Investigation of influences of air mass transport and chemical processes on the trace substance distribution in the lower atmosphere using chemical ionization mass spectrometry and a self-developed measurement drone
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Abstract
The distribution of trace substances in aerosol and gas phase varies with sources, transport mecha-nisms, and sinks, and influences air quality, and Earth’s climate. A deeper understanding of mete-orological influences on chemical processes is crucial to establish policies preventing hazards caused by humans and affecting them.
In this work, measurement devices and methods were developed to investigate chemical and mi-crophysical processes affecting trace substances and their distribution. A thermo desorption tech-nique for gases and aerosol particles for chemical analysis was developed as a complementary of-fline analysis method for an aerosol sampler that can collect aerosol and gas phase samples sepa-rately, differentiating depending on composition of the advected air. It was applied in-field near a wood‑fired pizza oven to investigate cooking emissions. The separated aerosol phases were ana-lyzed by chemical ionization mass spectrometry to identify characteristic cooking and bio-mass‑burning markers. The second developed device is the research measurement drone FLab (“Flying Laboratory”) to extend ground-based in-situ measurements into the vertical dimension to investigate transport of air masses in the planetary boundary layer (PBL). FLab was designed to measure the key variables relevant to tropospheric microphysical and chemical processes: wind, temperature and humidity, O3, CO2 and aerosol particle number concentration, size distribution, and black carbon concentrations. FLab was deployed in two 2.5-week long field campaigns in conti-nental Germany (BISTUM23 and BISTUM24). A case study demonstrates successful bridging of ground-based and airborne-based measurements. During BISTUM23, particle-phase samples were collected at multiple heights throughout the day, while FLab continuously profiled the vertical 500 m-range. Combining FLab data with external targeted and non‑targeted analyses of the particle phase’s chemical composition revealed oxidation and transport processes of biogenic and anthro-pogenic compounds and the general aerosol composition, respectively.
Across both campaigns, near‑hourly diurnal vertical profiling with FLab allowed detection of mixing layers in the lower troposphere. During nighttime, on average pollutant markers were more sensi-tive mixing layer height (MLH) tracers than meteorological variables; notably, specifically O3 and the potential temperature were overall the most effective markers. The dataset from 383 flights to 500 m above ground was used to evaluate how representative ground-based measurements are for the whole mixing layer. Meteorological variables are most reliably estimated with ground-based stations, while the distribution of pollutant markers was more affected by irradiance or local air mass advection, requiring vertical measurement to better characterize pollutant distribution and estimate representative pollutant concentrations. The influence of two synoptic frontal events on the local pollutant distribution in the PBL was analyzed by combining model analysis, drone-borne and ground-based measurements. Frontal events strongly influence air mass dynamics and the MLH which turns out to simultaneously influence local chemical processes due to oxidant depletion and entrainment in different altitudes. Synoptic air mass transport also altered the chemical com-position of air at the BISTUM24 measurement site as “marine-anthropogenic” influenced air led to significantly enhanced amounts of chlorine-containing species. Here, first-time reported tropo-spheric diel cycle of ClONO2 indicate high amounts of nighttime ClO radicals. Enhanced concentra-tions of ClNO2 and Cl2 after dawn suggest vertical mixing in the PBL, while high HOCl levels show significant contribution to primary OH radical production, especially in the morning.