Keywords

1 Introduction

Aerosols are solid particles or liquid substances in the atmosphere. Aerosols can be of natural or anthropological sources and have a significant impact on climate, cloud formation, air pollution, and human health. The most important sources of anthropogenic aerosols include urban pollution (Badarinath et al. 2007, 2008, 2009), industries (Nunes and Pio 1993), and biomass burning (Kant et al. 2000a, b; Prasad et al. 2002, 2003; Hayasaka et al. 2014; Vadrevu 2008; Vadrevu et al. 2006, 2008, 2013a, b, 2014; Vadrevu and Justice 2011; Le et al. 2014; Vay et al. 2011; Biswas et al. 2015). Aerosol scattering and absorption have effects on solar radiation, impacting energy balance between the Earth and atmosphere. Further, aerosols contribute to seeding for cloud and ice condensation, which may cause changes of cloud optical characteristics, microphysics, and lifetime (Kaufman et al. 2002a, b; Lau et al. 2008; Mielonen et al. 2011; Qi et al. 2013; Vadrevu et al. 2015). Aerosols in fine particle matter concentration can cause human respiratory diseases resulting in mortality (Cliff et al. 2005; Dominici et al. 2006). During the twentieth century, research has been conducted on the aerosol size and optical characteristics (Mie 1908), or aerosol size distribution of 0.01–10 μm particles derived from gases or smoke (Junge 1952) and of particles larger than 1 μm from soil or marine mineral (Hobbs 1993).

In recent decades, multispectral satellite images are utilized for estimation of Aerosol Optical Depth (AOD), a parameter measuring the extinction of the solar beam by dust and haze. The approach exploits spatial coverage of satellite images to provide information over land without the need for ground measurements. AOD methodologies were applied on different land surfaces including water, vegetation, desert, or urban (Martonchik et al. 1998; Kahn et al. 2001; Kant et al. 2000a, b; Kaufman and Sendra 1988; Kaufman et al. 2002a, b; Hsu et al. 2004, 2006; Kokhanovsky et al. 2007; Badarinath et al. 2008; Lee and Kim 2010; Wong et al. 2011). The methodologies are based on different multispectral bands to extract aerosol distribution from total reflectance recorded at Top Of Atmosphere (TOA). However, their accuracies are limited especially over complex land surfaces such as urban areas (King et al. 1999).

VIIRS (Visible Infrared Imaging Radiometer Suite) is a sensor on-board Suomi NPP (Suomi National Polar-orbiting Partnership). VIIRS orbits at a height of 829 km with swath of a 3060 km. VIIRS has 24 spectral bands from 0.412 μm đến12.01 μm in which five high-resolution Imagery channels (I-bands), 16 Moderate resolution channels (M-bands), and a Day/Night Band (DNB) have spatial resolutions at 357 and 750 m, respectively. VIIRS has different products for atmosphere, cloud, land surface and water, sea temperature, and sea color. The VIIRS aerosol data products are processed from the VIIRS Sensor Data Records (SDRs) and ancillary data on a granule-by-granule basis. Aerosol Optical Thickness/Depth (AOT/AOD) at 6 km Environmental Data Record (EDR) and 0.75 km for Intermediate Products (IPs) can be retrieved globally during daylight except for areas of clouds and bright surfaces. VIIRS AOD in EDRs have been compared to MODIS (MoDerate Resolution Imaging Spectroradiometer) and AERONET (AErosol RObotic NETwork) AOD at the global (Liu et al. 2012) or to GOCI (Geostationary Ocean Color Imager), MODIS Collection 6, AERONET in East Asia (Xiao et al. 2014).

