scholarly journals Occurrence and growth of sub-50 nm aerosol particles in the Amazonian boundary layer

2021 ◽  
Author(s):  
Marco A. Franco ◽  
Florian Ditas ◽  
Leslie Ann Kremper ◽  
Luiz A. T. Machado ◽  
Meinrat O. Andreae ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Meteorological Data ◽  
Cloud Condensation Nuclei ◽  
Deep Convection ◽  
Potential Temperature ◽  
Meteorological Conditions ◽  
Subsequent Growth ◽  
Wet Season ◽  
Particle Injection ◽  
Continuous Growth

Abstract. New particle formation (NPF), referring to the nucleation of molecular clusters and their subsequent growth into the cloud condensation nuclei (CCN) size range, is a globally significant and climate-relevant source of atmospheric aerosols. Classical NPF exhibiting continuous growth from a few nanometers to the Aitken mode around 60–70 nm is widely observed in the planetary boundary layer (PBL) around the world, but not in central Amazonia. Here, classical NPF events are rarely observed in the PBL, but instead, NPF begins in the upper troposphere (UT), followed by downdraft injection of sub-50 nm (CN< 50) particles into the PBL and their subsequent growth. Central aspects of our understanding of these processes in the Amazon have remained enigmatic, however. Based on more than six years of aerosol and meteorological data from the Amazon Tall Tower Observatory (ATTO, Feb 2014 to Sep 2020), we analyzed the diurnal and seasonal patterns as well as meteorological conditions during 254 of such Amazonian growth events on 217 event days, which show a sudden occurrence of particles between 10 and 50 nm in the PBL, followed by their growth to CCN sizes. The occurrence of events was significantly higher during the wet season, with 88 % of all events from January to June, than during the dry season, with 12 % from July to December, probably due to differences in the condensation sink (CS), atmospheric aerosol load, and meteorological conditions. Across all events, a median growth rate (GR) of 5.2 nm h−1 and a median CS of 0.0011 s−1 were observed. The growth events were more frequent during the daytime (74 %) and showed higher GR (5.9 nm h−1) compared to nighttime events (4.0 nm h−1), emphasizing the role of photochemistry and PBL evolution in particle growth. About 70 % of the events showed a negative anomaly of the equivalent potential temperature (∆θ'e) – as a marker for downdrafts – and a low satellite brightness temperature (Tir) – as a marker for deep convective clouds – in good agreement with particle injection from the UT in the course of strong convective activity. About 30 % of the events, however, occurred in the absence of deep convection, partly under clear sky conditions, and with a positive ∆θ'e anomaly. Therefore, these events do not appear to be related to downdraft injection and suggest the existence of other currently unknown sources of the sub-50 nm particles.

scholarly journals Boundary Layer Dynamics in a Simple Model for Convectively Coupled Gravity Waves

2009 ◽  
Vol 66 (9) ◽  
pp. 2780-2795 ◽  
Author(s):  
Michael L. Waite ◽  
Boualem Khouider
Keyword(s):  
Boundary Layer ◽  
Gravity Waves ◽  
Deep Convection ◽  
Potential Temperature ◽  
Phase Speed ◽  
Free Troposphere ◽  
Synoptic Scale ◽  
Basic States ◽  
Boundary Layer Dynamics ◽  
Baroclinic Modes

Abstract A simplified model of intermediate complexity for convectively coupled gravity waves that incorporates the bulk dynamics of the atmospheric boundary layer is developed and analyzed. The model comprises equations for velocity, potential temperature, and moist entropy in the boundary layer as well as equations for the free tropospheric barotropic (vertically uniform) velocity and first two baroclinic modes of vertical structure. It is based on the multicloud model of Khouider and Majda coupled to the bulk boundary layer–shallow cumulus model of Stevens. The original multicloud model has a purely thermodynamic boundary layer and no barotropic velocity mode. Here, boundary layer horizontal velocity divergence is matched with barotropic convergence in the free troposphere and yields environmental downdrafts. Both environmental and convective downdrafts act to transport dry midtropospheric air into the boundary layer. Basic states in radiative–convective equilibrium are found and are shown to be consistent with observations of boundary layer and free troposphere climatology. The linear stability of these basic states, in the case without rotation, is then analyzed for a variety of tropospheric regimes. The inclusion of boundary layer dynamics—specifically, environmental downdrafts and entrainment of free tropospheric air—enhances the instability of both the synoptic-scale moist gravity waves and nonpropagating congestus modes in the multicloud model. The congestus mode has a preferred synoptic-scale wavelength, which is absent when a purely thermodynamic boundary layer is employed. The weak destabilization of a fast mesoscale wave, with a phase speed of 26 m s−1 and coupling to deep convection, is also discussed.

