We conduct the first 4D‐Var inversion of NH₃ accounting for NH₃ bi‐directional flux, using CrIS satellite NH₃ observations over Europe in 2016. We find posterior NH₃ emissions peak more in springtime than prior emissions at continental to national scales, and annually they are generally smaller than the prior emissions over central Europe, but larger over most of the rest of Europe. Annual posterior anthropogenic NH₃ emissions for 25 European Union members (EU25) are 25% higher than the prior emissions and very close (<2% difference) to other inventories. Our posterior annual anthropogenic emissions for EU25, the UK, the Netherlands, and Switzerland are generally 10%–20% smaller than when treating NH₃ fluxes as uni‐directional emissions, while the monthly regional difference can be up to 34% (Switzerland in July). Compared to monthly mean in‐situ observations, our posterior NH₃ emissions from both schemes generally improve the magnitude and seasonality of simulated surface NH₃ and bulk NH_x wet deposition throughout most of Europe, whereas evaluation against hourly measurements at a background site shows the bi‐directional scheme better captures observed diurnal variability of surface NH₃. This contrast highlights the need for accurately simulating diurnal variability of NH₃ in assimilation of sun‐synchronous observations and also the potential value of future geostationary satellite observations. Overall, our top‐down ammonia emissions can help to examine the effectiveness of air pollution control policies to facilitate future air pollution management, as well as helping us understand the uncertainty in top‐down NH₃ emissions estimates associated with treatment of NH₃ surface exchange.

Fire emissions from Mexico and Central America are transported regularly to the U.S. Gulf Coast every spring under prevailing circulation patterns and affect U.S. air quality. Here we use a GEOS‐Chem passive tracer simulation to develop the climatology of transport pathways of fire emissions over a long‐term time period of April and May 2002–2015 and estimate their adverse air quality effects for urban areas along the Gulf Coast. A conceptual model is presented to describe the transport mechanisms, which involve southerly low‐level jets in the lower troposphere and warm conveyor belt in the middle troposphere. The warm conveyor belts and the southerly low‐level jets explain 31% and 69% of the interannual variability of the mid‐altitude (1.5–6 km) and low‐altitude events (0–1.5 km), respectively. Considering both transport and fire emissions, approximately 9% of the study period (59–88 days of 854 days) were identified as large pollution events during which Central American fire emissions adversely impacted surface air quality at several major urban centers along the Gulf Coast, including Houston and Corpus Christi in TX, New Orleans in LA, Mobile in AL, and Pensacola in FL. Compared to clean maritime flow from the Gulf of Mexico, these events were estimated to result in average enhancements of maximum daily average 8‐hr (MDA8) ozone and daily PM_2.5 (fine particulate matter <2.5 μm in diameter) in the Gulf Coast cities of 3–12 ppbv and 2–5 μg/m³, respectively. Only one ozone exceedance day (79 ppbv, on 18 May 2003) was found among the fire‐impact days for the Houston region.