Journal of Geo-information Science >
Classification for Spatial Patterns of Urban Ozone Pollution in Beijing Based on Semi-Supervised Few-Shot Learning
Received date: 2023-09-25
Revised date: 2023-11-06
Online published: 2024-03-31
Supported by
Fundamental Research Funds for the Central Universities(423062)
Ozone concentrations tend to be heterogeneous across a city's space due to the mixed land use and diverse landscapes. Studying spatial patterns of urban ozone pollution contributes to the knowledge of the mechanism of pollution formation and also provides scientific reference for pollution prevention and control. Nevertheless, most previous research focused on the averaged value of ozone concentration from monitoring sites, which cannot describe the spatial characteristics of the entire region's concentration surface. Additionally, the classification method was seldom used to analyze pollutants' spatial patterns, and thus very few studies paid attention to the varied types of patterns and their temporal variations. In this study, based on the distributions of ozone’s daily maximum 8-h moving average estimated from satellite data, an approach of semi-supervised few-shot learning was proposed to classify ozone's spatial patterns in Beijing. The self-training method considered the difficulty of data labeling and can utilize information from a large number of unlabeled samples to augment the training set iteratively. Three kinds of normalized features were involved in classification to describe the spatial variations of concentrations. Totally, there were 40 training samples and 249 test samples for the year of 2020, and the overall classification accuracy was 81.12% with a kappa coefficient of 0.741 6. This demonstrated the effectiveness of the semi-supervised classification method despite the small size of training samples. The classification results showed that, among the eight patterns of ozone distributions in Beijing, three of them were major patterns, including Pattern 1: high concentrations in the south/east/southeast and low in the north/west/northwest, Pattern 2: high concentrations in the north/northwest and low in the south/southeast, and Pattern 6: low concentrations in the center. They dominated the warm season (from Mar. to Oct.), the cold season (from Nov. to Feb.), and the transition period, respectively. These temporal variations of ozone's spatial patterns indicated the influence from the seasonality of regional transport and photochemical reactions. When training samples were transferred to the year of 2019, the overall classification accuracy reached 80.97%, and the kappa coefficient was 0.745 6, suggesting the high potential of sample migration. And the results of 2019 further confirmed the previous findings. Thus, the proposed classification method for spatial patterns of urban ozone pollution can not only benefit the identification of regions with heavy pollution but also support the study on mechanisms of different pollution events.
SUN Jin . Classification for Spatial Patterns of Urban Ozone Pollution in Beijing Based on Semi-Supervised Few-Shot Learning[J]. Journal of Geo-information Science, 2024 , 26(3) : 725 -735 . DOI: 10.12082/dqxxkx.2024.230581
表1 北京市臭氧分布数据经站点监测值验证结果Tab. 1 Validation of near-surface ozone concentrations in Beijing using monitoring data |
年份 | 总体 | 分站点 | |||||
---|---|---|---|---|---|---|---|
R2 | RMSE/ (μg/m3) | R2最 小值 | R2最 大值 | RMSE最小值/(μg/m3) | RMSE最大值/(μg/m3) | ||
2019 | 0.93 | 12.17 | 0.86 | 0.99 | 0.33 | 28.82 | |
2020 | 0.94 | 13.33 | 0.81 | 1.00 | 5.72 | 28.44 |
表2 北京市臭氧空间分布的8类模式Tab. 2 Eight types of ozone's spatial patterns in Beijing |
模式类型 | 模式描述 |
---|---|
1 | (东)南高(西)北低或东高西低 |
2 | (西)北高(东)南低 |
3 | 东北高西南低 |
4 | 西(南)高东(北)低 |
5 | 中心高 |
6 | 中心低 |
7 | 空间差异较小的均匀型 |
8 | 难以分入上述类型的混杂型 |
表3 半监督分类的混淆矩阵Tab. 3 Confusion matrix of semi-supervised classification |
模式类型 | 1 | 2 | 3 | 4 | 5 | 6 | 8 | 制图精度/% | 用户精度/% |
---|---|---|---|---|---|---|---|---|---|
1 | 99 | 0 | 0 | 0 | 2 | 1 | 1 | 96.12 | 89.19 |
2 | 0 | 55 | 4 | 0 | 1 | 0 | 3 | 87.30 | 90.16 |
3 | 0 | 0 | 14 | 0 | 0 | 0 | 0 | 100.00 | 58.33 |
4 | 2 | 0 | 0 | 8 | 1 | 0 | 0 | 72.73 | 72.73 |
5 | 7 | 0 | 0 | 0 | 4 | 0 | 0 | 36.36 | 44.44 |
6 | 0 | 5 | 1 | 2 | 0 | 20 | 5 | 60.61 | 90.91 |
8 | 3 | 1 | 5 | 1 | 1 | 1 | 2 | 14.29 | 18.18 |
总体分类精度/% | 81.12 | ||||||||
Kappa系数 | 0.741 6 |
表4 2020年北京市臭氧日最大8 h滑动均值分布模式分类结果统计Tab. 4 Statistics of spatial patterns of ozone's daily maximum 8-h moving averages in Beijing during 2020 |
模式类型 | 分类结果 | 实际 | |||
---|---|---|---|---|---|
天数 | 占比/% | 天数 | 占比/% | ||
1 | 121 | 33.06 | 113 | 30.87 | |
2 | 71 | 19.40 | 73 | 19.95 | |
3 | 29 | 7.92 | 19 | 5.19 | |
4 | 16 | 4.37 | 16 | 4.37 | |
5 | 15 | 4.10 | 17 | 4.64 | |
6 | 71 | 19.40 | 82 | 22.40 | |
7 | 32 | 8.74 | 32 | 8.74 | |
8 | 11 | 3.01 | 14 | 3.83 |
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