Regional modelling of aerosol interaction with deep convection and the effect on new particle formation over Amazonia

2020 ◽  
Author(s):  
Xuemei Wang ◽  
Daniel Grosvenor ◽  
Hamish Gordon ◽  
Meinrat O. Andreae ◽  
Ken Carslaw
Keyword(s):  
Boundary Layer ◽  
Cloud Condensation Nuclei ◽  
Deep Convection ◽  
Particle Formation ◽  
Nucleation Mechanism ◽  
Transport Processes ◽  
Radiative Properties ◽  
Free Troposphere ◽  
New Particle Formation ◽  
New Particles

&lt;p&gt;It has been estimated that over 50% of the present-day global low-level cloud condensation nuclei (CCN) are formed from new particle formation (NPF), and that this process has a substantial effect on the radiative properties of shallow clouds (Gordon et al. 2017). In contrast, we have a very limited understanding of how NPF affects deep convective clouds. Deep clouds could interact strongly with NPF because they extend into the high free troposphere where most new particles are formed, and they are responsible for most of the vertical transport of the nucleating vapours. Andreae et al. (2018) hypothesised from ACRIDICON-CHUVA campaign that organic gas molecules are transported by deep convection to the upper troposphere where they are oxidised and produce new particles, which are then be entrained into the boundary layer and grow to CCN-relevent sizes.&lt;/p&gt;&lt;p&gt;Here we study the interaction of deep convection and NPF using the United Kingdom Chemistry and Aerosols (UKCA) model coupled with the Cloud-AeroSol Interacting Microphyics (CASIM) embedded in the regional configuration of UK Met Office Hadley Centre Global Environment Model (HadGEM3). We simulate several days over a 1000 km region of the Amazon at 4 km resolution. We then compare the regional model, which resolves cloud up- and downdrafts, with the global model with parameterised convection and low resolution.&lt;/p&gt;&lt;p&gt;Our simulations highlight three findings. Firstly, solely using a binary H&lt;sub&gt;2&lt;/sub&gt;SO&lt;sub&gt;4&lt;/sub&gt;-H&lt;sub&gt;2&lt;/sub&gt;O nucleation mechanism strongly underestimates total aerosol concentrations compared to observations by a factor of 1.5-8 below 3 km over the Amazon. This points to the potential role of an additional nucleation mechanism, most likely involving biogenic compounds that occurs throughout more of the free troposphere. Secondly, deep convection transports insoluble gases such as DMS and monoterpenes vertically but not SO&lt;sub&gt;2&lt;/sub&gt;&amp;#160;or H&lt;sub&gt;2&lt;/sub&gt;SO&lt;sub&gt;4&lt;/sub&gt;. The time scale for DMS oxidation (~ 1 day) is much longer than for monoterpene (1-2 hours), which points to the importance of simulating biogenic nucleation over the Amazon in a cloud-resolving model, while lower-resolution global models may adequately capture DMS effects on H&lt;sub&gt;2&lt;/sub&gt;SO&lt;sub&gt;4&lt;/sub&gt; nucleation. Finally, we also examine the Andreae et al (2018) hypothesis of aerosol supply to the boundary layer by quantifying cloud-free and cloudy up- and downdraft transport. The transport of newly formed aerosols into the boundary layer is 8 times greater in cloud-free regions than in the clouds, but these transport processes are of similar magnitude for large aerosols.&lt;/p&gt;

scholarly journals Free troposphere as the dominant source of CCN in the Equatorial Pacific boundary layer: long-range transport and teleconnections

