Michael Geever

Coastal new particle formation: Environmental conditions and aerosol physicochemical characteristics during nucleation bursts

Nucleation mode aerosol was characterized during coastal nucleation events at Mace Head during intensive New Particle Formation and Fate in the Coastal Environment (PARFORCE) field campaigns in September 1998 and June 1999. Nucleation events were observed almost on a daily basis during the occurrence of low tide and solar irradiation. In September 1998, average nucleation mode particle concentrations were 8600 cm-3 during clean air events and 2200 cm-3 during polluted events. By comparison, during June 1999, mean nucleation mode concentrations were 27,000 cm-3 during clean events and 3350 cm-3 during polluted conditions. Peak concentrations often reached 500,000-1,000,000 cm-3 during the most intense events and the duration of the events ranged from 2 to 8 hours with a mean of 4.5 hours. Source rates for detectable particle sizes (d > 3 nm) were estimated to be between 104 and 106 cm-3 s-1 and initial growth rates of new particles were as high as 0.1-0.35 nm s-1 at the tidal source region. Recently formed 8 nm particles were subjected to hygroscopic growth and were found to have a growth factor of 1.0-1.1 for humidification at 90% relative humidity. The low growth factors implicate a condensable gas with very low solubility leading to detectable particle formation. It is not clear if this condensable gas also leads to homogeneous nucleation; however, measured sulphuric acid and ammonia concentration suggest that ternary nucleation of thermodynamically stable sulphate clusters is still likely to occur. In clear air, significant particle production (>105 cm-3) was observed with sulphuric acid gas-phase concentration as low as 2 × 10 6 molecules cm-3 and under polluted conditions as high as 1.2 × 108 molecules cm-3. Copyright 2002 by the American Geophysical Union.

Submicron sea spray fluxes

Eddy covariance aerosol flux measurements were conducted at the Mace Head coastal station in the North East Atlantic. Footprint and micrometeorological analysis under clean marine air mass conditions indicated that fluxes representative of open ocean conditions could be derived during high tide conditions and an oceanic fetch. Sea-spray fluxes were derived for total particle sizes larger than 10 nm and total particle sizes larger than 100 nm (i.e. covering the Aitken and Accumulation mode). The source fluxes (F) were found to be strongly correlated with both wind speed (U) and friction velocity (u*), following, by convention, an exponential relationship (Log F = a U + c) relationship. Comparison of source fluxes at sizes larger than 10 nm and larger than 100 nm demonstrates that approximately 50% of the number flux can be attributed to the accumulation mode and 50% to the Aitken mode. At 10 ms-1 wind speeds, the total primary marine aerosol flux is of the order of 2 × 106 m-2 s-1, increasing to 20 × 106 m-2 s-1 at 20 ms-1. Copyright 2005 by the American Geophysical Union.

Direct Measurements of New-Particle Fluxes in the Coastal Environment

Environmental Context. The formation of new secondary aerosol particles in the natural atmosphere is important in terms of controlling the background aerosol population, which significantly impacts on climate. The coastal zone is perhaps the strongest natural source of new secondary aerosol particles, driven by the release of biogenic vapours, which, after undergoing photochemical reactions, lead to the massive production of nucleation mode aerosols, with concentrations often reaching in excess of 10 6 cm −3 . Quantification of this source strength is important, particularly on a regional scale, in terms of estimating the impact of aerosols on climate. Abstract. Measurements of the flux of new secondary aerosol particles during nucleation events in the coastal environment using an eddy-correlation technique are reported for the first time. Events are divided into two types based on the prevailing wind direction. During tidal-related nucleation events, new-particle upward fluxes are typically of the order of 10 9 -10 10 particles m −2 s −1 . A close correlation (r 2 = 0.86) was found between total particle concentration and total (positive) flux when air masses were not affected by multiple sources. This would suggest that continuous measurements of particle number concentration at Mace Head can be translated into a flux using the resulting parameterization. It is expected that parameterizations obtained from similar data and analysis would be equally feasible at other coastal locations.

Relationships between condensation nuclei number concentration, tides, and standard meteorological variables at Mace Head, Ireland

The set of hourly averaged condensation nuclei (CN) data collected at Mace Head during 1991–1994 was examined for relationships that might exist between CN number concentrations and the more commonly measured meteorological variables, including tides. CN number concentrations at Mace Head can be characterized by typically low “background” levels (less than about 700 particles cm−3) when the wind is from the west, somewhat higher “background” levels (1000–4000 particles cm−3) when the wind is from the east, with sporadic bursts of short-lived discrete “events” of more than 10,000 cm −3 for several hours. These events occur typically during early afternoon and are normally associated with slack winds and anomalously warm, dry air. They appear to be independent of pressure, wind direction and precipitation. They can occur any time during the year, although the strongest events tend to occur during spring and autumn. Large-amplitude low tides also occur predominantly in the early afternoon during this observing period. We present evidence that large CN concentration events occur preferentially after exceptionally low tides during daylight. A neural network was employed to train the standard meteorological variables to predict CN concentrations. Baseline forecasts of CN counts for the final 180 days of the observing period were made using lagged values of all other variables. Further forecasts were made with some variables removed from the predictor set. The best correlation between the predicted values and the verifying data over the 180 days was 0.67, which was obtained from a 1-hour forecast using knowledge of all variables except temperature. Other variables whose removal improved the forecast (or whose presence degraded it) were pressure and wind speed. The best predictors of CN values were wind direction, relative humidity, and time of day. An elementary “nearest neighbor,” or “historical analogue” approach to predicting the same set of CN values generated lower correlations with the verifying data but generated a much more accurate probability distribution function.

Variations of CN number concentrations with respect to meteorological conditions at mace head, Ireland

During the Atmosphere/Ocean Experiment (AFROCE) measurements of condensation nuclei (CN) concentrations and meteorological parameters were conducted at the Mace Head field station (53“ 19’ N, 9’ 54’ W), located at the west coast of Ireland. The CN concentrations were measured with a TSI Model 3760 particle counter. The resulting time series cover the period from Duly 1991 to September 1994. All data were recorded at one minute intervals. In this contribution the dynamics of the measured CN concentrations are investigated in relation to the meteorological conditions. Emphasis is laid on variations in relative humidity favouring changes in CN numbers. Presented are cases of diurnal variations of relative humidity defining the course of CN concentrations. An increase of relative humidity under dry weather conditions can cause both an increase as well as a decrease of CN concentrations. Statistical analyses reveal a striking good correlation between the parameters in both cases, resulting in correlation coefficients r2 > 0.60 which are significant at the 95% confidence level. The correlation could be observed both under background conditions and in anthropogenic influenced air masses. A discussion of the CN dynamics includes the investigation of CN bursts in clean air masses described in a companion paper by Geever et al.. 3 dimensional backward air mass trajectories and AVHRR satellite images of cloudiness are considered for further interpretations.