Multi-dimensional satellite observations of aerosol
properties and aerosol types over three major urban
clusters in eastern China

Abstract. Using nine years (2007–2015) of data from passive (MODIS/Aqua) and active (CALIOP/CALIPSO) satellite measurements over China, we investigate (1) the temporal and spatial variation of aerosol properties over the Beijing-Tianjin-Hebei (BTH) region, the Yangtze River Delta (YRD) and the Pearl River Delta (PRD) and (2) the vertical distribution of aerosol types and extinction coefficients for different aerosol optical depth (AOD) and meteorological conditions. The results show the different spatial patterns and seasonal variations of the AOD over the three regions. Annual time series reveal the occurrence of AOD maxima in 2011 over the YRD and in 2012 over the BTH and PRD; thereafter the AOD decreases steadily. Using the CALIOP vertical feature mask, the contributions of different aerosol types to the AOD were analysed: contributions of dust and polluted dust decrease from north to south, contributions of clean ocean, polluted continental, clean continental and smoke aerosol increase from south to north. In the vertical, the peak frequency of occurrence (FO) for each aerosol type depends on region and season and varies with AOD and meteorological conditions. In general, three distinct layers are observed with the peak FO at the surface (clean continental and clean marine aerosol), at ~1 km (polluted dust and polluted continental aerosol) and at ~3 km (smoke aerosol), whereas dust aerosol may occur all over the altitude range considered in this study (from the surface up to 8 km). In this study nighttime CALIOP profiles were used. The comparison with daytime profiles shows substantial differences in the FO profiles with altitude which suggest effects of boundary layer dynamics and aerosol transport on the vertical distribution of aerosol types.


summer when the 9-year averaged seasonal AOD is lowest in the PRD (0.48) and highest in the BTH (0.81). In spring and autumn, the 9-year averaged AOD is similar in all three regions: around 0.69 in spring and about 0.51 in the autumn. In winter the AOD is similar to that in the autumn, with somewhat higher value in the BTH and a little lower in the other two regions. This seasonal variation is similar to 220 that observed using ATSR and MODIS-Terra (C6 DTDB) AOD data averaged over 2000(de Leeuw et al, 2018. Different processes contribute to the differences in AOD among the 3 regions. During the summer, the direct emission of aerosols and precursor gases (contributing to secondary formation of aerosols) from straw burning contribute to the high AOD over the BTH. The high relative humidity during the summer monsoon in the BTH results in the growth of aerosol particles and shift of 225 the particle size distribution to larger particles and thus an increase of the extinction and the AOD.
Furthermore, the larger boundary layer heights (BLHs) in the summer allow for mixing over a deeper layer resulting in elevated AOD. BLHs are greatest over the BTH and smallest over the PRD (Guo et al., 2016b). In the spring, the high AOD over the YRD and BTH may be due to the contribution of long-range transported dust, while over the YRD also hygroscopic growth during elevated RH early in 230 the monsoon season may contribute. The high AOD in the PRD in the spring may be related to long-range transport of pollutants from biomass burning in southeast Asia, which then mixes with moist air particles at the top of the boundary layer (Deng et al., 2008;Heese et al., 2017;Zhang et al., 2018). In the autumn the whole eastern region is dominated by westerly winds, which contributes to the diffusion of aerosols. Meanwhile, the impact of dust storms is relatively small in the autumn, which is also one of 235 the reasons why the AOD is relatively low in this season. Conversely, during the winter, northerly winds prevail, bringing dry and clean air. In this situation the aerosol tends to be transported to the south and thus the aerosol concentrations over the BTH are reduced (Qi et al., 2013;Si et al., 2018). Hou et al.  The monthly AOD, averaged over the 9-years 2007-2015, over the three regions ( Fig. 1(c)) shows that the largest differences between the regions occur from May to August. The summer AOD peak in the 245 BTH occurs in July (AOD of 0.86), whereas in the YRD it clearly occurs in June (0.93) with a fast decline thereafter. In both regions, the AOD is higher in the period before the summer (0.6-0.7) and declines from September (0.6) to December ( ̴ 0.4). In contrast, in the PRD the AOD peaks twice, in March (0.78) and in October (0.54), with much lower values in the summer and a clear minimum in July (0.4). The difference between the AOD variations in the three regions are due to processes discussed 250 above for the seasonal variation, while in addition the effect of the East Asian summer monsoon moving from the south of China in April to the North in July and then back to the south affects the month-to-month variations (Luo et al., 2014). The monsoon brings heavy rains which effectively washout aerosols, resulting in the monthly AOD variations over the PRD with one peak before the pre-summer rain in March, the minimum during the summer rain period in July and the second AOD 255 peak after the rainfall in October (Fig. 1(c)). With the seasonal progression of the monsoon to the north and weakening rainfall the monsoon arrives later in the year over the YRD and the BTH where the AOD https://doi.org/10.5194/acp-2020-1278 Preprint. Discussion started: 19 January 2021 c Author(s) 2021. CC BY 4.0 License. peaks occur in June and July, respectively.

