Diurnal evolution of negative atmospheric ions above the boreal forest: From ground level to the free troposphere

At SMEAR II research station in Hyytiälä, located in the Finnish boreal forest, the process of new particle formation and the role of ions has been investigated for almost 20 years near the ground and at canopy level. However, above SMEAR II, the vertical distribution and diurnal variation of these different atmospheric ions are poorly characterized. In this study, we assess the atmospheric ion composition in the stable boundary layer, residual layer, mixing layer and free troposphere, and the 5 evolution of these atmospheric ions due to photochemistry and turbulent mixing through the day. To measure the vertical profile of atmospheric ions, we developed a tailored setup for online mass spectrometric measurements, capable of being deployed in a Cessna 172 with minimal modifications. Simultaneously, instruments dedicated to aerosol properties measured in a second Cessna. We conducted a total of 16 measurement flights in May 2017, during the spring, which is the most active new particle formation season. A flight day typically consisted of three distinct flights through the day (dawn, morning and afternoon) to 10 observe the diurnal variation and at different altitudes (from 100 m to 3200 m above ground), and to capture the boundary layer development from stable boundary layer, residual layer to mixing layer, and the free troposphere. Our observations showed that the ion composition is distinctly different in each layer and depends on the air mass origin and time of the day. Before sunrise, the layers are separated from each other and have their own ion chemistry. We observed that the ions present within the stable layer are of the same composition as the ions measured at the canopy level. During daytime when the mixing layer evolved and the compounds are vertically mixed, we observed that highly oxidised organic molecules are distributed to the top of the boundary layer. The ion composition in the residual layer varies with each day, showing similarities with either the stable boundary layer or the free troposphere. Finally, within the free troposphere, we detected a variety of carboxylic acids and ions that are likely containing halogens, originating from the Arctic Sea.

Variations in Nocturnal Residual Layer Height and Its Effects on Surface PM2.5 over Wuhan, China

Large amounts of aerosols remain in the residual layer (RL) after sunset, which may be the source of the next day’s pollutants. However, the characteristics of the nocturnal residual layer height (RLH) and its effect on urban environment pollution are unknown. In this study, the characteristics of the RLH and its effect on fine particles with diameters <2.5 μm (PM2.5) were investigated using lidar data from January 2017 to December 2019. The results show that the RLH is highest in summer (1.55 ± 0.55 km), followed by spring (1.40 ± 0.58 km) and autumn (1.26 ± 0.47 km), and is lowest in winter (1.11 ± 0.44 km). The effect of surface meteorological factors on the RLH were also studied. The correlation coefficients (R) between the RLH and the temperature, relative humidity, wind speed, and pressure were 0.38, −0.18, 0.15, and −0.36, respectively. The results indicate that the surface meteorological parameters exhibit a slight correlation with the RLH, but the high relative humidity was accompanied by a low RLH and high PM2.5 concentrations. Finally, the influence of the RLH on PM2.5 was discussed under different aerosol-loading periods. The aerosol optical depth (AOD) was employed to represent the total amount of pollutants. The results show that the RLH has an effect on PM2.5 when the AOD is small but has almost no effect on PM2.5 when the AOD is high. In addition, the R between the nighttime mean RLH and the following daytime PM2.5 at low AOD is −0.49, suggesting that the RLH may affect the following daytime surface PM2.5. The results of this study have a guiding significance for understanding the interaction between aerosols and the boundary layer.

Substantiation of optimum relative humidity of slurry when applying the batch-type gravity flow system for its removal

The batch-type gravity flow system for slurry removal is most commonly installed on the new and reconstructed pig farms. The newly designed system is characterized by the presence of plastic sewing pipes under each slurry-collecting canal. Such a system allows for complete automation of slurry removal and, under proper operation, mitigates the negative impact of pig farms on the environment through the lower annual output of slurry. The paper presents the calculation of the main design parameter of the system, such as the depth of slurry pits and the burial depth of longitudinal and transverse slurry collectors. Under the increase of pig manure humidity from 88% to 90%, the height of the residual layer decreases owing to lower manure shear stress in the part, where the manure moisture content increases. Under the relative humidity above 91%, the height of the residual layer in the pit increases rapidly due to the intensive stratification of slurry into fractions. Thus, the minimum height of the residual layer in the pit is achieved at the relative moisture content of 89-91%. Slurry should be accumulated in the pits to the 3/4 or 4/5 of the height (depth) of the pit. The accumulation period must be a multiple of the operating cycles of housing of various age and gender groups of pigs.

The formation and mitigation of nitrate pollution: Comparison between urban and suburban environments

Ambient nitrate has been of increasing concern in PM2.5, while there are still large uncertainties in quantifying the formation of nitrate aerosol. The formation pathways of nitrate aerosol at an urban site and a suburban site in the Pearl River Delta (PRD) are investigated using an observation-constrained box model. Throughout the campaigns, aerosol pollution episodes were constantly accompanied with the increase of nitrate concentrations and fractions at both urban and suburban sites. The simulations demonstrate that chemical reactions in the daytime and at night both contributed significantly to formation of nitrate in the boundary layer at the two sites. However, nighttime reactions predominately occurred aloft in the residual layer at the urban site and downward transport from the residual layer in the morning are important source (53 %) for surface nitrate at the urban site, whereas similar amounts of nitrate were produced in the nocturnal boundary layer and residual layer at the suburban site, which results in little downward transport of nitrate from the residual layer to the ground at the suburban site. We show that nitrate formation was in the volatile organic compounds (VOCs)-limited regime at the urban site, and in the transition regime at the suburban site, identical to the response of ozone at both sites. The reduction of VOCs emissions can be an efficient approach to mitigate nitrate in both urban and suburban areas through influencing hydroxyl radical (OH) and N2O5 production, which will also be beneficial for the synergistic control of regional ozone pollution. The results highlight that the relative importance of nitrate formation pathways can be site-specific, and the quantitative understanding of various pathways of nitrate formation will provide insights for developing nitrate mitigation strategies.

