Long term MAX-DOAS measurements of NO<sub>2</sub>, HCHO and aerosols and evaluation of corresponding satellite data products over Mohali in the Indo-Gangetic plain

Abstract. We present comprehensive long term ground-based 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 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 molecules cm−2 and (13.2 ± 9.8) × 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. 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 on 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 meters. The vertical distribution of aerosol was found to be linked to the boundary layer height. The ground-based data set was also used for satellite validation. High-resolution MODIS AOD measurements correlate well but were systematically higher than MAX-DOAS AODs. 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 %. For surface volume mixing ratio (VMR), MAX-DOAS NO2 measurements show a good correlation but a slight overestimation compared to the in situ measurements. However, for HCHO, a larger bias was observed due to large measurement uncertainties. The difference in vertical representativeness was found to be crucial for the observed biases in NO2 and HCHO inter-comparisons. 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.