Aerosol Vertical Distribution Research Articles

Source mechanisms and transport patterns of tropospheric bromine monoxide: findings from long-term multi-axis differential optical absorption spectroscopy measurements at two Antarctic stations

Abstract. The presence of reactive bromine in polar regions is a widespread phenomenon that plays an important role in the photochemistry of the Arctic and Antarctic lower troposphere, including the destruction of ozone, the disturbance of radical cycles, and the oxidation of gaseous elemental mercury. The chemical mechanisms leading to the heterogeneous release of gaseous bromine compounds from saline surfaces are in principle well understood. There are, however, substantial uncertainties about the contribution of different potential sources to the release of reactive bromine, such as sea ice, brine, aerosols, and the snow surface, as well as about the seasonal and diurnal variation and the vertical distribution of reactive bromine. Here we use continuous long-term measurements of the vertical distribution of bromine monoxide (BrO) and aerosols at the two Antarctic sites Neumayer (NM) and Arrival Heights (AH), covering the periods of 2003–2021 and 2012–2021, respectively, to investigate how chemical and physical parameters affect the abundance of BrO. We find the strongest correlation between BrO and aerosol extinction (R=0.56 for NM and R=0.28 for AH during spring), suggesting that the heterogeneous release of Br2 from saline airborne particles (blowing snow and aerosols) is a dominant source for reactive bromine. Positive correlations between BrO and contact time of air masses, both with sea ice and the Antarctic ice sheet, suggest that reactive bromine is not only emitted by the sea ice surface but by the snowpack on the ice shelf and in the coastal regions of Antarctica. In addition, the open ocean appears to represent a source for reactive bromine during late summer and autumn when the sea ice extent is at its minimum. A source–receptor analysis based on back trajectories and sea ice maps shows that main source regions for BrO at NM are the Weddell Sea and the Filchner–Ronne Ice Shelf, as well as coastal polynyas where sea ice is newly formed. A strong morning peak in BrO frequently occurring during summer and that is particularly strong during autumn suggests a night-time build-up of Br2 by heterogeneous reaction of ozone on the saline snowpack in the vicinity of the measurement sites. We furthermore show that BrO can be sustained for at least 3 d while travelling across the Antarctic continent in the absence of any saline surfaces that could serve as a source for reactive bromine.

Lidar Profiling of Aerosol Vertical Distribution in the Urbanized French Alpine Valley of Annecy and Impact of a Saharan Dust Transport Event

The vertical aerosol layering of the troposphere is poorly documented in mountainous regions, particularly in the Alpine valleys, which are influenced by valley and mountain winds. To improve our knowledge of particulate matter trapped in the Annecy valley, synergetic measurements performed by a ground-based meteorological Raman lidar and a Rayleigh-Mie lidar aboard an ultralight aircraft were implemented as part of the Lacustrine-Water vApor Isotope inVentory Experiment (L-WAIVE) over Lake Annecy. These observations were complemented by satellite observations and Lagrangian modeling. The vertical profiles of aerosol optical properties (e.g., aerosol extinction coefficient (AEC), lidar ratio (LR), particle linear depolarization ratio (PDR)) are derived from lidar measurements at 355 nm during the period between 13 and 22 June 2019. The background aerosol content with an aerosol optical thickness (AOT) of 0.10 ± 0.05, corresponding to local–regional conditions influenced by anthropogenic pollution, has been characterized over the entirety of Lake Annecy thanks to the mobile ultralight payload. The aerosol optical properties are shown to be particularly variable over time in the atmospheric column, with mean LRs (PDRs) varying between 40 ± 8 and 115 ± 15 sr (2 ± 1 and 35 ± 2%). Those conditions can be disturbed by air masses that have recirculated over the valley, as well as by contributions from neighboring valleys. We have observed an important disruption in the atmospheric aerosol profiles by the arrival of an exceptionally dry air mass (RH ~ 30%), containing aerosols identified as coming from the Great Western Erg (AOT ~ 0.5, LR = 65 ± 10 sr, PDR = 20–35%) in the Sahara. These desert dust particles are shown to influence the entire atmospheric column in the Annecy valley. Such an experimental approach, coupling upward and downward lidar and spaceborne observation/Lagrangian modelling, was shown to be of significant interest for the long-term monitoring of the evolution of aerosol loads over deep valleys. It allows a better understanding of the influence of dust storms in the presence of severe convective weather processes.

