Terra Trends: A Global Slowdown in Decreasing Atmospheric CO and the
Regional Interpretation Using AOD
Atmospheric carbon monoxide (CO) has been decreasing globally for the
last two decades. Recently, positive fire trends in Northern Hemisphere
boreal regions may have impacted the decreasing CO. Additionally,
time-varying air quality policies will have different impacts on
atmospheric composition and related trends. Aerosols are co-emitted with
CO from both fires and anthropogenic sources. Consequently, a combined
trend analysis of CO and aerosol optical depth (AOD) measurements from
space can help elucidate the drivers of regional differences in the CO
trend. We use valuable long-term records from two instruments aboard the
Terra satellite. Measurements of Pollution in the Troposphere (MOPITT)
CO and AOD from the Moderate Resolution Imaging Spectroradiometer
(MODIS) instrument are examined hemispherically and in sub-regions to
determine trends between 2002 and 2018. The records are further split
into two sub-periods in order to examine temporal stability in the trend
values. We also assess the CO trends in monthly percentile values to use
seasonal information when interpreting trend contributions. Our focus is
on four major population centers: Southeast USA, Europe, Northeast China
and North India, as well as biomass burning regions in both hemispheres.
Our results show that globally, CO declines faster in the first half of
the record compared to the second half. Both atmospheric species are
important when interpreting trends in the smaller regions. Northern
Hemisphere boreal fire regions show a regime-shift in their seasonality
for both CO and AOD, which may counteract the downward trend in CO.
Anthropogenic regions with minimal air quality management such as North
India become more globally relevant as the global CO trend weakens. We
also find clear evidence of the atmospheric impact of policy choices.
Overall, we observe that local changes in biomass burning and air
quality can counteract the global downward trend in CO.