A Prediction Model for the Outbreak Date of Spring Pollen Allergy in Beijing Based on Satellite-Derived Phenological Characteristics of Vegetation Greenness

Abstract

Pollen allergies have a serious impact on people’s physical and mental health. Accurate and efficient prediction of the outbreak date of pollen allergies plays an important role in the conservation of people sensitive to allergenic pollen. It is a frontier research to combine new social media data and satellite data to develop a model to forecast the outbreak date of pollen allergies. This study extracted the real outbreak dates of spring pollen allergies from Sina Weibo records from 2011 to 2021 in Beijing and calculated five vegetation indices of three vegetation types as phenological characteristics within the 30 days before the average outbreak date. The sensitivity coefficients and correlation coefficients were used to screen the phenological characteristics that best reflected the outbreak date of spring pollen allergy. Based on the best characteristic, two kinds of prediction models for the outbreak date of spring pollen allergy in Beijing were established (the linear fit prediction model and the cumulative linear fit prediction model), and the root mean square error (RMSE) was calculated as the prediction accuracy. The results showed that (1) the date of EVI2 (2-band enhanced vegetation index) in evergreen forest first reaching 0.138 can best reflect the outbreak date of pollen allergies in spring, and (2) the cumulative linear fit prediction model based on EVI2 in evergreen forests can obtain a high accuracy with an average RMSE of 3.6 days, which can predict the outbreak date of spring pollen allergies 30 days in advance. Compared with the existing indirect prediction models (which predict pollen concentrations rather than pollen allergies), this model provides a new direct way to predict pollen allergy outbreaks by using only remote sensing time-series data before pollen allergy outbreaks. The new prediction model also has better representativeness and operability and is capable of assisting public health management.

1. Introduction

Pollen allergy is a disease that causes patients to present with IgE-induced (immunoglobulin E) allergy symptoms after inhalation or exposure to pollen allergens [1], such as allergic rhinitis, allergic conjunctivitis, asthma, urticaria, and allergic dermatitis. In Europe, more than 15% of the population suffers from pollen allergies, and the proportion of pollen allergy sufferers is much higher in urban areas [2]. In China, the probability of diseases caused by pollen allergies is 0.5–1.0%, reaching approximately 5% in densely populated areas [3], and the rate of incidence is increasing continuously [4]. For pollen-allergic patients, the most direct way of relieving the symptoms of pollen allergy is to suggest the use of protective gear during the pollen allergy period. Hence, effectively predicting the outbreak date of pollen allergies can help to inform patients for when protection is necessary.

Airborne pollen is an important cause of pollen allergies [5], and the outbreak date of pollen is closely related to vegetation phenology. Remote sensing data have been widely used in vegetation phenology monitoring [6,7]. Therefore, using remote sensing data to reveal the outbreak date of pollen allergies and the greenness characteristics before the outbreak date can facilitate the prevention of pollen allergy diseases.

Current related studies on pollen allergy monitoring based on remote sensing data mainly focus on detecting pollen sources [8,9], extracting the flowering date [10,11], and identifying spectral features of pollen [12,13,14]. Satellite-based monitoring of the flowering date is difficult due to the short flowering period (5–30 days) and the relatively weak spectral signals of flowers at large scales. Satellite data with coarse spatial resolution (e.g., Landsat and MODIS) have fewer applications for monitoring flowering dates due to their low ability to distinguish optical signals of flowering. However, the flowering date is closely related to the phenological characteristics of vegetation greenness [15]. Therefore, the satellite data with moderate and low spatial resolution are able to indirectly reflect the flowering date through the earlier greenness characteristics of vegetation despite its inability to directly detect the spectral signals of flowering, which provides a possibility to monitor the outbreak date of pollen allergy.

