Size-dependent particle activation properties in fog during the ParisFog 2012/13 field campaign

Abstract

Fog-induced visibility reduction is responsible for a variety of hazards in the transport sector. Therefore there is a large demand for an improved understanding of fog formation and thus improved forecasts. Improved fog forecasts require a better understanding of the numerous complex mechanisms during the fog life cycle. During winter 2012/13 a field campaign called ParisFog aiming at fog research took place at SIRTA (Instrumented Site for Atmospheric Remote Sensing Research). SIRTA is located about 20 km southwest of the Paris city center, France, in a semi-urban environment. In situ activation properties of the prevailing fog were investigated by measuring (1) total and interstitial (non-activated) dry particle number size distributions behind two different inlet systems; (2) interstitial hydrated aerosol and fog droplet size distributions at ambient conditions; and (3) cloud condensation nuclei (CCN) number concentration at different supersaturations (SS) with a CCN counter. The aerosol particles were characterized regarding their hygroscopic properties, fog droplet activation behavior and contribution to light scattering for 17 developed fog events. Low particle hygroscopicity with an overall median of the hygroscopicity parameter, κ, of 0.14 was found, likely caused by substantial influence from local traffic and wood burning emissions. Measurements of the aerosol size distribution at ambient RH re-vealed that the critical wet diameter, above which the hydrated aerosols activate to fog droplets, is rather large (with a median value of 2.6 µm) and is highly variable (ranging from 1 to 5 µm) between the different fog events. Thus, the number of activated fog droplets was very small and the nonactivated hydrated particles were found to contribute significantly to the observed light scattering and thus to the reduction in visibility. Combining all experimental data, the effective peak supersaturation, SS peak , a measure of the peak supersaturation during the fog formation, was determined. The median SS peak value was estimated to be in the range from 0.031 to 0.046 % (upper and lower limit estimations), which is in good agreement with previous experimental and modeling studies of fog.

Enhanced extinction of visible radiation due to hydrated aerosols in mist and fog

“…No direct measurements were made of such aerosols in ambient conditions, but one method is to convert available TSI SMPS measurements made in the dry state (e.g. Hammer et al, 2014b). New instrumentation may also provide interesting results (Renard et al, 2015a, b).…”

“…Consistently, Stolaki et al (2014) showed that the number concentration of aerosols included between 200 and 500 nm dry diameter, measured by a TSI SMPS particle counter, was of the same order of magnitude as the hydrated aerosols measured by the WELAS. Hammer et al (2014b) found a median transition diameter of 2.6 µm for the 2012-2013 ParisFog season. We then made computations for our transition diameter and for 2.5 µm, close to results by Hammer et al (2014b).…”

“…Hammer et al (2014b) found a median transition diameter of 2.6 µm for the 2012-2013 ParisFog season. We then made computations for our transition diameter and for 2.5 µm, close to results by Hammer et al (2014b). The hydrated aerosols larger than 2.5 µm were not numerous (35 ± 30 cm −3 , reaching sometimes 100 cm −3 ), but, as shown in Sect.…”

“…The transition over this threshold is difficult to observe because the threshold value depends on various factors (e.g. Hammer et al, 2014b) and uncertainties of RH measurements are usually too large. As stated by Clark et al (2008), visibility can be a more precise measurement of the impact of RH than the proper measurement of RH.…”

Aerosol-fog interaction and the transition to well-mixed radiation fog

“…We note here that we have excluded the lowest (1-2 µm) bin of the cloud-droplet probe from our analysis to avoid potential noise issues with large, dry aerosol affecting the counts. Because of the low supersaturation and therefore high activation diameter, hydrated aerosol particles within fog can grow larger than micrometer in size and can contribute up to 68% of total light-scattering during different fog periods (Hammer et al, 2014;Elias et al, 2015). However, most of these hydrated particles are still too small to considerably affect interaction with long-10 wave radiation after fog has developed.…”