Liisa Pirjola

Marine aerosol formation from biogenic iodine emissions

The formation of marine aerosols and cloud condensation nuclei—from which marine clouds originate—depends ultimately on the availability of new, nanometre-scale particles in the marine boundary layer. Because marine aerosols and clouds scatter incoming radiation and contribute a cooling effect to the Earths radiation budget, new particle production is important in climate regulation. It has been suggested that sulphuric acid—derived from the oxidation of dimethyl sulphide—is responsible for the production of marine aerosols and cloud condensation nuclei. It was accordingly proposed that algae producing dimethyl sulphide play a role in climate regulation, but this has been difficult to prove and, consequently, the processes controlling marine particle formation remains largely undetermined. Here, using smog chamber experiments under coastal atmospheric conditions, we demonstrate that new particles can form from condensable iodine-containing vapours, which are the photolysis products of biogenic iodocarbons emitted from marine algae. Moreover, we illustrate, using aerosol formation models, that concentrations of condensable iodine-containing vapours over the open ocean are sufficient to influence marine particle formation. We suggest therefore that marine iodocarbon emissions have a potentially significant effect on global radiative forcing.

Stable sulphate clusters as a source of new atmospheric particles

The formation of new atmospheric particles with diameters of 3–10 nm has been observed at a variety of altitudes and locations. Such aerosol particles have the potential to grow into cloud condensation nuclei, thus affecting cloud formation as well as the global radiation budget. In some cases, the observed formation rates of new particles have been adequately explained by binary nucleation, involving water and sulphuric acid, but in certain locations—particularly those within the marine boundary layer and at continental sites—observed ambient nucleation rates exceed those predicted by the binary scheme. In these locations, ambient sulphuric acid (H2SO4) levels are typically lower than required for binary nucleation, but are sufficient for ternary nucleation (sulphuric acid–ammonia–water). Here we present results from an aerosol dynamics model with a ternary nucleation scheme which indicate that nucleation in the troposphere should be ubiquitous, and yield a reservoir of thermodynamically stable clusters 1–3 nm in size. We suggest that the growth of these clusters to a detectable size (> 3 nm particle diameter) is restricted by the availability of condensable vapour. Observations of atmospheric particle formation and growth from a continental and a coastal site support this hypothesis, indicating that a growth process including ternary nucleation is likely to be responsible for the formation of cloud condensation nuclei.

On the formation, growth and composition of nucleation mode particles

Taking advantage of only the measured aerosol particles spectral evolution as a function of time, a new analytical tool is developed to derive formation and growth properties of nucleation mode aerosols. This method, when used with hygroscopic growth-factors, can also estimate basic composition properties of these recently-formed particles. From size spectra the diameter growth-rate can be obtained, and aerosol condensation and coagulation sinks can be calculated. Using this growth-rate and condensation sink, the concentration of condensable vapours and their source rate can be estimated. Then, combining the coagulation sink together with measured number concentrations and apparent source rates of 3 nm particles, 1 nm particle nucleation rates and concentration can be estimated. To estimate nucleation rates and vapour concentration source rates producing new particle bursts over the Boreal forest regions, three cases from the BIOFOR project were examined using this analytical tool. In this environment, the nucleation mode growth-rate was observed to be 2–3 nm hour−1, which required a condensable vapour concentration of 2.5–4×107 cm−3 and a source rate of approximately 7.5–11×104 cm−3 s−1 to be sustained. The formation rate of 3 nm particles was =1 particle cm−3 s−1 in all three cases. The estimated formation rate of 1 nm particles was 10–100 particles cm−3 s−1, while their concentration was estimated to be between 10,000 and 100,000 particles cm−3. Using hygroscopicity data and mass flux expressions, the mass flux of insoluble vapour is estimated to be of the same order of magnitude as that of soluble vapour, with a soluble to insoluble vapour flux ratio ranging from 0.7 to 1.4 during these nucleation events.

