Is the ocean surface a source of nitrous acid (HONO) in the marine boundary layer?

Nitrous acid, HONO, is a key net photolytic precursor to OH radicals in the atmospheric boundary layer. As OH is the dominant atmospheric oxidant, driving the removal of many primary pollutants and the formation of secondary species, a quantitative understanding of HONO sources is important to predict atmospheric oxidising capacity. While a number of HONO formation mechanisms have been identified, recent work has ascribed significant importance to the dark, ocean-surface-mediated conversion of NO2 to HONO in the coastal marine boundary layer. In order to evaluate the role of this mechanism, here we analyse measurements of HONO and related species obtained at two contrasting coastal locations – Cabo Verde (Atlantic Ocean, denoted Cape Verde herein), representative of the clean remote tropical marine boundary layer, and Weybourne (United Kingdom), representative of semi-polluted northern European coastal waters. As expected, higher average concentrations of HONO (70 ppt) were observed in marine air for the more anthropogenically influenced Weybourne location compared to Cape Verde (HONO < 5 ppt). At both sites, the approximately constant HONO/NO2 ratio at night pointed to a low importance for the dark, ocean-surface-mediated conversion of NO2 into HONO, whereas the midday maximum in the HONO/NO2 ratios indicated significant contributions from photo-enhanced HONO formation mechanisms (or other sources). We obtained an upper limit to the rate coefficient of dark, ocean-surface HONO-to-NO2 conversion of CHONO = 0.0011 ppb h−1 from the Cape Verde observations; this is a factor of 5 lower than the slowest rate reported previously. These results point to significant geographical variation in the predominant HONO formation mechanisms in marine environments and indicate that caution is required when extrapolating the importance of such mechanisms from individual study locations to assess regional and/or global impacts on oxidising capacity. As a significant fraction of atmospheric processing occurs in the marine boundary layer, particularly in the tropics, better constraint of the possible ocean surface source of HONO is important for a quantitative understanding of chemical processing of primary trace gases in the global atmospheric boundary layer and associated impacts upon air pollution and climate.

Application of local Monte Carlo method in neutronics calculation of EAST radial neutron camera

The Local Monte Carlo (LMC) method is used to solve the problems of deep penetration and long time in the neutronics calculation of the Radial Neutron Camera (RNC) diagnostic system on the Experimental Advanced Superconducting Tokamak (EAST), and the radiation distribution of the RNC and the neutron flux at the detector positions of each channel are obtained. Compared with the results calculated by the Global Variance Reduction (GVR) method, it is shown that the LMC calculation is reliable within the reasonable error range. The calculation process of LMC is analyzed in detail, and the transport process of radiation particles is simulated in two steps. In the first step, an integrated neutronics model considering the complex window environment and a neutron source model based on EAST plasma shape are used to support the calculation. The particle information on the equivalent surface is analyzed to evaluate the rationality of settings of equivalent surface source and boundary. Based on the characteristic that only a local geometric model is needed in the second step, it is shown that the LMC is an advantageous calculation method for the nuclear shielding design of tokamak diagnostic systems.

Effects of point source emission heights in WRF–STILT: a step towards exploiting nocturnal observations in models

An appropriate representation of point source emissions in atmospheric transport models is very challenging. In the Stochastic Time Inverted Lagrangian Transport model (STILT), all point source emissions are typically released from the surface, meaning that the actual emission stack height plus subsequent plume rise is not considered. This can lead to erroneous predictions of trace gas concentrations, especially during nighttime when vertical atmospheric mixing is minimal. In this study we use two WRF–STILT model approaches to simulate fossil fuel CO2 (ffCO2) concentrations: (1) the standard “surface source influence (SSI)” approach, and (2) an alternative “volume source influence (VSI)” approach, where nearby point sources release CO2 according to their effective emission height profiles. The comparison with 14C-based measured ffCO2 data from two-week integrated afternoon and nighttime samples collected at Heidelberg, 30 m above ground level, shows that the root-mean-square deviation (RMSD) between modelled and measured ffCO2 is indeed almost twice as high during night (RMSD = 6.3 ppm) compared to the afternoon (RMSD = 3.7 ppm) when using the standard SSI approach. In contrast, the VSI approach leads to a much better performance at nighttime (RMSD = 3.4 ppm), which is similar to its performance during afternoon (RMSD = 3.7 ppm). Representing nearby point source emissions with the VSI approach could, thus, be a first step towards exploiting nocturnal observations in STILT. To further investigate the differences between these two approaches, we conducted a model experiment in which we simulated the ffCO2 contributions from 12 artificial power plants with typical annual emissions of one million tons of CO2 and with distances between 5 and 200 km from the Heidelberg observation site. We find that such a power plant must be more than 50 km away from the observation site in order for the mean modelled ffCO2 concentration difference between the SSI and VSI approach to fall below 0.1 ppm.