2013 ◽  
Vol 13 (1) ◽  
pp. 1279-1326 ◽  
Author(s):  
A. D. Clarke ◽  
S. Freitag ◽  
R. M. C. Simpson ◽  
J. G. Hudson ◽  
S. G. Howell ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Marine Boundary Layer ◽  
Cloud Condensation Nuclei ◽  
Large Fraction ◽  
Deep Convection ◽  
Equatorial Pacific ◽  
Quasi Equilibrium ◽  
Free Troposphere ◽  
Low Carbon ◽  
Mixing Ratios

Abstract. Airborne aerosol measurements in the central equatorial Pacific during PASE (Pacific Atmospheric Sulfur Experiment) revealed that cloud condensation nuclei (CCN) activated in marine boundary layer (MBL) clouds were dominated by entrainment from the free troposphere (FT). About 65% entered at sizes effective as CCN in MBL clouds, while 25% entered the MBL too small to activate but subsequently grew via gas to particle conversion. The remaining 10% were inferred to be sea-salt aerosol; there was no discernable nucleation in the MBL. FT aerosols at low carbon monoxide (CO) mixing ratios (< 63 ppbv) were small and relatively volatile with a number mode around 30–40 nm dry diameter and tended to be associated with cloud outflow from distant deep convection (3000 km or more). Higher CO concentrations were commonly associated with trajectories from South America and the Amazon region (ca. 10 000 km away) and occurred in layers indicative of combustion sources partially scavenged by precipitation. These had number mode near 60–80 nm diameter with a large fraction already CCN.2 (those activated at 0.2% supersaturation and representative of MBL clouds) before entrainment into the MBL. Flight averaged concentrations of CCN.2 were similar for measurements near the surface, below the inversion and above the inversion, confirming that subsidence of FT aerosol dominated MBL CCN.2. Concurrent flight-to-flight variations of CCN.2 at all altitudes below 3 km imply MBL CCN.2 concentrations were in quasi-equilibrium with the FT over a 2–3 day time scale. This extended FT transport over thousands of kilometers indicates teleconnections between MBL CCN and cloud-scavenged sources of both natural and/or residual combustion origin. The low aerosol scattering and mass in such layers results in poor detection by satellite and this source of CCN is not represented in most current models. The measurements confirm nucleation in the MBL was not evident during PASE and argue against the CLAW hypothesis being effective in this region during PASE.

scholarly journals Methodology to determine the coupling of continental clouds with surface from lidar and meteorological data

2021 ◽  
Author(s):  
Tianning Su ◽  
Youtong Zheng ◽  
Zhanqing Li
Keyword(s):  
Remote Sensing ◽  
Boundary Layer ◽  
Great Plains ◽  
Meteorological Data ◽  
Potential Temperature ◽  
New Method ◽  
Persistent Problem ◽  
Low Clouds ◽  
Stratocumulus Clouds ◽  
Remote Sensing Methods

Abstract. The states of coupling between clouds and surface or boundary-layer have been investigated much more extensively for marine stratocumulus clouds than for continental low clouds, partly due to more complex thermodynamic structures over land. A manifestation is a lack of robust remote sensing methods to identify coupled and decoupled clouds over land. Here, we have generalized the concept of coupling and decoupling to low clouds over land, based on potential temperature profiles. Furthermore, by using ample measurements from a lidar and a suite of surface meteorological instruments at the U.S. Department of Energy’s Atmospheric Radiation Measurement Program’s Southern Great Plains site from 1998 to 2019, we have developed a method to simultaneously retrieve the planetary boundary layer (PBL) height (PBLH) and coupled states under cloudy conditions during the daytime. The coupled states derived from lidar show strong consistency with those derived from radiosondes. Retrieving the PBLH under cloudy conditions that has been a persistent problem in lidar remote sensing, is resolved in this study. Our method can lead to high-quality retrievals of the PBLH under cloudy conditions and the determination of cloud coupling states. With the new method, we find that coupled clouds are sensitive to changes in the PBL with a strong diurnal cycle, whereas decoupled clouds and the PBL are weakly related. Since coupled and decoupled clouds have distinct features, our new method offers an advanced tool to separately investigate them in climate systems.

scholarly journals Boundary layer nucleation as a source of new CCN in savannah environment

