341 anemometer (Lutron Electronic, Coopersburg, PA, USA). Measurements were obtained at a standardized height of 1.5 m, with data recorded at 1 h intervals over a continuous 24 h period. This protocol generated 216 datasets per residential area, culminating in 1968 exposure slides collected systemwide, of which 1960 slides met quality control criteria for subsequent microscopic analysis.

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Field Measurement Protocol for Vertical Dispersion

Field measurements were conducted to assess the vertical dispersion and distribution of allergenic pollen, focusing on the concentration variations at different heights and identifying the types of allergenic pollen that are more easily dispersed, along with their size ranges. Considering the urban heat island effect in the central area of Xi’an, residential areas were reselected based on the variability in building heights. The selected areas included Gongyuantianxia Community, Yingchun Community, Baimiao Community, and Fenglinlvzhou Community. Measurement points were established on the highest floors of buildings at intervals of five floors, facilitating the study of the impact of building height on allergenic pollen levels. Additionally, to explore the relationship between allergenic pollen at pedestrian height and vegetation, measurement points were placed at a height of 1.2 m (pedestrian level) and at an average plant height of 4.5 m (Figure 11).

3.2.2. Impact of Vegetation-Derived Pollutant Particle Planar Dispersion

Analysis of temporal pollen concentration profiles across four sampled residential communities (Jiujintai, Tianlang Junjincheng, Zhongtie Binfei, and Xingbi Chuanshuo) revealed significant spatiotemporal variations among green space typologies, with pronounced diurnal thermal amplitude effects. In Jiujintai, residential greening zones exhibited marked pollen concentration fluctuations (peak: 03:00; trough: 05:00), contrasting with stable levels in public green spaces. Tianlang Junjincheng demonstrated minimal overall variation, peaking at 05:00 and reaching its nadir at 15:00. Road verge plantings in Zhongtie Binfei maintained lower concentrations, though adjacent residential greenspaces showed considerable volatility. Xingbi Chuanshuo displayed extreme variability in residential greenspaces (peak: 05:00; trough: 15:00), while clustered green areas remained stable. Results demonstrate distinct diurnal pollen dispersion patterns across green space types, with residential greenspaces consistently exhibiting higher variability compared to public green spaces and road verges (Figure 12). These differential patterns likely stem from interactions between vegetation composition, microclimatic conditions, and anthropogenic activity intensities.

3.2.3. Impact of Vertical Dispersion of Plant-Derived Pollutant Particles

Analysis of 24 h vertical allergenic pollen dispersion data (480 valid datasets) across four residential communities (Gongyuan Tianxia, Jiaheyuan, Baimiao Xiaoqu, and Fenglin Lüzhou) revealed significant influences of vegetation height and wind conditions on vertical concentration profiles. In Gongyuan Tianxia, pollen concentration variability decreased with increasing elevation, stabilizing above 30 m. Near-ground (1.2 m) and high-altitude (>30 m) levels showed minimal fluctuations, while a distinct midday peak (10:00–14:00) occurred at 15 m (5th floor), potentially linked to diurnal temperature increases and wind speed variations. Jiaheyuan exhibited significant variability at 4.2 m (mean vegetation height) and 30 m, contrasting with stable concentrations at 1.2 m and 15 m, indicating direct pollen emission and aerodynamic modulation. Baimiao Xiaoqu displayed maximum variability and peak concentrations at 30 m, likely due to reduced building porosity enhancing wind-driven dispersion [19]. Fenglin Lüzhou maintained stable upper-level concentrations (60 m, 45 m, 30 m, 15 m), while lower strata (4.2 m, 1.2 m) showed midday/evening peaks (12:00, 18:00), correlating with pollen release phenology and microclimatic conditions. These vertical stratification patterns demonstrate biomechanical and aerodynamic interactions governing urban pollen dispersion (Figure 13).

The vertical dispersion of allergenic pollen is governed by three interconnected factors: (1) Vegetation height: lower atmospheric strata (<15 m) exhibit pronounced concentration fluctuations due to proximity to ground-level pollen emissions, while elevated zones (>30 m) demonstrate stabilized concentrations with reduced ground-source influence; (2) Aerodynamic conditions: reduced building porosity in uniform urban morphologies amplifies wind-driven dispersion, facilitating upward transport of ground-sourced particles to lower altitudes; (3) Microclimatic parameters: diurnal temperature increases and wind speed variations synchronize with peak pollen release and dispersion events, particularly observable during midday thermal maxima.

