Long-term trends of carbon monoxide over the North Pacific upper troposphere from in-situ airplane measurements 1991–2018

Carbon monoxide (CO) is not a greenhouse gas, but as the dominant sink of the hydroxyl radical it governs the oxidising capacity of the troposphere and so indirectly forces climate. Its trends in the North Pacific upper troposphere matter disproportionately: this is the principal outflow corridor for Asian anthropogenic emissions, the fastest-growing combustion source of the past half-century, yet it has almost no routine ground-based monitoring and satellite retrievals resolve it poorly. We therefore combine in-situ CO measurements from the In-service Aircraft for a Global Observing System (IAGOS; 2012-2018) with earlier in-situ data from NASA/NSF/NOAA campaigns (1991-2019), the Measurements of Pollution in the Troposphere (MOPITT) satellite record, and the Integrated Modelling System (IMS) global chemistry transport model. Trends are computed separately for the monthly maximum, percentile, minimum and mean statistics, using a 10,000-sample perturbation regression framework. Mean CO increases by 0.15 ppb month-1 (18.0 ppb decade-1; 95% CI: -0.02 to 0.32) over 150∘E-140∘W, 45∘N-60∘N and by 0.06 ppb month-1 (7.2 ppb decade-1; 95% CI: -0.04 to 0.16) over 25∘N-45∘N; both intervals include zero but lie predominantly positive. The monthly maximum rises significantly over 25∘N-45∘N, by 0.31 ppb month-1 (37.2 ppb decade-1; 95% CI: 0.06 to 0.57), while the monthly minimum declines weakly. The most polluted air masses over the North Pacific are thus becoming more polluted even as background CO falls. The North Atlantic upper troposphere shows significantly decreasing CO over the same period. The opposing signs track the opposing trajectories of the upwind inventories: Asian emissions rose through most of the record and are lifted efficiently by summertime convection and warm conveyor belts, whereas North American and European emissions fell steadily and the Atlantic receives a larger share of CO-poor stratospheric air.