Elevated Aerosol Layers Research Articles

Estimation of Surface Particulate Matter (PM2.5 and PM10) Mass Concentrations from Ceilometer Backscattered Profiles

ABSTRACT In this study, we used remotely sensed backscattered profiles from a ceilometer to characterize the vertical and horizontal mixing of aerosols in the polluted planetary boundary layer (PBL). These profiles revealed the structure of the boundary layer, which included the mixed layer, the nocturnal residual layer and the elevated aerosol layer far above the mixed layer over Delhi. The accumulation of aerosols near the surface during feeble turbulence and the mixing of aerosols from the residual layer into the surface layer during convection was captured very well by a ceilometer. The backscattered signal from a height of 45 m above the ground was strongly correlated (82%) with the observed surface PM2.5 and PM10 mass concentrations. We developed an empirical regression model based on this relationship, which was then tested and validated against independent measurements of the concentrations from November 2018. Although local meteorological conditions, particularly cloudiness and rain, influenced the strength of the correlation between the observed PM2.5 and PM10 mass concentrations and the backscattered signal, the magnitude of the mean bias between the observed and the values for PM2.5 (–21 µg m–3, RMSE = 75) and PM10 (31 µg m–3, RMSE = 118) indicated that the predicted values were fairly accurate. The model overestimated the PM2.5 by 7% and underestimated the PM10 by 6% on clear days.

Optical and physical properties of aerosols during active fire events occurring in the Indo-Gangetic Plains: Implications for aerosol radiative forcing

The climatological characteristics of biomass burning over the western Indo-Gangetic Plain (IGP) are examined during the post-monsoon (October-November) season from 2001-2018 using the Moderate resolution Imaging Spectrometer (MODIS) fire products. The intensity of the biomass burning are estimated from the fire pixels detected by the MODIS satellites in the form of fire counts, surface brightness temperature, and fire radiative power. Such biomass burning are mostly affected over Lahore and Patiala region of the western IGP. These fire products are examined along with optical and physical properties of aerosols and its radiative forcing estimated from AErosol RObotic NETwork (AERONET) data. Due to non-availability of long-term aerosol data, the study was performed only at Lahore (AERONET) and Patiala (MODIS) of the western IGP region. The current study shows increasing trends of average brightness temperature by 16% year−1 over Patiala and 14% year−1 over Lahore, which enhances the aerosol optical depth (AOD) by 3%–7% year−1 over the study region. The sphericity of the fine-mode aerosols are rising from 23% to 61% during non-fire to fire events and the dominance of fine-mode aerosols are observed during the active fire events. Further, the absorptivity of AOD (at 440 nm) has enhanced from 0.07 to 0.14 during non-fire to fire events. Asymmetric parameter (AS) of fine-mode aerosol increases from 0.65 to 0.68 during non-fire to active fire events. The characteristics of elevated aerosol layers due to the active fire events are clearly distinguished from the non-fire events within 0-2 km above the surface as noticed from high aerosol extinction and attenuated back scatter coefficients, obtained from the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO). Absorbing aerosols associated from biomass-burning contribute to the surface radiative forcing by -173.96 ± 47 W m−2 and atmospheric forcing by 123.57 ± 41 W m−2 over Lahore. The impact of the biomass-burning over Lahore has making the atmosphere more warming by 3.50 ± 1.14 K Day−1. The estimated heating rates (HR) in the atmosphere are rising during both fire and non-fire events by 3.79% and 3.44% per year, respectively.

Vertical distribution of aerosols and clouds over north-eastern South Asia: Aerosol-cloud interactions

Eleven years of CALIOP and MODIS data are used to investigate the vertical distribution of aerosols and clouds and their possible interactions over the north-eastern South Asia (22–30°N, 88–98°E). Distinct seasonality in the vertical aerosol structure with the presence of elevated aerosol layers (EALs) is observed. The EALs vary from ~1.4 to 4.8 km in post monsoon to ~4.8–7.4 km in monsoon. Strong convective activities mainly in pre-monsoon and monsoon and upper air transportation of aerosols contribute to the formation of EALs. The contribution of polluted dust, polluted continental/smoke and elevated smoke are found to be predominant in the vertical column during pre-monsoon and monsoon. Contrarily, clean continental, clean marine and dusty marine are dominant during winter and post monsoon. Small spherical particles are abundant during winter while in monsoon hygroscopically grown spherical particles predominate. Seasonally, the cloud occurrence frequency (COF) as a function of altitude is maximum during monsoon. An increase in cloud top height (CTH) is observed within this region corresponding to an increase in number of cloud layers, thus revealing invigoration effect. The occurrence of cirrus and deep convective clouds is maximum in monsoon and minimum in the dry season. Significant inhibition/invigoration is observed for mixed-phase/liquid clouds.