Overview of the international project on biogenic aerosol formation in the boreal forest (BIOFOR)

Aerosol formation and subsequent particle growth in ambient air have been frequently observed at a boreal forest site (SMEAR II station) in Southern Finland. The EU funded project BIOFOR (Biogenic aerosol formation in the boreal forest) has focused on: (a) determination of formation mechanisms of aerosol particles in the boreal forest site; (b) verification of emissions of secondary organic aerosols from the boreal forest site; and (c) quantification of the amount of condensable vapours produced in photochemical reactions of biogenic volatile organic compounds (BVOC) leading to aerosol formation. The approach of the project was to combine the continuous measurements with a number of intensive field studies. These field studies were organised in three periods, two of which were during the most intense particle production season and one during a non-event season. Although the exact formation route for 3 nm particles remains unclear, the results can be summarised as follows: Nucleation was always connected to Arctic or Polar air advecting over the site, giving conditions for a stable nocturnal boundary layer followed by a rapid formation and growth of a turbulent convective mixed layer closely followed by formation of new particles. The nucleation seems to occur in the mixed layer or entrainment zone. However two more prerequisites seem to be necessary. A certain threshold of high enough sulphuric acid and ammonia concentrations is probably needed as the number of newly formed particles was correlated with the product of the sulphuric acid production and the ammonia concentrations. No such correlation was found with the oxidation products of terpenes. The condensation sink, i.e., effective particle area, is probably of importance as no nucleation was observed at high values of the condensation sink. From measurement of the hygroscopic properties of the nucleation particles it was found that inorganic compounds and hygroscopic organic compounds contributed both to the particle growth during daytime while at night time organic compounds dominated. Emissions rates for several gaseous compounds was determined. Using four independent ways to estimate the amount of the condensable vapour needed for observed growth of aerosol particles we get an estimate of 2–10×107 vapour molecules cm−3. The estimations for source rate give 7.5–11×104 cm−3 s−1. These results lead to the following conclusions: The most probable formation mechanism is ternary nucleation (water-sulphuric acid-ammonia). After nucleation, growth into observable sizes (~3 nm) is required before new particles appear. The major part of this growth is probably due to condensation of organic vapours. However, there is lack of direct proof of this phenomenon because the composition of 1–5 nm size particles is extremely difficult to determine using the present state-of-art instrumentation.

Parameterizations for sulfuric acid/water nucleation rates

We present parameterized equations for calculation of sulfuric acid/water critical nucleus compositions and homogeneous nucleation rates. The parameterizations are in agreement with the thermodynamically consistent version of classical binary homogeneous nucleation theory [Wilemski, 1984] incorporating the hydration effect. The new parameterizations produce nucleation rates that differ by several orders of magnitude from the rates predicted by other parameterizations available in the literature. Model simulations of atmospheric aerosol formation show that the use of the new parameterizations may in some cases result in simulated nucleation mode particle number densities that are by a factor of 1000 lower than those obtained using the old parameterizations.

On the photochemical production of new particles in the coastal boundary layer

Concurrent measurements of ultra-fine (r<5 nm) particle (UFP) formation, OH and SO2 concentrations in the coastal environment are examined to further elucidate the processes leading to tidal-related homogeneous heteromolecular nucleation. During almost daily nucleation events, UFP concentration approached ≈300,000 cm−3 under conditions of solar radiation and low tide. Simultaneous measurements of OH illustrate that, as well as occurring during low tide, these events occur during conditions of peak OH concentration, suggesting that at least one of the nucleating species is photochemically produced. Derived H2SO4 production also exhibited remarkable coherence, although phase-lagged, with UFP formation, thus suggesting its involvement, although binary nucleation of H2SO4 and H2O can be ruled out as a plausible mechanism. Ternary nucleation involving NH3 seems most likely as a trigger mechanism, however, at least a fourth condensable species, X, is required for growth to detectable sizes. Since UFP are only observed during low tide events, it is thought that species X, or its parent, is emitted from the shore biota - without which, no nucleation is detected. Species X remains to be identified. Model simulations indicate that, in order to reproduce the observations, a nucleation rate of 107 cm−3 s−1, and a condensable vapour concentration of 5 × 107 cm−3, are required.

