Cn to ccn relationships and cloud microphysical properties in different air masses at a free tropospheric site

International audienceThe fraction of aerosol particles activated to droplets (CCN) is often derived from semi-empirical relationships that commonly tend to overestimate droplet number concentration leading to major uncertainties in global climate models. One of the difficulties in relating aerosol concentration to cloud microphysics and cloud albedo lies in the necessity of working at a constant liquid water path (LWP), which is very difficult to control. In this study we observed the relationships between aerosol number concentration (NCN), cloud droplet concentration (Nd) and effective radius (Reff), at the Puy de Dôme (France). A total of 20 cloud events were sampled representing a period of more than 250 h of cloud sampling. Samples are classified first according to air mass origins (Modified Marine, Continental and Polluted) and then according to their liquid water content (Thin, Medium and Thick clouds). The CCN fraction of aerosols appears to vary significantly according to the air mass origin. It is maximum for Continental air masses and minimum for Polluted air masses. Surprisingly, the CCN fraction of Modified Marine air masses fraction is lower than the continental air mass and from expected from previous studies. The limited number of activated particles in Modified Marine air masses is most likely the result of the presence of hydrophobic organic compounds. The limited activation effect leads to a 0.5 to 1 µm increase in Reff with respect to an ideal Marine case. This is significant and implies that the dReff/dNCN of low-continental clouds is higher than expected

Cloud chemistry at the Puy de Dôme: variability and relationships with environmental factors

The chemical composition of cloud water was investigated during the winter-spring months of 2001 and 2002 at the Puy de D&#244;me station (1465 m above sea level, 45&deg;46&prime;22&prime;&prime; N, 2&deg;57&prime;43&prime;&prime; E) in an effort to characterize clouds in the continental free troposphere. Cloud droplets were sampled with single-stage cloud collectors (cut-off diameter approximately 7 &micro;m) and analyzed for inorganic and organic ions, as well as total dissolved organic carbon. Results show a very large variability in chemical composition and total solute concentration of cloud droplets, ranging from a few mg l^-1 to more than 150 mg l^-1. Samplings can be classified in three different categories with respect to their total ionic content and relative chemical composition: background continental (BG, total solute content lower than 18 mg l^-1), anthropogenic continental (ANT, total solute content from 18 to 50 mg l^-1), and special events (SpE, total solute content higher than 50 mg l^-1). The relative chemical composition shows an increase in anthropogenic-derived species (NO₃^-, SO₄^2- and NH₄⁺) from BG to SpE, and a decrease in dissolved organic compounds (ionic and non-ionic) that are associated with the anthropogenic character of air masses. <P style='line-height: 20px;'> We observed a high contribution of solute in cloud water derived from the dissolution of gas phase species in all cloud events. This was evident from large solute fractions of nitrate, ammonium and mono-carboxylic acids in cloud water, relative to their abundance in the aerosol phase. The comparison between droplet and aerosol composition clearly shows the limited ability of organic aerosols to act as cloud condensation nuclei. The strong contribution of gas-phase species limits the establishment of direct relationships between cloud water solute concentration and LWC that are expected from nucleation scavenging

Parameterization of ion-induced nucleation rates based on ambient observations

Atmospheric ions participate in the formation of new atmospheric aerosol particles, yet their exact role in this process has remained unclear. Here we derive a new simple parameterization for ion-induced nucleation or, more precisely, for the formation rate of charged 2-nm particles. The parameterization is semi-empirical in the sense that it is based on comprehensive results of one-year-long atmospheric cluster and particle measurements in the size range ~1–42 nm within the EUCAARI (European Integrated project on Aerosol Cloud Climate and Air Quality interactions) project. Data from 12 field sites across Europe measured with different types of air ion and cluster mobility spectrometers were used in our analysis, with more in-depth analysis made using data from four stations with concomitant sulphuric acid measurements. The parameterization is given in two slightly different forms: a more accurate one that requires information on sulfuric acid and nucleating organic vapor concentrations, and a simpler one in which this information is replaced with the global radiation intensity. These new parameterizations are applicable to all large-scale atmospheric models containing size-resolved aerosol microphysics, and a scheme to calculate concentrations of sulphuric acid, condensing organic vapours and cluster ions

In-cloud processes of methacrolein under simulated conditions – Part 3: Hygroscopic and volatility properties of the formed secondary organic aerosol