Elevated aerosol layer over South Asia worsens the Indian droughts

Droughts have become more severe and recurrent over the Indian sub-continent during the second half of the twentieth century, leading to more severe hydro-climatic and socio-economic impacts over one of the most densely populated parts of the world. So far, droughts have mostly been connected to circulation changes concomitant with the abnormal warming over the Pacific Ocean, prevalently known as “El Niño”. Here, exploiting observational data sets and a series of dedicated sensitivity experiments, we show that the severity of droughts during El Niño is amplified (17%) by changes in aerosols. The model experiments simulate the transport of boundary layer aerosols from South Asian countries to higher altitudes (12–18 km) where they form the Asian Tropopause Aerosol Layer (ATAL) (~ 60–120°E, 20–40°N). During El Niño, the anomalous overturning circulation from the East Asian region further enriches the thickness of aerosol layers in the ATAL over the northern part of South Asia. The anomalous aerosol loading in the ATAL reduces insolation over the monsoon region, thereby exacerbating the severity of drought by further weakening the monsoon circulation. Future increases in industrial emissions from both East and South Asia will lead to a wider and thicker elevated aerosol layer in the upper troposphere, potentially amplifying the severity of droughts.

Vertical and horizontal distribution of submicron aerosol chemical composition and physical characteristics across northern India during pre-monsoon and monsoon seasons

<p><strong>Abstract.</strong> The vertical distribution in the physical and chemical properties of submicron aerosol has been characterised across northern India for the first time using airborne in situ measurements. This study focusses primarily on the Indo-Gangetic Plain, a low-lying area in the north of India which commonly experiences high aerosol mass concentrations prior to the monsoon season. Data presented are from the UK Facility for Airborne Atmospheric Measurements BAe-146 research aircraft that performed flights in the region during the 2016 pre-monsoon (11 and 12 June) and monsoon (30 June to 11 July) seasons.</p> <p>Inside the Indo-Gangetic Plain boundary layer, organic matter dominated the submicron aerosol mass (43 %) followed by sulfate (29 %), ammonium (14 %), nitrate (7 %) and black carbon (7 %). However, outside the Indo-Gangetic Plain, sulfate was the dominant species, contributing 44 % to the total submicron aerosol mass in the boundary layer, followed by organic matter (30 %), ammonium (14 %), nitrate (6 %) and black carbon (6 %). Chlorine mass concentrations were negligible throughout the campaign. Black carbon mass concentrations were higher inside the Indo-Gangetic Plain (2 <span class="inline-formula">µ</span>g m<span class="inline-formula"><sup>−3</sup></span>) compared to outside (1 <span class="inline-formula">µ</span>g m<span class="inline-formula"><sup>−3</sup></span>). Nitrate appeared to be controlled by thermodynamic processes, with increased mass concentration in conditions of lower temperature and higher relative humidity. Increased mass and number concentrations were observed inside the Indo-Gangetic Plain and the aerosol was more absorbing in this region, whereas outside the Indo-Gangetic Plain the aerosol was larger in size and more scattered in nature, suggesting greater dust presence, especially in north-western India. The aerosol composition remained largely similar as the monsoon season progressed, but the total aerosol mass concentrations decreased by <span class="inline-formula">∼50</span> % as the rainfall arrived; the pre-monsoon average total mass concentration was 30 <span class="inline-formula">µ</span>g m<span class="inline-formula"><sup>−3</sup></span> compared to a monsoon average total mass concentration of 10–20 <span class="inline-formula">µ</span>g m<span class="inline-formula"><sup>−3</sup></span>. However, this mass concentration decrease was less noteworthy (<span class="inline-formula">∼20</span> %–30 %) over the Indo-Gangetic Plain, likely due to the strength of emission sources in this region. Decreases occurred in coarse mode aerosol, with the fine mode fraction increasing with monsoon arrival. In the aerosol vertical profile, inside the Indo-Gangetic Plain during the pre-monsoon, organic aerosol and absorbing aerosol species dominated in the lower atmosphere (<span class="inline-formula"><1.5</span> km), with sulfate, dust and other scattering aerosol species enhanced in an elevated aerosol layer above 1.5 km with maximum aerosol height <span class="inline-formula">∼6</span> km. The elevated concentration of dust at altitudes <span class="inline-formula">>1.5</span> km is<span id="page5616"/> a clear indication of dust transport from the Great Indian Desert, also called the Thar Desert, in north-western India. As the monsoon progressed into this region, the elevated aerosol layer diminished, the aerosol maximum height reduced to <span class="inline-formula">∼2</span> km. The dust and sulfate-dominated aerosol layer aloft was removed upon monsoon arrival, highlighted by an increase in fine mode fraction throughout the profile.</p>