Vertical characteristics and potential sources of aerosols over northeast China using ground-based MAX-DOAS

Information on the vertical distribution of air pollutants is essential to understand the potential mechanisms of haze pollution. Multiaxial differential optical absorption spectrometry (MAX-DOAS) is an effective method to detect the vertical distribution of air pollutants. In this study, we used MAX-DOAS to continuously analyze the vertical aerosol distribution throughout the year (from January 2019 to December 2019) in Shenyang. In winter, the aerosol extinction coefficient (AEC) was the highest (0.63 ± 0.09 km−1), followed by summer (0.53 ± 0.08 km−1), spring (0.51 ± 0.08 km−1), and autumn (0.47 ± 0.09 km−1). The aerosol extinction coefficients in various seasons were affected by the weather type. The hygroscopic growth of small particles lead to a surface AEC reaching 1.45 ± 0.23 km−1 in summer, resulting in heavy haze. The coal burning for heating and automobile exhaust in winter increased the possibility of winter haze, wherein the surface AEC can reach 1.27 ± 0.19 km−1. Signs of aerosol transport can be observed at high latitudes in summer, and when combined with backward trajectories, the transport of pollutants from Inner Mongolia to Liaoning Province is evident. The possible sources of air pollutants were investigated using a concentration weighted trajectory (CWT) model. In addition to the nearby Liaoning Province, both Beijing and Tianjin were also the main sources of pollution in Shenyang (0.04 km; Shandong Province was the main source of pollution at 0.44 km).

Study on Influencing Factors of the Information Content of Satellite Remote-Sensing Aerosol Vertical Profiles Using Oxygen A-Band

Aerosol vertical distribution is decisive and hard to be constrained. It is of great significance for the study of atmospheric climate and environment. Oxygen absorption A-bands (755–775 nm) provide a unique opportunity to acquire vertical aerosol profiles from satellites over a large spatial coverage. To investigate the ability of O2 A-bands in retrieving aerosol vertical distribution, the dependence of retrieval on satellite observation geometry, spectral resolution, signal-to-noise ratio (SNR), size distribution, and a priori knowledge is quantified using information content theory. This work uses the radiative transfer model UNL to simulate four aerosol modes and the instrument noise model. The simulations show that a small scattering angle leads to an increase in the total amount of observed aerosol profile information, with the degrees freedom of signal (DFS) of a single band increasing from 0.4 to 0.85 at high spectral resolution (0.01 nm). The total DFS value of O2 A-bands varies accordingly between 1.2–2.3 to 3.8–5.1 when the spectral resolution increases from 1 nm to 0.01 nm. The spectral resolution has a greater impact on DFS value than the impact from SNR (an improvement of roughly 41–53% resulted from the change in spectral resolution and the SNR led to 13–18%). The retrieval is more sensitive to aerosols with a coarse-dominated mode. The improvement in spectral resolution on information acquisition is demonstrated using the DFS and the posterior error at various previous errors and resolutions.

Optical characteristics of aerosol and its potential sources over Nam Co in the Tibetan Plateau during Asian summer monsoon period

Aerosols over the Tibetan Plateau (TP) play an important role in the regional and global climate change and ecological environment. The Second Tibetan Plateau Scientific Expedition and Research Program (STEP) carried out observation experiment at the Nam Co Station for Multisphere Observation and Research, Chinese Academy of Sciences (Nam Co Station, 30°46′N, 90°59′E, 4730 m) in the central TP from August 5 to September 11, 2020. This paper investigated the aerosol optical properties and its potential sources over Nam Co region. The main results are as follow: the average aerosol optical depth was 0.025 during the observation period; the average black carbon (BC) concentration and aerosol number concentration was 37 ng/m3 and 129 N/cm3, respectively; the diurnal variation and vertical distribution of aerosol were affected by the local meteorological elements. Frequent southerly winds during the summer monsoon led to a certain contribution of South Asia to the BC concentration in Nam Co region. And particles from the Taklimakan Desert led to dramatic increasing of aerosol number concentration over Nam Co region. The aerosol optical characteristics and influencing factors over the central TP were analyzed, which is beneficial to further study the influence of aerosols over the TP on regional and global weather and climate.