For the lidar, CALIOP (Cloud-Aerosol Lidar with Orthogonal Polarization) on-board CALIPSO (Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation) launched in 2006 is used to observe cloud and aerosol characteristics for climate, weather forecast, and air quality (Winker et al. 2003, 2010). CALIPSO is the first polarization LiDAR satellite orbiting at 705 km and therefore, provides mostly global coverage between 82°W and 82°S, and crosses over the equator at 13:30 (PM time). Validation of cloud and aerosol products have been conducted, but aerosol subtypes were not considered yet (Lee and Kim 2010; Liu et al. 2009). Methodology for aerosol subtype classification for CALIOP data is based on LiDAR ratios investigated in (Omar et al. 2009) and is used in many dust and smoke studies later (Uno et al. 2008; Liu et al. 2009; Huang et al. 2008; Labonne et al. 2007; Jeong and Hsu 2008; Bhoi et al. 2009).

In Vietnam, utilization of satellite aerosol products has been previously investigated. For example, the analysis for Terra MODIS AOD and AERONET AOD at Bac Giang, Nghia Do, Nha Trang, and Bac Lieu stations have been done by Pham et al. (2015). Relationship between hotpots and atmospheric parameters such as UV Aerosol Index (UVAI), aerosol extinction absorption optical depth (AAOD), and carbon monoxide using satellite products is studied by Le et al. (2014). Estimation of Particulate Matter concentration (PM2.5) from Terra and Aqua MODIS AOD for air pollution monitoring over Vietnam has been studied by Nguyen et al. (2013, 2015).

In this study, we investigate VIIRS and CALIPSO aerosol products and their use in Vietnam. VIIRS and CALIPSO aerosol products are validated over three AERONET stations named Nghia Do, Nha Trang, and Bac Lieu representative for the North, Central, and South of Vietnam, respectively. The next sections present study area and validated datasets including methodology. Analysis of VIIRS and CALIPSO aerosol characteristics at Nghia Do, Nha Trang, and Bac Lieu stations will be shown in Sects. 23.5, 23.6, and 23.7. Finally, conclusions and future work were provided.

2 Study Area

Vietnam is the easternmost country on the Indochina peninsula in Southeast Asia. The total land area is nearly 332,210 km2 and extends from (8°27′N, 102°8′E) to (23°23′N, 109°27′E) with the population of 90.5 million as of 2014. Vietnam is divided into different climatic zones and characterized by seasons. The North has a rainy season from May to September, dry season from November to March, and transition in April and October. The Central and South’s climate is more stable with two seasons in a year. Rainy seasons start from September to December and May to October for the Central and South. The rest of the months are dry seasons (Table 23.1). Three AERONET stations named Nghia Do, Nha Trang, and Bac Lieu located in those regions are considered for analysis in relation to satellite aerosol products.

Table 23.1 Seasons in the North, Central, and South of Vietnam
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3 Datasets and Methodology

In this study, VIIRS AOD Level 2 product at 6 km is validated with AERONET AOD product at Nghia Do, Nha Trang, and Bac Lieu stations in Vietnam. The analysis on aerosol subtypes around AERONET sites is based on CALIPSO aerosol product Level 2 at 5 km. Table 23.2 presents detailed information for each dataset.

Table 23.2 Satellite and ground-based datasets
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The VIIRS aerosol EDR developed by NOAA contains the AOD for 11 wavelengths ranging from 0.412 to 2.25 and the Angstrom exponent. These values are stored as a 96 × 400 array of 16-bit integers with the corresponding scale and offset stored separately in the granule. The AOD quality flag is from 0 to 3 corresponding to four levels: Not Produced, Low, Medium, and High. The Aerosol EDR requires a corresponding Aerosol EDR geolocation for analysis. To match an Aerosol EDR to its corresponding Aerosol Geolocation file, the date, start time, end time, and orbit number in each filename are identical. In this study, VIIRS Aerosol Level 2 products (i.e., VIIRS AOD at 550 nm at all AOD quality levels) are gathered two times per day at 6 km spatial resolution at nadir from 2012 to 2015.