3.2.4. Impact of Climatic Factors on the Dispersion of Plant-Derived Pollution Under Warming Conditions

Allergenic pollen concentrations are intrinsically linked to plant phenology and pollination processes, which are significantly modulated by temperature, relative humidity, and wind speed. As evidenced by Figure 14 data analysis, pollen concentrations exhibit a significant positive correlation with ambient temperature. Diurnal temperature elevation accelerates pollen release and dispersion, with concentration peaks synchronizing with thermal maxima during 14:00–15:00, while nocturnal cooling corresponds to concentration minima. This diurnal pattern is mechanistically driven by solar radiation-induced pollen sac dehydration and subsequent dehiscence, coupled with wind-driven particle dispersion [20].

Temperature influences pollen concentrations through direct mechanisms (accelerating anther dehiscence rates) and indirect pathways (inducing localized airflow perturbations), processes intrinsically linked to plant phenological traits. These findings underscore that temperature elevation induced by the urban heat island (UHI) effect may exacerbate pollen pollution, necessitating strategic urban greening interventions. Prioritizing low-pollen-emission species in landscape design and optimizing ventilation corridor configurations to enhance aerodynamic dispersion efficiency are critical for mitigating allergenic pollen pollution risks in thermally stressed urban environments.

Empirical data analysis from four residential communities (Jiujintai, Tianlang Junjincheng, Xingbi Chuanshuo, and Zhongtie Binfei) (Figure 15) reveals a significant negative correlation between relative humidity (RH) and allergenic pollen concentrations. Pollen levels peak during diurnal humidity minima (14:00–16:00), exemplified by Jiujintai public green space recording 19 grains/1000 m³ at 16:00 (45% RH). This phenomenon is mechanistically linked to pollen hygroscopicity: under low humidity (<40% RH), accelerated surface moisture evaporation reduces particle mass, enhancing atmospheric suspension capacity, whereas high humidity (>60% RH) increases hygroscopic mass gain, elevating gravitational sedimentation rates [21].

To investigate these dynamics, monitoring stations were established across three surface types—vegetated green spaces, impervious surfaces, and aquatic zones. Comparative analysis (Figure 16) demonstrates that aquatic zones lower pollen concentrations more than vegetated and impervious areas, attributed to humidity elevation from water evaporation promoting particle deposition. Additionally, the porous surface morphology of pollen grains amplifies hygroscopic effects, accelerating sedimentation under high-humidity conditions through capillary condensation mechanisms.

Analysis of empirical data from four residential communities (Jiujintai, Tianlang Junjincheng, Xingbi Chuanshuo, and Zhongtie Binfei) demonstrates a significant negative correlation between wind speed and allergenic pollen concentrations (Figure 17). When wind speeds exceed 1.8 m/s, enhanced long-range pollen dispersion reduces localized concentrations while potentially elevating levels in downwind areas. Summer solar radiation intensification further amplifies this pattern: elevated surface temperatures induce thermal turbulence, increasing wind speeds that accelerate long-distance dispersal, resulting in abrupt declines in local pollen loads.

3.2.5. Impact of Spatial Layout Indicators on the Dispersion of Plant-Derived Pollution

To investigate the impact of spatial layout in residential areas on the dispersion of allergenic pollen, heat map analysis was conducted using ArcGIS10.7 software on actual measured pollen concentration data. Analysis of the data in Figure 18 reveals that in the comparison group based on greening rate, Jiujintai Community has a greening rate of 50%, while Tianlangjunjincheng Community has a rate of 40%. The overall pollen concentration in Jiujintai Community is higher than that in Tianlangjunjincheng Community. Specifically, the main concentration of allergenic pollen in Jiujintai Community is located in the central green space, while concentrations near water bodies are lower. In contrast, Tianlangjunjincheng Community features multiple large areas with low concentrations, particularly near water bodies and downwind of residential areas.