Multi‐Year CALIPSO Observations of Ubiquitous Elevated Aerosol Layer in the Free Troposphere Over South Asia: Sources and Formation Mechanism

AbstractThe vertical distribution of aerosols in the lower troposphere is critically important for assessing their impact on Earth's radiation budget and modulation of cloud microphysics. This study analyzed cloud‐free aerosol extinction coefficient (βext), aerosol subtypes, and particulate depolarization ratios obtained from CALIOP (Cloud‐Aerosol Lidar with Orthogonal Polarization) over the six regions of India during 2008–2018. We investigated unprecedented climatology of the physical and optical characteristics of elevated aerosol layers (EALs) along with their source and formation mechanism. The key findings include: (a) EALs over the Indian region were persistent between 4 and 6 km during all seasons, (b) geometrical layer thickness of EALs increased up to 36.7% and 25% from the annual mean during summer and fall seasons, respectively, compared to that of spring and winter, (c) dust and polluted dust accounted for up to 50%–80% from near‐surface to 6 km and up to 80%–90% of the EALs between 4 and 6 km, respectively for all the seasons and regions, (d) we anticipated that locally confined recirculation coupled with stratified stable layer capped within turbulent layers could be a possible mechanism of formation of stratified EALs between 4 and 6 km during winter‐spring‐fall, while in summer, vertical transport of pollutants from the PBL to mid‐troposphere due to enhanced deep convection served as a key formation mechanism of the EALs, (e) the second Modern‐Era Retrospective analysis for Research and Applications Global Modeling Initiative model reasonably simulated the shape and vertical gradient of βext with significant differences in magnitude below 4 km; however, it fails to reproduce EALs for all seasons and regions during the study period.

Aircraft observations on a continuous haze pollution event in Shijiazhuang area

A continuous haze event was recorded on November 14th∼17th, 2020 over Shijiazhuang. Two flights of King-air 350 meteorological research aircraft were performed on November 14th and 16th for the retrieval and observations of meteorological elements, aerosols, and black carbon. In this study, we combined airborne data with air pollution data (PM2.5), ground meteorological data, and ERA-5 reanalysis data to describe the vertical distribution of aerosols (namely 0.1–3.0 μm) and black carbon. We further explicated the formation of this haze event. PM2.5 pollution dominated this haze event, and the highest concentration of PM2.5 was 209 μg/m3. The intensity and height of thermal layers highly linked with the vertical transport of pollution. The highest number concentration of aerosols and black carbon was found below the thermal layers on both airborne sounding days. On the 14th, both BC and aerosol concentrations showed unimodal distribution, and the highest concentrations of BC and aerosols were 12683 ng/m3 and 6965.125#/L at 250 m within layer Ⅰ. The intensity of the thermal layer near-ground was weaker on the 16th that the number concentrations of BC and aerosols also remained at high levels in layer Ⅱ. Backward trajectories of air mass indicated the long-range transport of pollution contributed to the high level of pollution on the 16th. Vapor conditions were more favorable for aerosols growth through moisture absorption. The maximum concentration of 943.58#/L was recorded at particles with a diameter of 0.4 μm on the 16th, while 749.26#/L was reached at 0.14 μm on the 14th. The corresponding height was consistent with the height of maximum concentration in the vertical distribution.

Added Value of Aerosol Observations of a Future AOS High Spectral Resolution Lidar with Respect to Classic Backscatter Spaceborne Lidar Measurements

In the context of the Atmosphere Observing System (AOS) international program, a new-generation spaceborne lidar is expected to be in polar orbit for deriving new observations of aerosol and clouds. In this work, we analyze the added values of these new observations for characterizing aerosol vertical distribution. For this, synthetic observations are simulated using the BLISS lidar simulator in terms of the backscatter coefficient at 532 nm. We consider two types of lidar instruments, an elastic backscatter lidar instrument and a high spectral resolution lidar (HSRL). These simulations are performed with atmospheric profiles from a nature run (NR) modeled by the MOCAGE chemical transport model. In three case studies involving large events of different aerosol species, the added value of the HSRL channel (for measuring aerosol backscatter profiles with respect to simple backscatter measurements) is shown. Observations independent of an a priori lidar ratio assumption, as done typically for simple backscattering instruments, allow probing the vertical structures of aerosol layers without divergence, even in cases of intense episodes. A 5-day study in the case of desert dust completes the study of the added value of the HSRL channel with relative mean bias from the NR of the order of 1.5%. For low abundances, relative errors in the backscatter coefficient profiles may lay between +40% and −40%, with mean biases between +5% and −5%.