CALIPSO 5 km Aerosol Layer products (CALIPSO AL) at Level 2, version 3 provided vertical profile of AOD at wavelengths 532 and 1064 nm. For each aerosol layer, the product provides aerosol subtypes based on land surfaces (i.e., land or water), polarization ratio, extinction backscatter at the wavelength 532 nm, and aerosol layer’s height or lidar ratios at both mentioned wavelengths (Omar et al. 2009). Aerosol subtypes are defined as dust, polluted dust, clean continental, polluted continental, marine, and smoke. CALIPSO AL includes optical and spatial characteristics (i.e., scatter, extinction, vertical/horizontal resolution) of each aerosol layer at 5, 20, and 80 km horizontal resolution and 30 m vertical resolution in different height condition (Vaughan et al. 2009; Omar et al. 2009; Winker et al. 2009, 2010; Kittaka et al. 2011). CALIPSO AL, collected in the period of 2006–2015, has day and night images in every 16 days.

AERONET station network setup by NASA and PHOTONS provides global long-term, continuous and accessible aerosol data products for various research, satellite validation, and applications. AERONET AOD Level 2.0 (cloud-screened and quality-assured) at Nghia Do, Nha Trang, and Bac Lieu has been gathered for satellite AOD validation.

3.1 Data Integration

Since satellite and ground measurements have different spatial and temporal resolutions, they are first integrated based on location and time constraints (Ichoku et al. 2002). The spatial constraint is applied for VIIRS AOD and CALIPSO AL in which their pixels are only selected if the distances to AERONET station within a radius of “R”. Meanwhile, the time constraint “T” is specified to AERONET AOD to collect ground measurements in the period before and after T coinciding with the satellite overpasses.

Location and time constraints, in reality, vary by geographic areas, climate conditions, and the economy. Therefore, experiments are conducted to identify R and T based on the correlation coefficient between satellite and ground-based data. We employed threshold R as 25 and 80 km for VIIRS AOD and CALIPSO AL, respectively, and T as 30 min (i.e., duration of 60 min coinciding satellite overpasses) for AERONET AOD.

3.2 Validation

Validation of satellite aerosol products over Vietnam has been carried out by comparison of VIIRS and AERONET AOD to see absolute errors and seasonal trends. Data analysis is conducted for each AERONET site representative of each Vietnamese region (North, Central, and South).

VIIRS AOD at 550 nm is selected for comparison. Because AERONET AODs have been measured at 500 nm, we interpolated AERONET AOD at this wavelength to AOD at 550 nm using Angstrom exponent in range of 440–675 nm (Mielonen et al. 2011) by the following equation,

$$ {\tau}_{0.55\mu m}=\frac{\tau_{0.5\mu m}}{e^{-{\alpha}_{0.44\mu m-0.67}\ln \frac{0.5}{0.55}}} $$
(23.1)

in which τ0.55 μm and τ0.5 μm are AERONET AOD at 550 and 500 nm. α0.44 μm − 0.67 μm is Angstrom exponent in range of 440–675 nm.

VIIRS AOD and AERONET AOD are compared using arithmetic mean (\( \overline{x} \)), standard deviation (σ), and correlation coefficient (r 2) defined as follows:

$$ \overline{x}=\frac{1}{n}\sum \limits_{t=1}^n{x}_t $$
(23.2)
$$ \sigma =\sqrt{\frac{1}{n}\sum \limits_{t=1}^n{\left({x}_t-\overline{x}\right)}^2} $$
(23.3)
$$ {r}^2=\frac{{\left(\sum \limits_{t=1}^n\left({x}_t-\overline{x}\right)\left({y}_t-\overline{y}\right)\right)}^2}{\sum \limits_{t=1}^n{\left({x}_t-\overline{x}\right)}^2\sum \limits_{t=1}^n{\left({y}_t-\overline{y}\right)}^2} $$
(23.4)

where \( \overline{x\ } \), \( \overline{y\ } \) are arithmetic mean of AOD, xt, yt are AOD at time t, and n is number of samples.

3.3 Aerosol Subtype Classification

CALIOP LiDAR has advantages on aerosol subtype classification for different aerosol vertical layers. Layer numbers are identified by Surface Elevation Detection Frequency as 163 corresponding to 5-km averaging horizontal resolution and detection frequency as 100%. Aerosol subtypes are based on thresholds for LiDAR ratios at 532 and 1064 nm (Table 23.3) with CAD (Cloud Aerosol Discrimination) Score smaller than −70.

