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. 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 abovemean 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 anddistributions for the period from January 2013 to June 2017. We observedmean aerosol optical depth (AOD) at 360 nm, tropospheric NO2 verticalcolumn density (VCD), and tropospheric HCHO VCD for the measurement period tobe 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 tobe less polluted than urban and suburban locations of China and westerncountries, but comparable HCHO VCDs were observed. For the more than 4 years of measurements during which the region around the measurementlocation underwent significant urban development, we did not observe obviousannual trends in AOD, NO2, and HCHO. High tropospheric NO2 VCDswere observed in periods with enhanced biomass and biofuel combustion (e.g.agricultural residue burning and domestic burning for heating). Highesttropospheric HCHO VCDs were observed in agricultural residue burning periodswith favourable meteorological conditions for photochemical formation, whichin previous studies have shown an implication for high ambient ozone alsoover the IGP. Highest AOD is observed in the monsoon season, indicatingpossible hygroscopic growth of the aerosol particles. Most of the NO2is located close to the surface, whereas significant HCHO is present athigher altitudes up to 600 m during summer indicating activephotochemistry 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 highaltitudes similar to summer despite a shallow ERA5 BLH. Strong gradients inthe vertical profiles of HCHO are observed during the months when primaryanthropogenic sources dominate the formaldehyde production. High-resolutionMODIS AOD measurements correlate well but were systematically higher thanMAX-DOAS AODs. The ground-based MAX-DOAS measurements were used to evaluatethree NO2 data products and two HCHO data products of the OzoneMonitoring Instrument (OMI) for the first time over India and the IGP.NO2 VCDs from OMI correlate reasonably with MAX-DOAS VCDs but arelower by ∼30 %–50 % due to the difference in verticalsensitivities and the rather large OMI footprint. OMI HCHO VCDs exceed theMAX-DOAS VCDs by up to 30 %. We show that there is significant scope forimprovement in the a priori vertical profiles of trace gases, which are usedin OMI retrievals. The difference in vertical representativeness was foundto be crucial for the observed biases in NO2 and HCHO surface VMRintercomparisons. Using the ratio of NO2 and HCHO VCDs measured fromMAX-DOAS, we have found that the peak daytime ozone production regime issensitive to both NOx and VOCs in winter but strongly sensitive toNOx in other seasons.

atmospheric chemistry and physics

Long-term MAX-DOAS measurements of NO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt;, HCHO, and aerosols and evaluation of corresponding satellite data products over Mohali in the Indo-Gangetic Plain