Seasonal and Diurnal Characteristics of the Vertical Profile of Aerosol Optical Properties in Urban Beijing, 2017–2021

Seasonal and diurnal characteristics of the vertical profiles of aerosol properties are essential for detecting the regional transport and the climatic radiative effects of aerosol particles. We have studied the seasonal and diurnal characteristics of the vertical distribution of aerosols in urban Beijing from 2017 to 2021 based on long-term Raman–Mie LiDAR observations. The influence of the vertical distribution of aerosols, the meteorological conditions within the boundary layer, the optical–radiometric properties of aerosols, and their interconnections, were investigated during a heavy haze pollution event in Beijing from 8 to 15 February 2020 using both meteorological and sun photometer data. The aerosol extinction coefficient was highest in summer (0.4 km−1), followed by winter (0.35 km−1), and roughly equal in spring and autumn (0.3 km−1). The aerosol extinction coefficient showed clear daily variations and was different in different seasons as a result of the variation in the height of the boundary layer. During the haze pollution event, the particulate matter mainly consisted of scattered spherical fine particles and the accumulation time of pollutants measured via the AOD440nm and PM2.5 mass concentration was different as a result of the hygroscopic growth of the aerosol particles. This growth increased scattering and led to an increase in the aerosol optical depth. The vertical transport of particulate matter also contributed to the increase in the aerosol optical depth.

Analysis of the air quality in upper atmospheric boundary layer in a high-density city in Asia using 3-year vertical profiles measured by the 3-Dimensional Real-Time Atmospheric Monitoring System (3DREAMS).

Past studies focused on ground-level air quality, whereas air quality in upper atmospheric boundary layer (ABL) remains unclear due to lack of long-term and high time-resolution profile data. This study utilized the 3-Dimensional Real-Time Atmospheric Monitoring System (3DREAMS) to provide vertical profiles of aerosol backscatter coefficients and wind for three years (2019-2021), along with DustTrak to describe and analyze the characteristics of aerosols in the upper ABL in a high-density city in Asia (Hong Kong, China). It is the first study to assess the long-term record and spatial variations of upper-level aerosol in a high-density city using a LiDAR network. Results show an opposite diurnal profile of aerosol comparing between ground-level and upper ABL, which is different with the diurnal pattern observed in ground measurements (higher air pollutant concentration level in daytime and lower in nighttime). The co-location vertical wind measurements provided the explanation of the opposite diurnal patterns. The 3-year vertical profiles also show the significant spatial variation of vertical distribution of aerosol at different locations, whereas the temporal variations can be affected by various factors such as emissions and transboundary air pollution. Our episode analysis clearly demonstrated the capability of 3DREAMS to monitor transboundary air pollution with detailed information of horizontal transport and vertical convection. 3DREAMS is therefore proved as a suitable system for transboundary air pollution monitoring. Our findings provide a critical reference for atmospheric scientists and decisionmakers to understand transboundary air pollution.

Dynamics of Mixing Layer Height and Homogeneity from Ceilometer-Measured Aerosol Profiles and Correlation to Ground Level PM2.5 in New York City