The flowering period and pollen concentration are closely related to the symptoms of pollen allergy sufferers [16], so the current prediction models for pollen allergy outbreaks are mainly established to forecast the start date of flowering and the concentration of airborne pollen. Høgda et al. [17] predicted the start of the pollen season of Nordic birch based on the relationship between the normalized difference vegetation index (NDVI) and the phenology records of sites. Karlsen et al. [10] produced a map to characterize the onset of the birch pollen season utilizing NDVI satellite data. Khwarahm et al. [18] developed a technique to estimate the flowering phenophase of birch and grass from MERIS terrestrial chlorophyll index (MTCI) time-series data. Since the start date of the pollen season was recorded by monitoring the concentration of airborne pollen at sites, some studies started to predict the outbreak date of pollen allergies through the empirical relationship between pollen concentration and meteorological parameters [19]. For instance, He et al. [20] established a statistical model for pollen concentration prediction in Beijing combined with meteorological data. Iglesias et al. [21] developed a pollen concentration prediction model of sycamores in northwestern Spain based on temperature data. Myszkowska et al. [22] constructed a pollen concentration prediction model based on the relationship between the pollen concentration of multiple vegetation types and meteorological parameters in southern Poland. However, pollen monitors can only reflect pollen concentrations at the site scale because of their limited spatial coverage. Predicting pollen concentration based on the empirical relationship between pollen concentration and meteorological parameters tends to ignore the effects of other environmental factors on pollen concentration [23]. Therefore, some studies have developed pollen concentration prediction models based on remote sensing data, such as using machine learning methods to establish the relationship between pollen concentration and remote sensing data to predict future pollen concentrations [23,24,25,26]. However, neither the start date of flowering nor the concentration of pollen can fully reflect the outbreak date of pollen allergy, as they are only necessary conditions for pollen allergy rather than sufficient conditions. For example, the pollen concentration is also high in southern China in spring and autumn [27], but the concurrent number of pollen allergic patients is smaller than that in northern China [28]. This contradiction is because pollen allergy is caused by the allergenic pollen concentration rather than the total pollen concentration, and it is also related to seasonal changes in human immunity [29]. Therefore, it is necessary to establish a direct relationship between the incidence of pollen allergy (rather than flowering date or pollen concentration) and remote sensing data to predict the outbreak date of pollen allergy.

Beijing has various plants with allergenic pollen due to the warm temperate semi-humid climate [30], and the three main plants that cause pollen allergy in spring are cypress, poplar, and willow (especially cypress), whose flowering periods are all close to each other [31], resulting in a high risk of pollen allergy [32]. Shi [33] found that one-quarter to one-third of respiratory allergy patients in Beijing are allergic to pollen. Wang et al. [34] discovered that the pollen concentration in Beijing has two peaks, spring (March to April) and summer-autumn (August to September), where allergic pollen mainly comes from woody plants in spring and herbaceous plants in summer-autumn. Meanwhile, the incidence of pollen allergies also has an obvious seasonal pattern, with a high incidence mainly in spring and autumn [35]. To benefit pollen allergic patients in Beijing, researchers have established a pollen concentration prediction model based on meteorological parameters since 2016 to predict recent pollen concentrations in all districts of Beijing [36]. However, the prediction model can only forecast pollen concentration at sites and tends to ignore the effects of other environmental factors on pollen concentration due to the limit of pollen monitor amounts and the uncomprehensive representativeness of meteorological parameters. To address this problem, some scholars have started to apply satellite data to build pollen concentration prediction models in Beijing. Bian et al. [23] established a next-day pollen concentration prediction model based on the average vegetation leaf area index (LAI) and daily meteorological data of tree and grass growth areas in Beijing with a nonlinear autoregressive neural network model (NARXnet). Although the prediction model has a high accuracy, the forecast period is very short, i.e., only for the next day but not for the medium or long term (e.g., the next 10 days or one month), and the forecast content is the total pollen concentration rather than the incidence of pollen allergy. Therefore, it is still difficult to aid the prevention of pollen allergies.

