Impact of wildfire smoke on Arctic cirrus formation, part 1: analysis of MOSAiC 2019–2020 observations

Abstract. The number of wildfire smoke layers in the upper troposphere per fire season increased at mid and high northern latitudes during the last years. To consider smoke in weather and climate models appropriately, the influence of smoke on a variety of atmospheric processes needs to be explored in detail. In this study, we focus on the potential impact of wildfire smoke on cirrus formation. For the first time, state-of-the-art aerosol and cirrus observations with lidar and radar, presented in part 1 of a series of two articles, are closely linked to comprehensive modeling of gravity-wave-induced ice nucleation in cirrus evolution processes, presented in part 2. The complex study is based on aerosol and ice cloud observations in the central Arctic during the one-year MOSAiC (Multidisciplinary drifting Observatory for the Study of Arctic Climate) expedition. For almost a year (from the summer of 2019 to the spring of 2020), aged Siberian wildfire smoke polluted the tropopause region over the central Arctic and many cirrus systems developed in the polluted upper troposphere. Goal of the data analysis (part 1) is to provide observational evidence for a dominating impact of aged wildfire smoke (organic aerosol particles) on cirrus formation in the central Arctic (over the MOSAiC research icebreaker Polarstern) during the winter half year of 2019–2020. Aim of the simulations in part 2 is to gain a deeper and more detailed insight into the potential smoke impact on ice nucleation as a function of observed aerosol and meteorological conditions (temperature, relative humidity) and by considering realistic gravity wave characteristics (updraft speed, wave amplitude). Vertical movements of air parcels are essential to initiate cloud formation. The measurements presented in part 1 were conducted during the winter half year (October to March), aboard the ice breaker Polarstern. The research vessel Polarstern drifted with the pack ice in the central Arctic mainly at latitudes >85 °N during the winter half year. The cirrus statistics show typical properties of ice clouds of the synoptic cirrus category (top-down generation of cirrus structures). The ice clouds mostly started to evolve at heights close to the tropopause. Cirrus top temperatures accumulated between −60 and −75 °C. The cirrus optical thickness (COT at 532 nm) of the ice clouds covered a wide range of values from < 0.03 (subvisible cirrus fraction, 25 % out of all cases) over 0.03–0.3 (visible thin cirrus, 40 %) to > 0.3 (opaque cirrus fraction, 35 %). In about 30 % out of all high altitude lidar observations, cirrus signatures were detected, much more than expected (10 %). This fact may be taken as a first hint that wildfire smoke was significantly involved in Arctic cirrus formation. The smoke particle surface area concentration around the tropopause was of the order of 5–15 µm² cm⁻³ and indicated considerably enhanced levels of aerosol pollution in the upper troposphere. Based on the combined lidar-radar measurements, we analyzed 20 cirrus cases in terms of profiles of the ice water content (IWC), ice crystal number concentration (ICNC), and visible extinction coefficient. IWC mostly ranged from 0.001–0.01 g/m⁻³ and ICNC (in the ice virga) accumulated in the range from 0.01 to 10 L⁻¹. Three facts (in combination) corroborate our hypothesis that aged wildfire smoke triggered ice formation in many of the observed cirrus systems: (1) The low ICNC values point to heterogeneous ice nucleation, (2) the elevated smoke pollution levels (in terms of particle surface area concentrations) were high enough to significantly influence ice nucleation in the upper troposphere, and (3) the high ice saturation ratios accumulating around 1.3–1.4 in the upper part of the observed and analyzed cirrus decks indicate quite inefficient INPs as expected in the case of organic aerosol particles (wild fire smoke particles).