Changing aerosol loadings over Central Himalayan region (2007–2016) – A satellite perspective

The paper presents the variation of aerosol columnar variation and vertical profiles over the study area encompassing a portion of Central Himalayan region and the surrounding planar areas. The analysis is carried out using decadal (2007–2016) Level-2 CALIOP (onboard CALIPSO), Level-3 MODIS (onboard Aqua/Terra) and MISR (onboard Terra) datasets. Four seasons have been considered, namely, DJF (December-January-February), MAM (March-April-may), JJA (June-July-August) and SON (September-October-November). The CALIOP data has been screened using quality flags, i.e., CAD score, aerosol layer fraction and extinction quality checks. There are also signatures of elevated aerosol layers, especially during winter (DJF) and post-monsoon (SON) seasons. Latitudinal and longitudinal variations of the vertical structure have also been presented. The HYSPLIT back-trajectory analysis has been carried out in order to explore the changes in the sources of the aerosols during different seasons. It is seen that as we go higher up in the altitudes, the long range transport becomes more dominant. Compared to the other seasons, the air masses are more localized in JJA season. The changing aerosol amounts at different heights of the atmosphere have also been discussed in terms of decadal trends for the four seasons. These trends have been supplemented by the AOD trends from the passive satellite sensors. We find that aerosol amounts have increased in the last decade during DJF and SON seasons, when the atmospheric conditions are more stable. On the contrary, aerosol amounts have significantly decreased in JJA season throughout the vertical column. The effect of changing amounts of aerosols may have impacts on the snow cover variability as well. In this connection, the seasonal trends in snow cover have also been explored during the considered decadal period. Especially during the DJF and SON seasons, decreasing trends in snow cover are observed when the aerosol amounts have been found to be increased.

The evolution of an aerosol event observed from aircraft in Beijing: An insight into regional pollution transport

To investigate the transport and formation mechanism of pollution in Beijing, this study explored the vertical profiles of aerosol properties using aircraft measurement during a regional transport (RT) pollution event from 10th to 12th December 2016. The aerosol chemical composition and size distribution were characterized. Different vertical structures exhibited during three periods (defined as before RT, during RT, and after RT) for this pollution event. Before RT, PM2.5 and black carbon (BC) mass loading were at low level in the mixing layer (ML) when the clean northerly air mass dominated. During RT, an elevated aerosol layer, in which BC and PM2.5 mass concentrations were about 1.5 times higher than that at the ground level, was found in the upper ML (at 400–900 m) over Beijing. This elevated aerosol layer was advected by the prevailing southwesterly air mass which transported the pollutants from the intensively polluted southwestern region over Beijing. These aerosols through RT featured with pronounced secondary compositions and large coatings on BC particles. After RT, the pollutants were significantly diluted by the prevailing NW air mass, whereas the aerosol concentration almost maintained in the near surface, leading to notable vertical gradient. The continued surface pollution may result from the low wind speed and secondary aerosol formation in the next day. This study suggests that only the ground observation could not fully explain the pollution event, but the variation of vertical structure of aerosol properties should be considered to elucidate the formation mechanism of pollution over Beijing.

A novel post-processing algorithm for Halo Doppler lidars

Abstract. Commercially available Doppler lidars have now been proven to be efficient tools for studying winds and turbulence in the planetary boundary layer. However, in many cases low signal-to-noise ratio is still a limiting factor for utilising measurements by these devices. Here, we present a novel post-processing algorithm for Halo Stream Line Doppler lidars, which enables an improvement in sensitivity of a factor of 5 or more. This algorithm is based on improving the accuracy of the instrumental noise floor and it enables longer integration times or averaging of high temporal resolution data to be used to obtain signals down to −32 dB. While this algorithm does not affect the measured radial velocity, it improves the accuracy of radial velocity uncertainty estimates and consequently the accuracy of retrieved turbulent properties. Field measurements using three different Halo Doppler lidars deployed in Finland, Greece and South Africa demonstrate how the new post-processing algorithm increases data availability for turbulent retrievals in the planetary boundary layer, improves detection of high-altitude cirrus clouds and enables the observation of elevated aerosol layers.