The hygroscopic and volatility properties of secondary organic aerosol (SOA) produced from the nebulization of solutions after aqueous phase photooxidation of methacrolein was experimentally studied in a laboratory, using a Volatility-Hygroscopicity Tandem DMA (VHTDMA). The obtained SOA were 80% 100&amp;deg;C-volatile after 5 h of reaction and only 20% 100&amp;deg;C-volatile after 22 h of reaction. The Hygroscopic Growth Factor (HGF) of the SOA produced from the nebulization of solutions after aqueous-phase photooxidation of methacrolein is 1.34–1.43, which is significantly higher than the HGF of SOA formed by gas-phase photooxidation of terpenes, usually found almost hydrophobic. These hygroscopic properties were confirmed for SOA formed by the nebulization of the same solutions where NaCl was added. The hygroscopic properties of the cloud droplet residuals decrease with the reaction time, in parallel with the formation of more refractory compounds. This decrease was mainly attributed to the 250&amp;deg;C-refractive fraction (presumably representative of the highest molecular weight compounds), which evolved from moderately hygroscopic (HGF of 1.52) to less hygroscopic (HGF of 1.36). Oligomerization is suggested as a process responsible for the decrease of both volatility and hygroscopicity with time. The NaCl seeded experiments enabled us to show that 19&amp;plusmn;4 mg L&lt;sup&gt;&amp;minus;1&lt;/sup&gt; of SOA was produced after 9.5 h of reaction and 41&amp;plusmn;9 mg L&lt;sup&gt;&amp;minus;1&lt;/sup&gt; after 22 h of in-cloud reaction. Because more and more SOA is formed as the reaction time increases, our results show that the reaction products formed during the aqueous-phase OH-oxidation of methacrolein may play a major role in the properties of residual particles upon the droplet&apos;s evaporation. Therefore, the specific physical properties of SOA produced during cloud processes should be taken into account for a global estimation of SOA and their atmospheric impacts

Characterization of three new condensation particle counters for sub-3 nm particle detection during the Helsinki CPC workshop : the ADI versatile water CPC, TSI 3777 nano enhancer and boosted TSI 3010

In this study we characterized the performance of three new particle counters able to detect particles smaller than 3 nm during the Helsinki condensation particle counter (CPC) workshop in summer 2016: the Aerosol Dynamics Inc. (ADI; Berkeley, USA) versatile water condensation particle counter (vWCPC), TSI 3777 nano enhancer (TSI Inc., Shoreview, USA) and modified and boosted TSI 3010-type CPC from Universite Blaise Pascal called a B3010. The performance of all CPCs was first measured with charged tungsten oxide test particles at temperature settings which resulted in supersaturation low enough to not detect any ions produced by a radioactive source. Due to similar measured detection efficiencies, additional comparison between the 3777 and vWCPC were conducted using electrically neutral tungsten oxide test particles and with positively charged tetradodecylammonium bromide. Furthermore, the detection efficiencies of the 3777 and vWCPC were measured with boosted temperature settings yielding supersaturation which was at the onset of homogeneous nucleation for the 3777 or confined within the range of liquid water for the ADI vWCPC. Finally, CPC-specific tests were conducted to probe the response of the 3777 to various inlet flow relative humidities, of the B3010 to various inlet flow rates and of the vWCPC to various particle concentrations. For the 3777 and vWCPC the measured 50% detection diameters (d50s) were in the range of 1.3-2.4 nm for the tungsten oxide particles, depending on the particle charging state and CPC temperature settings, between 2.5 and 3.3 nm for the organic test aerosol, and in the range of 3.2-3.4 nm for tungsten oxide for the B3010.Peer reviewe

In-cloud processes of methacrolein under simulated conditions – Part 2: Formation of secondary organic aerosol

The fate of methacrolein in cloud evapo-condensation cycles was experimentally investigated. To this end, aqueous-phase reactions of methacrolein with OH radicals were performed (as described in Liu et al., 2009), and the obtained solutions were then nebulized and dried into a mixing chamber. ESI-MS and ESI-MS/MS analyses of the aqueous phase composition denoted the formation of high molecular weight multifunctional products containing hydroxyl, carbonyl and carboxylic acid moieties. The time profiles of these products suggest that their formation can imply radical pathways. These high molecular weight organic products are certainly responsible for the formation of secondary organic aerosol (SOA) observed during the nebulization experiments. The size, number and mass concentration of these particles increased significantly with the reaction time: after 22 h of reaction, the aerosol mass concentration was about three orders of magnitude higher than the initial aerosol quantity. The evaluated SOA yield ranged from 2 to 12%. These yields were confirmed by another estimation method based on the hygroscopic and volatility properties of the obtained SOA measured and reported by Michaud et al. (2009). These results provide, for the first time to our knowledge, strong experimental evidence that cloud processes can act, through photooxidation reactions, as important contributors to secondary organic aerosol formation in the troposphere

Aerosol mass and black carbon concentrations, a two year record at NCO-P (5079 m, Southern Himalayas)

