Biomass burning directly impacts air quality, nutrient cycles, weather, and climate by releasing large amounts of trace gases and particulate matter into the atmosphere (

Abstract. We present an overview of an optimal estimation algorithm to retrieve peroxyacetyl nitrate (PAN) from single-field-of-view Level 1B radiances measured by the Cross-Track Infrared Sounder (CrIS). CrIS PAN retrievals show peak sensitivity in the mid-troposphere, with degrees of freedom for signal less than or equal to 1.0. We show comparisons with two sets of aircraft measurements from the Atmospheric Tomography Mission (ATom), the PAN and Trace Hydrohalocarbon ExpeRiment (PANTHER) and the Georgia Tech chemical ionization mass spectrometer (GT-CIMS). We find a systematic difference between the two aircraft datasets, with vertically averaged mid-tropospheric values from the GT-CIMS around 14 % lower than equivalent values from PANTHER. However, the two sets of aircraft measurements are strongly correlated (R2 value of 0.92) and do provide a consistent view of the large-scale variation of PAN. We demonstrate that the retrievals of PAN from CrIS show skill in measurement of these large-scale PAN distributions in the remote mid-troposphere compared to the retrieval prior. The standard deviation of individual CrIS–aircraft differences is 0.08 ppbv, which we take as an estimate of the uncertainty of the CrIS mid-tropospheric PAN for a single satellite field of view. The standard deviation of the CrIS–aircraft comparisons for averaged CrIS retrievals (median of 20 satellite coincidences with each aircraft profile) is lower at 0.05 ppbv. This would suggest that the retrieval error is reduced with averaging, although not with the square root of the number of observations. We find a negative bias of the order of 0.1 ppbv in the CrIS PAN results with respect to the aircraft measurements. This bias shows a dependence on column water vapor. We provide a water-vapor-dependent bias correction for use with the CrIS PAN data.

Acyl peroxynitrates (PANs) are thermally unstable reservoir species for nitrogen oxide radicals (NO x = NO + NO 2 ) that can be transported over long distances to affect photochemical processes far from NO x emission sources (Sin-

Capsule Summary We present perspectives on current challenges and recommendations for future directions to advance predictive ability of interactions between air pollution and terrestrial ecosystems and impacts on human and Earth systems.

Wildfires are a major source of gas‐phase ammonia (NH3) to the atmosphere. Quantifying the evolution and fate of this NH3 is important to understanding the formation of secondary aerosol in smoke and its accompanying effects on radiative balance and nitrogen deposition. Here, we use data from the Western Wildfire Experiment for Cloud Chemistry, Aerosol Absorption, and Nitrogen (WE‐CAN) to add new empirical constraints on the e‐folding loss timescale of NH3 and its relationship with particulate ammonium (pNH4) within wildfire smoke plumes in the western U.S. during summer 2018. We show that the e‐folding loss timescale of NH3 with respect to particle‐phase partitioning ranges from ∼24 to ∼4000 min (median of 55 min). Within these same plumes, oxidation of nitrogen oxides is observed concurrent with increases in the fraction of pNH4 in each plume sampled, suggesting that formation of ammonium nitrate (NH4NO3) is likely. We find wide variability in how close our in situ measurements of NH4NO3 are to those expected in a dry thermodynamic equilibrium, and find that NH4NO3 is most likely to form in fresh, dense smoke plumes injected at higher altitudes and colder temperatures. In chemically older smoke we observe correlations between both the fraction of pNH4 and the fraction of particulate nitrate (pNO3) in the aerosol with temperature, providing additional evidence of the presence of NH4NO3 and the influence of injection height on gas‐particle partitioning of NH3.