Although there are two peak periods for pollen allergy in Beijing, spring and summer-autumn, we focused on the spring pollen allergy and its vegetation phenological characteristics for prediction models, considering that the vegetation phenology changes more obviously in spring. This study aims to address two issues: first, to reveal the satellite-derived phenological characteristics of vegetation greenness on and before the outbreak date of spring pollen allergy in Beijing; and second, to establish a direct prediction model for the outbreak date of spring pollen allergy in Beijing based on the satellite-derived phenological characteristics of vegetation greenness.

2. Materials and Methods

2.1. Study Area

Beijing is surrounded by mountains in the west and north, and its middle is a plain open to the southeast (Figure 1). The plain in the urban area of Beijing with a similar vegetation phenology was selected as the study area (elevation ≤ 100 m) because the local climates and vegetation phenology differ significantly between the plain and the surrounding mountains in Beijing. The study area is located within 39°28′N–40°53′N, 115°83′E–117°30′E, with a warm-temperate semi-humid monsoon climate, an average annual temperature of 12 °C [37], an average annual precipitation of 621 mm [38], and diverse vegetation types. Wang et al. [39] investigated the pollen sources in Beijing urban area and found that the main pollen comes from buttercup (Ranunculaceae), amaranth (Amaranthaceae), pine (Pinaceae), cypress (Cupressaceae), elm (Ulmaceae), birch (Betula), artemisia (Artemisia), quinoa (Chenopodium), willow (Salix), humulus (Humulopsis), and Planetree (Platanus), and the highly allergenic pollen comes from Chinese pine (Pinus tabuliformis), cork oak (Quercus variabilis), green ash (Fraxinus pennsylvanica), poplar (Populus tomentosa), tree of heaven (Ailanthus altissima), birch (Betula platyphylla), white elm (Ulmus pumila) and China savin (Juniperus chinensis). The types of allergenic pollen vegetation differ from season to season, with woody plants such as elm (Ulmaceae), cypress (Cupressaceae), pine (Pinaceae), and willow (Salix) in spring and herbaceous plants such as mulberry (Moraceae), chrysanthemum (Asteraceae), quinoa (Chenopodiaceae), and Graminae (Poaceae) in autumn [40].

2.2. Data

2.2.1. The Outbreak Dates of Spring Pollen Allergy

The outbreak dates of pollen allergies in Beijing were extracted from Sina Weibo records (see details in Appendix A). We obtained 13,404 pollen allergy Weibo records from 2011 to 2021 in Beijing in total, and 1803 valid records were manually screened for analysis. The outbreak dates of spring pollen allergies in Beijing were extracted from the maximum point on the second derivative of the fitted curve for Weibo data with the logistic function. The earliest date of the spring pollen allergy outbreak was on the 70th day (11th March), and the latest date was on the 91st day (1st April) in Beijing from 2011 to 2021. The extraction results were consistent with the outpatient records of pollen allergies in Beijing [34].

2.2.2. Remote Sensing Data

The satellite vegetation indices were downloaded from the Google Earth Engine (GEE). We selected the MODIS surface reflectance daily dataset (MOD09GA) with a resolution of 250 m from 2011 to 2021 to calculate different vegetation indices for different vegetation types during these 11 years.

2.2.3. Vegetation Classification Data

A vegetation classification map with a resolution of 500 m was produced based on satellite data in 2020 and 2021 and the FROM-GLC10 land cover map released by Tsinghua University, which classified the study area into evergreen forest, deciduous forest, grassland, cropland, and non-vegetation areas (Figure 1). The details of specific vegetation classification data and methods are listed in Appendix B. This study focuses on two vegetation types, evergreen forest and deciduous forest, which are related to spring pollen allergens.

2.3. Methods

2.3.1. Extraction of Satellite-Derived Phenological Characteristics of Vegetation Greenness within the 30 Days before the Spring Average Pollen Allergy Outbreak Date during 2011–2021

The concentration and type of pollen are related to the vegetation type in a region [41]. Given that the allergenic vegetation in Beijing is mainly woody plants in spring [42], we selected the vegetation indices in forests to calculate the greenness characteristics before the outbreak date of pollen allergy in spring.