Retrieval of Vertical Mass Concentration Distributions—Vipava Valley Case Study

Aerosol vertical profiles are valuable inputs for the evaluation of aerosol transport models, in order to improve the understanding of aerosol pollution ventilation processes which drive the dispersion of pollutants in mountainous regions. With the aim of providing high-accuracy vertical distributions of particle mass concentration for the study of aerosol dispersion in small-scale valleys, vertical profiles of aerosol mass concentration for aerosols from different sources (including Saharan dust and local biomass burning events) were investigated over the Vipava valley, Slovenia, a representative hot-spot for complex mixtures of different aerosol types of both anthropogenic and natural origin. The analysis was based on datasets taken between 1–30 April 2016. In-situ measurements of aerosol size, absorption, and mass concentration were combined with lidar remote sensing, where vertical profiles of aerosol concentration were retrieved. Aerosol samples were characterized by SEM-EDX, to obtain aerosol morphology and chemical composition. Two cases with expected dominant presence of different specific aerosol types (mineral dust and biomass-burning aerosols) show significantly different aerosol properties and distributions within the valley. In the mineral dust case, we observed a decrease of the elevated aerosol layer height and subsequent spreading of mineral dust within the valley, while in the biomass-burning case we observed the lifting of aerosols above the planetary boundary layer (PBL). All uncertainties of size and assumed optical properties, combined, amount to the total uncertainty of aerosol mass concentrations below 30% within the valley. We have also identified the most indicative in-situ parameters for identification of aerosol type.

An overview of airborne measurement in Nepal – Part 1: Vertical profile of aerosol size, number, spectral absorption, and meteorology

Abstract. The paper provides an overview of an airborne measurement campaign with a microlight aircraft over the Pokhara Valley region, Nepal, a metropolitan region in the central Himalayan foothills. This is the first aerial measurement in the central Himalayan foothill region, one of the polluted but relatively poorly sampled regions of the world. Conducted in two phases (in May 2016 and December 2016–January 2017), the goal of the overall campaign was to quantify the vertical distribution of aerosols over a polluted mountain valley in the Himalayan foothills, as well as to investigate the extent of regional transport of emissions into the Himalayas. This paper summarizes results from the first phase where test flights were conducted in May 2016 (pre-monsoon), with the objective of demonstrating the potential of airborne measurements in the region using a portable instrument package (size with housing case: 0.45 m × 0.25 m × 0.25 m, 15 kg) onboard an ultralight aircraft (IKARUS-C42). A total of five sampling test flights were conducted (each lasting for 1–1.5 h) in the Pokhara Valley to characterize vertical profiles of aerosol properties such as aerosol number and size distribution (0.3–2 µm), total particle concentration (>14 nm), aerosol absorption (370–950 nm), black carbon (BC), and meteorological variables. Although some interesting observations were made during the test flight, the study is limited to a few days (and only a few hours of flight in total) and thus the analysis presented may not represent the entire pollution–meteorology interaction found in the Pokhara Valley. The vertical profiles of aerosol species showed decreasing concentrations with altitude (815 to 4500 m a.s.l.); a steep concentration gradient below 2000 m a.s.l. in the morning; and mixed profiles (up to ca. 4000 m a.s.l.) in the afternoon. The near-surface (<1000 m a.s.l.) BC concentrations observed in the Pokhara Valley were much lower than pre-monsoon BC concentrations in the Kathmandu Valley, and similar in range to Indo-Gangetic Plain (IGP) sites such as Kanpur in India. The sampling test flight also detected an elevated polluted aerosol layer (around 3000 m a.s.l.) over the Pokhara Valley, which could be associated with the regional transport. The total aerosol and black carbon concentration in the polluted layer was comparable with the near-surface values. The elevated polluted layer was also characterized by a high aerosol extinction coefficient (at 550 nm) and was identified as smoke and a polluted dust layer. The observed shift in the westerlies (at 20–30∘ N) entering Nepal during the test flight period could be an important factor for the presence of elevated polluted layers in the Pokhara Valley.