2012 ◽  
Vol 12 (3) ◽  
pp. 8503-8531 ◽  
Author(s):  
L. Laakso ◽  
J. Merikanto ◽  
V. Vakkari, ◽  
H. Laakso ◽  
M. Kulmala ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Biomass Burning ◽  
Growth Rates ◽  
Seasonal Cycle ◽  
Cloud Condensation Nuclei ◽  
Winter Season ◽  
Particle Formation ◽  
Large Contribution ◽  
Wet Season ◽  
High Particle

Abstract. The South African savannah region is complex environment of air pollution and natural emissions influenced by a strong seasonal cycle in biomass burning and strong precipitation. However, the scarcity of long-term observations means that our knowledge of controlling aerosol processes in this environment is very poor. Here we use a new dataset of 18 months of aerosol observations to understand the factors that control aerosol properties, and in particular cloud condensation nuclei. We find that biomass burning produces a strong source of primary CCN-sized particles during the dry winter season. However, measured CCN-sized particle concentrations remain high during the wet summer season despite the lack of burning and high wet removal rates. We show that during the wet season, a substantial fraction of CCN-sized particles originate from boundary layer new particle formation, whereas primary sources dominate during the dry winter season. The large contribution of boundary layer nucleation to CCN concentrations during the wet season is found to be due to high particle formation and growth rates and low pre-existing particle concentration in the beginning of particle formation. Based on the estimated seasonal cycle of condensable sulphuric acid and organic vapours, higher growth rates during the wet season are attributed to vapours of biogenic origin. Global model results for this region have the same seasonal cycle in nuclei growth rates but the opposite cycle in particle formation rates, and both rates are much lower than observed. In contrast, the same model tends to capture the seasonal cycle in particle concentrations at many other global sites where nucleation is an important process. These results point to deficiencies in our understanding of biogenic emissions and the factors controlling nucleation and growth in such dynamic environments.

Convective Bursts, Downdraft Cooling, and Boundary Layer Recovery in a Sheared Tropical Storm

2013 ◽  
Vol 141 (3) ◽  
pp. 1048-1060 ◽  
Author(s):  
John Molinari ◽  
Jaclyn Frank ◽  
David Vollaro
Keyword(s):  
Boundary Layer ◽  
Deep Convection ◽  
Potential Temperature ◽  
Tangential Velocity ◽  
Vertical Wind Shear ◽  
Surface Heat ◽  
Tropical Storm ◽  
Convective Bursts ◽  
Moisture Fluxes ◽  
Heat And Moisture

Abstract Tropical Storm Edouard (2002) experienced episodic outbreaks of convection downshear within the storm core in the presence of 11–15 m s−1 of ambient vertical wind shear. These outbreaks lasted 2–6 h and were followed by long periods with no deep convection. Flights from U.S. Air Force reconnaissance aircraft within the boundary layer were used to investigate the cause of one such oscillation. Low equivalent potential temperature θe air filled the boundary layer as convection ceased, creating a 4–6-K deficit in θe within the convective region. Soundings within 110 km of the center were supportive of convective downdrafts, with midlevel relative humidity below 15% and large downdraft CAPE. Deep convection ceased within 75 km of the center for more than 8 h. Tangential velocity reached hurricane force locally during the convective outbreak, then became nearly symmetric after convection stopped, arguably as a result of axisymmetrization, and the storm weakened. Nevertheless, the corresponding lack of convective downdrafts during this period allowed surface heat and moisture fluxes to produce substantial increases in boundary layer entropy. A new burst of convection followed. Consistent with recent papers it is argued that tropical cyclone intensification and decay can be understood as a competition between surface heat and moisture fluxes (“fuel”) and low-entropy downdrafts into the boundary layer (“antifuel”).

scholarly journals Simple Multicloud Models for the Diurnal Cycle of Tropical Precipitation. Part I: Formulation and the Case of the Tropical Oceans

2011 ◽  
Vol 68 (10) ◽  
pp. 2169-2190 ◽  
Author(s):  
Yevgeniy Frenkel ◽  
Boualem Khouider ◽  
Andrew J. Majda
Keyword(s):  
Boundary Layer ◽  
Diurnal Cycle ◽  
Deep Convection ◽  
Potential Temperature ◽  
Bowen Ratio ◽  
Tropical Convection ◽  
Heat Fluxes ◽  
Tropical Precipitation ◽  
Shallow Cumulus ◽  
Early Afternoon