Age Spectra and Other Transport Diagnostics in the North American Monsoon UTLS from SEAC⁴RS In Situ Trace Gas Measurements

The upper troposphere and lower stratosphere (UTLS) during the summer monsoon season over North America (NAM) is influenced by the transport of air from a variety of source regions over a wide range of time scales (hours to years). Age spectra are useful for characterizing the transport into such a region and in this study we use and build on recently developed techniques to infer age spectra from trace gas measurements with photochemical lifetimes from days to centuries. We show that the measurements taken by the Whole Air Sampler instrument during the SEAC4RS campaign can be used to not only derive age spectra, but also path-integrated lifetimes of each of the trace gases and surface source regions. The method used here can also clearly identify and adjust for measurement outliers that were influenced by polluted surface source regions. The results are generally consistent with expected transport features of the NAM but also provide a range of transport diagnostics that have not previously been computed solely from in situ measurements. These methods may be applied to many other existing in situ datasets and the transport diagnostics can be compared with chemistry-climate model transport in the UTLS.

Is the ocean surface a source of nitrous acid (HONO) in the marine boundary layer?

Nitrous acid, HONO, is a key net photolytic precursor to OH radicals in the atmospheric boundary later. As OH is the dominant atmospheric oxidant, driving the removal of many primary pollutants and the formation of secondary species, a quantitative understanding of HONO sources is important to predict atmospheric oxidising capacity. While a number of HONO formation mechanisms have been identified, recent work has ascribed significant importance to the dark, ocean-surface mediated conversion of NO2 to HONO in the coastal marine boundary layer. In order to evaluate the role of this mechanism, here we analyse measurements of HONO and related species obtained at two contrasting coastal locations – Cape Verde (Atlantic Ocean), representative of the clean remote tropical marine boundary layer, and Weybourne (United Kingdom), representative of semi-polluted Northern European coastal waters. As expected, higher average concentrations of HONO (70 ppt) were observed in marine air for the more anthropogenically influenced Weybourne location compared to Cape Verde (HONO < 5 ppt). At both sites, the approximately constant HONO/NO2 ratio at night pointed to a low importance for the dark ocean-surface mediated conversion of NO2 into HONO, whereas the midday maximum in the HONO/NO2 ratios indicated significant contributions from photo-enhanced HONO formation mechanisms (or other sources). We obtained an upper limit to the rate coefficient of dark ocean-surface HONO-to-NO2 conversion of CHONO = 0.0011 ppb hr−1 from the Cape Verde observations; this is a factor of 5 lower than the slowest rate reported previously. These results point to significant geographical variation in the predominant HONO formation mechanisms in marine environments and indicate that caution is required when extrapolating the importance of such mechanisms from individual study locations to assess regional and/or global impacts on oxidising capacity. As a significant fraction of atmospheric processing occurs in the marine boundary layer, particularly in the tropics, better constraint of the possible ocean surface source of HONO is important for a quantitative understanding of chemical processing of primary trace gases in the global atmospheric boundary layer and associated impacts upon air pollution and climate.

An Improved Method for Computing Broadband Green’s Functions of Surface Sources and Its Application to Inverting the Processes of the 2017 Xinmo Landslide

Landslides are dramatic and complex surface processes that can result in extensive casualties and property damage. The broadband seismic signals generated by landslides provide datasets essential for understanding time-dependent sliding processes. However, traditional methods for computing Green’s functions based on wavenumber integration converge very slowly for surface sources, especially at high frequencies. Usually, long-period synthetic waves with a cutoff k-integral for an approximated near-surface source are adopted for landslide studies, which may lead to artifacts. Thus, the development of efficient methods for computing the broadband Green’s functions of surface sources is important. The generalized reflection and transmission method with the peak-trough averaging technique can overcome the difficulties in wavenumber integration for surface sources, quickly converging even for high-frequency calculations. We use this improved method to compute Green’s functions for surface single-force sources and invert the force histories of the 2017 devastating Xinmo landslide in different frequency bands. The results indicate that the complex sliding process of this drastic event can be revealed by broadband signals (0.02–0.5 Hz), and that the initiation stage of this event shows a dominant frequency up to 0.2 Hz.

Stress Analysis Using Direct-S Wavelets Produced by a Vertical Vibrator

We demonstrate how to extract the azimuth of maximum horizontal stress (Shmax) in deep rocks by doing a simple, 360-degree, mathematical rotation of a down-going, direct-S, wavelet generated at the base plate of a surface-based vertical vibrator. We worked with direct-S wavelets that travel through stressed rocks to a deep, horizontal, VSP geophone. We show that the azimuth where a polarity reversal occurs in mathematical rotations of this down-going direct-S wavelet defines the azimuth of Shmax in the rocks between the receiver and the surface source. We tested this direct-S wavelet rotation method for determining Sh-max azimuth at a site in the Illinois Basin using legacy VSP data acquired in 2013. SHmax azimuths indicated by this simple wavelet-rotation method were determined when vertical vibrators were stationed at zero-offset, at far-offset, and at different azimuths around a VSP receiver well. These VSP-based SHmax azimuths agreed with the azimuth of SHmax found by traditional mini-frack tests in the VSP receiver well. This simple VSP procedure seems to not be discussed in geophysical literature. The publication of this technical finding should be of interest to the worldwide geophysical community, especially to those who need to monitor how stress fields shift when fluids are injected into, or extracted from, deep porous reservoirs.