With reference to the study results of pollen identification based on remote sensing spectral features by Peng et al. [12], we used the normalized difference vegetation index (NDVI), enhanced vegetation index (EVI), 2-band enhanced vegetation index (EVI2), the sum of blue, red, and near-infrared reflectance (NIR + R + B) and the ratio of green to red reflectance (G/R) to analyze the characteristic.

For a certain vegetation type, the satellite-derived phenological characteristic of vegetation greenness is defined as a combination of a certain vegetation index value and its corresponding date. The technical process for the extraction of the greenness characteristics before pollen allergy outbreak is shown in Figure 2, which includes the following three main steps: (1) The preprocessing of remote sensing vegetation index time-series data; (2) the extraction of satellite-derived phenological characteristics of vegetation greenness; and (3) the analysis of the representative capacity of the greenness characteristics to the outbreak date of pollen allergy.

(1) Preprocessing of remote sensing vegetation index time-series data

The daily remote sensing images from 2011 to 2021 were used to calculate the vegetation index value for each vegetation type. Since there are many noises in the daily time-series data, we adopted two methods to reduce the noises. First, the spatial 5% trimmed mean of each vegetation index value in every vegetation type was taken as the daily phenological characteristics for each year. Second, the data were smoothed by taking the mean of the higher 6 values within the previous 11-day filtering window for each day.

Since we have set the forecast of pollen allergies to be 30 days in advance and the earliest date of spring pollen allergies in Beijing from 2011 to 2021 is the 70th day and the latest date is the 91st day, we only need to consider the vegetation index time-series data from the 50th to 90th day of each year (Figure 2c). Based on the shape of the vegetation index time-series curve from the 50th to the 90th day, as shown in Figure 2c, we chose a linear function to fit the daily time-series curve and the cumulative time-series curve of the vegetation index.

$$Y\left(t\right)=m +nt$$

(1)

where $Y\left(t\right)$ is the daily vegetation index value or cumulative vegetation index value; $t$ is DOY 50 ≤ $t$ ≤ 90; and $m$, $n$ are the parameters that need to be fitted.

(2) The extraction of satellite-derived phenological characteristics of vegetation greenness

The outbreak dates of spring pollen allergy extracted from the Weibo data were used to find the corresponding vegetation index value on the linear fitted curve. For each vegetation type, the value of a certain vegetation index on the outbreak date of pollen allergy is regarded as the satellite-derived phenological characteristic of vegetation greenness in this study (right plots in Figure 2c).

(3) Analysis of the representative capacity of the greenness characteristics to the outbreak date of pollen allergy

To determine the greenness characteristic that can best reflect spring pollen allergy outbreaks, we calculated the sensitivity coefficients of each characteristic to the outbreak date of spring pollen allergies and the correlation coefficients between each characteristic and the outbreak date of spring pollen allergies.

The sensitivity coefficient can reflect the sensitivity of each satellite-derived phenological characteristic of vegetation greenness to the outbreak date of spring pollen allergy. It is calculated as follows:

$$Sv=\frac{(y_2-y_{1)}/y_1}{(x_2-x_1)/x_1}$$

(2)

where $x_1$ is the DOY 20 days earlier than the earliest breakout date of spring pollen allergy during 2011–2021, i.e., the 50th day; $x_2$ is the average DOY of spring pollen allergy outbreak during 2011–2021, i.e., the 81st day; $y_1$ is the vegetation index value corresponding to $x_1$ on the fitted curve of the smoothed multiyear average daily curve of a vegetation index for a vegetation type; and $y_2$ is the vegetation index value corresponding to $x_2$ on the fitted curve of the smoothed multiyear average daily curve of a vegetation index for a vegetation type (the right plot in Figure 2d).