A dedicated study of new particle formation and fate in the coastal environment (PARFORCE): overview of objectives and achievements

A dedicated study into the formation of new particles, New Particle Formation and Fate in the Coastal Environment (PARFORCE), was conducted over a period from 1998 to 1999 at the Mace Head Atmospheric Research Station on the western coast of Ireland. Continuous measurements of new particle formation were taken over the 2-year period while two intensive field campaigns were also conducted, one in September 1998 and the other in June 1999. New particle events were observed on ∼90% of days and occurred throughout the year and in all air mass types. These events lasted for, typically, a few hours, with some events lasting more than 8 hours, and occurred during daylight hours coinciding with the occurrence of low tide and exposed shorelines. During these events, peak aerosol concentrations often exceeded 106 cm−3 under clean air conditions, while measured formation rates of detectable particle sizes (i.e., d > 3 nm) were of the order of 104–105 cm−3 s−1. Nucleation rates of new particles were estimated to be, at least, of the order of 105–106 cm−3 s−1 and occurred for sulphuric acid concentrations above 2 × 106 molecules cm−3; however, no correlation existed between peak sulphuric acid concentrations, low tide occurrence, or nucleation events. Ternary nucleation theory of the H2SO4-H2O-NH3 system predicts that nucleation rates far in excess of 106 cm−3 s−1 can readily occur for the given sulphuric acid concentrations; however, aerosol growth modeling studies predict that there is insufficient sulphuric acid to grow new particles (of ∼1 nm in size) into detectable sizes of 3 nm. Hygroscopic growth factor analysis of recently formed 8-nm particles illustrate that these particles must comprise some species significantly less soluble than sulphate aerosol. The nucleation-mode hygroscopic data, combined with the lack of detectable VOC emissions from coastal biota, the strong emission of biogenic halocarbon species, and the fingerprinting of iodine in recently formed (7 nm) particles suggest that the most likely species resulting in the growth of new particles to detectable sizes is an iodine oxide as suggested by previous laboratory experiments. It remains an open question whether nucleation is driven by self nucleation of iodine species, a halocarbon derivative, or whether first, stable clusters are formed through ternary nucleation of sulphuric acid, ammonia, and water vapor, followed by condensation growth into detectable sizes by condensation of iodine species. Airborne measurements confirm that nucleation occurs all along the coastline and that the coastal biogenic aerosol plume can extend many hundreds of kilometers away from the source. During the evolution of the coastal plume, particle growth is observed up to radiatively active sizes of 100 nm. Modeling studies of the yield of cloud-condensation nuclei suggest that the cloud condensation nuclei population can increase by ∼100%. Given that the production of new particles from coastal biogenic sources occurs at least all along the western coast of Europe, and possibly many other coastlines, it is suggested that coastal aerosols contribute significantly to the natural background aerosol population.

FORMATION OF SULPHURIC ACID AEROSOLS AND CLOUD CONDENSATION NUCLEI: AN EXPRESSION FOR SIGNIFICANT NUCLEATION AND MODEL COMPRARISON

Abstract A new analytical expression has been derived to predict atmospheric conditions where homogeneous water–sulphuric acid nucleation will have a significant effect on aerosol and cloud condensation nuclei population. In the expression, the condensational sink due to pre-existing aerosol particles and source due to chemical production of sulphuric acid have been taken into account. The analytical expression has been derived using a sectional aerosol dynamic model including nucleation, condensation, coagulation, deposition and sulphuric acid formation in the gas phase. In the present study we have also compared the sectional model with modal and monodisperse models. All models may be used for predicting the onset of significant new particle formation. However, the computationally more efficient models—monodisperse, modal, and sectional with low number of sections—over- or underpredict particle formation in some situations.