Table 23.3 Rules of LiDAR ratio at 532 and 1064 nm for aerosol subtype classification (ASDC)
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Aerosol subtypes are defined as follows (ASDC): Marine primarily consists of sea-salt (i.e., NaCl). Desert dust is mostly mineral soil; Clean continental (i.e., background or rural aerosol) is light loaded aerosol such as sulfates (SO4 2−), nitrates (NO3 −), organic carbon (OC), and ammonium (NH4 +); Polluted continental is background aerosol with a substantial fraction of urban pollution while polluted dust is a mixture of desert dust and smoke or urban pollution; Biomass burning refers to aged smoke which mainly includes soot and OC.

4 Analysis at the Nghia Do Station in the North of Vietnam

4.1 VIIRS AOD Validation

Nghia Do station is located in Hanoi city characterized by the northern region climate which has four seasons per year. The North has a long dry season from November to March, strongly affected by North-East (NE) monsoon and a rainy season from May to September coinciding with the South-West (SW) monsoon. Transitions between dry and rainy seasons are in April and October (Pham and Pham 1993; Nguyen and Nguyen 2004).

During the period of 2012–2015, 123 matches between AERONET AOD and VIIRS AOD are observed. Table 23.4 presents AOD at 550 nm averaged by month and season for both VIIRS and AERONET datasets and Table 23.5 presents frequency by magnitudes for VIIRS and AERONET AOD. A number of matched observations is small, especially in February (i.e., the end of the winter) when the North is often covered by dense clouds and drizzles.

Table 23.4 Number of observations (#Obs), VIIRS AOD average (AODVIIRS), AERONET AOD average (AODAER), standard deviation (σVIIRS), and absolute error average by month and season of VIIRS AOD to AERONET AOD at Nghia Do station from 2012 to 2015
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Table 23.5 Frequency by magnitudes for VIIRS and AERONET AOD
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The peak values are observed in March and October with averaged VIIRS AOD at 1.43 and 1.09 corresponding to AERONET AOD at 1.12 and 0.96. In October, the NE monsoon flows from continental China to the North rising to a height of 850 hPa, which creates a stable temperature layer and causes dry weather in the area. Dust elevates in the dry air and is kept at that height. In March, wind from East of China goes through the Gulf of Tonkin to the North and reaches a similar height; therefore, air with vapor saturation forms clouds and drizzles. Aerosols existing during the dry season are moved up to cloud creating a high dense aerosol cloud layer (Pham et al. 2015). Effected by monsoon circulation, aerosol observation is high for both VIIRS and AERONET in October and March at the Nghia Do station. March is also the peak month of biomass burning in Southeast Asia (Huang et al. 2012) as well as in Vietnam (Le et al. 2014), with the highest AOD in this month.

Averaged AODs are 0.76 for VIIRS and 0.68 for AERONET in the dry season meanwhile they are slightly lower with 0.67 and 0.54 for VIIRS and AERONET in the rainy season. During the transition time (i.e., April and October), VIIRS AOD is 0.97 corresponding to 0.90 for AERONET AOD. The absolute errors are small in transition month (0.07–7.69%), larger in the dry season (0.08–11.76%), and largest in rainy season (0.13–24.07%). We observed AOD’s in dry season higher than AODs in rainy season. It can be explained by influence of the high pressure concurring with NE monsoon and cold weather, which leads to poor atmospheric dispersion and therefore enhances a high buildup of air pollutants. Otherwise, during the summer from May to August, air is cleaned by summer rains (Nguyen et al. 2015). Another experiment on monthly averaged AOD of all VIIRS and AERONET observations from 2012 to 2015 also show trend consistence (Fig. 23.1). However, VIIRS AOD is often larger than the AERONET AOD. The relationship of VIIRS and AERONET AOD is in accordance with the previous study for MODIS AOD Collection 5 and AERONET AOD in the North (Pham et al. 2015).

