Using ALMA observations of the 13CN and DCN lines in the massive star-forming region G33.92+0.11A, we investigate the CN/HCN abundance ratio, which serves as a tracer of photodissociation chemistry, over the whole observed region. Even considering the uncertainties in calculating the abundance ratio, we find high ratios (》1) in large parts of the source, especially in the outer regions of star-forming clumps A1, A2, and A5. Regions with high CN/HCN ratios coincide with the in infows of accreted gas suggested by Liu et al. (2015). We conclude that we found strong evidence for interaction between the dense gas clumps and the accreted ambient gas which may have sequentially triggered the star formation in these clumps.

Abstract
1. INTRODUCTION
2. DATA
3. RESULTS AND DISCUSSION
    3.1. Spatial Distribution of Emission
    3.2. Abundances
    3.3. The CN/HCN Abundance Ratio
4. SUMMARY
REFERENCES

• Young Chol Minh(Korea Astronomy and Space Science Institute) Corresponding author
• Hauyu Baobab Liu(Academia Sinica Institute of Astronomy and Astrophysics)