Long-term MAX-DOAS measurements of NO2, HCHO, and aerosols and evaluation of corresponding satellite data products over Mohali in the Indo-Gangetic Plain
Authors: Kumar Vinod , Beirle Steffen , Dörner Steffen , Mishra Abhishek Kumar , Donner Sebastian , Wang Yang , Sinha Vinayak , Wagner Thomas 
Published: November 2020

Abstract. We present comprehensive long-term ground-based multi-axis differential optical absorption spectroscopy (MAX-DOAS)
measurements of aerosols, nitrogen dioxide (NO2), and formaldehyde
(HCHO) from Mohali (30.667∘ N, 76.739∘ E; ∼310 m above
mean sea level), located in the densely populated Indo-Gangetic Plain (IGP)
of India. We investigate the temporal variation in tropospheric columns,
surface volume mixing ratio (VMR), and vertical profiles of aerosols,
NO2, and HCHO and identify factors driving their ambient levels and
distributions for the period from January 2013 to June 2017. We observed
mean aerosol optical depth (AOD) at 360 nm, tropospheric NO2 vertical
column density (VCD), and tropospheric HCHO VCD for the measurement period to
be 0.63 ± 0.51, (6.7 ± 4.1) × 1015, and (12.1 ± 7.5) × 1015 molecules cm−2,
respectively. Concerning the tropospheric NO2 VCDs, Mohali was found to
be less polluted than urban and suburban locations of China and western
countries, but comparable HCHO VCDs were observed. For the more than 4 years of measurements during which the region around the measurement
location underwent significant urban development, we did not observe obvious
annual trends in AOD, NO2, and HCHO. High tropospheric NO2 VCDs
were observed in periods with enhanced biomass and biofuel combustion (e.g.
agricultural residue burning and domestic burning for heating). Highest
tropospheric HCHO VCDs were observed in agricultural residue burning periods
with favourable meteorological conditions for photochemical formation, which
in previous studies have shown an implication for high ambient ozone also
over the IGP. Highest AOD is observed in the monsoon season, indicating
possible hygroscopic growth of the aerosol particles. Most of the NO2
is located close to the surface, whereas significant HCHO is present at
higher altitudes up to 600 m during summer indicating active
photochemistry at high altitudes. The vertical distribution of aerosol,
NO2, and HCHO follows the change in boundary layer height (BLH), from the ERA5 dataset of European Centre for Medium-Range Weather Forecasts, between summer and winter.
However, deep convection during the monsoon transports the pollutants at high
altitudes similar to summer despite a shallow ERA5 BLH. Strong gradients in
the vertical profiles of HCHO are observed during the months when primary
anthropogenic sources dominate the formaldehyde production. High-resolution
MODIS AOD measurements correlate well but were systematically higher than
MAX-DOAS AODs. The ground-based MAX-DOAS measurements were used to evaluate
three NO2 data products and two HCHO data products of the Ozone
Monitoring Instrument (OMI) for the first time over India and the IGP.
NO2 VCDs from OMI correlate reasonably with MAX-DOAS VCDs but are
lower by ∼30 %–50 % due to the difference in vertical
sensitivities and the rather large OMI footprint. OMI HCHO VCDs exceed the
MAX-DOAS VCDs by up to 30 %. We show that there is significant scope for
improvement in the a priori vertical profiles of trace gases, which are used
in OMI retrievals. The difference in vertical representativeness was found
to be crucial for the observed biases in NO2 and HCHO surface VMR
intercomparisons. Using the ratio of NO2 and HCHO VCDs measured from
MAX-DOAS, we have found that the peak daytime ozone production regime is
sensitive to both NOx and VOCs in winter but strongly sensitive to
NOx in other seasons.