4.1. The Recommended Vertical Correction Schemes
For the whole of China, Table 2
shows the seasonal and annual correlations between ambient PM2.5
and original satellite-derived AOD, ambient PM2.5
and vertically revised AOD by PBLH, and ambient PM2.5
and vertically revised AOD by CALIOP ratios. Considering the influential factors of terrain, climate, meteorology, and aerosol transmission, the effectiveness of the vertical correction on satellite-derived AOD has been discussed according to different regions divided by the geographical distributions of monsoons, temperature, humidity and air density, as well as the interrelationships between various types of natural geographic phenomena [49
In northwestern China, AOD vertically revised by PBLH is more closely related to ground-measured PM2.5
than the original relationship, and the relationship after vertical correction by the CALIOP ratio. In our collected PBLH datasets of the two years, the PBLH in northwest possessed a large span from 50 to 2800 m and had the largest standard deviation when compared to other regions of china, which is consistent with previous research finding that the boundary layer height varied dramatically due to high frequencies of temperature variation and surface pressure change, combined with the climatologically strong near-surface wind and intense solar radiation in northwestern China [50
]. Assuming that aerosols are homogeneous under PBL, changes in PBLH could stretch or compress the surface aerosol layer, further influencing the density of aerosols, which led to the discrepancies of PM2.5
with column AOD [26
]. Therefore, after vertical correction via PBLH, the influence of the dramatic changes in boundary layer height could be eliminated to a certain extent, and it is suggested that the AOD vertically revised via PBLH should be used for the estimation of ground-level PM2.5
in the northwest.
In the North China Plain, the relationship between ambient PM2.5
and AOD vertically revised by CALIOP ratio was optimal in Tianjin (TJC). Based on two-year CALIOP aerosol profiles data, the existence of valid extinction coefficient values in higher atmospheric layer indicated that the elevated aerosol layer was inclined to appear in this region. Considering aerosol transmission in the North China Plain, Tianjin was significantly affected by air masses that originated from Mongolia and the North China Plain regions, which led to the emergence of elevated aerosol levels, further forming the combined influence of exogenous aerosols and local aerosols in this region [52
]. The vertical correction by CALIOP ratio could remove the elevated aerosols from the total columnar aerosol layers and separate elevated aerosol layers from the near-surface aerosol layers. This eliminates the effect of elevated aerosols on near-surface aerosol layers, while the vertical correction via PBLH failed to consider and solve the issues of elevated aerosols. Applying the CALIOP ratio for vertical correction on satellite AOD when estimating ambient PM2.5
in Tianjin is, therefore, suggested. However, it was identified that the relationship between AOD after vertical correction and ground-level PM2.5
decreased when compared to the original relationship in Beijing (BJC). In two-year CALIOP aerosol profiles data, there were few valid values of extinction coefficient in higher atmospheric layer, indicating that the aerosol almost accumulated in near-surface in Beijing. Because there are mountains surrounding three sides of Beijing, the diffusion of air pollutants would be hindered and further leading to an agglomeration and settlement of aerosols, which led to aerosols accumulating in the near-surface area in Beijing [53
]. Thus, the original satellite-derived AOD is recommended for estimating PM2.5
in Beijing, while the AOD vertically revised by CALIOP is suggested for Tianjin.
