Observations of glyoxal and formaldehyde as metrics for the anthropogenic impact on rural photochemistry

Abstract

We present simultaneous fast, in-situ measurements of formaldehyde and glyoxal from two rural campaigns, BEARPEX 2009 and BEACHON-ROCS, both located in Pinus Ponderosa forests with emissions dominated by biogenic volatile organic compounds (VOCs). Despite considerable variability in the formaldehyde and glyoxal concentrations, the ratio of glyoxal to formaldehyde, R<sub>GF</sub>, displayed a very regular diurnal cycle over nearly 2 weeks of measurements. The only deviations in R<sub>GF</sub> were toward higher values and were the result of a biomass burning event during BEARPEX 2009 and very fresh anthropogenic influence during BEACHON-ROCS. Other rapid changes in glyoxal and formaldehyde concentrations have hardly any affect on R<sub>GF</sub> and could reflect transitions between low and high NO regimes. The trend of increased R<sub>GF</sub> from both anthropogenic reactive VOC mixtures and biomass burning compared to biogenic reactive VOC mixtures is robust due to the short timescales over which the observed changes in R<sub>GF</sub> occurred. Satellite retrievals, which suggest higher R<sub>GF</sub> for biogenic areas, are in contrast to our observed trends. It remains important to address this discrepancy, especially in view of the importance of satellite retrievals and in situ measurements for model comparison. In addition, we propose that R<sub>GF</sub> represents a useful metric for biogenic or anthropogenic reactive VOC mixtures and, in combination with absolute concentrations of glyoxal and formaldehyde, furthermore represents a useful metric for the extent of anthropogenic influence on overall reactive VOC processing via NO<sub>x</sub>. In particular, R<sub>GF</sub> yields information about not simply the VOCs dominating reactivity in an airmass, but the VOC processing itself that is directly coupled to ozone and secondary organic aerosol production

Modeling of HCHO and CHOCHO at a semi-rural site in southern China during the PRIDE-PRD2006 campaign

“…However, based on the in situ observations, DiGangi et al (2012) found that the anthropogenic emissions have positive impact on R GF . Our model study at the BG site indicate that the influence of anthropogenic emissions on R GF is rather complicated.…”

“…5), high R GF is usually found in early morning and a sharp decrease of R GF is identified shortly after sunrise. This DiGangi et al (2012) at a rural site in the USA. Our model shows that P CHOCHO /P HCHO of aromatic compounds (e.g., benzene, toluene, m,p-xylene) are more than 3 times larger than that of isoprene.…”

“…As illustrated by Vrekoussis et al (2010) and DiGangi et al (2012), the CHOCHO to HCHO ratio (R GF ) can be used as an indicator of anthropogenic or biogenic impact on photochemistry. When HCHO and CHOCHO are entirely photochemically formed, R GF is determined by the relative strength of production and destruction of HCHO and CHOCHO.…”

“…Ambient concentration of CHOCHO ranges from tens of ppt in remote and rural areas to ≈1 ppb in heavy polluted urban regions (e.g., Volkamer et al, 2007;Washenfelder et al, 2011;DiGangi et al, 2012). Compared to HCHO, CHOCHO has nearly no primary sources except biomass burning and biofuel combustion.…”

“…Average R GF up to 0.2-0.4 were observed by MacDonald et al (2012) in an Asian tropic forest. However, after analyzing the measured R GF and relevant trace gases at a rural site, DiGangi et al (2012) found that higher R GF corresponded to increased anthropogenic impact on local photochemistry.…”