How significantly does coagulational scavenging limit atmospheric particle production

A model study was performed to investigate the initial fate of nanometer-size nuclei formed in the atmosphere by homogeneous nucleation. Two important results were obtained: (1) coagulational scavenging into preexisting aerosol particles is an extremely strong sink for nuclei formed recently in the atmosphere, and (2) the growth of fresh nuclei to detectable sizes and above requires in most cases the presence of nonvolatile condensable vapors other than H2SO4(g). These vapors are probably secondary organics, and their total required concentration level ranges from <107 up to 108 molecules cm−3 depending on the preexisting particle population. The fact that newly formed particles are scavenged significantly or even totally before reaching detectable sizes has important consequences when interpreting atmospheric measurements. First, atmospheric nucleation events are probably much more frequent than what has been believed based on earlier observations, and second, the nucleation rates derived from atmospheric observations are likely to be substantially lower than the rates at which new particles are really formed by nucleation. Since newly formed nuclei will be scavenged away unless they grow sufficiently fast, there must be some lower limit for the nuclei growth rate below which the nucleation event will not be recorded using the current measurement techniques. The mimimum growth rate was estimated to be below 1 nm h−1 in “clean” environments and of the order of several nm h−1 in “polluted” environments. This requirement may explain why observed atmospheric particle formation events are accompanied frequently by the growth of new particles to sizes typical of the Aitken mode.

Can New Particle Formation Occur in the Clean Marine Boundary Layer

An analysis of new particle formation probability in the marine boundary layer (MBL) is conducted using a detailed aerosol dynamics and gas-phase chemistry model, thermodynamically correct classical binary (H2O-H2SO4) nucleation theory, and recently developed ternary (H2O-H2SO4-NH3) nucleation theory. Additionally, the effect of boundary-layer meteorology (i.e., adiabatic cooling, small scale fluctuations, and entrainment) in enhancing nucleation is also examined. The results indicate that for typical marine conditions, binary nucleation does not occur for any realistic conditions regardless of adiabatic cooling, turbulent fluctuations, or entrainment. For polar marine conditions, binary nucleation does occur due to lower temperatures, and is enhanced due to turbulent fluctuations. An increase in detectable particle sizes (N3>3 nm), is only seen after multiple boundary layer circulations for conditions of high dimethyl sulphide (DMS) concentrations (400 ppt). Under extreme conditions of entrainment of free-troposphere layers containing very low aerosol condensation sinks and extraordinary high sulphuric acid concentrations (>108 molecules cm−3), increases in detectable particles up to 10,000 cm−3 are predicted only in polar marine air, but are viewed as unlikely to occur in reality. Comparison of model simulations with observed values of DMS and sulphuric acid in polar marine air masses suggest that binary nucleation may lead to an enhancement of ≈ 1000 cm−3 in N3 particle concentration, but not to enhancements of ≈ 10,000 cm−3. Ternary nucleation is predicted to occur under realistic sulphuric acid (1.2×107 molecules cm−3) and ammonia (>5 ppt) concentrations; however, significant growth to detectable sizes (N3) only occurs for DMS concentrations of the order of 400 ppt and very low aerosol condensation sinks, but these conditions are thought to be very infrequent in the MBL and are unlikely to make a significant contribution to the general MBL aerosol concentration. It is plausible that the background MBL aerosol concentration could be maintained by a slow, almost undetectable production rate, and not by noticeable nucleation events where large enhancements in N3 concentrations are observed. The former requires sustained DMS concentrations of the order of 100 ppt which seems unlikely. In summary, the occurrence of new particles in the unperturbed MBL would be difficult to explain by DMS emissions alone. DMS emissions can explain the occurrence of thermodynamically stable sulphate clusters, but under most conditions, to grow these clusters to detectable sizes before they are scavenged by coagulation, an additional condensable species other than DMS-derived sulphuric acid would be required. In the event, however, of significant removal of the preexisting aerosol due to precipitation, the MBL aerosol can be replenished through growth of new particle formed through ternary nucleation under moderately high DMS concentrations.