Study of the interrelationships between NO2, SO2, HCHO and CHOCHO vertical column densities derived by car MAX-DOAS observations in and around the megacity of Lahore, Pakistan

We measured the four trace species (NO₂, SO₂, HCHO, and CHOCHO) during two extensive car MAX-DOAS measurement campaigns in and around the megacity of Lahore. To our knowledge, CHOCHO was successfully measured from car-MAX-DOAS observations for the first time. The car-MAX-DOAS measurements were performed in summer 2017 and spring 2018. We retrieved the vertically integrated concentration (the so-called tropospheric vertical column density, VCD) of the trace gases along the driving routes from the measured spectra by using the so-called geometric approximation method. We present the results of the correlation analyses performed for all possible trace gas pairs. Strong correlations were found between the NO₂-SO₂ and HCHO-CHOCHO pairs.  The results indicate that NO₂ and SO₂ have common sources, most probably emissions from fossil fuel burning. Also, HCHO and CHOCHO have common sources, most probably secondary formation from common precursors like volatile organic compounds (VOCs).