First airborne in situ SO2 observations of two coal-fired power plants in Serbia and Bosnia-Herzegovina: Potential for top-down emission estimate and satellite validation&#160;
Authors: Klausner Theresa , Huntrieser Heidi , Aufmhoff Heinfried , Baumann Robert , Fiehn Alina , Gottschaldt Klaus-Dirk , Hedelt Pascal , Ilić Predrag , Jöckel Patrick , Kurilić Sanja Mrazovac , Loyola Diego , Makroum Ismail , Mertens Mariano , Podraščanin Zorica , Roiger Anke 
Published: March 2021

&lt;p&gt;Sulfur dioxide (SO&lt;sub&gt;2&lt;/sub&gt;) is known as a major air pollutant harmful to human health. Furthermore, it is a precursor gas of sulfate aerosol, which exerts a direct negative radiative forcing and thus leads to climate cooling. Anthropogenic SO&lt;sub&gt;2&lt;/sub&gt; sources are primarily associated with the combustion of sulfur-rich fossil fuels. While the operation of flue gas desulfurization devices has led to large SO&lt;sub&gt;2&lt;/sub&gt; reductions in western Europe, a hotspot of anthropogenic SO&lt;sub&gt;2&lt;/sub&gt; sources remains in the Balkan region as recently observed from space by the TROPOMI instrument on the Sentinel-5P satellite. Large coal-fired power plants with no or only incomplete SO&lt;sub&gt;2&lt;/sub&gt; removal cause these high emissions.&lt;/p&gt;&lt;p&gt;Targeting these strong emitters, the DLR Falcon 20 aircraft was equipped with an isotopically on-line calibrated Chemical Ionization Ion Trap Mass Spectrometer (CI-ITMS) to obtain detailed in situ SO&lt;sub&gt;2&lt;/sub&gt; observations during the METHANE-To-Go-Europe aircraft campaign in autumn 2020. These SO&lt;sub&gt;2&lt;/sub&gt; measurements were complemented by in situ observations of greenhouse gases (CO&lt;sub&gt;2&lt;/sub&gt;, CH&lt;sub&gt;4&lt;/sub&gt;), aerosol number concentrations, and other short-lived pollutants (CO, NO, NO&lt;sub&gt;y&lt;/sub&gt;). Two flights, on November 2&lt;sup&gt;nd&lt;/sup&gt; and 7&lt;sup&gt;th&lt;/sup&gt; 2020, focused on characterizing the pollution plumes downwind of two coal-fired power plants located in Bosnia-Herzegovina (Tuzla) and Serbia (Nikola Tesla), respectively. These power plants belong to the ten strongest SO&lt;sub&gt;2&lt;/sub&gt; emitters in Europe, and according to the World Health Organization, both countries are among the most polluted ones in Europe.&lt;/p&gt;&lt;p&gt;We present a detailed analysis of the two DLR Falcon flights with strongly enhanced SO&lt;sub&gt;2&lt;/sub&gt; mixing ratios (exceeding 50 ppb), which were observed at low flight altitude (&lt;1 km). Respective flight patterns were designed to allow for the evaluation of the TROPOMI vertical SO&lt;sub&gt;2&lt;/sub&gt; column densities, and both flights were performed during cloud-free conditions. The airborne measurements and satellite data will also be complemented by hourly ground-based SO&lt;sub&gt;2&lt;/sub&gt; measurements near both power plants. In addition, measurements are combined with state-of-the art model simulations from (i) the regional atmospheric chemistry climate model MECO(n); (ii) the atmospheric transport and dispersion model HYSPLIT; and (iii) the chemistry coupled Weather Research and Forecasting model WRF-Chem to improve the emission quantification of these power plants.&lt;/p&gt;