Aerosol particle formation in the upper residual layer

According to current estimates, atmospheric new particle formation (NPF) produces a large fraction of aerosol particles and cloud condensation nuclei in the Earth's atmosphere, which have implications for health and climate. Despite recent advances, atmospheric NPF is still insufficiently understood in the lower troposphere, especially above the mixed layer (ML). This paper presents new results from co-located airborne and ground-based measurements in a boreal forest environment, showing that many NPF events (∼42 %) appear to start in the topmost part of the residual layer (RL). The freshly formed particles may be entrained into the growing mixed layer (ML) where they continue to grow in size, similar to the aerosol particles formed within the ML. The results suggest that in the boreal forest environment, NPF in the upper RL has an important contribution to the aerosol load in the boundary layer (BL).

Guiding Chart for Initial Layer Choice with Nanoimprint Lithography

When nanoimprint serves as a lithography process, it is most attractive for the ability to overcome the typical residual layer remaining without the need for etching. Then, ‘partial cavity filling’ is an efficient strategy to provide a negligible residual layer. However, this strategy requires an adequate choice of the initial layer thickness to work without defects. To promote the application of this strategy we provide a ‘guiding chart’ for initial layer choice. Due to volume conservation of the imprint polymer this guiding chart has to consider the geometric parameters of the stamp, where the polymer fills the cavities only up to a certain height, building a meniscus at its top. Furthermore, defects that may develop during the imprint due to some instability of the polymer within the cavity have to be avoided; with nanoimprint, the main instabilities are caused by van der Waals forces, temperature gradients, and electrostatic fields. Moreover, practical aspects such as a minimum polymer height required for a subsequent etching of the substrate come into play. With periodic stamp structures the guiding chart provided will indicate a window for defect-free processing considering all these limitations. As some of the relevant factors are system-specific, the user has to construct his own guiding chart in praxis, tailor-made to his particular imprint situation. To facilitate this task, all theoretical results required are presented in a graphical form, so that the quantities required can simply be read from these graphs. By means of examples, the implications of the guiding chart with respect to the choice of the initial layer are discussed with typical imprint scenarios, nanoimprint at room temperature, at elevated temperature, and under electrostatic forces. With periodic structures, the guiding chart represents a powerful and straightforward tool to avoid defects in praxis, without in-depth knowledge of the underlying physics.

The Relationship Between Atmospheric Boundary Layer Structure, Brown Haze, and Air Pollution in Auckland, New Zealand

&lt;p&gt;&lt;span&gt;&lt;span&gt;A brown air pollution haze that forms over some international cities during the winter has been found to be associated with negative health outcomes and high surface air pollution levels. Previous research has demonstrated a well-established link between the structure of the atmospheric boundary layer (ABL) and surface air quality; however, the degree to which the structure of the ABL influences for formation of local-&lt;/span&gt;&lt;/span&gt;&lt;span&gt;&lt;span&gt;scale&lt;/span&gt;&lt;/span&gt;&lt;span&gt;&lt;span&gt; brown haze is unknown. Using continuous ceilometer data covering seven consecutive winters, we investigate the influence of the structure of the ABL in relation to surface air pollution and brown haze formation over an urban area of complex coastal terrain in the Southern Hemisphere city of Auckland, New Zealand. Our results suggest the depth and evolution of the ABL has a strong influence on severe brown haze formation. When days with severe brown haze are compared with those when brown haze is expected but not observed (based on favorable meteorology and high surface air pollution levels), days with severe brown haze are found to coincide with significantly shallower daytime convective boundary layers (~ 48% lower), and the nights preceding brown haze formation are found to have significantly shallower nocturnal boundary layers (~ 28% lower). On severe brown haze days the growth rate during the morning transition phase from a nocturnal boundary layer to a convective daytime boundary layer is found to be significantly reduced (70 m h&lt;/span&gt;&lt;/span&gt;&lt;sup&gt;&lt;span&gt;&lt;span&gt;-1&lt;/span&gt;&lt;/span&gt;&lt;/sup&gt;&lt;span&gt;&lt;span&gt;) compared to days on which brown haze is expected but not observed (170 m h&lt;/span&gt;&lt;/span&gt;&lt;sup&gt;&lt;span&gt;&lt;span&gt;-1&lt;/span&gt;&lt;/span&gt;&lt;/sup&gt;&lt;span&gt;&lt;span&gt;). Compared with moderate brown haze, severe brown haze conditions are found to be associated with a significantly higher proportion of days with a distinct residual layer present in the ceilometer profiles, suggesting the entrainment of residual layer pollutants may contribute to the severity of the haze. This study illustrates the complex interaction between the ABL structure, air pollution, and the presence of brown haze, and demonstrates the utility of a ceilometer instrument in understanding and predicting the occurrence of brown haze events. &lt;/span&gt;&lt;/span&gt;&lt;/p&gt;