Fig. 23.1
figure 1

Monthly averaged AOD of all available (a) AERONET and (b) VIIRS AOD at Nghia Do station from 2012 to 2015. Numbers of monthly samples are presented at tops of columns

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Regarding frequency of two datasets, most AODs are observed in range of 0.25–0.5 (32.52% and 34.15%) and 0.5–0.75 (27.64% and 23.58%) for VIIRS and AERONET, respectively. AERONET can observe much small AODs (10.57% for AOD 0.25) while VIIRS see larger AODs (17.07% for AOD > 1.25) (Table 23.5).

We calculated the correlation coefficient for VIIRS and AERONET AOD for all data, dry season, rainy season, and transition as shown in Fig. 23.2. Correlation Coefficient (r 2) for all 123 matched data is moderate at 0.647 (Fig. 23.2a) which is consistent with similar study for VIIRS in Asia (Xiao et al. 2014). Fifty-five observations in dry season show r 2 as 0.624 (Fig. 23.2b) while 41 data in rainy season suggest r 2 as 0.58 (Fig. 23.2c). Transition months have 27 matching data with r 2 as 0.647. VIIRS AOD has strong relationship with AERONET AOD in dry season with high r 2 and regression slope and intercept reaching 0.95 and 0.08.

Fig. 23.2
figure 2

Correlation coefficient of VIIRS AOD and AERONET AOD from 2012 to 2015 for (a) all months, (b) dry season, (c) rainy season, and (d) transition months

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4.2 CALIPSO Aerosol Subtypes

Figure 23.3 presents monthly aerosol subtype distribution within 80 km of the Nghia Do station. The location is considered as peri-urban which is a hybrid landscape of fragmented urban and rural areas. Therefore, biomass burning and polluted dust make up a large portion of the aerosol. Biomass burning aerosols are observed frequently in March and October. It relates to biomass burning in Vietnam in March (Le et al. 2014) and rice straw burning after harvest in October for summer-autumn paddy rice crop (i.e., from June to September) (Lin et al. 2013). Polluted continental aerosol is characteristic for urban pollution which is higher in dry season than in rainy season, reflecting seasonal effects at the North of Vietnam. In the dry season, influence of the high pressure concurring with North-East monsoon and cold weather leads to poor atmospheric dispersion and therefore enhances a high buildup of air pollutants. Otherwise, rain washes dust in the air during rainy season (Nguyen et al. 2015). Meanwhile, polluted dust is more frequently observed from April to August when background temperatures are high. Clean continental corresponding to rural aerosols occurs mostly in periods of January/February, June/July, and October/November when farmers start and finish paddy rice for winter-spring crop (from February to May) and summer-autumn crop (from June to September). Desert dust is observed more frequently in April and May, caused by airborne soils and desert dust arrived at Hanoi by wind trajectories extending back over inland areas of northern and western China and Mongolia (Hien et al. 2004). In China, more than ten dust storms each year, occurring mainly during the period from March to May ingest about 800 MT of dust into the atmosphere annually from Taklamakan and Gobi desert regions (Cohen et al. 2010, Wang et al. 2008). Considering monthly mean of AOD values, urban background aerosol (polluted continental) is dominant followed by biomass burning and polluted dust (Fig. 23.4). Polluted continental is high from October to next April (dry and transition seasons). Biomass burning peaks in March as evident from hotpots detected from the satellite in Vietnam and Southeast Asia regions in this month (Le et al. 2014; Pham et al. 2015). However, polluted dust maximum in February and desert dust extreme in October months need more investigation.

Fig. 23.3
figure 3

Aerosol subtypes at Nghia Do area primarily consist of polluted dust (blue), biomass burning (green), polluted continental (gray), clean continental (yellow), and desert dust (orange). Monthly subtype observation numbers are presented in corresponding column

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Fig. 23.4
figure 4

Monthly subtype AOD means at 532 nm at Nghia Do

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5 Analysis at the Nha Trang Station in the Central of Vietnam

5.1 Seasonal Trend

Nha Trang station, representative of the Central region, has a tropical climate with the dry season from January to August and the rainy season from September to December (Pham and Pham 1993; Nguyen and Nguyen 2004).

