Characterization of positive air ions in boreal forest air at the Hyytiälä SMEAR station

International audienceThe behavior of the concentration of positive small (or cluster) air ions and naturally charged nanometer aerosol particles (aerosol ions) has been studied on the basis of measurements carried out in a boreal forest at the Hyytiälä SMEAR station, Finland, during the BIOFOR III campaign in spring 1999. Statistical characteristics of the concentrations of cluster ions, two classes of aerosol ions of the sizes of 2.5?8 nm and 8?ca. 20 nm and the quantities that determine the balance of small ions in the atmosphere have been given for the nucleation event days and non-event days. The dependence of small ion concentration on the ion loss (sink) due to aerosol particles was investigated applying a model of bipolar diffusion charging of particles by small ions. The small ion concentration and the ion sink were closely correlated (correlation coefficient ?87%) when the fog events and the hours of high relative humidity (above 95%), as well as nocturnal calms and weak wind (wind speed ?1) had been excluded. However, an extra ion loss term presumably due to small ion deposition on coniferous forest with a magnitude equal to the average ion loss to pre-existing particles is needed to explain the observations. Also the hygroscopic growth correction of measured aerosol particle size distributions was found to be necessary for proper estimation of the ion sink. In the case of nucleation burst events, variations in the concentration of small positive ions were in accordance with the changes caused by the ion sink due to aerosols; no clear indication of positive ion depletion by ion-induced nucleation was found. The estimated average ionization rate of the air at the Hyytiälä station in early spring, when the ground was partly covered with snow, was about 6 ion pairs cm?3 s?1. The study of the charging state of nanometer aerosol particles (2.5?8 nm) revealed a strong correlation (correlation coefficient 88%) between the concentrations of particles and positively charged particles (positive air ions) during nucleation bursts. The estimated charged fraction of particles, which varied from 3% to 6% considering various nucleation event days, confirms that these particles are almost quasi-steady state charged. Also the particles and air ions in the size range of 8?ca. 20 nm showed a good qualitative consistency; the correlation coefficient was 92%

SMEAR Estonia : Perspectives of a large-scale forest ecosystem – Atmosphere research infrastructure

Non peer reviewe

Nucleation events for the formation of charged aerosol particles at a tropical station - Preliminary results

Simultaneous measurements of atmospheric ions in the mobility range of 3.16-0.00133cm2v-1s-1 (diameter range 0.46-50nm) made with a Neutral Cluster and Air Ion Spectrometer (NAIS) at a tropical station, Pune (18° 31' N, 73° 55' E) over a period of 47days, revealed that nucleation events for the formation of ions occurred on 17days. These events generally occurred between 0800 and 1000LT and enhanced concentrations of Aitken particles continued to be observed up to 1700-1800LT. During these events, particles grew at rates of 3.1±0.8 to 11.2±3.5nmh-1 for different particle size ranges. Three cases of nucleation events, two with large and one with small particle concentrations, are examined and differences in ion characteristics between the different cases are pointed out. Characteristics and contributions of opposite polarity of ions to the increase in total ion concentration have been examined. A unique type of nucleation event in which particles grew to a size of ~41nm at 0800LT diameter and then decreased to a size of 8.64nm diameter, is also reported. In this event, increase in total ion concentration occurred due to the increase of intermediate-, light large-, and heavy large-ions, but the decrease was mainly due to the decrease in only light and heavy large ions. Ratio of the positive to negative ions decreased during the period of this unique event as compared to before and after the event. Sulfur dioxide (SO2) concentration is positively correlated with the concentrations of intermediate, light large and heavy large ions but negatively correlated with the concentration of cluster ions

