基于改进Apriori算法的环境空气NO2浓度变化的关联因素分析

THE CORRELATION FACTORS OF THE CHANGE OF AMBIENT AIR NO2 CONCENTRATION: AN ANALYSIS BASED ON IMPROVED APRIORI ALGORITHM

关键词:  空气污染物NO2  关联因素  Apriori算法改进  交通流和气象

Key words:  air pollutant NO2  correlation factors  improved Apriori algorithm  traffic flow and meteorology

摘要:

环境空气污染是一种复杂的非线性动态现象，受道路网络上的交通流、气象条件等因素的影响，定量揭示这些因素与空气污染物浓度的变化关系，是空气质量预测与改善的重要基础。以广东省佛山市南海区气象局空气质量监测站点周边1.5 km的半径区域为研究范围，利用改进后的关联规则算法对监测站点不同方位的道路交通流，引入气象条件，定量分析对空气污染物NO2浓度的影响，并通过线性相关分析验证关联规则的结果。(1) 传统的Apriori算法计算效率低，得到的规则多为无效、不可靠。将原算法对数据库多次扫描，改变为对上一频繁K项集的元素进行扫描。并引入新的可靠性衡量指标“提升度”，及加入对关联规则结果的筛选过程。改进后的算法只需扫描数据库一次，新增的两类改进方法使Apriori算法计算效率得到提升，且增强了强关联规则挖掘的可靠性。(2) 从强关联规则得出：影响NO2浓度变化的主要因素是风速、温度和气压，风速和温度与NO2浓度的变化呈负相关，气压与NO2浓度变化呈正相关。(3) 引入道路交通流，结合气象因素对NO2的影响：道路交通流量大且扩散条件好，不会导致污染物迅速上升，具体表现为：当道路交通流量较大时，伴随气压较低、风速较大或者温度较高，NO2浓度处于低浓度等级，置信度较高(90%～100%)；而道路交通流小且气象条件差，会导致污染物逐渐累积，具体表现为：道路交通流较小时，伴随气压较高、风速较低或气温较低，不利于NO2扩散，且量化的置信度存在一定的偏差；考虑风向条件时，道路位于上下风向，对NO2浓度的影响也不同。(4) 基于强关联规则识别的关键影响因素，与NO2进行线性拟合并计算皮尔逊相关系数，所得结果与关联规则算法的结论一致。通过以上结果，表明关联规则算法在挖掘定量关系具有较高的效率性和准确性，可为区域空气治理和预测提供技术支持。

Abstract:

Ambient air pollution is a complex non-linear dynamic phenomenon. It is affected by factors such as the traffic flow on road networks and meteorological conditions. Quantitatively revealing the relationship between these factors and the concentration of air pollutants is an important basis for air quality prediction and improvement. This paper focuses on the 1.5 km road network around the air quality monitoring station at the Meteorological Bureau of Nanhai District, Foshan. It uses the improved association rule algorithm to monitor the road traffic flow in both directions on the road around the station, introduces meteorological conditions to quantitatively analyzes NO2 concentration, and verify the results of association rules through linear correlation analysis. The results show that: (1) The traditional Apriori algorithm has low computational efficiency, and the rules obtained are mostly invalid and unreliable. To improve the algorithm, the present study changes from scanning the database for multiple times to scanning the elements of the last frequent K item sets. It also introduces a new reliability measurement index "lift" and a process to screen the results of the association rules. The database only needs to be scanned once when the improved algorithm is used. The two new methods increase the calculation efficiency of the Apriori algorithm and enhance the reliability of strong association rule mining. (2) According to the strong association rule, the main factors affecting the change of NO2 concentration are wind speed, temperature, and air pressure. Wind speed and temperature are negatively correlated with changes in NO2 concentration, whereas air pressure is positively correlated with changes in NO2 concentration. (3) In the present research, road traffic flow is introduced as a parameter and combined with meteorological factors to assess their influence on NO2. It is found that when the road traffic flow is large, the accompanying air pressure is low, the wind speed is high or the temperature is high, the NO2 concentration will be at a low concentration level, and the confidence level will be high (90%~100%). When the road traffic flow is small, the pressure is high, the wind speed is low, or the temperature is low, the diffusion of NO2 will be affected, and there will be certain deviation in the quantified confidence level. As for wind direction, the road is located in the upwind and downwind directions, which have different effects on NO2 concentration. (4) Based on the key influencing factors identified by the strong association rule, linear fitting is performed with NO2 and the Pearson correlation coefficient is calculated. The results obtained are consistent with those of the association rule algorithm. The above results show that the association rule algorithm has high efficiency and accuracy in mining quantitative relationships and can provide technical support for regional air governance and forecasting.