During 2012–2015, 270 matching measurements between AERONET AOD and VIIRS AOD at 550 nm were averaged by month and season are presented in Table 23.6. A number of matching observations is especially small in transition period of two seasons (i.e., August, September). Different from Nghia Do station, there is no large difference of AODs observed in dry season (0.33 and 0.20) and rainy season (0.36 and 0.19) for both VIIRS and AERONET measurements at Nha Trang station. However, errors are very large with 0.13 (~65%) for dry period and 0.18 (~95%) for rainy season because of small AOD values. The peaks in AOD AERONET are observed in October (0.38) and March (0.28) as an effect from the North (Table 23.6 and Fig. 23.5b). However, similar seasonal trend and peaks are not found in VIIRS AOD (Fig. 23.5a). The large errors between two measurements should explain the differences.

Table 23.6 Number of observations (#Obs), VIIRS AOD average (AODVIIRS), AERONET AOD average (AODAER), standard deviation (σVIIRS), and absolute error average by month and season of VIIRS AOD to AERONET AOD at Nha Trang station from 2012 to 2015
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Fig. 23.5
figure 5

Monthly averaged AOD of all available (a) AERONET and (b) VIIRS data at Nha Trang station from 2012 to 2015. Numbers of monthly samples are presented at top of columns

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Most AODs are observed in a range smaller than 0.25 (48.15% and 72.22%) and then from 0.25 to 0.5 (36.67% and 24.07%) for VIIRS and AERONET, respectively. The number of AOD values larger than 0.75 are few. AERONET can observe smaller AOD than VIIRS (Table 23.7).

Table 23.7 Frequency by magnitudes for VIIRS and AERONET AOD
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Correlation coefficients of VIIRS and AERONET AODs are low at Nha Trang station (r 2 are 0.349, 0.331, and 0.504 on all dataset (270 samples), dry months (210 samples), and rainy season (60 samples) (Fig. 23.6)). Relatively low AERONET AOD has been observed as large values during the dry season; therefore, the lower correlation coefficient is found in those months and in rainy months. However, slopes and intercepts show VIIRS AOD as slightly underestimated in the dry season and overestimated in the rainy season.

Fig. 23.6
figure 6

Correlation coefficient of VIIRS AOD and AERONET AOD from 2012 to 2015 for (a) all months, (b) dry season, and (c) rainy season

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5.2 Aerosol Subtype Classification

Figure 23.7 presents monthly aerosol subtypes distribution averaged for 80 km from the Nha Trang station. Located on the coast, the area has a large portion of marine aerosol during the year. Marine aerosol is highest in October, November, December, and January since the NE monsoon goes through the sea and brings the aerosol to the coastal region during those months. However, light East and South-East monsoon blow in Nha Trang from March to September and therefore, marine influence is less (Fig. 23.7). High-temperature heating background together with breeze winds move air pollution up and limit dispersion in May, June, July, and August, which may cause observations of polluted dust and polluted continental. Biomass burning appears in February and March coinciding with farmers living around (i.e., in Nam Noc, Khanh Hoa and in Song Hinh, Phu Yen) practicing agricultural residue burning. Khanh Hoa province, where Nha Trang station is located, has a small agriculture sector in which plantation is dominant instead of rice. It can explain for few observations of clean continental characterized for rural aerosol in the study area. Figure 23.8 present monthly subtype AOD means at Nha Trang station. Biomass burning and polluted dust are higher than other subtype aerosols all months. Desert dust peaks in June; however, no trend and appropriate explanation have been found yet.

Fig. 23.7
figure 7

Aerosol subtypes at Nha Trang station primarily consist of marine, polluted continental (gray), polluted dust (blue), biomass burning (green), and desert dust (orange)

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Fig. 23.8
figure 8

Monthly subtype AOD means at 532 nm at Nha Trang

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6 Analysis at the Bac Lieu Station in the South of Vietnam

6.1 Seasonal Trend

The Bac Lieu station is located in Bac Lieu city characterized by the Southern climate condition which has a dry season from November to the next April still effected by North-East monsoon, a rainy season from May to October coinciding with South-West monsoon (Pham and Pham 1993; Nguyen and Nguyen 2004).