Abstract The variation of tropical precipitation due to the diurnal cycle of solar heating is examined here in the context of two simple models for tropical convection. The models utilize three cloud types—congestus, deep, and stratiform—that are believed to characterize organized tropical convection and are based on the two first baroclinic modes of vertical structure plus a boundary layer mode. The two models differ mainly in the way they treat the boundary layer dynamics. The first one is purely thermodynamical and is reduced to a single equation for the equivalent potential temperature θe connecting the boundary layer to the upper troposphere through downdrafts and to the surface through evaporation while the second uses full bulk boundary layer (FBBL) dynamics with a careful separation between sensible and latent heat fluxes and parameterization of nonprecipitating shallow cumulus. It turns out that in the case of the precipitation over the ocean where the Bowen ratio is small, both models yield a qualitatively similar solution, characterized by an overnight initiation and early morning peak in precipitation consistent with observations. The modeled diurnal cycle of precipitation over the ocean is divided into four cyclic phases: 1) a CAPE (re)generation phase characterized by the enhancement of the boundary layer θe and moisture fluxes during midday and early afternoon that is followed by 2) a (re)moistening phase dominated by congestus heating during the late afternoon and moistening from downdrafts (due to detrainment of shallow cumulus, specifically in the FBBL model) and radiative cooling that lasts until midnight. 3) Deep convection is initiated around midnight when the midtroposphere is sufficiently moist and cool and (re)establishes the precipitation level near its radiative convective equilibrium (1 K day−1) and then 4) peaks with sunrise at 0600 LST to yield a precipitation maximum of roughly 2 K day−1 at around 0900 LST that dries the troposphere and consumes CAPE and closes the cycle.

scholarly journals Ship-Based Observations of the Diurnal Cycle of Southeast Pacific Marine Stratocumulus Clouds and Precipitation

2013 ◽  
Vol 70 (12) ◽  
pp. 3876-3894 ◽  
Author(s):  
Casey D. Burleyson ◽  
Simon P. de Szoeke ◽  
Sandra E. Yuter ◽  
Matt Wilbanks ◽  
W. Alan Brewer
Keyword(s):  
Boundary Layer ◽  
Diurnal Cycle ◽  
Meteorological Data ◽  
Potential Temperature ◽  
Areal Density ◽  
Cell Frequency ◽  
Marine Stratocumulus ◽  
Spatial Continuity ◽  
Stratocumulus Clouds ◽  
Ensemble Effect

Abstract The diurnal cycle of marine stratocumulus in cloud-topped boundary layers is examined using ship-based meteorological data obtained during the 2008 Variability of American Monsoon Systems (VAMOS) Ocean–Cloud–Atmosphere–Land Study Regional Experiment (VOCALS-REx). The high temporal and spatial continuity of the ship data, as well as the 31-day sample size, allows the diurnal transition in degree of coupling of the stratocumulus-topped boundary layer to be resolved. The amplitude of diurnal variation was comparable to the magnitude of longitudinal differences between regions east and west of 80°W for most of the cloud, surface, and precipitation variables examined. The diurnal cycle of precipitation is examined in terms of areal coverage, number of drizzle cells, and estimated rain rate. East of 80°W, the drizzle cell frequency and drizzle area peaks just prior to sunrise. West of 80°W, total drizzle area peaks at 0300 local solar time (LST), 2–3 h before sunrise. Peak drizzle cell frequency is 3 times higher west of 80°W compared to east of 80°W. The waning of drizzle several hours prior to the ramp up of shortwave fluxes may be related to the higher peak drizzle frequencies in the west. The ensemble effect of localized subcloud evaporation of precipitation may make drizzle a self-limiting process where the areal density of drizzle cells is sufficiently high. The daytime reduction in vertical velocity variance in a less coupled boundary layer is accompanied by enhanced stratification of potential temperature and a buildup of moisture near the surface.

scholarly journals Boundary layer nucleation as a source of new CCN in savannah environment

2013 ◽  
Vol 13 (4) ◽  
pp. 1957-1972 ◽  
Author(s):  
L. Laakso ◽  
J. Merikanto ◽  
V. Vakkari ◽  
H. Laakso ◽  
M. Kulmala ◽  
...  
Keyword(s):  
Boundary Layer ◽  
South African ◽  
Biomass Burning ◽  
Particle Formation ◽  
Dry Season ◽  
Aerosol Size Distribution ◽  
Subsequent Growth ◽  
The South ◽  
Wet Season ◽  
Aerosol Model

