Atmospheric aerosols absorb and reflect solar radiation causing surface cooling and heating of the atmosphere. The interaction between aerosols and radiation depends on their complex index of refraction, which is related to the particles' chemical composition. The contribution of light absorbing organic compounds, such as HUmic-LIke Substances (HULIS) to aerosol scattering and absorption is among the largest uncertainties in assessing the direct effect of aerosols on climate. Using a Cavity Ring Down Aerosol Spectrometer (CRD-AS), the complex index of refraction of aerosols containing HULIS extracted from pollution, smoke, and rural continental aerosols, and molecular weight-fractionated fulvic acid was measured at 390 nm and 532 nm. The imaginary part of the refractive index (absorption) substantially increases towards the UV range with increasing molecular weight and aromaticity. At both wavelengths, HULIS extracted from pollution and smoke particles absorb more than HULIS from the rural aerosol. Sensitivity calculations for a pollution-type aerosol containing ammonium sulfate, organic carbon (HULIS), and soot suggests that accounting for absorption by HULIS leads in most cases to a significant decrease in the single scattering albedo and to a significant increase in aerosol radiative forcing efficiency, towards more atmospheric absorption and heating. This indicates that HULIS in biomass smoke and pollution aerosols, in addition to black carbon, can contribute significantly to light absorption in the ultraviolet and visible spectral regions.

The Multiple Chamber Aerosol Chemical Aging Study (MUCHA-CHAS) tested the hypothesis that hydroxyl radical (OH) aging significantly increases the concentration of first-generation biogenic secondary organic aerosol (SOA). OH is the dominant atmospheric oxidant, and MUCHACHAS employed environmental chambers of very different designs, using multiple OH sources to explore a range of chemical conditions and potential sources of systematic error. We isolated the effect of OH aging, confirming our hypothesis while observing corresponding changes in SOA properties. The mass increases are consistent with an existing gap between global SOA sources and those predicted in models, and can be described by a mechanism suitable for implementation in those models.atmospheric chemistry | biosphere-atmosphere interactions O rganic aerosol (OA) comprises a large fraction of fine-particle mass (PM 2.5 ) (1). In the developed world, 1-2% of deaths are blamed on inhalation of PM 2.5 (2), and the leading uncertainty in climate forcing is the interplay between the number of fine particles large enough to nucleate cloud droplets and the amount of sunlight reflected by those clouds (3). Oxidation and condensation of organics play a major but uncertain role in both phenomena.Traditional models treat most OA as nonvolatile primary OA (POA), augmented by secondary OA (SOA) (4), and they underpredict OA concentrations by a factor of 3-10 (5). α-Pinene is a major biogenic SOA source, sometimes used to represent all SOA in global models (4, 6). However, less than 20% of the carbon from fresh α-pinene oxidation condenses in chambers at room temperature; (7) the remainder is gaseous (Fig. 1A). This "chamber" SOA is modestly oxidized, with an oxygen to carbon ratio ðO∶CÞ < 0.4 (7). It is unambiguously semivolatile: Yields rise with increasing SOA mass loading (8, 9) and decreasing temperature (10), and the SOA evaporates upon heating (11-13) and after isothermal dilution (14).In contrast, ambient OA is highly oxidized (0.5 ≤ O∶C ≤ 1.0) (1, 15) and not very volatile (16). Ambient SOA is much less volatile than ambient POA (16). Consequently, "chamber" SOA does not represent the atmosphere. Our hypothesis is that homogeneous gas-phase aging by OH is a major missing process connecting chamber studies to the atmosphere. Considerable attention has been paid to heterogeneous uptake of oxidants to particles (17, 18), and recently gas-phase oxidation of semivolatile primary emissions (19), but the degree to which gas-phase oxidation can age chamber SOA is uncertain (1,4,18,20).OA resides in the atmosphere for about one week (21), while the gas-phase lifetimes of major semivolatile SOA constituents are far shorter. Typical α-pinene products pinonaldehyde, cispinonic acid, and pinic acid all have lifetimes of only a few hours for summertime conditions (22). Without question, oxidation of semivolatile SOA vapors will perturb the equilibrium phase partitioning of these constituents. Because almost all of the first-generation products are less volatile than α...