Aerosol mass and the absorbing fraction are important variables, needed to constrain the role of atmospheric particles in the Earth radiation budget, both directly and indirectly through CCN activation. In particular, their monitoring in remote areas and mountain sites is essential for determining source regions, elucidating the mechanisms of long range transport of anthropogenic pollutants, and validating regional and global models. Since March 2006, aerosol mass and black carbon concentration have been monitored at the Nepal Climate Observatory-Pyramid, a permanent high-altitude research station located in the Khumbu valley at 5079 m a.s.l. below Mt. Everest. The first two-year averages of PM&lt;sub&gt;1&lt;/sub&gt; and PM&lt;sub&gt;1−10&lt;/sub&gt; mass were 1.94 μg m&lt;sup&gt;−3&lt;/sup&gt; and 1.88 μg m&lt;sup&gt;−3&lt;/sup&gt;, with standard deviations of 3.90 μg m&lt;sup&gt;−3&lt;/sup&gt; and 4.45 μg m&lt;sup&gt;−3&lt;/sup&gt;, respectively, while the black carbon concentration average is 160.5 ng m&lt;sup&gt;−3&lt;/sup&gt;, with a standard deviation of 296.1 ng m&lt;sup&gt;−3&lt;/sup&gt;. Both aerosol mass and black carbon show well defined annual cycles, with a maximum during the pre-monsoon season and a minimum during the monsoon. They also display a typical diurnal cycle during all the seasons, with the lowest particle concentration recorded during the night, and a considerable increase during the afternoon, revealing the major role played by thermal winds in influencing the behaviour of atmospheric compounds over the high Himalayas. The aerosol concentration is subject to high variability: in fact, as well as frequent "background conditions" (55% of the time) when BC concentrations are mainly below 100 ng m&lt;sup&gt;−3&lt;/sup&gt;, concentrations up to 5 μg m&lt;sup&gt;−3&lt;/sup&gt; are reached during some episodes (a few days every year) in the pre-monsoon seasons. The variability of PM and BC is the result of both short-term changes due to thermal wind development in the valley, and long-range transport/synoptic circulation. At NCO-P, higher concentrations of PM&lt;sub&gt;1&lt;/sub&gt; and BC are mostly associated with regional circulation and westerly air masses from the Middle East, while the strongest contributions of mineral dust arrive from the Middle East and regional circulation, with a special contribution from North Africa and South-West Arabian Peninsula in post-monsoon and winter season

Seasonal variations in aerosol particle composition at the puy-de-Dôme research station in France

Detailed investigations of the chemical and microphysical properties of atmospheric aerosol particles were performed at the puy-de-Dôme (pdD) research station (1465 m) in autumn (September and October 2008), winter (February and March 2009), and summer (June 2010) using a compact Time-of-Flight Aerosol Mass Spectrometer (cToF-AMS). Over the three campaigns, the average mass concentrations of the non-refractory submicron particles ranged from 10 μg m&lt;sup&gt;&amp;minus;3&lt;/sup&gt; up to 27 μg m&lt;sup&gt;&amp;minus;3&lt;/sup&gt;. Highest nitrate and ammonium mass concentrations were measured during the winter and during periods when marine modified airmasses were arriving at the site, whereas highest concentrations of organic particles were measured during the summer and during periods when continental airmasses arrived at the site. The measurements reported in this paper show that atmospheric particle composition is strongly influenced by both the season and the origin of the airmass. The total organic mass spectra were analysed using positive matrix factorisation to separate individual organic components contributing to the overall organic particle mass concentrations. These organic components include a low volatility oxygenated organic aerosol particle (LV-OOA) and a semi-volatile organic aerosol particle (SV-OOA). Correlations of the LV-OOA components with fragments of &lt;i&gt;m/z&lt;/i&gt; 60 and &lt;i&gt;m/z&lt;/i&gt; 73 (mass spectral markers of wood burning) during the winter campaign suggest that wintertime LV-OOA are related to aged biomass burning emissions, whereas organic aerosol particles measured during the summer are likely linked to biogenic sources. Equivalent potential temperature calculations, gas-phase, and LIDAR measurements define whether the research site is in the planetary boundary layer (PBL) or in the free troposphere (FT)/residual layer (RL). We observe that SV-OOA and nitrate particles are associated with air masses arriving from the PBL where as particle composition measured from RL/FT airmasses contain high mass fractions of sulphate and LV-OOA. This study provides unique insights into the effects of season and airmass variability on regional aerosol particles measured at an elevated site

Chemical precursors of new particle formation in coastal New Zealand

To reduce uncertainties in climate predictions, we need to better understand aerosol formation in different environments. An important part of this is studying which chemical species are responsible for particle formation. While many advances have been made in this field, measurements are especially lacking from marine environments. Here, we measured the chemical composition of ambient anions over 7 months at Baring Head station, located in coastal New Zealand. This adds to our previous work which reported the aerosol size distribution measurements and investigated new particle formation and environmental conditions favouring new particle formation at the station. By combining the information on ion chemical composition with our previous work, we were able to study the chemical precursors of new particle formation. Our results showed that while over land new particle formation is likely driven by sulfuric acid and organic species, in clean marine air iodine oxoacids and sulfur species are likely important drivers of particle formation processes. These data were also used to characterise the diurnal and seasonal cycles of the most important anion groups and their geographical source regions. Bisulfate anions displayed a clear daytime maximum where as iodine oxoacids had morning and evening maximums. Highly oxygenated organic molecules, on the other hand, were most abundant during the night when the air was land-influenced. This data set is highly valuable and our results provide important information on the chemical species driving new particle formation at a remote Southern Hemisphere coastal site.</p