Observations of mixing layer height (MLH), its dynamics and the extent of homogeneity of the mixing layer are important in the study of air pollution as well as meteorological and air quality model validation. They can contribute to improvements in the application of satellite aerosol optical depth (AOD) products to ground fine particulate matter (PM2.5) estimation that can potentially provide synoptic-scale monitoring of fine particulate matter. Ceilometers have shown great potential for continuous monitoring of MLH and aerosol profiles within the boundary layer. In this study, we report an automated quality control/quality assurance (QC/QA) method that improves the consistency of MLH retrievals from ceilometer measurements and present measurements of MLH variation in New York City (NYC) as observed by ceilometers in summer and winter seasons. Distinct issues due to SNR and quality of overlap correction are addressed within the QC/QA method. We also analyze the diurnal and seasonal correlations between ceilometer-attenuated backscatter and ground-level PM2.5 as a function of height, time of day, and season, to shed light on the homogeneity of aerosol vertical mixing within the MLH, as well as the correlation between aerosol optical properties and PM2.5. The results show that the overall correlation in summer is better than in winter. This correlation decreases with increasing height but the degradation is less severe in summer than in winter, which is qualitatively consistent with urban heating models of convective mixing. However, no significant diurnal variation of the correlation coefficient was observed for both seasons. We also found that the linear regression slope between ceilometer-attenuated backscatter coefficients and ground PM2.5 shows seasonal variation, which can be partially explained by the difference in aerosol size distribution and aerosol species between summer and winter. Finally, we investigated the homogeneity of aerosol vertical distribution within the mixing layer (ML) during the daytime. The results indicate that the aerosols are well-mixed within the lower part of ML up to 500 m.

Seasonal Variation of Dust Aerosol Vertical Distribution in Arctic Based on Polarized Micropulse Lidar Measurement

This study investigates the seasonal variation of dust aerosol vertical distribution using polarized Micropulse lidar (MPL) measurements at the Atmospheric Radiation Measurement (ARM) North Slope of Alaska (NSA) observatory from January 2013 to September 2017. For the first time, multi-year aerosol backscatter coefficients are retrieved at the ARM NSA site from MPL measurements and are consistent with co-located high spectral resolution lidar (HSRL) measurements. The high-quality aerosol backscatter coefficient retrievals are used to derive the particle depolarization ratio (PDR) at the wavelength of 532 nm, which is used to identify the presence of dust aerosols. The annual cycles of the vertical distributions of dust backscatter coefficient and PDR and dust aerosol optical depth (DAOD) show that aerosol loading has a maximum in late winter and early spring but a minimum in late summer and early autumn. Vertically, dust aerosol occurs in the entire troposphere in spring and winter and in the low and middle troposphere in summer and autumn. Because dust aerosols are effective ice nuclei, the seasonality of dust aerosol vertical distribution has important implications for the Arctic climate through aerosol–cloud–radiation interactions, primarily through impacting mixed-phase cloud processes.

Impact of Relative Humidity on the vertical distribution of aerosols over India

Water uptake by aerosol particles significantly changes its light-scattering characteristics, alters the physical and optical properties of aerosols. In this article, we present the effect of ambient relative humidity (RH) on the optical properties of surface and columnar aerosols under different RH conditions during different seasons over India. In particular, relation between surface RH and surface aerosol properties, surface RH and aerosol optical depth (AOD) and profile of RH on the AOD is presented. An increase in the surface aerosol scattering coefficient (σscat) and total aerosol number concentration (N(d)) is observed under high humidity (50–100%) condition than low humidity (0–30%). This is more prominent in the winter and pre-monsoon seasons. An increase in the AOD is observed as surface RH increase in all the seasons except in monsoon season where it showed an inverse relation. The profile measurements (Lidar and radiosonde) also showed a consistent increase in AOD (0-6 km) from low to high RH (0-6 km) conditions. This relationship is significant during winter followed by the post-monsoon, pre-monsoon, and monsoon seasons. An increase in AOD with RH is observed regardless of altitude in all seasons, except during the monsoon at higher altitudes (2-6 km). Our findings clearly demonstrated that during the winter and monsoon (pre- and post-monsoon) seasons, aerosols within (above) the boundary layer are more sensitive to the RH. This relationship has been tested in different surface conditions across India using CALIPSO and IMD radiosonde measurements. These measurements also confirmed an increase in AOD with increasing RH in all the regions except East Coast (EC) region. Irrespective of the region, this feature is more prominent at lower altitudes (0–1.5 km) during winter and higher altitudes (1.5-6 km) during other seasons. At higher altitudes, only a few regions (Central India, Indo-Gangetic Plain, and West Coast) showed an inverse relationship between AOD and RH during the monsoon and post-monsoon seasons. Further, our results illustrated the contribution of hygroscopic particles to the RH increases the AOD in a given season. This finding implies that hygroscopic growth caused by water-soluble particles in the lower troposphere can significantly alter the magnitude of aerosol radiative forcing both at the surface and at the top of the atmosphere.

