Long term changes in fog frequency at Swedish airports and its potential drivers

Fog can substantially impact air traffic by inhibiting or aggravating take-off or landing. This can result in large economical costs and even loss of human lives. In this study we investigate how fog frequency has changed at Swedish airports over time. The large north-south extent of Sweden with strong gradients in aerosol concentrations makes it an interesting study area for aerosol impact on fog. We base the study on visibility data from 14 airports. Most visibility data from the airports start in the 1970s but some stations have data before this and some have measurements that start in the 1980s. The visibility measurements are combined with data on air temperature, wind speed, wind direction, and air pressure from the airports. In the study we also include measurements aerosol proxies, namely SO₂ in the air from 4 stations and SO₄^2- in rainwater from 10 stations in Sweden. Moreover, emission data of SO₂ from Europe from 1970 to present day has been analysed. 
The analysis shows that the fog frequency changes in Sweden vary with location of the airport. At four airports in southern Sweden, the fog frequency show a statistically significant decrease when comparing the periods before and after 1995. The most prominent changes has occurred at the airports close to Malmö and Gothenburg. The annual fog frequency for these stations changes from 5-6 % in the 1980s to 3-4 % in the 2010s with higher changes during winter. For the airports located further north there is no decrease in fog frequency. Some airports in the northern part of Sweden show a statistically significant increase in fog frequency, though the fog frequencies are lower there than in southern Sweden. 
We find that the fog frequency changes in Southern Sweden correlate well with changes in air of SO₂, rainwater SO₄^2- concentrations, which both show strong decreases since the 1970 and 1980s in southern Sweden. The changes in these concentrations are much weaker further north in Sweden. The fog frequency changes in southern Sweden thus seems to be driven by changes in the load of hygroscopic aerosol. The fog changes in northern Sweden correlate well with temperature, which is increasing at all airports. The rising temperatures in the north could contribute with more favorable conditions for fog formation at these airports where it previously was to cold for fog formation during parts of the year. Out results indicate that the work on air pollution mitigation in Europe over the past 50 years has reduced fog impact on air traffic in southern Sweden.