Variation and balance of positive air ion concentrations in a boreal forest

Air ions are characterized on the basis of measurements carried out in a boreal forest at the Hyytiälä SMEAR station, Finland, during the BIOFOR III campaign in spring 1999. The air ions were discriminated as small ions (charged molecular aggregates of the diameter of less than 2.5 nm), intermediate ions (charged aerosol particles of the diameter of 2.5–8 nm), and large ions (charged aerosol particles of the diameter of 8–20 nm). Statistical characteristics of the ion concentrations and the parameters of ion balance in the atmosphere are presented separately for the nucleation event days and non-event days. In the steady state, the ionization rate is balanced with the loss of small ions, which is expressed as the product of the small ion concentration and the ion sink rate. The widely known sinks of small ions are the recombination with small ions of opposite polarity and attachment to aerosol particles. The dependence of small ion concentration on the concentration of aerosol particles was investigated applying a model of the bipolar diffusion charging of particles by small ions. When the periods of relative humidity above 95% and wind speed less than 0.6 m s<sup>−1</sup> were excluded, then the small ion concentration and the theoretically calculated small ion sink rate were closely negatively correlated (correlation coefficient −87%). However, an extra ion loss term of the same magnitude as the ion loss onto aerosol particles is needed for a quantitative explanation of the observations. This term is presumably due to the small ion deposition on coniferous forest. The hygroscopic growth correction of the measured aerosol particle size distributions was also found to be necessary for the proper estimation of the ion sink rate. In the case of nucleation burst events, the concentration of small positive ions followed the general balance equation, no extra ion loss in addition to the deposition on coniferous forest was detected, and the hypothesis of the conversion of ions into particles in the process of ion-induced nucleation was not proved. The estimated average ionization rate of the air at the Hyytiälä station in early spring, when the ground was partly covered with snow, was about 6 ion pairs cm<sup>−3</sup> s<sup>−1</sup>. The study of the charging state of nanometer aerosol particles (diameter 2.5–8 nm) in the atmosphere revealed a strong correlation (correlation coefficient 88%) between the concentrations of particles neutralized in the aerosol spectrometer and naturally positively charged particles (air ions) during nucleation bursts. The charged fraction of particles varied from 3% to 6% in accordance with the hypothesis that the particles are quasi-steady state charged

Results of the first air ion spectrometer calibration and intercomparison workshop

The Air Ion Spectrometer (AIS) measures mobility and size distributions of atmospheric ions. The Neutral cluster and Air Ion Spectrometer (NAIS) can additionally measure neutral particles. The number of the (N)AIS instruments in the world is only 11. Nevertheless, they are already widely used in atmospheric ion studies, particularly related to the initial steps of new particle formation. There is no standard method applicable for calibrating the ion spectrometers in the sub-3 nm ion range. However, recent development of high resolution DMAs has enabled the size separation of small ions with good mobility resolution. For the first time, the ion spectrometers were intercompared and calibrated in a workshop, held in January–February 2008 in Helsinki, Finland. The overall goal was to experimentally determine the (N)AIS transfer functions. Monomobile mobility standards, 241-Am charger ions and silver particles were generated and used as calibration aerosols. High resolution DMAs were used to size-separate the smaller (1–10 nm) ions, while at bigger diameters (4–40 nm) the size was selected with a HAUKE-type DMA. Negative ion mobilities were detected by (N)AISs with slightly better accuracy than positive, nonetheless, both were somewhat overestimated. A linear fit of slope of one to the whole dataset of mobilities suggested that (N)AISs measured the negative mobilities 1.36±0.16 times larger compared with the reference instruments. Similarly, positive mobilities were measured 1.39±0.15 times larger compared with the reference instruments. The completely monomobile mobility standards were measured with the best accuracy. The (N)AIS concentrations were compared with an aerosol electrometer (AE) and a condensation particle counter (CPC). At sizes below 1.5 nm (positive) and 3 nm (negative) the ion spectrometers detected higher concentrations while at bigger sizes they showed similar concentrations as the reference instruments. The total particle concentrations measured by the NAISs were within ±50% of the reference CPC concentration at 4–40 nm sizes. The lowest cut-off size of the NAIS in neutral particle measurements was determined to be between 1.5 and 3 nm, depending on the measurement conditions and the polarity

Direct effect of aerosols on solar radiation and gross primary production in boreal and hemiboreal forests

The effect of aerosol loading on solar radiation and the subsequent effect on photosynthesis is a relevant question for estimating climate feedback mechanisms. This effect is quantified in the present study using ground-based measurements from five remote sites in boreal and hemiboreal (coniferous and mixed) forests of Eurasia. The diffuse fraction of global radiation associated with the direct effect of aerosols, i.e. excluding the effect of clouds, increases with an increase in the aerosol loading. The increase in the diffuse fraction of global radiation from approximately 0.11 on days characterized by low aerosol loading to 0.2-0.27 on days with relatively high aerosol loading leads to an increase in gross primary production (GPP) between 6% and 14% at all sites. The largest increase in GPP (relative to days with low aerosol loading) is observed for two types of ecosystems: a coniferous forest at high latitudes and a mixed forest at the middle latitudes. For the former ecosystem the change in GPP due to the relatively large increase in the diffuse radiation is compensated for by the moderate increase in the light use efficiency. For the latter ecosystem, the increase in the diffuse radiation is smaller for the same aerosol loading, but the smaller change in GPP due to this relationship between radiation and aerosol loading is compensated for by the higher increase in the light use efficiency. The dependence of GPP on the diffuse fraction of solar radiation has a weakly pronounced maximum related to clouds.Peer reviewe

Global analysis of continental boundary layer new particle formation based on long-term measurements

International audienc