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The spatial variation of the AOD over the three urban clusters, averaged over the seasons in the years from 2007 to 2015, is shown in Fig. 2. The spatial patterns over the three regions are similar in all seasons, but the values of the AOD vary from season to season. The AOD over the BTH is low over the mountains in Shanxi province in the Northwest and high in the Southeast over Hebei and Shandong.
The mountains separate the North China Plan (NCP) in the east, with a very high degree of 265 industrialization and a very high population density resulting in very high pollution, from the cleaner areas in the west. The mountains prevent the transport of pollution which accumulates along the ridge in meteorological conditions when the wind is from south-easterly directions, as observed all seasons.
The heavy industries and power plants in the NCP are responsible for the high AOD. Meanwhile, the AOD in the summer may also be enhanced by emissions of aerosols and precursor gases from 270 straw-burning (Kang et al., 2016a;Kumar et al., 2015;Si et al., 2018). Over the YRD, the AOD is lower in the Zhejiang and southern Anhui provinces as compared to other areas, during all seasons. The AOD is highest in the eastern part of the YRD, especially Shanghai and Jiangsu. There is a line with enhanced AOD going from Shanghai to the southwest of Zhejiang, i.e. over the Jin-Qu basin with high population density and much industrial activity. The AOD is lower over the mountains on both sides of 275 the basin. The AOD spatial distribution over the PRD shows a ring-shaped pattern, with the highest values in the center and decreasing toward the outside of the ring. The highest AOD areas cover the busy industrial centers with much economic activity and a high number of vehicles, leading to elevated anthropogenic pollutants from coal, biomass burning and industrial emissions (Chen et al., 2014;Mai et al., 2018;Zhang et al., 2018).

General distribution of aerosol types over major urban clusters
Aerosol types were obtained from the CALIOP VFM files (nighttime) over the three regions. and 27%, respectively. In contrast to the other two regions, clean marine aerosol contributes substantially (14%) over the PRD and dust contributes only 3%. The contribution of clean continental aerosol is higher (6%) over the PRD than the other two regions. Although local anthropogenic pollution exerts a major influence on aerosols over the PRD, the northwest winter monsoon may 305 transport continental aerosols (Heese et al., 2017), and the southeast summer monsoon may transport marine aerosols to this region (Wu et al., 2013;Heese et al., 2017).
These data show the large differences between the PRD and the other two regions. The frequencies of occurrence of clean marine, polluted continental, clean continental and smoke aerosol are lowest over the BTH and highest over the PRD. In contrast, polluted dust and dust contribute most to the aerosol 310 over the BTH and very little over the PRD.

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The occurrence of aerosol depends on local sources, i.e. direct production and secondary formation from precursor gases, and processes affecting their transformation and dispersion, as well as long-range transport from remote sources. Atmospheric circulation and the resulting weather conditions affect the formation and transformation of aerosol particles (Zhang et al., 2008;Cao et al., 2013).

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The thermal stability of the atmosphere is closely related to the diffusion and accumulation of aerosols (Kipling et al., 2016). The stability of the lower atmosphere is one of the common atmospheric thermal conditions, which is used to describe the increase or decrease of the vertical motion of the atmosphere.
The lower tropospheric stability (LTS) is calculated from the difference of the potential temperature in the free atmosphere (700hPa) and near the surface (1000hPa), indicating a measure of the atmospheric 525 thermodynamic state (Klein and Hartmann, 1993). The larger the LTS, the more stable the atmosphere and the tendency to deter vertical motion; and vice versa, the smaller the LTS, the more unstable the atmosphere and the tendency to facilitate vertical motion. In this study, all aerosol samples were divided into three equally sized subsets from the lowest to the highest LTS. Mean vertical distributions of the FO of the aerosol subtypes, averaged over the years 2007-2015, for each subset are presented in 530 Fig. 9. In unstable atmospheric conditions (LTS=28.93), dust aerosol dominates at altitudes higher than 4 km and polluted dust dominants below 4 km. In contrast, polluted continental is the dominant aerosol type below 2 km and smoke dominates above 2 km during stable atmospheric conditions. Figure 9 also shows that the peak FO of smoke aerosols around 3 km (i.e. in the free troposphere above the atmospheric boundary layer). Due to the heat released by fossil fuel combustion and biomass burning, 535 the temperature near the ground will rise, and the updraft results in the transport of smoke aerosols into the higher atmosphere, i.e. layer C. The data in Fig. 9 show that the FO of polluted continental aerosol in layer B increases when the atmosphere becomes more stable, which indicates that, when turbulence and convection are restrained, aerosol particles tend to accumulate in the near surface layer (Tian et al., 2017). In contrast, the peak FOs of polluted dust and dust aerosol in layer B gradually decreases with 540 the increase of LTS. This may imply that convection can lift dust aerosols to higher altitudes.