In Central China, the Pearson correlation coefficient of AOD, vertically revised by CALIOP ratio, and PM2.5
has a noticeable increase over the original relationship, and a slight increase over the relationship following vertical correction via PBLH, except for summer, which demonstrated that the vertical correction by CALIOP ratio is optimal for estimating ambient PM2.5
. In two-year CALIOP aerosol profiles data, the existence of valid extinction coefficient values in higher atmospheric layer in spring, autumn and winter, indicated that the elevated aerosol layer was inclined to appear in Central China in specific seasons. The statistical result was in accord with previous researches that Central China is more susceptible to elevated aerosols, due to long-distance transportation of air masses, which primarily originated in the northern parts of China or Mongolia and travelled through the highly polluted Jing-Jin-Ji regions, before arriving in Central China, combined with air masses originating from the northwest with dust aerosols [54
]. After vertical correction by the CALIOP ratio, the influence of elevated aerosol could be eliminated to some extent, improving the relationship between AOD and PM2.5
. Therefore, performing vertical correction by the CALIOP ratio when estimating ground-level PM2.5
via satellite-derived AOD in spring, autumn, and winter in Central China is, therefore, recommended. However, the correlation between AOD after vertical correction and ambient PM2.5
was poorer than the original situation in summer, suggesting that the original satellite-derived AOD is optimal for estimating ambient PM2.5
in summer in Central China. As Central China is in a subtropical zone with a subtropical monsoon climate, the summer monsoon enhanced the transmission of clean air masses originating from the Eastern and Southern China Seas, which enhanced the diffusion of local aerosols due to the scour effect by precipitation and atmospheric convection, both horizontally and vertically [54
]. Since air pollution tended to be mainly from local aerosols emerging in the near-surface area, the original satellite-derived AOD without vertical correction is suggested to be useful when estimating ambient PM2.5
concentrations in summer.
In the southeastern coast, the original satellite-derived AOD was more closely related to ground-measured PM2.5
than AOD after vertical correction, while the relationship after CALIOP ratio vertical correction is optimal in the spring. Based on two-year CALIOP aerosol profiles data, there were many valid extinction coefficient values in higher atmospheric layer only in spring, indicating that the aerosol was inclined to accumulate in near-surface at most of the time in this region. As the southeast coast is greatly influenced by air masses originating from the northern desert and desertified regions, this leads to the occurrence of elevated aerosols in spring, consistent with previous observations that elevated aerosols accounted for approximately two fifths of the total aerosol layer during spring via CALIOP [56
]. Since vertical correction by CALIOP ratio could eliminate the influence of elevated aerosols on the near-surface aerosol layers to some extent, the revised AOD by CALIOP ratio is recommended for estimating ambient PM2.5
in the spring in the southeastern coast. However, due to slight convection and a relatively thin mixed layer in summer, autumn, and winter, large proportions of aerosols aggregated in the near-surface area in these three seasons, indicating that air pollution was mainly from local aerosols throughout the year, except for spring in the southeastern coast, consistent with previous measurement results that observed larger percentages of near-surface aerosols in the total aerosol layer in summer, autumn, and winter via CALIOP [56
]. Thus, estimating ambient PM2.5
via original satellite-derived AOD is recommended in the summer, autumn, and winter of the southeastern coast. It should be noted that an optimal relationship between the AOD vertically revised by CALIOP ratio and ground-level PM2.5
was observed most in Shanghai (SHH), while the relationship between the AOD after vertical correction and ground-level PM2.5
decreased in winter. According to the two-year CALIOP aerosol profiles data, there were many valid extinction coefficient values in higher atmospheric layer in spring, summer and autumn, which indicated that Shanghai was inclined to be influenced by the elevated aerosol layer in these three seasons. Other research suggests that air masses that originate from western and northern China could bring particles from the Jiangsu and Zhejiang Provinces into Shanghai, so Shanghai would be influenced by exogenous aerosols most of the time [58
]. This is consistent with previous measurements that elevated aerosol layers accounted for one-third of the total aerosol layers in the spring, summer, and autumn via a depolarization-sensitive micropulse lidar (MPL) [59
]. Dense local aerosols from emissions from the combustion of carbonaceous fuels for heating, combined with low wind speed and frequent static wind due to weaker East Asia winter monsoons, would led to the agglomeration of a large percentage of aerosols in near-surface layers during the winter in Shanghai [60
]. This is consistent with previous observations that larger proportions of near-surface aerosols against the total column aerosols were observed via MPL in winter [59
]. Since the vertical correction by CALIOP ratio could separate the elevated aerosols layer from the near-surface aerosols layer, it is recommended that the AOD vertically revised by CALIOP ratio should be used for PM2.5
estimation for most of the year in Shanghai, while the original satellite-derived AOD without vertical correction is suggested for winter.