Vertical and horizontal distribution of sub-micron aerosol chemical composition and physical characteristics across Northern India, during the pre-monsoon and monsoon seasons

<p><strong>Abstract.</strong> The vertical distribution in the physical and chemical properties of submicron aerosol has been characterised across northern India for the first time using airborne in-situ measurements. This study focusses primarily on the Indo-Gangetic Plain, a low-lying area in the north of India which commonly experiences high aerosol mass concentrations prior to the monsoon season. Data presented are from the UK Facility for Airborne Atmospheric Measurements BAe-146 research aircraft that performed flights in the region during the 2016 pre-monsoon (11<sup>th</sup> and 12<sup>th</sup> June) and monsoon (30<sup>th</sup> June to 11<sup>th</sup> July) seasons.</p> <p> Inside the Indo-Gangetic Plain boundary layer, organic matter dominated the submicron aerosol mass (43 %) followed by sulphate (29 %), ammonium (14 %), nitrate (7 %) and black carbon (7 %). However, outside the Indo-Gangetic Plain, sulphate was the dominant species contributing 44 % to the total submicron aerosol mass in the boundary layer, followed by organic matter (30 %), ammonium (14 %), nitrate (6 %) and black carbon (6 %). Chlorine mass concentrations were negligible throughout the campaign. Black carbon mass concentrations were higher inside the Indo-Gangetic Plain (2 µg/m<sup>3</sup> std) compared to outside (1 µg/m<sup>3</sup> std). Nitrate appeared to be controlled by thermodynamic processes, with increased mass concentration in conditions of lower temperature and higher relative humidity. Increased mass and number concentrations were observed inside the Indo-Gangetic Plain and the aerosol was more absorbing in this region, whereas outside the Indo-Gangetic Plain the aerosol was larger in size and more scattering in nature, suggesting greater dust presence especially in northwest India. The aerosol composition remained largely similar as the monsoon season progressed, but the total aerosol mass concentrations decreased by ~ 50 % as the rainfall arrived; the pre-monsoon average total mass concentration was 30 µg/m<sup>3</sup> std compared to a monsoon average total mass concentration of 10–20 µg/m<sup>3</sup> std. However, this mass concentration decrease was less noteworthy (~ 20–30 %) over the Indo-Gangetic Plain, likely due to the strength of emission sources in this region. Decreases occurred in coarse mode aerosol, with the fine mode fraction increasing with monsoon arrival. In the aerosol vertical profile, inside the Indo-Gangetic Plain during the pre-monsoon, organic aerosol and absorbing aerosol species dominated in the lower atmosphere (< 1.5 km) with sulphate, dust and other scattering aerosol species enhanced in an elevated aerosol layer above 1.5 km with maximum aerosol height ~ 6 km. As the monsoon progressed into this region, the elevated aerosol layer diminished, the aerosol maximum height reduced to ~ 2 km and the total mass concentrations decreased by ~ 50 %. The dust and sulphate-dominated aerosol layer aloft was removed upon monsoon arrival, highlighted by an increase in fine mode fraction throughout the profile.</p>

Combining airborne in situ and ground-based lidar measurements for attribution of aerosol layers

Abstract. Understanding the distribution of aerosol layers is important for determining long-range transport and aerosol radiative forcing. In this study we combine airborne in situ measurements of aerosol with data obtained by a ground-based high spectral resolution lidar (HSRL) and radiosonde profiles to investigate the temporal and vertical variability of aerosol properties in the lower troposphere. The HSRL was deployed in Hyytiälä, southern Finland, from January to September 2014 as a part of the U.S. DOE ARM (Atmospheric Radiation Measurement) mobile facility during the BAECC (Biogenic Aerosols – Effects on Cloud and Climate) Campaign. Two flight campaigns took place in April and August 2014 with instruments measuring the aerosol size distribution from 10 nm to 5 µm at altitudes up to 3800 m. Two case studies with several aerosol layers present were selected from the flight campaigns for further investigation: one clear-sky and one partly cloudy case. During the clear-sky case, turbulent mixing ensured small temporal and spatial variability in the measured aerosol size distribution in the boundary layer, whereas mixing was not as homogeneous in the boundary layer during the partly cloudy case. The elevated layers exhibited larger temporal and spatial variability in aerosol size distribution, indicating a lack of mixing. New particle formation was observed in the boundary layer during the clear-sky case, and nucleation mode particles were also seen in the elevated layers that were not mixing with the boundary layer. Interpreting local measurements of elevated layers in terms of long-range transport can be achieved using back trajectories from Lagrangian models, but care should be taken in selecting appropriate arrival heights, since the modelled and observed layer heights did not always coincide. We conclude that higher confidence in attributing elevated aerosol layers to their air mass origin is attained when back trajectories are combined with lidar and radiosonde profiles.