Day/night variation of the vertical distributions of aerosol types
The two CALIPSO overpasses, at 1:30 am and pm local time, provide information on the day/night differences of the vertical distribution of the FO of the aerosol types derived from the CALIOP observations. Figure 10 shows the vertical distributions of the FO of the aerosol types during the daytime overpasses over each of the three regions, for each season averaged over the years 2007-2015.

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Comparison of the daytime vertical distributions in Fig. 10 with the nighttime vertical distributions in Fig. 5 shows substantial differences which depend on altitude, aerosol type, season and region. To clearly illustrate these differences, difference plots (nightday) are presented in Fig. 11. Fig. 11 shows, for instance, that in the summer, in all three regions, the maximum FO of the polluted dust layers is larger during the day than during the night (negative night-day difference). The difference is 555 much larger over the PRD than over the YRD which in turn is larger than over the BTH. In contrast, for smoke the maximum FO during the day over the PRD and YRD is substantially smaller than during the night (positive night-day difference), while over the BTH the day/night difference is rather small. For clean marine aerosol the FO is larger during the night in the PRD and negligible in the other two regions. For polluted continental aerosol, however, the difference profiles in Fig. 11 clearly show that the 565 daytime FO is highest in a layer adjacent to the surface (night minus day negative) which is clearly separated from the layer above where the FO is higher during the night. The distribution of polluted continental aerosol over two layers is not evident from either the daytime or the nighttime FO distributions, although a closer look shows some small discontinuities in the vertical distributions.
The higher daytime FO indicates the accumulation of this aerosol type in a turbulent mixed layer 570 https://doi.org/10.5194/acp-2020-1278 Preprint. Discussion started: 19 January 2021 c Author(s) 2021. CC BY 4.0 License.
which expands under the influence of solar heating. After sunset, radiative cooling of the surface results in the formation of a stable nocturnal boundary attached to the surface which lifts the mixed layer into a disconnected residual layer (Stull, 1988), including the polluted continental aerosol which is thus observed at higher elevation.
This separation is most clearly observed over the PRD during the summer and autumn, although the 575 distribution over two different layers is also suggested by the profiles in winter and spring. The profiles over the YRD in the summer and autumn behave similarly, although weaker than over the PRD. In all these cases, polluted continental is the dominant aerosol type, or one of the most dominant aerosol types, in the lower 2 km. Polluted continental aerosol is emitted, or formed from precursor gases, near the surface and its transport to higher elevations is prohibited by the temperature 580 inversion at the top of the mixed layer. The formation from precursor gases often involves a photo-chemical reaction, i.e. requires the availability of solar radiation and thus occurs during daytime.
In contrast, marine aerosol is directly emitted from the ocean in high wind conditions when waves break (de Leeuw et al., 2011). Marine aerosol is confined to the mixed layer (layer A) and significant 585 FOs of clean marine and dusty marine aerosol are mainly observed over the PRD, in all four seasons. Fig. 11 shows that the FO of dusty marine aerosol is higher during the day than during the night, whereas in contrast, the FO of clean marine aerosol is higher during the night. Over the ocean the air-sea temperature difference does not change strongly as it does over land and thus nocturnal boundary layers are not formed over the ocean. Marine aerosol is transported to the study area, which 590 is over land, in on-shore wind and hence marine aerosol is well distributed over the lower boundary layer.
Dust is long-range transported from the deserts in the north and west of China where it is emitted to high elevations before is passes over the mountains to east China (

