Transpacific Transport of Asian Peroxyacetyl Nitrate (PAN) Observed from Satellite: Implications for Ozone

Peroxyacetyl nitrate (PAN) is produced in the atmosphere by photochemical oxidation of non-methane volatile organic compounds in the presence of nitrogen oxides (NOx), and it can be transported over long distances at cold temperatures before decomposing thermally to release NOx in the remote troposphere. It is both a tracer and a precursor for transpacific ozone pollution transported from East Asia to North America. Here, we directly demonstrate this transport with PAN satellite observations from the infrared atmospheric sounding interferometer (IASI). We reprocess the IASI PAN retrievals by replacing the constant prior vertical profile with vertical shape factors from the GEOS-Chem model that capture the contrasting shapes observed from aircraft over South Korea (KORUS-AQ) and the North Pacific (ATom). The reprocessed IASI PAN observations show maximum transpacific transport of East Asian pollution in spring, with events over the Northeast Pacific offshore from the Western US associated in GEOS-Chem with elevated ozone in the lower free troposphere. However, these events increase surface ozone in the US by less than 1 ppbv because the East Asian pollution mainly remains offshore as it circulates the Pacific High.

Formation of peroxyacetyl nitrate (PAN) and its impact on ozone production in the coastal atmosphere of Qingdao, North China

Peroxyacetyl nitrate (PAN), acting as a relatively long-lived reservoir for both NO_x and radicals, plays a crucial role in ozone (O₃) formation in the troposphere. However, its quantitative impacts on radical concentrations and O₃ production were rarely studied in the coastal atmosphere. In this study, ambient concentrations of PAN, O₃, and related species were simultaneously measured from October 5 to November 10, 2018 (autumn), and July 14 to August 24, 2019 (summer) at a rural coastal site in Qingdao, North China. The formation mechanism of PAN and its impact on in-situ O₃ production were explored with an observation-based chemical box model. Photochemical formation of PAN and O₃ was controlled by both NO_x and VOCs, and acetaldehyde and methylglyoxal were the main contributors to PAN formation. However, the sensitivities of PAN to precursors were larger than that of O₃ in autumn while smaller in summer, which was mainly caused by the rapid decomposition of PAN at high temperatures. Zero-out sensitivity simulation showed that PAN could either promote or inhibit the in-situ O₃ formation by affecting the radical chemistry. It tended to suppress O₃ production by competing with precursors and terminating radical chain reactions under low-NO_x and low-RO_x circumstances but enhanced O₃ production by supplying RO₂ radicals under conditions with sufficient NO_x. This study provides some new complementary insights into the formation mechanism of PAN and its impacts on O₃ production, and has implications for the formulation of control policy to mitigate regional photochemical pollution in northern China.

Atmospheric peroxyacetyl nitrate (PAN): a global budget and source attribution

Peroxyacetyl nitrate (PAN) formed in the atmospheric oxidation of non-methane volatile organic compounds (NMVOCs) is the principal tropospheric reservoir for nitrogen oxide radicals (NOx = NO + NO2). PAN enables the transport and release of NOx to the remote troposphere with major implications for the global distributions of ozone and OH, the main tropospheric oxidants. Simulation of PAN is a challenge for global models because of the dependence of PAN on vertical transport as well as complex and uncertain NMVOC sources and chemistry. Here we use an improved representation of NMVOCs in a global 3-D chemical transport model (GEOS-Chem) and show that it can simulate PAN observations from aircraft campaigns worldwide. The immediate carbonyl precursors for PAN formation include acetaldehyde (44% of the global source), methylglyoxal (30 %), acetone (7 %), and a suite of other isoprene and terpene oxidation products (19 %). A diversity of NMVOC emissions is responsible for PAN formation globally including isoprene (37 %) and alkanes (14 %). Anthropogenic sources are dominant in the extratropical Northern Hemisphere outside the growing season. Open fires appear to play little role except at high northern latitudes in spring, although results are very sensitive to plume chemistry and plume rise. Lightning NOx is the dominant contributor to the observed PAN maximum in the free troposphere over the South Atlantic.