In the northeastern and southwestern regions, the AOD vertically revised by CALIOP ratio was more closely related to the ground-measured PM2.5
than the original satellite-derived AOD, and the AOD after vertical correction by PBLH. Based on two-year CALIOP aerosol profiles data, the existence of valid extinction coefficient values in higher atmospheric layer indicated that the elevated aerosol layer was inclined to appear in these two regions. Because the southwest is significantly influenced by air masses originating from western desert regions and northern desert regions due to the East Asian and Indian monsoons [62
], while the northeast is more affected by air masses from the North China region travelling through intensive industrial areas with heavy pollution [63
]. These extraneous aerosols lead to the occurrence of elevated aerosol layers in Central China; therefore, the AOD vertically corrected by CALIOP ratio is recommended for PM2.5
estimation in northeastern and southwestern regions.
4.2. Model Performance and Validation
To verify the reliability of our discussions and conclusions, validation experiments using the three vertical correction methods above, respectively, in six regions, were carried out in 2016. The results were displayed in Table 3
. In northwest, the best Pearson correlation coefficient appeared in the experiments using vertical correction via PBLH. In northeast, North China Plain, southwest and Central China (except in summer), ambient PM2.5
had the best correlation coefficients with the revised AOD by CALIOP ratio. In southeast coast, the correlations of the original AOD and ground PM2.5
is best most of the time. In other words, the results of correlation coefficients demonstrated that the optimal vertical correction method in 2016 agreed well with our recommended vertical correction scheme.
Based on the discussions and conclusions above, different vertical correction schemes were recommended in different regions, thus one typical provincial capital for each region was selected as a representative to verify the effectiveness of the recommended vertical correction scheme using LME model fitting. Considering the low temporal resolution of CALIPSO (16 days), seasonal CALIOP ratio was selected to vertically correct MODIS AOD. For each season, after removing the maximum and minimum five percentiles, the remaining daily CALIOP ratios were averaged to calculate seasonal CALIOP ratio. The daily AOD could therefore be revised by multiplying by the according seasonal correction factor.
The CV results are shown in Figure 4
. In southeastern coast, since it is recommended that the AOD vertically revised by CALIOP ratio should be used for PM2.5
estimation in Shanghai (SHH), the comparison between the original situation and the revised situation after vertical correction via CALIOP ratio was conducted. As a result, after vertical correction via CALIOP ratio, CV R2
value reached 0.85, which slightly improved when compared to the original situation of 0.82, and the RMSE value decreased from 17.30 μg/m3
to 16.24 μg/m3
, demonstrating that the later possessed more aggregation distribution. The regression equation after vertical correction via CALIOP ratio was closer to the 1:1 reference line, as the slope displayed a trend closer to 1 and the intercept declined from 11.53 to 10.39 when compared to the original situation. Similarly, in the northeast, southwest, North China Plain and Central China where have been recommended vertical correction via CALIOP ratio when estimating satellite-based PM2.5
, Harbin, Chengdu, Tianjin, and Xi’an were accordingly selected as representatives to verify the vertical correction scheme. It is shown in Figure 4
a–h that all CV R2
values achieved 0.77 after vertical correction and the highest value even approximately achieved 0.90 (in Harbin). Compared to the original situation, the CV R2
values slightly improved from 0.01 to 0.03, and all RMSE values decreased to some extent, with the regression lines closer to 1:1 reference lines after vertical correction via CALIOP ratio. Besides, in northwestern China where vertical correction via PBLH is suggested before estimating ambient PM2.5
using satellite AOD, the comparison between the original situation and the revised situation after vertical correction by PBLH was conduct in Hohhot (HOH) using LME model fitting. The fitting result showed that the CV R2
value slightly increased from 0.76 to 0.77 and RMSE value decreased from 13.28 μg/m3
to 13.11 μg/m3
, with the linear regression equation closer to 1:1 reference line, after vertical correction by PBLH process.
Overall, after vertical correction via CALIOP ratio or PBLH, all CV R2 values reached up to 0.77 and the CV RMSE values decreased to 13.11 μg/m3, demonstrating that the estimates from LME model fitting after vertical correction agreed well with the measured values. Moreover, compared to the original situation, all CV R2 values improved a little, and CV RMSE values declined, with the regression equations closer to 1:1 reference lines, which proved that the estimation accuracy of PM2.5 can be further improved through our recommended vertical correction scheme.