The results indicate that as plants serve as sources of pollen, higher greening rates correspond to increased concentrations of allergenic pollen. Additionally, concentrations of allergenic pollen are generally lower near water bodies, which may relate to the hygroscopic nature of pollen particles. In the comparison group based on floor area ratio, Xingbichuanshuo Community has a ratio of 6, while Zhongtiebinfen Community has a ratio of 3.5. Despite having the same green space ratio, Zhongtiebinfen Community exhibits a significantly higher proportion of allergenic pollen concentration compared to Xingbichuanqiao Community. This may be attributed to how densely built high-rise areas alter local airflow patterns, making it difficult for pollen to accumulate in one location. Furthermore, increased wind speeds between high-rise buildings may prevent pollen from remaining in lower air layers. However, it is also possible that a higher floor area ratio intensifies the urban heat island effect, thereby extending the flowering period of plants and increasing allergenic pollen concentrations.

From the perspective of building layout indicators, allergenic pollen concentrations are higher in linear residential areas compared to clustered residential areas.

To further investigate the impact of building height and surrounding spatial layout on allergenic pollen concentrations, a comparative analysis was conducted using vertical measurements from residential areas (Figure 19). The findings show that in four residential areas, when the height reaches 4.5 floors (the average height of vegetation), allergenic pollen concentrations are generally higher than at other floors, indicating direct influence from nearby plants. Additionally, the overall pollen concentration tends to decrease as the average building height increases. For instance, in the Baimiao Community residential area, the average building height is 21 m, while in Jiaheyuan Community, it is 35 m. Baimiao Community has a lower average building height and more open space, with a large hard-surfaced square in front of the measurement location, facilitating air movement and increasing pollen dispersal. This results in higher pollen concentrations at higher floors. In contrast, Jiaheyuan Community is surrounded by buildings, which obstruct air movement.

When comparing the building height variation index, Fenglinlvzhou Community has a value of 0.47, while Park World has a value of 0.23. Fenglinlvzhou Community shows higher allergenic pollen concentrations than Park World, suggesting that variations in building height can cause local wind speed instability, forming vortices or stagnation zones. In such cases, pollen may accumulate in certain areas rather than dispersing evenly, leading to increased pollen concentrations. In terms of building layout, cluster-style layouts, such as in Fenglinlvzhou Community, may help disperse pollen, reducing the risk of localized high concentrations. On the other hand, row-style layouts, such as in Park World, may allow pollen to accumulate in specific locations more easily.

4. The Optimization of Residential Area Spatial Layout and Plant Configuration Under the Influence of Urban Heat Island

4.1. Urban Residential Planning and Vegetation Configuration

(1) Floor Area Ratio (FAR): Analysis reveals that a high FAR is associated with greater building density and height. As building density and height increase, airflow becomes restricted, causing allergenic pollen to accumulate in the area. This results in higher pollen concentrations at lower levels and near the ground. Therefore, reducing building height and density can help improve air circulation and reduce the accumulation of allergenic pollen.

(2) Green Space Ratio: Horizontal and vertical measurement data show that allergenic pollen tends to accumulate near large public green spaces. These areas have a higher diversity and density of plants, concentrating pollen allergens. Unlike smaller green spaces adjacent to buildings, public green spaces are less affected by building obstructions, which can influence pollen concentrations in the surrounding environment. Considering the environmental factors, underlying surfaces, and resident activity levels around different types of green spaces, targeted strategies have been proposed (

Table 5. Table of optimization strategies for different types of green spaces.

Types of Green Spaces	Allergenic Range	Layout Optimization	Management Measures
Public Green Space	Larger area, widely distributed, with a broad allergenic range	Consider resident activity density and reduce areas with high activity concentration.	Set up barriers between green spaces and residential activity areas
		Select and combine plant species that are as non-allergenic as possible	Regularly spray and clean to reduce pollen concentration
Residential Green Space	Distributed in a scattered manner, with a small area, forming various small allergenic ranges in a point-like distribution within the residential area	Select downwind locations for planting greenery, and minimize allergenic planting in upwind areas.	Timely pruning and cleaning of plants
		Select and combine plant species that are as non-allergenic as possible
Roadside Green Space	Mainly connects various entrances and central activity areas	Select and combine plant species that are as non-allergenic as possible	Increase ground spraying and daily maintenance methods

). Additionally, based on allergenicity ratings and phenological information of tree species in Xi’an residential areas, a vegetation configuration plan has been developed (

Table 6. Selected plant species for Xi’an.