Role of dynamics on the formation and maintenance of the elevated aerosol layer during monsoon season over south-east peninsular India

The existence of elevated aerosol layer is common over India during monsoon season. Though its sources are well explained through long-range transport, its formation and maintenance is not explained to date. The formation and maintenances of an elevated aerosol layer, starting from ∼2 km and extending up to ∼5.5 km noticed is explained using two nearby lidars located in peninsular India. Existence of a cleaner environment with low aerosol loading below 2 km is attributed to the wet scavenging and existence of no strong local source. The low level jet (LLJ) from Arabian Sea persisting between 2 and 3 km is the main mechanism suggesting strong role of dynamics in the formation of these elevated layers. Persistent strong shears existing between LLJ and tropical easterly jet during this season restrict the up-liftment of aerosols to the higher altitudes. Observed features are explained in the light of dynamics, meteorology and long-range transport.

Impact and Suggestion of Column-to-Surface Vertical Correction Scheme on the Relationship between Satellite AOD and Ground-Level PM2.5 in China

As China is suffering from severe fine particle pollution from dense industrialization and urbanization, satellite-derived aerosol optical depth (AOD) has been widely used for estimating particulate matter with an aerodynamic diameter less than 2.5 μm (PM2.5). However, the correlation between satellite AOD and ground-level PM2.5 could be influenced by aerosol vertical distribution, as satellite AOD represents the entire column, rather than just ground-level concentration. Here, a new column-to-surface vertical correction scheme is proposed to improve separation of the near-surface and elevated aerosol layers, based on the ratio of the integrated extinction coefficient within 200–500 m above ground level (AGL), using the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP)) aerosol profile products. There are distinct differences in climate, meteorology, terrain, and aerosol transmission throughout China, so comparisons between vertical correction via CALIOP ratio and planetary boundary layer height (PBLH) were conducted in different regions from 2014 to 2015, combined with the original Pearson coefficient between satellite AOD and ground-level PM2.5 for reference. Furthermore, the best vertical correction scheme was suggested for different regions to achieve optimal correlation with PM2.5, based on the analysis and discussion of regional and seasonal characteristics of aerosol vertical distribution. According to our results and discussions, vertical correction via PBLH is recommended in northwestern China, where the PBLH varies dramatically, stretching or compressing the surface aerosol layer; vertical correction via the CALIOP ratio is recommended in northeastern China, southwestern China, Central China (excluding summer), North China Plain (excluding Beijing), and the spring in the southeast coast, areas that are susceptible to exogenous aerosols and exhibit the elevated aerosol layer; and original AOD without vertical correction is recommended in Beijing and the southeast coast (excluding spring), where the elevated aerosol layer rarely occurs and a large proportion of aerosol is aggregated in near-surface. Moreover, validation experiments in 2016 agreed well with our discussions and conclusions drawn from the experiments of the first two years. Furthermore, suggested vertical correction scheme was applied into linear mixed effect (LME) model, and high cross validation (CV) R2 (~85%) and relatively low root mean square errors (RMSE, ~20 μg/m3) were achieved, which demonstrated that the PM2.5 estimation agreed well with the measurements. When compared to the original situation, CV R2 values and RMSE after vertical correction both presented improvement to a certain extent, proving that the suggested vertical correction schemes could further improve the estimation accuracy of PM2.5 based on sophisticated model in China. Estimating PM2.5 with better accuracy could contribute to a more precise research of ecology and epidemiology, and provide a reliable reference for environmental policy making by governments.

An intercomparison of AOD-converted PM2.5 concentrations using different approaches for estimating aerosol vertical distribution

Due to the limited spatial coverage of surface PM2.5 monitoring sites, satellite AOD (aerosol optical depth) products have been widely used to estimate surface PM2.5 in different parts of the world. A major difficulty as well as source of uncertainty in converting AOD to PM2.5 is the determination of aerosol vertical distribution, usually represented by the boundary layer height (BLH). In this study, we evaluate the performance of different approaches of estimating aerosol vertical distributions in the AOD-PM2.5 conversion process, using long-term and multi-source data acquired at a super station, Yuen Long, Hong Kong. The monthly climatology of aerosol vertical distribution and BLH products derived from lidar, radiosonde, and MERRA reanalysis data are respectively applied for converting AOD to surface aerosol extinction coefficients. Seasonal empirical hygroscopic growth functions are constructed to convert aerosol extinction to dry PM2.5 mass concentration. Results indicate that different vertical distribution estimation approaches can have highly varying effect on the converted PM2.5 concentration. Using lidar-derived BLHs shows the best agreement, with a correlation coefficient of 0.73 and a relative bias of 30.6% between retrievals and observations. Since continuous lidar measurements are not available for most regions, the climatology pattern of aerosol structure and radiosonde-derived BLHs are found to be suitable alternatives with a correlation coefficient of ∼0.6, and considerably outperform the results using BLHs derived from reanalysis data. Elevated aerosol layers appear to be the major source of uncertainty and result in an overestimate of satellite results, especially during the spring and summer seasons.

