We present observations of HCO+ 1–0 absorption lines toward two extragalactic compact radio sources, NRAO 150 and BL Lac with the Korean VLBI Network in order to investigate their time variation over 20 years by Galactic foreground clouds. It is found that the line shape of –17 km s−1 component changed marginally during 1993–1998 period and has remained unaltered thereafter for NRAO 150. Its behavior is different from that of H2CO 110–111, suggesting chemical differentiation on ∼ 20 AU scale, the smallest ever seen. On the other hand, BL Lac exhibits little temporal variation for the HCO+ and H2CO lines. Our observation also suggests that Korea VLBI Network performs reliably in the spectrum mode in that the shapes of the new HCO+ 1–0 spectra are in good agreement with the previous ones to an accuracy of a few percent except the time varying component toward NRAO 150.

Abstract
1. INTRODUCTION
2. OBSERVATION AND DATA REDUCTION
3. RESULTS
4. DISCUSSIONS
    4.1. Temporal Variation
    4.2. Reliability of KVN in Spectrum Mode
5. SUMMARY
REFERENCES

• Junghwan Han(School of Integrated Technology, Yonsei University)
• Youngjoo Yun(Korea Astronomy and Space Science Institute,)
• Yong-Sun Park(Department of Physics and Astronomy, Seoul National University) Corresponding author