Urban trees function as passive biosensors, accumulating biochemical records of atmospheric pollution exposure that instrumental monitoring cannot capture at the city scale. This dissertation presents a biochemical and gravimetric approach to assessing anthropogenic air pollution in urban trees, using Ginkgo biloba L. and Hedera helix L. as bioindicator species across study sites in Debrecen and Budapest, Hungary. Four interrelated studies address atmospheric particulate matter dynamics, seasonal pigment physiology, development of a novel bioindicator index, and spatial–seasonal pollution tolerance assessment. The first study quantified PM₁₀ concentrations at three monitoring stations in Debrecen across pre-pandemic (2018 - March 2020), pandemic (March 2020 - February 2022), and post-pandemic (after March 2022) periods. Meteorological variables - wind speed, wind direction, and boundary layer stability - frequently exerted stronger control over PM₁₀ dynamics than the emission reductions imposed by COVID-19 lockdown measures, producing heterogeneous inter-station patterns that underscore the decisive influence of local meteorology on pollutant accumulation at urban hotspots. The second study characterised seasonal dynamics of photosynthetic pigments – chlorophyll a, chlorophyll b, carotenoids, and pheophytins – in G. biloba leaves sampled monthly from July to October 2022 along an urban traffic gradient in Debrecen. Progressive chlorophyll decline correlated significantly with traffic-derived pollutants (NOₓ, NO₂, PM₁₀), confirming the suitability of G. biloba pigment profiles as biochemical indicators of cumulative exposure to urban air pollution. The third study introduced the Pigment Integrity-to-Dust Ratio (PIDR), a novel bioindicator index developed within this doctoral research. PIDR couples the chlorophyll-to-pheophytin ratio - a marker of photosynthetic membrane integrity - with gravimetrically measured leaf surface dust load, integrating internal physiological stress and external particulate exposure within a single dimensionless index. Pilot validation using G. biloba at three sites in Budapest (2023-2024) demonstrated that PIDR provided greater spatial discrimination of pollution stress than the Air Pollution Tolerance Index (APTI) and exhibited pollutant-specific response patterns consistent with established mechanisms of particulate and oxidative damage. Both component measurements are technically accessible, making PIDR practically applicable for routine urban biomonitoring. The fourth study examined spatial and seasonal variation in dust deposition, photosynthetic pigments, ascorbic acid content, relative water content, and APTI in H. helix L. growing on standardised structures at 36 bus and tram stops across Debrecen. APTI consistently classified H. helix as pollution-sensitive. Dust loads and pigment concentrations declined systematically with increasing distance from the city centre, while ascorbic acid content increased, confirming a coherent pollution gradient response. Redundancy analysis identified significant associations between leaf biochemical parameters and traffic-derived pollutants, supporting H. helix as a sensitive year-round bioindicator. Collectively, this dissertation demonstrates that biochemical and gravimetric leaf parameters provide mechanistically interpretable, cumulative records of urban pollution exposure. PIDR represents a methodological advance in plant-based biomonitoring, offering process-specific sensitivity not found in existing indices, with direct implications for urban greenbelt planning and the design of cost-effective pollution monitoring networks.