Long-term changes of tropospheric NO2 over megacities derived from multiple satellite instruments Institute of Environmental Physics, University of Bremen, P.O. Box 330 440, 28334 Bremen, Germany
18 Apr 2013
Received: 24 October 2012 – Published in Atmos. Chem. Phys. Discuss.: 11 December 2012 Abstract. Tropospheric NO2, a key pollutant in particular in cities, has been
measured from space since the mid-1990s by the GOME, SCIAMACHY, OMI, and
GOME-2 instruments. These data provide a unique global long-term dataset of
tropospheric pollution. However, the observations differ in spatial
resolution, local time of measurement, viewing geometry, and other details.
All these factors can severely impact the retrieved NO2 columns.
Revised: 11 March 2013 – Accepted: 12 March 2013 – Published: 18 April 2013
In this study, we present three ways to account for instrumental differences
in trend analyses of the NO2 columns derived from satellite
measurements, while preserving the individual instruments' spatial
resolutions. For combining measurements from GOME and SCIAMACHY into one
consistent time series, we develop a method to explicitly account for the
instruments' difference in ground pixel size (40 × 320 km2
vs. 30 × 60 km2). This is especially important when analysing
NO2 changes over small, localised sources like, e.g. megacities. The
method is based on spatial averaging of the measured earthshine spectra and
extraction of a spatial pattern of the resolution effect. Furthermore, two
empirical corrections, which summarise all instrumental differences by
including instrument-dependent offsets in a fitted trend function, are
developed. These methods are applied to data from GOME and SCIAMACHY
separately, to the combined time series, and to an extended dataset
comprising also GOME-2 and OMI measurements.
All approaches show consistent trends of tropospheric NO2 for a
selection of areas on both regional and city scales, for the first time
allowing consistent trend analysis of the full time series at high spatial
resolution. Compared to previous studies, the longer study period leads to
significantly reduced uncertainties.
We show that measured tropospheric NO2 columns have been strongly
increasing over China, the Middle East, and India, with values over east-central China tripling from 1996 to 2011. All parts of the developed
world, including Western Europe, the United States, and Japan, show
significantly decreasing NO2 amounts in the same time period. On a
megacity level, individual trends can be as large as
+27.2 ± 3.9% yr−1 and +20.7 ± 1.9% yr−1 in Dhaka
and Baghdad, respectively, while Los Angeles shows a very strong decrease of
−6.00 ± 0.72% yr−1. Most megacities in China, India, and the
Middle East show increasing NO2 columns of +5 to 10% yr−1,
leading to a doubling to tripling within the study period.
Citation: Hilboll, A., Richter, A., and Burrows, J. P.: Long-term changes of tropospheric NO2 over megacities derived from multiple satellite instruments, Atmos. Chem. Phys., 13, 4145-4169, doi:10.5194/acp-13-4145-2013, 2013.