Regional and Urban Column CO Trends and Anomalies as Observed by MOPITT Over 16 Years

Atmospheric carbon monoxide (CO) concentrations have decreased since the beginning of the century, and the rate of decrease depends on the region. Depending on how regions are chosen, their boundaries may not always align with where there are differences in trends. To address this, we calculate trends within 0.4 degrees x 0.4 degrees grid cells independently throughout the globe using satellite CO retrievals from the Measurements Of Pollution In The Troposphere (MOPITT) satellite instrument from 2002 to 2017. These trends are found with the caveat that there are large singular biomass burning events somewhere nearly every year, and we include examples of large column CO anomalies during sporadic but large burning events in the North American and Eurasian boreal forests, the Amazon, Africa, and Indonesia. CO trends behave similarly within regions that span about a few thousand kilometers. Using TransCom region definitions, we find average trends between -0.9 and 0.1 ppb year(-1) (about -0.9 to 0.1% year(-1)) for 2002-2017. Over 5-year subsets, trends in TransCom regions vary between -3.6 and 1.8 ppb year(-1). This substantial spatial and temporal variability in trends is in agreement with other studies. With an understanding of regional trends, we compare with trends from urban areas. Generally, CO trends within urban areas are indistinguishable from regional trends. This may be because of a combination of noise in the data, the large footprint for MOPITT, or because anthropogenic CO reduction measures were implemented before the MOPITT record began. We provide case studies for a few cities, such as Los Angeles, and find long-term variation in the rate of change of column CO.

Automated summary

Atmospheric carbon monoxide concentrations have decreased since the beginning of the century, with varying rates of decrease depending on the region. Trends in CO concentrations within regions that span about a few thousand kilometers behave similarly. On average, trends between -0.9 and 0.1 ppb year(-1) were found for 2002-2017, with variability in trends observed in different TransCom regions over 5-year subsets. Urban CO trends were generally indistinguishable from regional trends, possibly due to noise in the data or early implementation of CO reduction measures. Regional trends were compared with trends from urban areas, and case studies of cities such as Los Angeles showed long-term variations in column CO change rates.