| The rapid rising anthropogenic emissions driven by economic growth over China have been documented by satellite observations and bottom-up inventories. The large abundance of nitrogen oxides (NOx ≡ NO + NO 2) and volatile organic compounds (VOCs) over the region has led to elevated secondary photochemical pollutants, e.g. peroxy acetyl nitrates (PANs) and O3, and caused severely degraded air quality. Furthermore, the increased emissions over China might also have contributed to the recent upward trends of tropospheric O3 over the regions downwind of China. Nevertheless, recent studies suggest that the chemistry over polluted regions in China could not be fully understood by current knowledge of atmospheric chemistry and emission estimates over the region.;In order to understand the apparently unique photochemical environment over China, multi-scale modeling analyses facilitated by ground-level, aircraft and satellite observations have been conducted. Summertime photochemistry at Beijing is analyzed using a 1-D chemical transport model (REAM-1D) constrained by measurements of a full-suite of chemical tracers during CAREBeijing-2007 Experiment in August of 2007. During the experiment, PAN and HONO (especially during daytime) were observed with outstandingly high abundance. REAM-1D modeling results suggest that reactive aromatic VOCs are the major source (~75%) of PAN and methylglyoxal (MGLY) is the predominant intermediate that produces peroxy acetyl (PAN) radicals; downward transport of PAN from the upper boundary layer contributes as much as chemistry near the surface in the observed PAN concentrations. Detailed radical budget analyses reveal the very fast RO x (OH + HO2 + RO2) production, recycling and destruction driven by VOC oxidation and heterogeneous processes. Photolysis of HONO and oxygenated VOCs (OVOCs) are the major primary ROx sources, and aerosol uptake of HO2 is a large and uncertain radical loss term due to high loading of aerosols. O3 production is also very fast and is found to be in the transition regime, where reduction of NOx or VOCs could both reduce O3. Given the important role of nitrous acid (HONO) and aromatic VOCs in the chemical system, their sources and regional implications are investigated. Analysis of the observations and REAM-1D modeling consistently suggest that photoenhanced aerosol surface uptake of NO 2 is the predominant HONO source during daytime (~70%), followed by ground uptake of NO2 and the gas-phase HONO source. Results from REAM-3D modeling show that such an aerosol heterogeneous HONO source could lead to notable (up to 50%) enhancement of regional hydroxyl radical (OH) level over central and southern China. REAM-3D modeling analyses of tropospheric vertical column densities of glyoxal (CHOCHO) from SCIAMACHY show that anthropogenic emissions of aromatic VOCs are substantially underestimated (by a factor of 5 – 6, regionally varied) over China. Such an underestimation is the main cause of a large missing source of CHOCHO over the region in current global models, and could also partly explain the underestimation of organic aerosols in previous modeling studies. |