Observational study of the influences of thermal and dynamic boundary layer on the vertical distribution of black carbon aerosol in ShouXian county in wintertime, 2016

Using vertical observation data of black carbon aerosol and meteorological parameters in the ShouXian area of Anhui Province from 14 December 2016 to 3 January 2017, the thermal and dynamic effects of the boundary layer on the vertical distribution structure of black carbon were studied. The results show that 1) of 82 vertical sounding profiles obtained during the observation period, there were 72 boundary layer sounding profiles dominated by heat and 10 profiles dominated by dynamics. 2) When thermal effects were dominant, the concentration of black carbon aerosol was significantly affected by diurnal changes in the boundary layer. In the unstable boundary layer, black carbon aerosol was uniformly distributed in the vertical direction; during the transition from an unstable to a stable boundary layer, the concentrations of black carbon aerosol were significantly higher in the lower layer than in the upper layer; in the stable boundary layer, the concentrations of black carbon aerosol decreased continuously with height; and during the transition from a stable to an unstable boundary layer, the black carbon aerosol concentrations exhibited high values in the upper layer, with the concentration difference reaching 4 μg m−3. 3) When the dynamic effect was dominant, the structure of the vertical distribution of black carbon aerosol was affected by wind and by diurnal changes in the boundary layer simultaneously. The high winds (>4 m/s) removed the black carbon aerosol. In the unstable boundary layer, the black carbon aerosol uniformly distributed in the vertical direction had significantly lower concentrations in the high-wind range; when stable boundary layers occurred and during transitions from stable to unstable boundary layers, the black carbon aerosol concentrations were higher in the lower layer and lower in the upper layer. The stratification was more obvious than that observed under thermal control.

Polar Aerosol Vertical Structures and Characteristics Observed with a High Spectral Resolution Lidar at the ARM NSA Observatory

Aerosol vertical distributions impact both the direct and indirect radiative effects of aerosols. High Spectra Resolution Lidar (HSRL) separates between atmospheric molecular signals and aerosol particle signals and therefore can provide reliable measurements of aerosol properties. Six years of HSRL measurements between 2014 and 2019 from the Department of Energy (DOE) Atmospheric Radiation Measurement (ARM) North Slope of Alaska (NSA) atmospheric observatory at Utqiaġvik are used to statistically analyze Arctic aerosol vertical distributions. The annual cycle of aerosol vertical distributions in terms of aerosol particulate backscatter coefficient (βp), lidar scattering ratio (SR), and aerosol particulate depolarization ratio (δp) profiles at the wavelength of 532 nm shows that Arctic Haze events are prevalent in later winter and spring at the NSA site. Mineral dust is frequently presented in strong aerosol layers in the spring, fall, and winter seasons. Over the summer season, the NSA site has large aerosol loadings that are dominated by small spherical aerosol particles.

The Influence of Temperature Inversion on the Vertical Distribution of Aerosols

Temperature inversion plays an important role in the accumulation and diffusion of aerosols. In this study, the relationship between temperature inversion and the vertical distribution of aerosols is investigated based on Raman lidar observations taken from January 2010 to September 2015 at the Atmospheric Radiation Measurement site in the Southern Great Plains, USA. First, the diurnal and seasonal variations of the surface-based inversion (SBI) and elevated temperature inversion (EI) are investigated. The results indicate that the occurrence frequency of SBI and EI have different seasonal trends. SBI has the highest frequency in summer, while EI has the highest frequency in winter. The diurnal variation of SBI is obvious, with a higher frequency in nighttime and a lower frequency in daytime. The inversion intensity (ΔT) and inversion depth (ΔZ) of SBI and EI have consistent diurnal and seasonal trends. The effects of SBI and EI on the vertical distribution of aerosols are then analyzed. The mean aerosol optical depth (AOD) below the SBI height shows a clear seasonal variation, which is consistent with the seasonal trends of ΔT and ΔZ. This phenomenon also occurs on the AOD below EI top height. The sensitivity analysis shows that the mean AOD below SBI height or EI top height increases with an increase of the ΔT and ΔZ of SBI (EI). It indicates that ΔT and ΔZ are the key factors affecting the vertical distribution of aerosols. In addition, the variation of AOD below and above EI top height is opposite to that of AOD below and above EI bottom height under different ΔT and ΔZ conditions. The correlation coefficients between ΔT (ΔZ) of EI with AOD in EI were 0.62 (0.65). These results indicate that the space between EI bottom height and EI top height can store aerosols. The larger the ΔZ of EI, the more aerosols are stored. These findings contribute to our understanding of the effect of temperature inversion on the vertical distribution of aerosols.

