Identifying different source contributions of formaldehyde using long-term MAX-DOAS measurements of atmospheric trace gases

Multi-AXis (MAX)-Differential Optical Absorption Spectroscopy (DOAS) measurements use spectra of scattered sun light recorded under different elevation angles. Such measurements allow the retrievals of tropospheric vertical column densities (VCDs) and aerosol optical depths (AODs) as well as vertical profiles of atmospheric trace gases and aerosols for the lower troposphere. Further, this kind of measurement enables the simultaneous observation of multiple trace gases, e.g. formaldehyde (HCHO), glyoxal (CHOCHO) and nitrogen dioxide (NO₂), with one measurement setup. Together with international partners, we run several long-term MAX-DOAS measurements at different places around the globe and conducted intensive measurement campaigns at various locations. These campaign data sets include both stationary and mobile (car and ship MAX-DOAS) measurements. For our measurements self-built so-called Tube MAX-DOAS instruments were used which cover a wavelength range of approximately 302 to 465 nm with a FWHM of around 0.65 nm.

In the presented study, we focus on measurements of tropospheric formaldehyde which is mainly secondarily produced by reactions from precursor substances. However, in small amounts it can also be emitted directly by anthropogenic and biogenic activities. Further, HCHO plays an important role in atmospheric chemistry. As secondarily produced HCHO is an intermediate product of basic oxidation cycles of other hydrocarbons (also referred to as volatile organic compounds (VOCs)) observations of HCHO can be used as an indicator for VOCs. Since our measurements were taken at different places with different underlying meteorological and environmental conditions, our large data set allows to gain insights into the contributions from different sources and chemical processes covering various geographic and environmental conditions. Here, it is important to note that compared to satellite instruments, MAX-DOAS instruments have a much higher sensitivity to boundary layer HCHO (by a factor of 10 or more).

In this presentation we try to identify different pollution levels, source contributions and chemical regimes of formaldehyde by combining HCHO VCDs, surface values and profiles with the same properties of other trace species such as NO₂, CHOCHO and aerosols. The results will be compared for four measurement sites, namely the stations at Mainz/Germany, Bayfordbury/United Kingdom, Mohali/India and the Amazonian Tall Tower Observatory (ATTO) measurement site/Brasil.