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Based on the above findings, the regional and seasonal variations of the spatial and vertical distributions of aerosol properties are discussed in the following. -In the summer, the AOD is highest over the BTH and YRD (Fig. 1b), which may be attributable to more abundant water vapor and higher temperatures in the summer resulting in strong convection causing deeper boundary layers. The moist air results in higher RH and thus in the swelling of 615 aerosol particles, i.e. a shift of the particle size distribution to larger sizes which in turn results in higher extinction and AOD. The higher temperature results in faster chemical reactions and thus formation of secondary aerosol. With the increased RH, the peak FOs of smoke and clean continental aerosol increase and the peak FO of polluted continental aerosol reaches its maximum value under humid conditions (Fig. 7). Dynamically, the altitude of the peak FO of smoke 620 increases when the ascending motion of air masses occurs (Fig. 9). In addition, biomass burning is the main source of smoke aerosol in the summer. This is in line with smoke aerosol being the second dominant aerosol type above 2 km over the BTH in the summer. Over the YRD, smoke is the dominant aerosol type above 2 km during the summer (Fig. 5). In contrast, the AOD is lowest over the PRD, which may be due to wet removal by East Asian summer monsoon precipitation.
625 Fig. 5 also shows that the FO of clean marine aerosol over the PRD is highest in the summer.
Moreover, polluted continental aerosol dominates the aerosol below 3 km over the YRD and PRD in the summer.
-In the spring, the AOD is highest over the PRD (Fig. 1b), which may be related to long-range transport of pollutants from biomass burning in southeast Asia. This is consistent with the 630 observation that smoke is the dominant aerosol type above 2 km and the FO extends to higher altitudes than in other seasons (Fig. 5). Over the BTH and YRD, dust and polluted dust dominate from near the surface to the upper troposphere, leading to the higher AOD over these two areas.
Over the YRD, the altitude of the peak FO of dust in MAM and DJF is substantially higher (5 km) than over the BTH (Fig. 5), which may be due to the long-range transport of dust aerosol by 635 westerly winds from north-west China.
-In the autumn, the AOD over the three regions is relatively low throughout the year (Fig. 1b). In this season, the whole eastern region is dominated by westerly winds, which transports relatively clean air to the study regions and contributes to the diffusion of aerosols. The impact of dust storms is relatively small in the autumn.

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-In the winter, the prevailing northerly wind brings dry and clean air to the study regions and hence the aerosol concentrations, and thus AOD, are low (Qi et al., 2013;Si et al., 2018). Nevertheless, high AOD does occur frequently in the winter, which is reflected in the average AOD of 0.7 over the BTH in February. Very high values occur during weather conditions conducive of the formation of haze (low wind speed, low BLH, stable stratification). The peak FOs of smoke, 645 polluted continental and clean continental aerosol get larger and that of dust and polluted dust aerosol gets smaller when the atmosphere become stable (Fig. 9).
-With regard to the altitude of the peak FOs of the aerosol types over the three regions, the order from low to high altitude is overall as follows: dust > polluted dust > clean continental/smoke > polluted continental > clean marine/dusty marine. with increasing AOD in the three regions. In heavily polluted conditions, the peak FO of smoke aerosol at an altitude of ~ 2.5 km is largest over the three regions. The FOs of smoke and dust aerosol in low AOD conditions occur at higher altitude than during the other two AOD conditions. The extinction coefficient of the aerosols below 6 km is lowest over the PRD and highest over the YRD.
The variation of the aerosol vertical distribution was also analysed in terms of relative humidity and 695 dynamic and thermodynamic boundary layer conditions. Overall, the FOs of clean continental and smoke aerosol gradually increase with the increase of RH. In addition, the altitude of the peak FO of smoke aerosol is largest during conditions with high relative humidity. Dynamically, the downward motion of air parcels can increase the FOs of polluted dust and polluted continental aerosol at 1 km.
With regard to thermal stability and vertical mixing, using LTS as a proxy, the peak FOs of smoke and 700 polluted continental aerosol increase when the atmosphere becomes more stable. Conversely, the peak FOs of polluted dust and dust aerosol around 1 km gradually decrease with the increase of LTS.
In this study, nighttime CALIOP observations were used to study the vertical distribution of aerosol types and extinction coefficients. During the night, meteorological conditions, atmospheric chemistry and aerosol processes are different from those during the day. The two CALIPSO overpasses, at 1:30 705 am and pm local time, were used to evaluate daytime/nighttime differences between the vertical distributions of the frequency of occurrence of CALIOP-derived aerosol types. These differences depend on the aerosol type, altitude, season and location and provide information on effects of aerosol transport and boundary layer processes on the vertical distribution of different aerosol types.
In summary, the aerosol properties, aerosol types and vertical profiles in different AOD and 710 meteorological conditions over three representative regions over eastern China were described, using synergetic use of aerosol products from active and passive sensors. This work can be used to improve model assessment of the direct and indirect aerosol effects in eastern China (Wang et al., 2011;Wu et al., 2016).

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All data used in this study are publicly available. The satellite data from the MODIS instrument used in this study were obtained from https:// ladsweb.modaps.eosdis.nasa.gov/search/ (last access: 19 January 2021). All data used in this study are publicly available. The satellite data from the CALIOP instrument used in this study were obtained from https://subset.larc.nasa.gov/calipso/ (last access: 19 January 2021). The ECMWF ERA-Interim data were collected from the ECMWF data server 720 http://apps.ecmwf.int/datasets/data/interim-full-daily/levtype=pl/ (last access: 19 January 2021).

Author contributions
YL and TL designed the research. YL led the analyses. YL and GL wrote the manuscript with major input from JH and further input from all other authors. All authors contributed to interpreting the results and to the finalization and revision of the manuscript.