Serial Number	Common Name	Latin Name	Family Name	Allergenic Potential
1	Chinese Arborvitae	Platycladus orientalis (L.) Franco	Cupressaceae	Medium
2	Spruce	Picea asperata Mast	Pinaceae	Medium
3	Cedar	Cedrus deodara (Roxb.) G. Don	Pinaceae	Medium
4	Chinese White Pine	Pinus bungeana Zucc. ex Endl.	Pinaceae	Medium
5	Chinese Pine	Pinus tabuliformis Carrière	Pinaceae	Medium
6	Walnut	Juglans regia L.	Juglandaceae	Medium
7	Hazel	Corylus heterophylla Fisch. ex Trautv.	Betulaceae	Medium
8	Paper Mulberry	Broussonetia papyrifera (L.) L’Hér. ex Vent.	Moraceae	Medium
9	Mulberry	Morus alba L.	Moraceae	Medium
10	Chinese Ash	Fraxinus chinensis Roxb.	Oleaceae	Medium
11	Ginkgo	Ginkgo biloba L.	Ginkgoaceae	Low
12	Japanese Pagoda Tree	Styphnolobium japonicum (L.) Schott	Fabaceae	Low
13	Black Locust	Robinia pseudoacacia L.	Fabaceae	Low
14	Chinaberry	Melia azedarach L.	Meliaceae	Low
15	Glossy Privet	Ligustrum lucidum W. T. Aiton	Oleaceae	Low
16	Windmill Palm	Trachycarpus fortunei (Hook.) H. Wendl.	Arecaceae	Low
17	Silk Tree or Persian Silk Tree	Albizia julibrissin Durazz.	Fabaceae	Low
18	Chinese Boxwood	Buxus sinica (Rehder & E. H. Wilson) M. Cheng	Buxaceae	Low
19	Kentucky Bluegrass	Poa annua L.	Poaceae	Low
20	Mondo Grass	Ophiopogon japonicus (L. f.) Ker Gawl.	Liliaceae	Low

).

4.2. Building Layout Patterns and Vertical Variability in Building Heights

A well-planned building layout and variation in building heights can help leverage natural wind flow to carry away some pollen, reducing localized pollen concentrations. Based on the analysis of both horizontal and vertical measurement data, cluster-style grid layouts are more conducive to air circulation, dispersing pollen and lowering localized concentrations compared to other spatial layouts. Additionally, increasing building spacing and vertical variability in building heights can further prevent the accumulation of allergenic pollen due to building obstructions, thereby reducing the allergen exposure risk for residents on lower floors and near the ground.

5. Conclusions

This study systematically reveals the spatiotemporal differentiation patterns of allergenic pollen pollution in urban residential areas and its relationships with climatic factors and spatial layouts. The results indicate that allergenic pollen pollution in Xi’an’s residential areas exhibits significant seasonal characteristics, with the highest pollen concentration occurring in spring, dominated by arboreal pollen, forming a pollution peak, while herbaceous pollen dominates in summer and autumn. The vertical distribution of pollen pollution presents a stratified pattern, with the near-ground region being significantly influenced by diurnal temperature variations and local microclimate regulation, and a concentration enrichment layer identified at a height of 13 m. In relation to climatic factors, temperature reaches its daily peak between 14:00 and 15:00, and higher temperatures directly promote pollen release. Conversely, high humidity and strong wind speed show a negative correlation with pollen concentration by enhancing pollen deposition and accelerating air circulation, thereby reducing pollen levels. Spatially, elevated floor area ratios (FARs) and reduced building porosity exacerbate near-surface pollen accumulation, while clustered or linear layouts with appropriate building spacing significantly reduce localized concentrations through optimized ventilation efficiency. Based on these findings, this study proposes avoiding high FAR and excessive green space ratios in residential planning, minimizing densely planted public green spaces near allergenic sources, and prioritizing low-allergenicity vegetation configurations to mitigate pollen exposure risks, thereby providing a scientific basis for optimizing human settlements.