An intercomparison of long‐term planetary boundary layer heights retrieved from CALIPSO, ground‐based lidar, and radiosonde measurements over Hong Kong

AbstractThe planetary boundary layer height (PBLH) is a very important parameter in the atmosphere, because it determines the range where the most effective dispersion processes take place, and serves as a constraint on the vertical transport of heat, moisture, and pollutants. As the only space‐borne lidar, Cloud‐Aerosol Lidar with Orthogonal Polarization onboard Cloud‐Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) measures the vertical distribution of aerosol signals and thus offers the potential to retrieve large‐scale PBLH climatology. In this study, we explore different techniques for retrieving PBLH from CALIPSO measurements and validate the results against those obtained from ground‐based micropulse lidar (MPL) and radiosonde (RS) data over Hong Kong, where long‐term MPL and RS measurements are available. Two methods, namely maximum standard deviation (MSD) and wavelet covariance transform (WCT), are used to retrieve PBLH from CALIPSO. Results show that the RS‐ and MPL‐derived PBLHs share similar interannual variation and seasonality and can complement each other. Both MSD and WCT perform reasonably well compared with MPL/RS products, especially under sufficient aerosol loading. Uncertainties increase when aerosol loading is low and the CALIPSO signal consequently becomes noisier. Overall, CALIPSO captures the general PBLH seasonal variability over Hong Kong, despite a high bias in spring and a low bias in summer. The spring high bias is likely associated with elevated aerosol layers due to transport, while the summer low bias can be attributed to higher noise level associated with weaker aerosol signal.

The impact of seasonalities on direct radiative effects and radiative heating rates of absorbing aerosols above clouds

The impact of seasonalities on direct radiative effects (DREs) and radiative heating rates (RHRs) of absorbing aerosols above clouds in the southeast Atlantic is examined using radiative transfer calculations. For an aerosol optical thickness of 0.6 located between 0 and 4 km, a cloud optical thickness of 9.0 and a cloud effective radius of 12.8 µm at 0.55 µm located between 1 and 2 km, the diurnally averaged RHR at noon in the aerosol layer increases from ∼6.6 K day−1 in June to ∼8.9 K day−1 in October. In June (October), the RHR in the cloud layer at noon is 1.3 (1.7) K day−1 higher than the case of pristine clouds. However, an elevated aerosol layer (2–4 km) reduces the RHR by ∼0.2 K day−1 in the cloud layer relative to a pristine cloudy case. The DRE at top‐of‐atmosphere (TOA) reaches its peak when the solar zenith angle (SZA) is 54°. The DRE increases (decreases) with SZA for SZA less (greater) than 54°. The primary peak DRE is ∼29.5 W m−2 at 5.0°S 5.0°E, occurring at 0800 UTC. At noon, the DRE at TOA is ∼18.9, ∼20.5 and ∼23.1 W m−2 at 5.0°S, 15.0°S and 25.0°S along 5.0°E, respectively. This study provides data and theoretical understanding to help positioning science flights that target measurements of above‐cloud aerosol radiative effects.

Cross-Sectional View of Atmospheric Aerosols over an Urban Location in Central India

ABSTRACTSurface, column and vertically resolved variations of physical and optical properties of atmospheric aerosol over Hyderabad, a tropical urban location in central India are explored on the basis of ground based and satellite retrieved data. Annual mean aerosol optical depth (τ) observed with Microtops sun-photometer is 0.61 ± 0.07 and seasonally it varied from 0.71 ± 0.06 in pre-monsoon to 0.55 ± 0.05 in winter. Aerosol types are categorized based on Angstrom exponent (α) and τ relations; revealed that the study region is dominated by mixed type (MT) aerosol followed by urban/industrial aerosols under high τ (HUI) category. A consistent diurnal variation of black carbon (BC) is observed irrespective of seasonal variation with annual BC mass concentration is found to be 9.7 ± 1.9 µg m–3. During Telangana Survey day, which was the least pollutant day showed a reduction of 75% BC concentration during day time in comparison to five years average values, indicating the influence of anthropogenic effect over Hyderabad. Vertical information’s on aerosol are analyzed using Cloud Aerosol Lidar Pathfinder Satellite Observations (CALIPSO) and ground based Lidar (LAMP) data. LAMP data analysis shows a significant elevated aerosol layer up to 4 km during pre-monsoon while aerosols are confined below 3 km during post-monsoon and winter. Long term CALIPSO observations revealed that during post-monsoon to winter, the study area is dominated (~60%) by ‘urban’ aerosol; while during pre-monsoon period ~75% of the aerosol type belongs to ‘dusty mix’ category. A decline in short wave flux at the top of the atmosphere (0.66 Wm–2 yr–1) is observed, as revealed by long term Clouds and Earths Radiant Energy System (CERES) data analysis with higher decline rate observed in winter (1 Wm–2 yr–1) followed by pre-monsoon (0.8 Wm–2 yr–1).