During 2012–2015, 163 matches between VIIRS AOD and AERONET AOD are observed by months and season (Table 23.8). A number of matching observations is very small in rainy months. Monthly averaged VIIRS and AERONET AODs are 0.42 and 0.23 in the dry season and 0.53 and 0.16 in the rainy seasons, respectively. The seasonal trend has not been captured by VIIRS measurements as a result of the extremely high error of AODs in rainy month (0.37–231.25%) and then in dry months (0.19–82.61%).

Table 23.8 Number of observations (#Obs), satellite AOD average (AODVIIRS), AERONET AOD average (AODAER), standard deviation (σVIIRS), and absolute error average by month and season of VIIRS AOD to AERONET AOD at Bac Lieu station from 2012 to 2015
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In Fig. 23.9, VIIRS AOD and AERONET AOD in November–January (i.e., dry months), and October (i.e., the last month of the rainy season) are high together because of NE monsoon (Pham et al. 2015). However, VIIRS AOD is much different to AERONET AOD in July, a middle month of the rainy season (Fig. 23.9).

Fig. 23.9
figure 9

Monthly averaged AOD of all available data of (a) AERONET and (b) VIIRS at Bac Lieu station from 2012 to 2015. Numbers of monthly samples are presented at top of columns

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Regarding frequency of two datasets, most of the small AODs are observed in range of 0–0.25 (25.15% and 63.80%) and of 0.25–0.5 (50.92% and 30.06%) for VIIRS and AERONET, respectively. AERONET can observe small AOD while VIIRS captures larger AOD (Table 23.9). Correlation coefficients (r 2) of VIIRS and AERONET AOD are 0.299, 0.378, and 0.76 for all dataset (163 samples), dry months (128 samples), and rainy month (35 samples). Correlation coefficient in dry season is lower than in rainy season, which is mostly opposite to normal seasonal trend. Besides, VIIRS AOD is extremely overestimated in dry season (i.e., slope is 2.69).

Table 23.9 Frequency by magnitudes for VIIRS and AERONET AOD
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6.2 Aerosol Subtype Classification

Figure 23.10 presents monthly aerosol subtypes distribution in the area of 80 km radius around the Bac Lieu station. The marine aerosol is observed in all months, especially high from November to April when NE monsoons blow through the sea and bring marine aerosol to continental (Pham et al. 2015). The hot weather in June, July, and August combined with high density urban environment as in Bac Lieu, Soc Trang, and Can Tho cities creates heat island phenomenon. Therefore, polluted dust is at-large. Polluted continental is quite consistent during a year. Bac Lieu station is located in the Mekong River Delta which has three rice crops per year. The spring crop is from November to December and harvest in February to March. The winter crop starts in rainy months (i.e., June or July) and ends in November or December. Meanwhile, the autumn crop begins in May or June and is harvested in mid-August or September. In February–March and November–December, biomass burning aerosol is observed more frequently, which coincides with agriculture burning of the spring and winter crops in Mekong River Delta. Clean continental appears in August and September corresponding to the time of autumn rice crop harvest when farmers often leave rice straw residue at field instead of burning as in spring and winter crops. Figure 23.11 shows monthly subtype AOD means at Bac Lieu station. Biomass burning and polluted continental are higher than other subtype aerosols in all months. Biomass burning peaks in March (see Fig. 23.11). The pollution continental is high in dry season from November to April except a peak in June (Fig. 23.12).

Fig. 23.10
figure 10

Correlation coefficient of VIIRS AOD versus AERONET AOD from 2012 to 2015 for (a) all months, (b) dry season, (c) rainy season

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Fig. 23.11
figure 11

Aerosol subtypes at Bac Lieu area primarily consist of marine aerosol (light blue), polluted continental (gray), polluted dust (blue), biomass burning (green), and clean continental (yellow). Monthly subtype observation numbers are presented in corresponding columns

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Fig. 23.12
figure 12

Monthly subtype AOD means at 532 nm in Bac Lieu

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7 Conclusions

In this study, VIIRS (2012–2015) and CALIPSO (2006–2015) aerosol products are validated at Nghia Do, Nha Trang, and Bac Lieu stations corresponding to the North, Central, and South regions of Vietnam. VIIRS AOD is compared with CALIPSO aerosol and analyzed for aerosol subtype classification for different months.