Investigating temporal changes in Jupiter’s aerosol structure with rotationally-averaged 2015–2020 HST WFC3 images

Annual imaging observations of Jupiter from 2015 onward by the Hubble Space Telescope (HST) Wide Field Camera 3 (WFC3) as part of the Outer Planet Atmospheres Legacy (OPAL) program (Simon et al., 2015) have provided a valuable data set from which Jupiter’s temporal variability can be characterized and modeled. Spectral samples at discrete filter bands from the UV to short near-IR wavelengths also provide an opportunity to connect temporal changes in reflectivity with temporal changes in cloud structure. To improve constraints on that cloud structure we used center-to-limb variations that were noise-reduced by rotational averaging during each annual OPAL observation sequence. Fitting observations at multiple and distinctly different observer zenith angles helps to make up for the lack of multiple methane-band filters in discerning the vertical distribution of aerosols. The short-wavelength limit of the OPAL observations also provides better constraints than ground-based observation on the properties of Jupiter’s chromophore, i.e., the compound that creates Jupiter’s variety of red colorations to clouds and hazes composed mainly of colorless condensates. We chose four latitudes to investigate with radiative transfer models based on a simple 3-layer aerosol structure consisting of a deep putative NH3 cloud layer, with a physically thin chromophore layer that has a fitted location at or near the top of the deep cloud, and an overlying stratospheric haze layer. Our chosen planetocentric latitudes were 5.5°S (Equatorial Zone South,EZS), 0° (Equatorial Zone, EZ), 10°N (North Equatorial Belt, NEB), and 21.75°N (North Temperate Belt, NTB). Two major color transformations occurred during the 2015–2020 period we analyzed. One is a dramatic dimming of the center of the Equatorial Zone at short wavelengths, especially after 2017, and a significant reddening of that region. The second is a significant reddening of part of the North Temperate Belt (near 22°N) between 2016 and 2017. Both reddening events can be modeled by doubling of the optical depth of the chromophore layer during the transitions. Comparison with prior 2017 model results for the NEB, EZ, and NTB have revealed agreement with Braude et al. (2020) on the vertical location of the chromophore layer but disagreements on cloud-top pressures and the vertical distribution of stratospheric haze particles. We identified what seems to be common tendency of models based on HST/WFC3 observations to either infer very little stratospheric haze opacity or to put that opacity at too high a pressure.

Method to evaluate errors of Mie-Lidar in the transition zone with a dual CCD laser system

Mie Scattering Light Detection and Ranging (Mie-Lidar) is the most frequently used tool for obtaining the vertical profile of aerosol extinction coefficients. However, Mie-Lidar has an area that is not fully overlapping (called a transition zone) several hundred meters above the ground. There is uncertainty, although a geometric overlap factor (GOF) correction is normally used. To specifically evaluate the error of Mie-Lidar in the transition zone, a dual charge-coupled device (CCD) laser system (DCLS) is developed in this study. Multiple joint measurements as well as a continuous detection experiment are conducted. The extinction coefficient profiles obtained with the DCLS and Mie-Lidar are compared. The results indicate that the extinction coefficients are nearly consistent above the transition zone, and Pearson’s correlation coefficient remains above 0.9. However, the extinction coefficients have specific differences in the transition zone, and the linear fit results are relatively bad. The average relative difference of the extinction coefficients between the Mie-Lidar and DCLS is nearly 20%. Compared Mie-Lidar, the blind zone of the DCLS is merely approximately 8 m. The DCLS shows more details of the near-surface atmosphere, and its spatial resolution of up to 1 × 10–3 m can provide a useful reference for Mie-Lidar. The DCLS contributes to obtaining a more accurate vertical distribution of aerosols in the atmospheric boundary layer.