The correlation coefficient can reflect the degree of consistency between the interannual variation of each characteristic and the outbreak date of spring pollen allergy. A higher value of the correlation coefficient indicates a more stable ability to represent the phenological characteristics of spring pollen allergies. To calculate the correlation coefficient, the average outbreak date (T) of spring pollen allergy (T₂₀₁₁, T₂₀₁₂, …, T₂₀₂₁) during 2011–2021 was used to find the corresponding DOY (t₂₀₁₁, t₂₀₁₂, …, t₂₀₂₁) in each vegetation index value time series of different vegetation types every year. Then, the Pearson correlation coefficient between the real outbreak date (T₂₀₁₁, T₂₀₁₂, …, T₂₀₂₁) and the corresponding satellite DOY (t₂₀₁₁, t₂₀₁₂, …, t₂₀₂₁) was calculated for each combination of vegetation indices and vegetation types (the left plot in Figure 2d).

2.3.2. Establishment and Accuracy Assessment of the Prediction Models

We established prediction models based on the satellite-derived phenological characteristics of vegetation greenness within the 20 days before the earliest date of the outbreak dates of spring pollen allergy in Beijing during 2011–2021 (Figure 3a,b). The prediction models were developed as follows.

$$Y=mt+n$$

(3)

$$W=\mathsf{\Delta }t=t-t_0=\left(Y-n\right)/m-81$$

(4)

where $Y$ is the preprocessed multiyear average daily fit curve of the remote sensing vegetation index when establishing the prediction model or the preprocessed daily vegetation index in a given year when predicting; $m$ and $n$ are the coefficient of the linear fit of $Y$; $W$ is the prediction function, $\mathsf{\Delta }t$ is the predicted number of days to the outbreak date of spring pollen allergy in Beijing (a negative value indicates that the outbreak date has not yet arrived, and a positive value indicates that the outbreak date has passed); t is DOY of the current date; and $t_0$ is DOY of the average outbreak date of spring pollen allergy in Beijing during 2011–2021, i.e., the 81st day. The prediction models for the outbreak date of spring pollen allergy were named the linear fit prediction model and cumulative linear fit prediction model, respectively, according to the different preprocessing of remote sensing vegetation index data.

The accuracy of the prediction models was evaluated with the root mean square error (RMSE), which reflects the deviation of the forecast date from the real date. A smaller RMSE means a higher forecast accuracy. To test the performance of each prediction model (the linear fit prediction model and cumulative linear fit prediction model), the vegetation index on the 50th day of each year during 2011–2021 (the DOY of the 20 days before the earliest outbreak date of spring pollen allergy in these 11 years) was put into every prediction model to calculate the countdown to the outbreak date of spring pollen allergy in Beijing, and then the predicting outbreak date of pollen allergy for each year in Beijing was determined (Figure 3c,d). Finally, the RMSE between forecasted dates and real dates was calculated to select the best prediction model.

To give an objective and fair evaluation of the models, we established the models by randomly selecting 7 years from 2011–2021 and predicted outbreak dates of pollen allergy for the remaining 4 years. A total of 330 training sessions ($C_{11}^7=330$) were performed for each model (the linear fit prediction model and cumulative linear fit prediction model).

3. Results

3.1. Satellite-Derived Phenological Characteristics of Vegetation Greenness within 30 Days before the Spring Average Pollen Allergy Outbreak Date during 2011–2021

The value range of each greenness characteristic derived from different vegetation indices was diverse in each vegetation type within the 30 days before the pollen allergy outbreak date (

Table 1. The value range of each greenness characteristic derived from different vegetation indices within the 30 days before the spring average pollen allergy outbreak date during 2011–2021.

