Browsing by Subject "aerosol optics"

As global concern about climate change grows, particularly in meeting the Paris Agreement’s goal of limiting global warming below 2 °C, studying and observing the effects of light-absorbing aerosol particles in the atmosphere becomes a necessity. This study determined 38 years of equivalent black carbon (eBC) concentrations in Joensuu airport from 1965 to 2003. Samples were originally collected on either cellulose or glass fibre filters for radioactivity monitoring. Measuring black carbon optically from filters comes with its own challenges, such as artefacts caused by scattering aerosol particles and filter material itself. Thus, a calibration setup was built to study the effects of apparent absorption caused by scattering aerosols and the loading effect in various types of filters. First, in the calibration experiments, we examined how different filter materials affect measured transmittance when pure black carbon is deposited, showing non-linear calibration functions. Additionally, we investigated the impact of apparent absorption by depositing various-sized ammonium sulfate particles onto different filter materials, e.g. cellulose and glass fibre. Results showed that apparent absorption depended on the size of the scattering aerosol particles and the filter material. Smaller particles resulted in a 5–38 % overestimation of absorption, while larger particles showed 1–14 %, depending on the transmittance values. In the latter part of the thesis, real sample filters were analyzed with Particle Soot Absorption Photometer for transmittance, and with the conversion functions derived from the first part of the thesis, transmittance values were converted into eBC. For some samples, ion chromatography was used to determine ion concentrations of some scattering particles that alter optical measurements. From 1965 until 1971, cellulose filters were used for collecting aerosol particles, and glass fibre filters were used onwards. Conversion functions obtained from this study were shown to be inadequate for cellulose filters. However, for glass fibre filters, the calculated eBC concentration values were comparable to other studies. Depending on the season, cellulose filters (1965–1971) had 0.25–12 μgm⁻³ of eBC, and glass fibre filters (1972–2003) had eBC of 0.48–3.8 μgm⁻³. For those filters where ions were analyzed, ions contributed up to 30 % overestimation of eBC, making analysis of ions in samples crucial information. Statistically decreasing trends were found in both filters, where in winter had a rate of -22 ngm⁻³yr⁻¹ and -6 ngm⁻³yr⁻¹ in summer. Calculated trends was similar to what was observed in Helsinki. The decrease in eBC can be attributed to the technological advancement in emission control. This comprehensive study enabled the long-term estimation of eBC concentrations solely from transmittance measurements, and provided insights in optical measurements in filters.