Columnar aerosol characteristics and radiative forcing over the Doon Valley in the Shivalik range of northwestern Himalayas.

Spectral aerosol optical depth (AOD) measurements obtained from multi-wavelength radiometer under cloudless conditions over Doon Valley, in the foothills of the western Himalayas, are analysed during the period January 2007 to December 2012. High AOD values of 0.46±0.08 and 0.52±0.1 at 500nm, along with low values of Ångström exponent (0.49±0.01 and 0.44±0.03) during spring (March-May) and summer (June-August), respectively, suggest a flat AOD spectrum indicative of coarse-mode aerosol abundance compared with winter (December-February) and autumn (September-November), which are mostly dominated by fine aerosols from urban/industrial emissions and biomass burning. The columnar size distributions (CSD) retrieved from the King's inversion of spectral AOD exhibit bimodal size patterns during spring and autumn, while combinations of the power-law and unimodal distributions better simulate the retrieved CSDs during winter and summer. High values of extinction coefficient near the surface (∼0.8-1.0km-1 at 532nm) and a steep decreasing gradient above are observed via CALIPSO profiles in autumn and winter, while spring and summer exhibit elevated aerosol layers between ∼1.5 and 3.5km due to the presence of dust. The particle depolarisation ratio shows a slight increasing trend with altitude, with higher values in spring and summer indicative of non-spherical particles of dust origin. The aerosol-climate implications are evaluated via the aerosol radiative forcing (ARF), which is estimated via the synergy of OPAC and SBDART models. On the monthly basis, the ARF values range from ∼ -30 to -90Wm-2 at the surface, while aerosols cause an overall cooling effect at the top of atmosphere (approx. -5 to -15Wm-2). The atmospheric heating via aerosol absorption results in heating rates of 1.2-1.6Kday-1 during March-June, which may contribute to changes in monsoon circulation over northern India and the Himalayas.

Spaceborne observations of low surface aerosol concentrations in the Stockholm region

This article investigates the feasibility of using spaceborne observations of aerosol optical thickness (AOT) derived with the Moderate Resolution Imaging Spectroradiometer (MODIS) for monitoring of fine particulate matter (PM2.5) in an environment of low aerosol loading. Previous studies of the AOT-to-PM2.5 relationship benefit from the large range of observed values. The Stockholm region features a comprehensive network of ground-based monitoring stations that generally show PM2.5 values <20 µg m−3. MODIS AOT at 555 nm is usually <0.20 and in good agreement with ground-based sun photometer observations in this region. We use MODIS Collection 5 AOT data with a horizontal resolution of 10 km×10 km and ground-based in-situ PM2.5 observations to derive an AOT-to-PM2.5 relationship that can be used to estimate fields of PM2.5. This has been carried out with respect to the months from April to September of the period 2000–2013. Relative average absolute deviations of 33–55 % (mean of 45 %) are obtained between MODIS-retrieved and ground-based PM2.5. The root mean square error is 0.2159 µg m−3 between retrieved and measured PM2.5. From spaceborne lidar observations, it is found that elevated aerosol layers are generally sparse in the Stockholm region. This favours remote sensing of PM2.5 from space. The deviations found between measured and retrieved PM2.5 are mainly attributed to infrequent situations of inhomogeneous aerosol layering for which column-integrated observations cannot be connected to surface conditions. Using MODIS Collection 6 data with a resolution of 3 km×3 km in a case study actually gives far fewer results than the coarser Collection 5 product. This is explained by the complex geography of the Stockholm region with a coastline and an abundance of lakes, which seems to induce biases in the retrieval of AOT at higher spatial resolution.