Vegetation Type	NDVI	EVI	EVI2	NIR + R + B	G/R
Evergreen Forest	0.257–0.309	0.125–0.131	0.113–0.138	2.074–2.217	0.967–0.974
Deciduous Forest	0.196–0.227	0.107–0.123	0.095–0.126	1.800–2.061	0.971–0.976
Evergreen + Deciduous Forest	0.233–0.260	0.117–0.135	0.113–0.124	2.008–2.139	0.969–0.975

). During spring, the NIR + R + B values of each vegetation type had the most obvious magnitude of variation among all indices during the 30 days before the outbreak date, and the most obvious change in its value occurred in deciduous forest, from 1.800 to 2.061. The G/R values of each vegetation type changed the least, among which the G/R value of evergreen + deciduous forest changed by only 0.006 during the 30 days.

In terms of sensitivity coefficients (

Table 2. The sensitivity coefficients of each greenness characteristic to the vegetation greenness date of spring pollen allergy.

Vegetation Type	NDVI	EVI	EVI2	NIR + R + B	G/R
Evergreen Forest	0.253	0.060	0.270	0.086	0.009
Deciduous Forest	0.198	0.187	0.116	0.181	0.006
Evergreen+ Deciduous Forest	0.145	0.192	0.122	0.082	0.008

Note: Bold font indicates the maximum value of the sensitivity coefficient in each row.

), the sensitivity coefficient of EVI2 to the outbreak date of spring pollen allergy was the largest at 0.270 in evergreen forest, followed by NDVI of evergreen forest with a sensitivity coefficient of 0.253, and the sensitivity coefficients of G/R of all vegetation types were smaller than 0.01.

The correlation coefficients between each greenness characteristic and the outbreak date of pollen allergy are shown in

Table 3. The correlation coefficients between the interannual variations in each greenness characteristic and the outbreak dates of pollen allergy.

Vegetation Type	NDVI	EVI	EVI2	NIR + R + B	G/R
Evergreen Forest	0.724 *	0.289	0.693 *	0.159	0.612 *
Deciduous Forest	0.762 *	0.429	0.633 *	0.485	0.538
Evergreen+ Deciduous Forest	0.717 *	0.675	0.705 *	0.280	0.519

Note: Bold font indicates the maximum value of the correlation coefficient in each row; * indicates significance at p = 0.05.

. The NDVI and EVI2 of each vegetation type were significantly correlated (p < 0.05) with the outbreak date of spring pollen allergy, with the largest correlation coefficient of 0.762 for the NDVI in deciduous forest, while the EVI and NIR + R + B of each vegetation type were not significantly correlated with the outbreak date.

Combining the sensitivity coefficients and correlation coefficients of each greenness characteristic, we found that the date of EVI2 in evergreen forest first reaching 0.137 can best reflect the outbreak date of pollen allergy in spring.

3.2. The Prediction Models and Their Accuracies

Based on the EVI2 of evergreen forest during 2011–2021, the linear fit prediction model and cumulative linear fit prediction model were developed, which best reflects the outbreak date of spring pollen allergy in Beijing (Figure 4). The expressions of each prediction model were obtained as follows:

(1) The linear fit prediction model

W =1666.67Y₁ − 219.5

(5)

where Y₁ is the smoothed daily EVI2 value of evergreen forest for a given forecast year and W is the number of days to the outbreak date of spring pollen allergy in Beijing.

(2) The cumulative linear fit prediction model

W =8.628Y₂ − 78.69

(6)

where Y₂ is the cumulative smoothed daily EVI2 value of evergreen forest for a given forecast year and W is the number of days to the outbreak date of spring pollen allergy in Beijing.

The prediction accuracies obtained from 330 training sessions for each model based on EVI2 of evergreen forest are shown in

Table 4. Cross-test results of the prediction models based on EVI2 of evergreen forest, NDVI of evergreen forest and EVI2 of deciduous forest for 330 training sessions.