Impact of urban heat island on inorganic aerosol in the lower free troposphere: a case study in Hangzhou, China

Abstract. Urban heat island (UHI) and urban air pollution are two major environmental problems faced by many metropolises. The UHI affects air pollution by changing the local circulation and the chemical reaction environment, e.g. air temperature and relative humidity. In this study, the WRF-CMAQ (Weather Research and Forecasting Community Multiscale Air Quality) model was used to investigate the impact of an UHI on the vertical distribution of aerosol particles, especially secondary inorganic aerosol (SIA), taking the strong UHI in Hangzhou, China, as an example. The results show that during the day with the strongest UHI case of the 8 d simulation, the UHI effect resulted in a decrease in the PM2.5 concentrations in the boundary layer (BL) by about 33 %, accompanied by an increase in the lower free troposphere (LFT) by about 19 %. This is mostly attributed to the UHI circulation (UHIC) effect, which accounted for 91 % of the UHI-induced variations in PM2.5, rather than the UHI temperature or humidity effects, which contributed only 5 % and 4 %, respectively. The UHIC effect plays a dominant role, ranging from 72 % to 93 %, in UHI-induced PM2.5 variation in all eight UHI cases. The UHIC not only directly transports aerosol particles from ground level to the LFT but also redistributes aerosol precursors. During the strongest UHI case, about 80 % of the UHIC-induced increase in the aerosol particles in the LFT is due to direct transport of aerosol particles, whereas the other 20 % is due to secondary aerosol formation resulting from the transport of aerosol precursor gases. Of this 20 %, 91 % is contributed by SIA, especially ammonium nitrate aerosol formed from ammonia and nitric acid. In the atmosphere, ammonium nitrate is in equilibrium with ammonia and nitric acid, and the equilibrium depends on the ambient temperature. In the lower urban BL, the temperature is higher than in the LFT, and the ammonium nitrate equilibrium in the lower BL is more toward the gas phase than in the LFT; when these gases are transported by the UHIC into the colder LFT, the equilibrium shifts to the aerosol phase. Hence, the UHIC changes the vertical distribution of SIA, which may have potential implications on the radiation budget, cloud formation, and precipitation in the urban and surrounding areas.

Estimation of the atmospheric boundary layer height by means of machine learning techniques using ground-level meteorological data

Aerosols affect air quality, weather and climate through many mechanisms and are dangerous to human health. They are mostly concentrated within the atmospheric boundary layer (ABL) its height is affected by the radiation emitted by the surface, causing turbulence and evolving along the day, influencing the vertical mixing of the air pollutants generated near the surface and therefore, their ground-level concentration from local sources. Lidars have demonstrated their capabilities to study the aerosol vertical distribution and their spatio-temporal evolution can provide very complete information on the ABL dynamics. In this work, machine learning techniques are employed to predict the ABL height. The meteorological variables measured at ground-level are used as features of the algorithm and the ABL height estimated by the STRATfinder algorithm using ceilometer profiles, a small lidar instrument with enhanced characteristics for unassisted continuous operation, are considered the truth in the supervised regression algorithm. The machine learning models allow considering combination of features in the regression algorithm and also allow characterizing the importance of each of the predictors to determine the final result. This property is used to study different boundary layer regimes. The ABL is difficult to study in certain parts of the day due to transitions between atmospheric regimes. In order to improve the performance of the model, each day was divided in four parts (nighttime, morning, daytime and evening). The Madrid ceilometer profile database has been studied for the year 2020, splitting the training datasets for the machine learning algorithm into season and part of the day, and the importance of predictors analyzed. Major influence of temperature and relative humidity is found in most of the situations, but also wind velocity in certain circumstances and pressure. The influence of radiation is small, contrary to expected. The main advantage of the proposed method is that MLHs and ABLHs can be retrieved directly from widely available ground-level meteorological data. Future work will focus on more relevant predictors, as latent heat or turbulence.