At Nghia Do station, VIIRS AOD has the best quality in terms of the seasonal trend, low errors, and high correlation coefficients. VIIRS and AERONET AOD have the largest averaged values in transition months of April and October (0.97 and 0.90), large values during the dry season from November to March (0.76 and 0.68), and lower values in the rainy season from May to September (0.67 and 0.54). Errors corresponding to those seasonal trend are 7.69%, 11.76%, and 24.07%. The peak months of both VIIRS and AERONET AOD are in March and October as a result of the NE monsoon, cold weather, and vegetation fires. Based on CALIPSO aerosol subtypes, aerosol subtypes at Nghia Do primarily consist of polluted dust, biomass burning, polluted continental, clean continental, and desert dust. Biomass burning peaks during March and October is attributed to local rice straw burning. Polluted dust is more frequently observed from April to August when summer high temperatures heat the background atmosphere. Polluted continental is higher in the dry season than in rainy season, which reflects seasonal effects at the North of Vietnam. Clean continental corresponding to rural aerosols occur mostly in January/February, June/July, and October/November which are periods of starting and harvest of rice crops in the Red River Delta. Desert dust is sometimes observed in April and May as a result of long-range transport of airborne soils and desert dust from northern and western China and Mongolia during spring time.

For the Nha Trang station, VIIRS AOD quality is degraded. VIIRS and AERONET AOD are 0.33 and 0.20 in the dry season from January to August and 0.36 and 0.19 in the rainy season from September to December, respectively. Therefore, large errors have been found as 0.13 (~65%) for the dry period and 0.18 (~95%) for the rainy period. The VIIRS AOD cannot capture seasonal trend found in AERONET AOD at Nha Trang station. From CALIPSO, aerosol subtypes include marine, polluted continental, polluted dust, biomass burning, and desert dust. The marine aerosol is dominant all months especially during October–January since the NE monsoon goes through the sea and brings the aerosol to the coastal regions. Meanwhile, moderate East and South-East monsoons appear in Nha Trang during March and September and therefore marine aerosols are less observed. Polluted dust and polluted continental are observed in May–August as a result of high temperature heating background together with breeze winds moving air pollution up and limiting dispersion. Biomass burning appears in February and March coinciding with agriculture burning time in the study area. At Bac Lieu station, VIIRS AOD has been at worst in comparison with AERONET AOD. Monthly averaged VIIRS and AERONWT AODs are 0.42 and 0.23 in the dry season and 0.53 and 0.16 in rainy seasons, respectively. The seasonal trend has not been captured by VIIRS measurements as a result of extremely high error of AODs in rainy month (0.37–231.25%) and then in dry months (0.19–82.61%). Correlation coefficients of VIIRS and AERONET AOD are 0.299, 0.378, and 0.76 for all dataset (163 samples), dry months (128 samples), and rainy month (35 samples) but VIIRS AOD is extremely overestimated in the dry season. Based on CALIPSO aerosol products, aerosol subtypes at Bac Lieu area primarily consist of marine aerosol, polluted continental, polluted dust, biomass burning, and clean continental. Marine aerosol is observed in all months, especially high from November to April when NE monsoons flow through the sea and bring marine aerosol to continental. Polluted dust during June, July, and August, may be related to heat island effect. Polluted continental is quite consistent during a year. Biomass burning aerosol is observed frequently during February–March and November–December when agricultural residues are burnt during spring and winter in the Mekong River Delta. Clean continental appears in August and September and the season doesn’t coincide with biomass burning.

The validation and analysis of VIIRS and CALIPSO aerosol product with AERONET AOD over Vietnam highlights their data qualities and application capacities in different climate conditions and regions in Vietnam. More such validation exercises are being planned to cover different regions in Vietnam.