Years for Model Building	Years for Model Test	RMSE for the Linear Fit Prediction Model			RMSE for the Cumulative Linear Fit Prediction Model
		EVI2 of Evergreen Forest	NDVI of Evergreen Forest	EVI2 of Deciduous Forest	EVI2 of Evergreen Forest	NDVI of Evergreen Forest	EVI2 of Deciduous Forest
2011–2017	2018–2021	52.991	61.538	42.497	3.369	18.177	14.123
2012–2018	2011, 2019–2021	9.832	242.953	55.808	2.434	19.811	14.123
2013–2019	2011–2012, 2020–2021	48.862	78.272	56.464	1.766	15.813	11.989
2014–2020	2011–2013, 2021	11.399	53.608	18.232	2.853	11.285	10.225
2015–2021	2011–2014	40.172	88.491	14.385	3.073	31.525	6.865
…	…	…	…		…	…
Average of RMSEs (mean ± sd)	/	95.549 ± 197.572	92.636 ± 134.075	26.673 ± 21.725	3.589 ± 1.101	22.519 ± 10.184	10.450 ± 2.689

. The average RMSEs of the linear fit prediction model and the cumulative linear fit prediction model were 95.549 days and 3.589 days, respectively, which indicated that the linear fit prediction model has little predictive power while the cumulative linear fit prediction model has a very good prediction ability. It should be noted that the first training session in Table 4 with the years 2011–2017 for model building and the years 2018–2021 for model test can also get a low RMSE (i.e., 3.369 days) for the cumulative linear fit prediction model, though there is a dramatic increase in the number of valid pollen allergy Weibo records since 2018 (Figure A1), which indicated that the dramatic increase in the number of valid records since 2018 did not affect the prediction accuracy.

To further verify the validity of the screened vegetation phenological greenness characteristic that best reflects the date of pollen allergy outbreak (i.e., EVI2 of evergreen forest), we also tested the prediction accuracies for the models based on NDVI of evergreen forest and EVI2 of deciduous forest (Table 4), respectively. Controlling the variable in model construction (i.e., change EVI2 to NDVI or change evergreen forest to deciduous forest), we found that the prediction accuracy of each model built by the above two vegetation characteristics was lower than that of the prediction model built by EVI2 of evergreen forest.

4. Discussion

4.1. Phenological Characteristics of Remote Sensing Vegetation Greenness at the Beginning and Early Stages of the Spring Pollen Allergy Outbreak in Beijing

We found that the date of EVI2 in evergreen forest first reaching 0.137 can best reflect the outbreak date of pollen allergy in spring. Evergreen forest can better reflect spring pollen allergy outbreaks in Beijing than other vegetation types because cypress, the main allergenic vegetation in Beijing in spring, is an evergreen plant [31]. EVI2 can better indicate vegetation greenness before flowering because it can reduce atmospheric and soil disturbances [43,44], making it more sensitive to vegetation greenness in areas with high background noises [44,45].

EVI2 at 0.137 of evergreen forest has a specific phenological indication. The remote sensing vegetation index time-series curve for a complete vegetation growth season generally has four key transition dates: greenup (the date of onset of photosynthetic activity), maturity (the date at which plant green leaf area achieves maximum), senescence (the date at which photosynthetic activity and green leaf area begin to rapidly decrease), and dormancy (the date at which physiological activity becomes near zero) [6]. In this study, the satellite-derived greenness characteristic that best reflected the outbreak date of spring pollen allergy corresponded to the key greenup transition date: the date of EVI2 at 0.137 in the growth period of evergreen forest in spring corresponded to the greenup date of evergreen forest (e. g. the date of EVI2 at 0.136 of evergreen) (Figure 5). Actually, the allergenic pollens in Beijing in spring mainly come from cypress, poplar, and willow, especially cypress [40]. As an evergreen forest, cypress flowerings at the time of needle-leaf flush, while poplar and willow flowering first and then leaf out immediately. However, if the flowering and leaf spreading periods of the allergenic plant are separated by a long time, its greenness characteristics cannot reflect its flowering, and the method will be not applicable.

