Calibration Uncertainty of 23nm Engine Exhaust Condensation Particle Counters with Soot Generators: a European Automotive Laboratory Comparison

Calibration of condensation particle counters (CPC) to measure non-volatile particle number (PN) from vehicle emissions is a significant source of uncertainty of the regulated particle number measurements. In this work, the calibration uncertainty of automotive and calibration laboratories was determined in a first-of-its-kind comparison. For this purpose, the counting efficiency of a reference CPC for automotive exhaust emission measurements was determined at seven participants across Europe with ten soot aerosol generators. Calibration uncertainty was found to be very different in the CPC’s cut-off regime (around the D50 of 23nm) with a coefficient of variation (CoV) of 11% and the plateau regime (from the D90 of 41nm upwards) with a CoV of 4.5%. The uncertainty was higher for a group of soot generators with poorly optimized operating points with a CoV of 31% at 23nm and 5.8% at ≥41nm. Specific influence factors on the calibration uncertainty (measured as the inter-lab variability) could be identified. The calibration of the laboratories’ reference counters accounted for most of the variability in the plateau regime, while 20% of the variability was attributed to the sample flow measurement. Differences between soot generators were the main cause of variability in the cut-off regime due to the increased material sensitivity of the CPC at this particle size but had only secondary relevance in the plateau regime. The calibration uncertainty found in this inter-laboratory exercise should be a guideline for users and legislators, as it provides a typical value for the expected measurement uncertainty of a CPC for automotive exhaust PN.

Coating material-dependent differences in modelled lidar-measurable quantities for heavily coated soot particles

<p>The depolarisation ratio of heavily coated soot particles was previously found to be sensitive to the chemical composition of the coating material, which is reflected by the refractive index. Employing the Discrete Dipole Approximation code ADDA optical calculations were performed with a set of heavily coating soot aggregates with two different coating materials at 355 nm, 532 nm, and 1064 nm. As coating materials sulphate and a toluene-based material were assumed. The soot aggregates were modelled based on results reported from in-situ field measurements and using a coating model, which allows for a tunable transition between film coating and spherical shell coating. The aggregates’ size was varied by increasing the number of soot monomers inside each aggregate from 26 to 1508 in linearly equidistant steps.</p><p>Size-averaged lidar-measureable quantities for the coated aggregates, such as the linear depolarisation ratio, the extinction-to-backscatter ratio (lidar ratio), and the Ångström exponents of the extinction coefficient, the backscatter coefficient, and the extinction-to-backscatter ratio were calculated, and the error of the simulations was estimated. With the exception of the linear depolarisation ratio at 1064 nm these observables do not overlap within the estimated error bounds. As the coating materials result in clearly distinguishable lidar observables, information on the chemical composition of coated soot aerosol can potentially be inferred from lidar measurements.</p><p> </p><p> </p>

Multiple-scattering correction factor of quartz filters and the effect of filtering particles mixed in water: implications for analyses of light absorption in snow samples

The deposition of light-absorbing aerosol (LAA) onto snow initiates processes that lead to increased snowmelt. Measurements of LAA, such as black carbon (BC) and mineral dust, have been observed globally to darken snow. Several measurement techniques of LAA in snow collect the particulates on filters for analysis. Here we investigate micro-quartz filters' optical response to BC experiments in which the particles are initially suspended in air or in a liquid. With particle soot absorption photometers (PSAPs) we observed a 20 % scattering enhancement for quartz filters compared to the standard PSAP Pallflex filters. The multiple-scattering correction factor (Cref) of the quartz filters for airborne soot aerosol is estimated to be ∼3.4. In the next stage correction factors were determined for BC particles mixed in water and also for BC particles both mixed in water and further treated in an ultrasonic bath. Comparison of BC collected from airborne particles with BC mixed in water filters indicated a higher mass absorption cross section by approximately a factor of 2 for the liquid-based filters, which is probably due to the BC particles penetrating deeper in the filter matrix. The ultrasonic bath increased absorption still further, roughly by a factor of 1.5, compared to only mixing in water. Application of the correction functions to earlier published field data from the Himalaya and Finnish Lapland yielded mass absorption coefficient (MAC) values of ∼7–10 m2 g−1 at λ=550 nm, which is in the range of the published MAC of airborne BC aerosol.