Abstract. During the measurement campaign FROST (FReezing Of duST), LACIS (Leipzig Aerosol CloudInteraction Simulator) was used to investigate the immersion freezing behavior of size selected, coated and uncoated Arizona Test Dust (ATD) particles with a mobility diameter of 300 nm. Particles were coated with succinic acid (C 4 H 6 O 4 ), sulfuric acid (H 2 SO 4 ) and ammonium sulfate ((NH 4 ) 2 SO 4 ). Ice fractions at mixed-phase cloud temperatures ranging from 233.15 K to 239.15 K (±0.60 K) were determined for all types of particles. In this temperature range, pure ATD particles and those coated with C 4 H 6 O 4 or small amounts of H 2 SO 4 were found to be the most efficient ice nuclei (IN). ATD particles coated with (NH 4 ) 2 SO 4 were the most inefficient IN. Since the supercooled droplets were highly diluted before freezing occurred, a freezing point suppression due to the soluble material on the particles (and therefore in the droplets) cannot explain this observation. Therefore, it is reasonable to assume that the coatings lead to particle surface alterations which cause the differences in the IN abilities. Two different theoretical approaches based on the stochastic and the singular hypotheses were applied to clarify and parameterize the freezing behavior of the particles investigated. Both approaches describe the experimentally determined results, yielding parameters that can subsequently be used to compare our results to those from other studies. HowCorrespondence to: D. Niedermeier (niederm@tropos.de) ever, we cannot clarify at the current state which of the two approaches correctly describes the investigated immersion freezing process. But both approaches confirm the assumption that the coatings lead to particle surface modifications lowering the nucleation efficiency. The stochastic approach interprets the reduction in nucleation rate from coating as primarily due to an increase in the thermodynamic barrier for ice formation (i.e., changes in interfacial free energies). The singular approach interprets the reduction as resulting from a reduced surface density of active sites.

Abstract. The aim of this study was to investigate oxidation of SOA formed from ozonolysis of α-pinene and limonene by hydroxyl radicals. This paper focuses on changes of particle volatility, using a Volatility Tandem DMA (VTDMA) set-up, in order to explain and elucidate the mechanism behind atmospheric ageing of the organic aerosol. The experiments were conducted at the AIDA chamber facility of Karlsruhe Institute of Technology (KIT) in Karlsruhe and at the SAPHIR chamber of Forchungzentrum Jülich (FZJ) in Jülich. A fresh SOA was produced from ozonolysis of α-pinene or limonene and then aged by enhanced OH exposure. As an OH radical source in the AIDA-chamber the ozonolysis of tetramethylethylene (TME) was used while in the SAPHIR-chamber the OH was produced by natural light photochemistry. A general feature is that SOA produced from ozonolysis of α-pinene and limonene initially was rather volatile and becomes less volatile with time in the ozonolysis part of the experiment. Inducing OH chemistry or adding a new portion of precursors made the SOA more volatile due to addition of new semi-volatile material to the aged aerosol. The effect of OH chemistry was less pronounced in high concentration and low temperature experiments when lower relative amounts of semi-volatile material were available in the gas phase. Conclusions drawn from the changes in volatility were confirmed by comparison with the measured and modelled chemical composition of the aerosol phase. Three quantified products from the α-pinene oxidation; pinonic acid, pinic acid and methylbutanetricarboxylic acid (MBTCA) were used to probe the processes influencing aerosol volatility. A major conclusion from the work is thatCorrespondence to: K. Salo (kent.salo@chem.gu.se) the OH induced ageing can be attributed to gas phase oxidation of products produced in the primary SOA formation process and that there was no indication on significant bulk or surface reactions. The presented results, thus, strongly emphasise the importance of gas phase oxidation of semi-or intermediate-volatile organic compounds (SVOC and IVOC) for atmospheric aerosol ageing.

Abstract. The ability of coated mineral dust particles to act as ice nuclei (IN) was investigated at LACIS (Leipzig Aerosol Cloud Interaction Simulator) during the FROST1-and FROST2-campaigns (Freezing of dust). Sulphuric acid was condensed on the particles which afterwards were optionally humidified, treated with ammonia vapour and/or heat. By means of aerosol mass spectrometry we found evidence that processing of mineral dust particles with sulphuric acid leads to surface modifications of the particles. These surface modifications are most likely responsible for the observed reduction of the IN activation of the particles. The observed particle mass spectra suggest that different treatments lead to different chemical reactions on the particle surface. Possible chemical reaction pathways and products are suggested and the implications on the IN efficiency of the treated dust particles are discussed.