4.2. Advantages of the Prediction Models

The cumulative linear fit prediction model has a very good prediction ability for the outbreak date of spring pollen allergy in Beijing based on the EVI2 of evergreen forest. It not only attenuates the fluctuation of the original data before the pollen allergy outbreak by cumulating daily data, but also has a higher fitting accuracy compared with the linear fit prediction model, in which the cumulative linear fitting curve almost coincides with the cumulative EVI2 time-series curve at the prediction period (Figure 4).

The existing pollen allergy-related prediction models mainly forecast pollen allergy outbreaks by forecasting the airborne pollen concentration through the empirical relationships between pollen concentrations and meteorological elements [46,47]. There are also models utilizing the flowering period of allergenic vegetation estimated by the blooming period of earlier flowering vegetation [48] to predict pollen allergies, but these models only predict vegetation characteristics related to pollen allergies and do not directly predict the date when people will be affected by allergic pollens. In addition, the existing prediction models require a substitution of data related to multiple meteorological factors for actual forecasting, which greatly limits the application of the prediction models. Moreover, the forecasting accuracy of the existing models is reduced by this indirect relationship between forecasting features and pollen allergy outbreaks.

The newly developed cumulative linear fit prediction model is not only capable of directly forecasting pollen allergy outbreaks, but also has a high data availability to operate, which only requires remote sensing vegetation index time-series data in the preliminary period of pollen allergy outbreaks. Moreover, the cumulative linear fit prediction model is more representative (directly reflecting pollen allergy rather than pollen concentration) and operational, and the accuracy of this direct forecast is higher (RMSE of 3.5 days) than that of the indirect forecast.

4.3. The Importance of Data Preprocessing for the Daily Vegetation Index Time-Series Data and Limitations for the Application of the Prediction Models

There are many noises in the daily vegetation index time-series data because of the disturbances from cloud contamination and atmospheric conditions [49]. Therefore, it is needed to reduce the noises before the application of the prediction models. In this study, the spatial 5% trimmed mean of each vegetation index value in every vegetation type was taken as the daily phenological characteristics for each year, and then the data were further smoothed by taking the mean of the higher six values within the previous 11-day filtering window for each day. Other noise reduction methods may affect the forecast accuracy since the prediction models are easily limited to the quality of remote sensing data. In extreme cases, if the study area is long affected by cloud cover, which causes degradation in the quality of remote sensing data, the forecast accuracy will be significantly reduced.

5. Conclusions

This study revealed the satellite-derived phenological characteristics of vegetation greenness before the spring pollen allergy outbreak in Beijing based on 11-year Sina Weibo data and corresponding satellite data, and established a prediction model to forecast the outbreak date of spring pollen allergies in Beijing based on the phenological characteristics that best reflect spring pollen allergies. The main conclusions are as follows.

(1) The satellite-derived phenological characteristics of vegetation greenness are obvious during the early period of the spring pollen allergy outbreak in Beijing. The date of EVI2 in evergreen forest first reaching 0.138 can best reflect the outbreak date of pollen allergy in spring. Moreover, it has a specific phenological indication: the date of EVI2 in evergreen forest first reaching 0.138 in spring basically corresponds to the greenup date.

(2) The cumulative linear fit prediction model based on EVI2 of evergreen forest has a very good prediction ability for forecasting the spring pollen allergy outbreak date in Beijing, and it can predict 30 days in advance, with a low average RMSE of 3.6 days. The existing forecast models of pollen allergy outbreak are mainly based on indirect forecasting of pollen concentrations, while the newly developed model can forecast pollen allergy outbreak directly. It only requires the time-series data of remote sensing vegetation index before pollen allergy outbreak to achieve the forecast, which is more representative (directly reflecting pollen allergy rather than pollen concentration) and operable. However, the forecast accuracy of the cumulative linear fit prediction model is easily limited by the quality of remote sensing data, and if the study area is affected by cloud cover for a long time, which results in degradation of the quality of satellite remote sensing data, the forecast accuracy will be significantly reduced.