Atmospheric Chemistry of the Suburban Forest Airborne Observations – ACROSS-AO

To quantify the evolution of gases and aerosols within the plume of Paris as it leaves the metropolitan region, and is exposed to biogenic emissions (e.g. from the Rambouillet Forest) then is transported downwind 100 kilometers or more, carefully designed flight patterns are employed allowing the on-board instruments to collect data to be used to improve scientific understanding.
The intensive measurement period consists of about 16 scientific flights of the ATR-42 aircraft from the French SAFIRE fleet, with each flight having a duration of about 3.5 hours.
The observational campaign enables: (i) quantification of a wide range of variables in Paris urban outflow, (ii) maximization of the airborne payload, (iii) capitalization of aircraft capabilities to complement surface observations with 3-dimenstional information,
The payload can be grouped into four categories:

Standard and Aircraft State. These instruments provide data on the basic state of the atmosphere (temperature, pressure, dew and frost point, relative humidity, liquid water content) and the state of the aircraft (location, altitude, ground and air speed).

Radiation. Since daytime atmospheric chemistry is driven by photochemical processes, measurements of solar radiation are important. The instruments in this category include an NO2 photolysis radiometer and three irradiance radiometers that respond in different wavelength regions.

Gas-Phase. These instruments quantify a wide range of gaseous chemical components including ozone, water vapor, CO, CO2, CH4, several reactive nitrogen compounds and a variety of VOCs. In the reactive nitrogen category, there are measurements of NO, NO2, HNO3, the sum of organic alkyl nitrates (ANs) and the sum of peroxy nitrates (PNs). Two approaches are used to quantify VOCs that include non-methane hydrocarbons, oxygenated VOCs and biogenic VOCs.

Aerosol Phase. Measurements of aerosol properties are conducted with several instruments. Sub- and super-micron aerosol size distributions are measured with three probes provided by SAFIRE and another instrument. Submicron size distributions are measured by two separate instruments. The composition of aerosols is provided by several instruments including a single particle mass spectrometer system for inorganic and organic content, an instrument for black carbon content and mixing state, and impactor and filter samples collected during flights and analysed in the laboratory using supercritical fluid extraction(SFE)-GC-MS coupled to BSTFA derivatisation. Instruments are included to measure hygroscopicity and optical properties.

Since the campaign was only completed in July 2022, final data has not yet been produced, and so detailed analysis is not possible. Initial results show that the flight patterns indeed allowed sampling of the Paris plume as it encountered nearby forested areas with biogenic volatile organic carbon emissions. Evolution of species were monitored downwind as the aircraft made multiple transects of the plume. Occasionally, it was also possible, in the same flight, to monitor biogenic emissions without significant urban influence. Sometimes these situations seemed to indicate new particle formation (as evidenced by large numbers of very small particles). The vertical profiling conducted on many flights indicates the indicates the differences in composition and properties between the boundary layer, where most of the emissions reside, and the free troposphere, which is generally fairly clean.

It is premature to assess a single outstanding feature of the data and its analysis. We do hope to reveal such aspects of the data over the next months of data reduction and analysis.

While the production of scientific papers has been limited so far, we expect this to greatly increase as the data are reduced and analyzed.

Anthropogenic activities but also the biosphere lead to the emission of many compounds into the atmosphere. They evolve there according to chemistry and nonlinear physics that leads to the formation of complex secondary constituents. The ACROSS (Atmospheric ChemistRy Of the Suburban foreSt) megaproject is an integrative, innovative and large-scale initiative aimed at improving understanding of the impacts of the mixture of urban air masses on the one hand, and biogenic, on the other hand. evolution of pollution plumes: in particular on the oxidation of organic compounds, aerosol, the formation of photo-oxidants. The ACROSS hypothesis is that this mixture of air masses leads to modifications in the production of secondary compounds whose physical properties modify the chemistry of the troposphere and the air quality. It is also based on the fact that the Paris agglomeration as an intense source of pollution inserted in a relatively sparsely urbanized region, heavily wooded and with moderate orography, constitutes a space of choice to study the impact of the forest suburban study on air pollution at regional level.

ACROSS-AO is the airborne component of ACROSS. By using the ATR-42 of the National Infrastructure of research aircraft, it projects the in-situ characterization of the composition of the Parisian plume in synergy with ground measurements (outside the project). These measurements relate to a broad spectrum of carbonaceous, nitrogenous and photo-oxidant species in the gas and particulate phases. Particular interest is paid to the chemical processes leading to secondary pollutants and to the properties of aerosols. The project bringing together five partners is organized into six work packages which respectively focus on the coordination of activities, the preparation of the campaign, the use of data to answer priority scientific questions and the exploitation of this data by a community of modellers. extending beyond this project.

The project will advance science through high-quality observations and analyses, and it will lead to the development of improved chemical mechanisms to eventually incorporate them into air quality models. They will lead to more reliable forecasts and more effective mitigation strategies. After more than twelve years without comparable campaigns, these results are possible due to recent advances in our understanding of atmospheric chemistry but also thanks to advances in instrumentation. They are made necessary by the recent insufficient performance of operational modelling of air quality in the context of climate change and changes in anthropogenic emissions.