Urban mixing layer height and aerosol concentration in Sofia, Bulgaria

The characteristics of mixing layer height (MLH) and its role to aerosol number concentration in Sofia, Bulgaria, are analyzed in this study. Monitoring of MLH diurnal evolution over the city is performed during ten days measurement campaign in summer of 2015 by Jenoptik ceilometer CHM15k. Radiosonde thermodynamic and wind profiles are used as a reference in reliability assessment of ceilometer determined MLHs and it is shown that ceilometer underestimate MLH by about 110m. The temporal variations of aerosols number concentration are investigated and correlated to MLH and ventilation coefficient (VC). The influence of MLH and VC evolution and car traffic intensity on air quality is investigated. Two clearly observed peaks in aerosol number concentration are found. The first peak (7-10 LT) is associated with morning rush hour in combination with low MLH. The second one is at 20-21 LT in the evening about 1-2 hours after the heavy traffic as a result of decreasing MLH during evening transition. The presented results emphasize important role of MLH in determination of surface particulate matter concentration.