Abstract. The South African savannah region is a complex environment of air pollution and natural emissions influenced by a strong seasonal cycle in biomass burning and strong precipitation. However, the scarcity of long-term observations means that the knowledge of controlling aerosol processes in this environment is limited. Here we use a recent dataset of 18 months of aerosol size distribution observations trying to understand the annual cycle of cloud condensation nuclei (CCN). Our observations show that the concentration of CCN-sized particles remains, in line with previous studies, high throughout the year with the highest concentrations during the dry winter and the lowest during the wet summer. During the wet season with reduced anthropogenic and biomass burning primary emissions, this pool of CCN is partly filled by boundary layer nucleation with subsequent growth. The enhanced importance of formation and growth during the wet season is addressed to increased biogenic activity together with enhanced free tropospheric removal decreasing the concentration of pre-existing CCN. During the dry season, while frequent new particle formation takes place, particle growth is reduced due to reduced condensing vapour concentrations. Thus in the dry season particles are not able to grow to sizes where they may act as CCN nearly as efficiently as during the wet season. The observations are compared to simulations by a global aerosol model GLOMAP. To our surprise, the global aerosol model utilized to explain the observations was not capable of re-producing the characteristics of particle formation and the annual CCN cycle, despite earlier good performance in predicting the particle concentrations in a number of diverse environments, including the South African savannah region. While the average yearly CCN concentrations of modelled CCN is close to observed concentrations, the characteristics of nucleation bursts and subsequent growth are not captured satisfactory by the model. Our sensitivity tests using different nucleation parameterizations and condensing organic vapour production rates show that neither of these is likely to explain the differences between observed and modelled nucleation and growth rates. A sensitivity study varying 28 modelling parameters indicates that the main uncertainties in the result are due to uncertainties in biomass burning emissions during the dry season, and anthropogenic sulphur emissions during the wet season, both in terms or emitted mass and particle sizes. The uncertainties appear to be mostly related to uncertainties in primary particle emissions, including the emissions variability not captured by monthly emission inventories. The results of this paper also highlights the fact that deficiencies in emissions estimates may result in deficiencies in particle production fluxes, while the end product such as modelled CCN concentration may be in line with observations.

scholarly journals Vertical Profiles of Ozone Concentration Collected by an Unmanned Aerial Vehicle and the Mixing of the Nighttime Boundary Layer over an Amazonian Urban Area

Atmosphere ◽  
2019 ◽  
Vol 10 (10) ◽  
pp. 599 ◽  
Author(s):  
Guimarães ◽  
Ye ◽  
Batista ◽  
Barbosa ◽  
Ribeiro ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Urban Area ◽  
Unmanned Aerial Vehicle ◽  
Ozone Concentration ◽  
Potential Temperature ◽  
Specific Humidity ◽  
Vertical Profiles ◽  
Wet Season ◽  
Aerial Vehicle ◽  
Median Height

The nighttime boundary layer was studied in an urban area surrounded by tropical forest by use of a copter-type unmanned aerial vehicle (UAV) in central Amazonia during the wet season. Fifty-seven vertical profiles of ozone concentration, potential temperature, and specific humidity were collected from surface to 500 m above ground level (a.g.l.) at high vertical and temporal resolutions by use of embedded sensors on the UAV. Abrupt changes in ozone concentration with altitude served as a proxy of nighttime boundary layer (NBL) height for the case of a normal, undisturbed, stratified nighttime atmosphere, corresponding to 40% of the cases. The median height of the boundary layer was 300 m. A turbulent mixing NBL constituted 28% of the profiles, while the median height of the boundary layer was 290 m. The remaining 32% of profiles corresponded to complex atmospheres without clear boundary layer heights. The occurrence of the three different cases correlated well with relative cloud cover. The results show that the standard nighttime model widely implemented in chemical transport models holds just 40% of the time, suggesting new challenges in modeling of regional nighttime chemistry. The boundary layer heights were also somewhat higher than observed previously over forested and pasture areas in Amazonia, indicating the important effect of the urban heat island.

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