Multiple scattering correction factor of quartz filters and the effect of filtering particles mixed in water: implications to analyses of light-absorption in snow samples

The deposition of light-absorbing aerosols (LAA) onto snow initiates processes that lead to increased snowmelt. Measurements of LAA, such as black carbon (BC) and mineral dust, have been observed globally to darken snow. Several measurement techniques of LAA in snow collects the particulates on filters for analysis. Here we investigate micro-quartz filters optical response to BC experiments where the particles initially are suspended in air or in a liquid. With particle soot absorption photometers (PSAP) we observed a 20 % scattering enhancement for quartz filters compared to the standard PSAP Pallflex filters. The multiple-scattering correction factor (Cref) of the quartz filters for airborne soot aerosol is estimated to ~3.4. In the next stage correction factors were determined for BC particles mixed in water and also for BC particles both mixed in water and further treated in an ultrasonic bath. Comparison of BC collected from airborne particles with BC mixed in water filters indicated approximately a factor of two higher mass absorption cross section for the liquid based filters, probably due to the BC particles penetrating deeper in the filter matrix. The ultrasonic bath increased absorption still further, roughly by a factor of 1.5 compared to only mixing in water. Application of the correction functions to earlier published field data from the Himalaya and Finnish Lapland yielded MAC values of ~7–10 m2 g−1 at λ= 550 nm which is in the range of published MAC of airborne BC aerosol.

On transient climate change at the Cretaceous−Paleogene boundary due to atmospheric soot injections

Climate simulations that consider injection into the atmosphere of 15,000 Tg of soot, the amount estimated to be present at the Cretaceous−Paleogene boundary, produce what might have been one of the largest episodes of transient climate change in Earth history. The observed soot is believed to originate from global wildfires ignited after the impact of a 10-km-diameter asteroid on the Yucatán Peninsula 66 million y ago. Following injection into the atmosphere, the soot is heated by sunlight and lofted to great heights, resulting in a worldwide soot aerosol layer that lasts several years. As a result, little or no sunlight reaches the surface for over a year, such that photosynthesis is impossible and continents and oceans cool by as much as 28 °C and 11 °C, respectively. The absorption of light by the soot heats the upper atmosphere by hundreds of degrees. These high temperatures, together with a massive injection of water, which is a source of odd-hydrogen radicals, destroy the stratospheric ozone layer, such that Earth’s surface receives high doses of UV radiation for about a year once the soot clears, five years after the impact. Temperatures remain above freezing in the oceans, coastal areas, and parts of the Tropics, but photosynthesis is severely inhibited for the first 1 y to 2 y, and freezing temperatures persist at middle latitudes for 3 y to 4 y. Refugia from these effects would have been very limited. The transient climate perturbation ends abruptly as the stratosphere cools and becomes supersaturated, causing rapid dehydration that removes all remaining soot via wet deposition.

A New Ice Nucleation Active Site Parameterization for Desert Dust and Soot

Based on results of 11 yr of heterogeneous ice nucleation experiments at the Aerosol Interaction and Dynamics in the Atmosphere (AIDA) chamber in Karlsruhe, Germany, a new empirical parameterization framework for heterogeneous ice nucleation was developed. The framework currently includes desert dust and soot aerosol and quantifies the ice nucleation efficiency in terms of the ice nucleation active surface site (INAS) approach. The immersion freezing INAS densities nS of all desert dust experiments follow an exponential fit as a function of temperature, well in agreement with an earlier analysis of AIDA experiments. The deposition nucleation nS isolines for desert dust follow u-shaped curves in the ice saturation ratio–temperature (Si–T) diagram at temperatures below about 240 K. The negative slope of these isolines toward lower temperatures may be explained by classical nucleation theory (CNT), whereas the behavior toward higher temperatures may be caused by a pore condensation and freezing mechanism. The deposition nucleation measured for soot at temperatures below about 240 K also follows u-shaped isolines with a shift toward higher Si for soot with higher organic carbon content. For immersion freezing of soot aerosol, only upper limits for nS were determined and used to rescale an existing parameterization line. The new parameterization framework is compared to a CNT-based parameterization and an empirical framework as used in models. The comparison shows large differences in shape and magnitude of the nS isolines especially for deposition nucleation. For the application in models, implementation of this new framework is simple compared to that of other expressions.