Country-scale complementary passive and active biomonitoring of airborne trace elements for environmental risk assessment

•Passive and active biomonitoring was performed in an innovative national assessment.•Concentrations of seven air pollutant metals were measured in 410 sampling sites.•High pollution was measured for 4.7% passive and 9.8% active monitoring samples.•Element concentrations were significantly higher in the moss bag samples.•Active biomonitoring may be more suitable for large-scale studies. Despite the growing list of elements assessed and recognised as environmental and human health hazards, pollution monitoring and mitigation strategies are mandatory only for a limited number of metals, for which ambient air quality limits are defined by national and international policies. In order to overcome this deficiency, and promote cost-effective strategies and methods to assess airborne pollution, biomonitoring has been employed on several occasions using various protocols and organisms. Here we present a complex national monitoring study based on a combined approach that integrates air pollution data obtained through passive biomonitoring using the foliar content of various tree species and active biomonitoring with Hylocomium splendens moss bags. Concentrations of seven air pollutant metals, Al, Cr, Co, Cu, Fe, Mn and Zn, not regularly monitored and lacking established legal limit values, were measured in 410 passive and 142 active biomonitoring sampling sites across Romania, in two seasons, representing the largest study reported up-to-date applying complementary passive and active biomonitoring. Literature data reporting concentrations from uncontaminated areas was used to establish the concentration limits for four pollution levels (no, low, medium and high pollution), in order to develop a colour-coded warning system. High pollution levels were measured for 4.7% of passive and 9.8% of active monitoring samples. The largest proportions of samples showing high pollution levels were obtained for zinc (38.4%, for active monitoring) and chromium (18.9%, for passive monitoring). A significantly higher element content was found in moss bag samples than in leaves/needles. Though we emphasize the potential of both passive and active biomonitoring in assessing airborne pollution, our experiences and findings show that the more standardized methodology of moss-bag biomonitoring is sounder and better suited for such large-scale studies than a study conducted on multiple tree species. Some of the difficulties encountered during the two biomonitoring studies are also report